Scholarly article on topic 'The metabolic evaluation of the child with an intellectual developmental disorder: Diagnostic algorithm for identification of treatable causes and new digital resource'

The metabolic evaluation of the child with an intellectual developmental disorder: Diagnostic algorithm for identification of treatable causes and new digital resource Academic research paper on "Clinical medicine"

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{"Inborn errors of metabolism" / Algorithm / Diagnosis / Treatable / App / Cognition}

Abstract of research paper on Clinical medicine, author of scientific article — Clara D.M. van Karnebeek, Michael Shevell, Johannes Zschocke, John B. Moeschler, Sylvia Stockler

Abstract Intellectual developmental disorders (IDD), characterized by significant impairment of cognitive functions, with limitations of learning, adaptive behavior and skills, are frequent (2.5% of the population affected) and present with significant co-morbidity. The burden of IDD, in terms of emotional suffering and associated health care costs, is significant; prevention and treatment therefore are important. A systematic literature review, updated in 2013, identified 89 inborn errors of metabolism (IEMs), which present with IDD as prominent feature and are amenable to causal therapy. Therapeutic effects include improvement and/or stabilization of psychomotor/cognitive development, behavior/psychiatric disturbances, seizures, neurologic and systemic manifestations. The levels of available evidence for the various treatments range from Level 1b, c (n=5); Level 2a, b, c (n=14); Level 4 (n=53), and Levels 4–5 (n=27). For a target audience comprising clinical and biochemical geneticists, child neurologists and developmental pediatricians, five experts translated....this data into a 2-tiered diagnostic algorithm: The first tier comprises metabolic “screening” tests in urine and blood, which are relatively accessible, affordable, less invasive, and have the potential to identify 60% of all treatable IEMs. The second tier investigations for the remaining disorders are ordered based on individual clinical signs and symptoms. This algorithm is supported by an App www.treatable-id.org, which comprises up-to-date information on all 89 IEMs, relevant diagnostic tests, therapies and a search function based on signs and symptoms. These recommendations support the clinician in early identification of treatable IEMs in the child with IDD, allowing for timely initiation of therapy with the potential to improve neurodevelopmental outcomes. The need for future studies to determine yield and usefulness of these recommendations, with subsequent updates and improvements to developments in the field, is outlined.

Academic research paper on topic "The metabolic evaluation of the child with an intellectual developmental disorder: Diagnostic algorithm for identification of treatable causes and new digital resource"

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The metabolic evaluation of the child with an intellectual developmental disorder: Diagnostic algorithm for identification of treatable causes and new digital resource

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Clara D.M. van Karnebeek a,b'*, Michael Shevellc,d, Johannes Zschocke e, John B. Moeschlerf, Sylvia Stockler

a Division of Biochemical Diseases, Department of Pediatrics, BC Children's Hospital, Child & Family Research Institute, University of British Columbia, Vancouver, Canada b Treatable Intellectual Disability Endeavor in British Columbia (TIDE-BC (TIDE-BC, www.tidebc.org)), Canada c Department ofPediatrics, Montreal Children's Hospital, McGill University, Montreal, Canada d Department ofNeurology/Neurosurgery, Montreal Children's Hospital, McGill University, Montreal, Canada e Division of Human Genetics, Medical University of Innsbruck, Innsbruck, Austria f Geisel School of Medicine at Dartmouth College, Lebanon, NH, USA

ARTICLE INFO

Article history:

Received 27 November 2013 Received in revised form 19 January 2014 Accepted 19 January 2014 Available online 24 January 2014

Keywords:

Inborn errors of metabolism

Algorithm

Diagnosis

Treatable

Cognition

ABSTRACT

Intellectual developmental disorders (IDD), characterized by significant impairment of cognitive functions, with limitations of learning, adaptive behavior and skills, are frequent (2.5% of the population affected) and present with significant co-morbidity. The burden of IDD, in terms of emotional suffering and associated health care costs, is significant; prevention and treatment therefore are important. A systematic literature review, updated in 2013, identified 89 inborn errors of metabolism (IEMs), which present with IDD as prominent feature and are amenable to causal therapy. Therapeutic effects include improvement and/or stabilization of psychomotor/ cognitive development, behavior/psychiatric disturbances, seizures, neurologic and systemic manifestations. The levels of available evidence for the various treatments range from Level 1b, c (n = 5); Level 2a, b, c (n = 14); Level 4 (n = 53), and Levels 4-5 (n = 27). For a target audience comprising clinical and biochemical geneticists, child neurologists and developmental pediatricians, five experts translated....this data into a 2-tiered diagnostic algorithm: The first tier comprises metabolic "screening" tests in urine and blood, which are relatively accessible, affordable, less invasive, and have the potential to identify 60% of all treatable IEMs. The second tier investigations for the remaining disorders are ordered based on individual clinical signs and symptoms. This algorithm is supported by an App www.treatable-id.org, which comprises up-to-date information on all 89 IEMs, relevant diagnostic tests, therapies and a search function based on signs and symptoms. These recommendations support the clinician in early identification of treatable IEMs in the child with IDD, allowing for timely initiation of therapy with the potential to improve neurodevelopmental outcomes. The need for future studies to determine yield and usefulness of these recommendations, with subsequent updates and improvements to developments in the field, is outlined.

© 2014 The Authors. Published by Elsevier Inc. All rights reserved. This is an open access article under the CC BY-

NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

Contents

1. Introduction............................................................................................................................429

1.1. Intellectual developmental disorders (IDD)..........................................................................................429

1.2. IDD etiology and diagnostic approach..............................................................................................429

1.3. Treatable inborn errors of metabolism and IDD......................................................................................429

2. Systematic literature review for treatable inborn errors of metabolism........................................................................430

2.1. Treatable IEMs in the year 2011 and updated in the year 2013 ........................................................................430

Abbreviations: CMA, chromosome micro-array; CSF, cerebrospinal fluid; EEG, electro-encephalogram; ICD, international classification of diseases; IDD, intellectual developmental disorder; IEM, inborn error of metabolism; MRI, magnetic resonance imaging; OTC, ornithine transcarbamylase deficiency.

☆ This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited.

* Corresponding author at: Division of Biochemical Diseases, Rm K3-201, Department of Pediatrics, BC Children's Hospital, Child and Family Research Institute, Centre for Molecular Medicine and Therapeutics, University of British Columbia, 4480 Oak Street, Vancouver, BC V6H 3V4, Canada. Fax: +1 604 875 2349. E-mail address: cvankarnebeek@cw.bc.ca (C.D.M. van Karnebeek).

http://dx.doi.org/10.10167j.ymgme.2014.01.011

1096-7192/$ - see front matter © 2014 The Authors. Published by Elsevier Inc. All rights reserved. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

2.2. Clinical features..................................................................................................................430

2.3. Diagnostic testing................................................................................................................430

2.4. Causal treatments................................................................................................................431

3. Diagnostic recommendation for the identification of treatable causes of IDD..................................................................432

3.1. The algorithm....................................................................................................................432

3.2. First tier investigations............................................................................................................432

3.3. Practical considerations..........................................................................................................433

3.4. Second tier investigations........................................................................................................433

4. Resources..............................................................................................................................433

4.1. New online resource..............................................................................................................433

4.2. Other resources..................................................................................................................434

5. Using this recommendation for metabolic diagnostic evaluation in practice..................................................................434

5.1. General considerations............................................................................................................434

5.2. Yield............................................................................................................................434

5.3. Newborn screening..............................................................................................................436

5.4. Treatable IEMs as a cause of unspecific IDD ........................................................................................436

5.5. 'Late-onset' or atypical variants of conditions........................................................................................436

5.6. Primary gene analysis............................................................................................................436

6. Costs..................................................................................................................................436

7. Metabolic testing for "non-treatable" inborn errors of metabolism..........................................................................437

8. Limitations and future directions ........................................................................................................437

Funding....................................................................................................................................437

Conflict of interest............................................................................................................................437

Acknowledgments ..........................................................................................................................437

References..................................................................................................................................437

1. Introduction

1.1. intellectual developmental disorders (IDD)

In 2011, the World Health Organization International Classification of Diseases (ICD) Working Group on the Classification of Intellectual Disabilities proposed the term intellectual developmental disorders (IDD) to encompass a group of developmental conditions characterized by significant impairment of cognitive functions associated with limitations of learning, adaptive behavior and life skills [1]. IDD comprises both 'Intellectual Disability' (defined as an IQof <70, at age 5 years or older) [2,3] and 'Global Developmental Delay' (term used at age < 5 years, defined as deficits in 2 or more developmental domains, e.g., fine/gross motor skills, speech and interaction) [4]; IDD is further defined as existing over the course of an individual's life span, requiring consideration of ongoing developmental stages and life transitions. Additionally, it is frequently associated with behavioral problems (autistic features, hyperactivity, aggressive and self-injurious behaviors), as well as neurological symptoms such as epilepsy [5,6]. In the present article we apply the term IDD to both intellectual disability and global developmental delay.

1.2. IDD etiology and diagnostic approach

Affecting 2-3% of children and adults worldwide, IDD is common and associated with the highest life-time health care and economic costs of any disease—nearly equaling the economic impact of stroke, heart disease and cancer combined [7]. The etiology of IDD is diverse and has been conceptualized by the ICD Working Group as a 'meta-syndromic' health condition with infectious, traumatic and toxic origins. However, genetic etiologies represent the most frequent cause of IDD [8,9], and range from numeric and structural chromosomal abnormalities and submicroscopic rearrangements, to methylation abnormalities and single gene defects [10,11].

Presently, recommendations aimed at structuring the evaluation of genetic causes of IDD are based on the frequencies of single conditions and yield of diagnostic methods and procedures [12]. Consequently, karyotyping and array-comparative genomic hybridization are standard practice as part of the first-line investigation and yield a causal diagnosis in up to 20% of cases [13,14]. Such diagnoses provide

opportunities for better genetic counseling for the family, modified management strategies and targeted screening for medical complications (e.g. congenital heart disease and tumors) with significant impact on quality and quantity of life in the affected child [15,16]. However, for most of the conditions identified by these investigations, medical intervention targeting the underlying defect and/or pathogenesis is not currently available. Therefore, treatment is symptomatic rather than causal and, while essential, in most cases the therapeutic benefits are limited.

1.3. Treatable inborn errors of metabolism and IDD

Inborn errors of metabolism (IEMs) are uniquely amenable to beneficial causal treatment, defined as a medical intervention targeting the underlying defect and/or pathogenesis. Treatments include dietary restriction/supplement, co-factor/-enzyme, vitamin, substrate inhibition, (small molecule) substrate reduction, enzyme replacement, bone marrow and hematopoietic stem cell transplant, and gene therapy (see Table 1 for definitions). Several reviews have been published regarding the metabolic causes of IDD, most of which are based on individual expertise in the field of IEMs [17-19]. Further, while technologies for better recognition have been introduced into clinical practice, these have yet to be incorporated into diagnostic practice recommendations or parameters for the evaluation of children with IDD, such as those of the American College of Medical Genetics (1997) [20], the American Academy of Pediatrics (2006) [21], and the American Academy of Neurology (2011) [22].

The target audience of this article includes developmental pediatricians, biochemical and clinical geneticists and neurologists, i.e. all specialists who are faced with the important and often difficult challenge of timely, accurate and expeditious diagnoses of treatable IEMs associated with IDD in children and adolescents. What is especially important in this challenge is that the identification of these conditions is the necessary precondition to the implementation of specific therapeutic interventions known to improve outcomes. The recommendations outlined herein represent an international collaborative effort that integrates available evidence and expert opinion into a two-tiered algorithm supported by a digital application. The recommendations are designed to aid the pediatric specialist in the evaluation of children with IDD. Clinical skills and differential diagnosis

Table 1

Definitions used in systematic literature review and current recommendation.

Global developmental delay (DD): applied to age < 5 years; significant delay (defined as performance 2 standard deviations or more below the mean on age-appropriate, standardized norm-referenced testing) in 2 or more of developmental domains including gross/fine motor skills, speech/language, cognition, social/personal, and activities of daily living [2].

Intellectual developmental disorders (IDD) [1]: a group of developmental conditions characterized by significant impairment of cognitive functions, which are associated with limitations of learning, adaptive behavior and skills.

Main descriptors

• IDD is characterized by a marked impairment of core cognitive functions necessary for the development of knowledge, reasoning, and symbolic representation of the level expected of one's age peers, and cultural and community environment. Nevertheless, very different patterns of cognitive impairments appear for particular conditions of IDD.

• In general, persons with IDD have difficulties with verbal comprehension, perceptual reasoning, working memory and processing speed.

• The cognitive impairment in persons with IDD is associated with difficulties in different domains of learning, including academic and practical knowledge.

• Persons with IDD typically manifest difficulties in adaptive behavior; that is, meeting the demands of daily life expected for one's age peers, cultural, and community environment. These difficulties include limitations in relevant conceptual, social, and practical skills.

• Persons with IDD often have difficulties in managing their behavior, emotions, and interpersonal relationships, and maintaining motivation in the learning process.

• IDD is a life span condition requiring consideration of developmental stages and life transitions.

Intellectual disability (ID): applied to age >5 years and manifesting before age 18 years, historically referred to as 'mental retardation'; intellectual functioning level (IQ) less than 70- 75 and significant limitations in two or more adaptive skills [1,5].

Inborn error of metabolism (IEM): genetic disease involving a disorder of metabolism with confirmation based on the internationally accepted diagnostic test(s) for that IEM (gene mutations, enzyme deficiency, or specific biochemical marker). This term excludes endocrine disorders such as hypothyroidism and hyperinsulinism.

Causal of IDD: sufficient evidence in literature from bench and/or clinical research to make a pathophysiological relationship between IEMs and ID/DD highly likely.

Treatable IEMs: if a particular therapeutic modality based on pathogenesis is capable of preventing or improving the IDD phenotype, or halting/slowing neurocognitive decline (with acceptable adverse effects) in the IEM, i.e. positively influencing the 'outcome measures'.

Therapeutic modalities: dietary restriction/supplement, co-factor/-enzyme, vitamin, substrate inhibition, (small molecule) substrate reduction, enzyme replacement, bone marrow and hematopoietic stem cell transplant, gene therapy.

Outcome measure/effect: primary = IQ developmental testing score/performance, survival; secondary = epilepsy, behavior, psychiatric, neurological deficit (e.g. movement disorder), neuro-imaging, systemic symptoms influencing developmental/cognitive performance (e.g. ichthyosis, liver disease).

Levels of evidence: Level 1a = Systematic Review of RCTs, 1b = Individual RCT, 1c = 'All or None' [=(prolongation of) survival with therapy]; Level 2a = Systematic Review of Cohort Studies, 2b = Individual Cohort Study, 2c = 'Outcomes Research' [focussed on end results of therapy for chronic conditions, including functioning and quality of life (http://www.ahrq.gov/clinic.outfact.htm)]; Level 3 = Systematic Review of Case-Control Studies; Level 4 = Individual Case-Control Study or Case-series/report; Level 5 = Expert opinion without critical appraisal; based on physiology, bench research or first principles.

Standard of care: A formal treatment process a physician will follow for a patient with a specific illness, which experts generally accept as 'best clinical practice'.

Individual patient basis: Decision to start specific treatment depends on patient characteristics (i.e. disease stage), physician's opinion, availability of treatment, and potential side-effects.

based on the specific patient presentation remain paramount in this process, especially as the evaluation of usefulness and yield of the algorithm are pending.

2. Systematic literature review for treatable inborn errors of metabolism

The authors (CvK, SS) performed a literature review, following the Cochrane Collaboration methodology (http://www.cochrane.org/ training/cochrane-handbook) as closely as possible, to identify all IEMs for which a particular therapeutic modality exists and can prevent or improve the IDD phenotype, or halt/slow neurocognitive decline (with acceptable side-effects), i.e. positively influence 'outcome

measures' [23]. Subsequently, the clinical and diagnostic recognition patterns were characterized, as were treatment modalities pertinent to the identified IEMs. An attempt was made to assess the level of available evidence and effect of the various treatments on clinical outcome measures. As this literature review forms the basis of the current recommendations, we provide a summary of the relevant results.

2.1. Treatable IEMs in the year 2011 and updated in the year 2013

Applying the above criteria in 2011, we identified 81 treatable IEMs causing ID, which are listed in Table 2a including MIM number, biochemical deficiency and corresponding gene(s). Since then, 8 novel treatable IEMs have been reported which meet the criteria, and thus were added to the list: dihydrofolate reductase deficiency (oral folinic acid supplements) [24], VMAT2 deficiency (dopamine agonists) [25], SC4MOL deficiency (oral cholesterol supplements) [26], Brown-Vialetto-Van Laere/Fazio Londe (BVVL/FL) syndromes (oral riboflavin) [27], Lesch-Nyhan syndrome (hematopoietic stem cell transplantation) [28], carbonic anhydrase VA deficiency (Carglumic acid, sick day formula) [29], HMPDC (hypermanganesemia with dystonia, polycythemia, and cirrhosis) syndrome (chelation therapy) [30], and MEDNIK (mental retardation, enteropathy, deafness, neuropathy, ichthyosis, keratodermia) syndrome (zinc acetate) [31].

These 89 disorders include disorders of amino acids (n = 12); cholesterol and bile acids (n = 3); creatine (n = 3); fatty aldehydes (n = 1); glucose homeostasis and transport (n = 2); hyperhomocys-teinemia (n = 7); lysosomes (n = 12); metals (n = 5); mitochondria (n = 2); neurotransmission (n = 8); organic acids (n = 19); peroxisomes (n = 1); purines and pyrimidines (n = 3); urea cycle (n = 8); and vitamins/co-factors (n = 10). Although amenable to treatment, fatty acid oxidation disorders are not included in our list because of their clinical presentation, which is a metabolic crisis with hypoglycemia (and subsequent neurologic sequelae) rather than unexplained IDD. These conditions will not be missed by this protocol however, as the acylcarnitine profile is included as first tier test.

2.2. Clinical features

Nearly all of these conditions are associated with additional neurological and/or non-neurological features. Neurologic features include ataxia, behavioral disturbance, dementia, dystonia, encephalopathic crisis, epilepsy, hearing loss, hypotonia/myopathy, neuro-imaging abnormalities (basal ganglia, cerebellum, cerebrum, cysts/dysgenesis, white matter, mixed), neuropathy, ocular movement abnormality, psychiatric disturbance, sensorineural hearing loss, spasticity, stroke, and vision loss. All IEMs except one (Tyrosinemia type II) are associated with at least one additional prominent neurologic feature, of which the most frequent are epilepsy and various types and degrees of movement disorders (e.g. spasticity, dyskinesia and ataxia). However, many of these conditions can present with IDD as sole feature for a considerable time prior to manifestation of the full phenotype (e.g. disorders of creatine synthesis and transport).

The non-neurologic features affect the following anatomic/organ systems: bones and joints; dermatology; endocrinology; eye; facial dysmorphism; growth and stature; heart; gastrointestinal; hematology; immunology; kidney; liver; and odor. For 57 of the 89 ( 64%) treatable IEMs, a non-neurologic feature is a prominent part of the phenotype.

2.3. Diagnostic testing

54 of the 89 disorders (60%) are identified by metabolic screening tests in blood (plasma amino acids, homocysteine, copper, ceruloplasmin) and urine (creatine metabolites, glycosaminoglycans, oligosaccharides, organic acids, pyrimidines). For the remaining 35 disorders (40%)

Table 2a

Overview of the first tier metabolic screening tests denoting all diseases (with OMIM# and gene(s)) potentially identified per individual test.

Diagnostic test Disease OMlM# Gene

Blood tests

Plasma amino acids l.o. Argininemia 207800 ARG1 (AR)

Plasma amino acids l.o. Argininosuccinic aciduria 207900 ASL (AR)

Plasma amino acids l.o. Citrullinemia 215700 ASS1 (AR)

Plasma amino acids Citrullinemia type II 605814 SLC25A13 (AR)

Plasma amino acids l.o. CPS deficiency 237300 CPS1 (AR)

Plasma amino acids HHH syndrome (hyperornithinemia, hyperammonemia, homocitrullinuria) 238970 SLC25A15 (AR)

Plasma amino acids Maple syrup urine disease (variant) 248600 BCKDHA/BCKDHB/DBT (AR)

Plasma amino acids l.o. NAGS deficiency 237310 NAGS (AR)

Plasma amino acids (& UOA incl orotic acid) l.o. OTC deficiency 311250 OTC (X-linked)

Plasma amino acids Phenylketonuria 261600 PAH (AR)

Plasma amino acids (& UOA) Tyrosinemia type II 276600 TAT (AR)

Plasma amino acids (tHcy) l.o. MTHFR deficiency 236250 MTHFR (AR)

Plasma total homocysteine Cobalamin E deficiency 236270 MTRR (AR)

Plasma total homocysteine Cobalamin G deficiency 250940 MTR (AR)

Plasma total homocysteine (& UOA) Cobalamin F deficiency 277380 LMBRD1 (AR)

Plasma total homocysteine (& OUA) Cobalamin C deficiency 277400 MMACHC (AR)

Plasma total homocysteine (& OUA) Homocystinuria 236200 CBS (AR)

Plasma total homocysteine (& PAA) l.o. MTHFR deficiency 236250 MTHFR (AR)

Plasma total homocysteine (& UOA) Cobalamin D deficiency 277410 MMADHC (AR)

Serum ceruloplasmin & copper (& serum iron & ferritin) Aceruloplasminemia 604290 CP (AR)

Serum copper & ceruloplasmin (& urine copper) MEDNIK diseases 609313 AP1S1 (AR)

Serum copper & ceruloplasmin (urine deoxypyridonoline) Menkes disease/occipital horn syndrome 304150 ATP7A (AR)

Serum copper & ceruloplasmin (& urine copper) Wilson disease 277900 ATP7B (ar)

Urine tests

Urine creatine metabolites AGAT deficiency 612718 GATM (AR)

Urine creatine metabolites Creatine transporter defect 300352 SLC6A8 (X-linked)

Urine creatine metabolites GAMT deficiency 612736 GAMT (AR)

Urine glycosaminoglycans Hunter syndrome (MPS II) 309900 IDS (X-linked)

Urine glycosaminoglycans Hurler syndrome (MPS I) 607014 IDUA (AR)

Urine glycosaminoglycans Sanfilippo syndrome A (MPS IIIa) 252900 SGSH (AR)

Urine glycosaminoglycans Sanfilippo syndrome B (MPS 111b) 252920 NAGLU (AR)

Urine glycosaminoglycans Sanfilippo syndrome C (MPS IIIc) 252930 HGSNAT (AR)

Urine glycosaminoglycans Sanfilippo syndrome D (MPS 111d) 252940 GNS (AR)

Urine glycosaminoglycans Sly syndrome (MPS VII) 253220 GUSB (AR)

Urine oligosaccharides a-Mannosidosis 248500 MAN2B1 (AR)

Urine oligosaccharides Aspartylglucosaminuria 208400 AGA (AR)

Urine organic acids 3-Ketothiolase deficiency 203750 ACAT1 (AR)

Urine organic acids Cobalamin A deficiency 251100 MMAA (AR)

Urine organic acids Cobalamin B deficiency 251110 MMAB (ar)

Urine organic acids l.o. Glutaric acidemia 1 231670 GCDH (AR)

Urine organic acids Glutaric acidemia 11 231680 ETFA, ETFB, ETFDH (AR)

Urine organic acids HMG-CoA lyase deficiency 246450 HMGCL (AR)

Urine organic acids Holocarboxylase synthetase deficiency 253270 HLCS (AR)

Urine organic acids 3-Methylglutaconic aciduria type 1 250950 AUH (AR)

Urine organic acids MHBD deficiency 300438 HSD17B10 (X-linked recessiv

Urine organic acids mHMG-CoA synthase deficiency 605911 HMGCS2 (AR)

Urine organic acids SCOT deficiency 245050 OXCT1 (AR)

Urine organic acids SSADH deficiency 271980 ALDH5A1 (AR)

Urine organic acids (& ACP) Ethylmalonic encephalopathy 602473 ETHE1 (AR)

Urine organic acids (& ACP) l.o. 1sovaleric acidemia 243500 IVD (AR)

Urine organic acids (& ACP) 3-Methylcrotonylglycinuria 210200 MCC1/MCC2 (AR)

Urine organic acids (& ACP) l.o. Methylmalonic acidemia 251000 MUT (AR)

Urine organic acids (& tHcy) Cobalamin C deficiency 277400 MMACHC (AR)

Urine organic acids (& tHcy) Cobalamin D deficiency 277410 MMADHC (AR)

Urine organic acids (& tHcy) Homocystinuria 236200 CBS(AR)

Urine organic acids incl orotic acid (& PAA) l.o. OTC deficiency 311250 OTC (X-linked)

Urine organic acids (& PAA) Tyrosinemia type 11 276600 TAT (AR)

Urine organic acids (& ACP) l.o. Propionic acidemia 606054 PCCA/PCCB (AR)

Urine organic acids (tHcy) Cobalamin F deficiency 277380 LMBRD1 (AR)

Urine purines & pyrimidines Lesch-Nyhan syndrome 300322 HPRT (AR)

Urine purines & pyrimidines Molybdenum cofactor deficiency type A 252150 MOCS1, MOCS2, (AR)

Urine purines & pyrimidines Pyrimidine 5-nucleotidase superactivity 606224 NT5C3 (AR)

specific tests are required including primary molecular analysis (Tables 2a and 2b).

2.4. Causal treatments

The therapeutic modalities available for these lEMs include: "sick-day" management; diet; co-factor/vitamin supplements; substrate inhibition; stem cell transplant; and gene therapy. Therapeutic effects

include improvement and/or stabilization of psychomotor/cognitive development; behavior/psychiatric disturbances; seizures; and neurologic and systemic manifestations. The levels of available evidence for the effect of various treatments vary from Level 1b, c (n = 5); Level 2a, b, c (n = 14); Level 4 (n = 53), to Levels 4-5 (n = 27) [32]. In clinical practice more than 60% of treatments with evidence Levels 4-5 are internationally accepted as 'standard of care'. This situation, with limited evidence levels guiding clinical practice, is inherent to rare diseases

Table 2b

Overview of all diseases (in alphabetical order) requiring second tier biochemical testing, i.e. a specific test per disease approach; for each disease the OMIM# and gene(s) are listed.

Disease OMIM# Gene(s) Diagnostic test

(X-linked) Adrenoleukodystrophy 300100 ABCD1 (X-linked) Plasma very long chain fatty acids

Biotin responsive basal ganglia disease 607483 SLC19A3 (AR) Gene analysis

Biotinidase deficiency 253260 BTD (AR) Biotinidase enzyme activity

Cerebral folate receptor-a deficiency 613068 FOLR1 (AR) CSF 5'-methyltetrahydrofolate

Cerebrotendinous xanthomatosis 213700 CYP27A1 (AR) Plasma cholestanol

Co-enzyme Q10 deficiency 607426 COQ2, APTX, PDSS1, PDSS2, Co-enzyme Q (fibroblasts) & gene analysis

CABC1, COQ9 (most AR)

Congenital intrinsic factor deficiency 261000 GIF (AR) Plasma vitamin B12 & folate

Dihydrofolate reductase deficiency 613893 DHFR (AR) CSF 5'-methyltetrahydrofolate

DHPR deficiency (biopterin deficiency) 261630 QDPR (AR) CSF neurotransmitters & biopterin loading test

Gaucher disease type III 231000 GBA (AR) Glucocerebrosidase enzyme activity (lymphocytes)

GLUT1 deficiency syndrome 606777 SLC2A1 (AR) CSF: plasma glucose ratio

GTPCH1 deficiency 233910 GCH1 (AR) CSF neurotransmitters & biopterin loading test

Hypermanganesemia with dystonia, polycythemia, 613280 SLC30A10 Whole blood manganese

and cirrhosis (HMDPC)

Hyperinsulinism hyperammonemia syndrome 606762 GLUD1 (AR) Gene analysis (& ammonia, glucose, insulin)

Imerslund Grasbeck syndrome 261100 CUBN & AMN (AR) Plasma vitamin B12 & folate

MELAS 540000 MTTL1, MTTQ, MTTH, MTTK, Mitochondrial DNA mutation testing

MTTC, MTTS1, MTND1, MTND5,

MTND6, MTTS2 (Mt)

l.o. Metachromatic leukodystrophy 250100 ARSA (AR) Arylsulfatase-a enzyme activity

Niemann-Pick disease type C 257220 NPC1 NPC2 (AR) Filipin staining test (fibroblasts) & gene analyses

l.o. Non-ketotic hyperglycinemia 605899 AMT/GLDC/GCSH (AR) CSF amino acids (& PAA)

PCBD deficiency (biopterin deficiency) 264070 PCBD1 (AR) CSF neurotransmitters & biopterin loading test

PDH complex deficiency OMIM# according to each PDHA1 (X-linked), DLAT (AR), Serum & CSF lactate:pyruvate ratio enzyme activity,

enzyme subunit deficiency: PDHX (AR) gene analysis

312170; 245348; 245349

PHGDH deficiency (serine deficiency) 601815 PHGDH (AR) CSF amino acids (& PAA)

PSAT deficiency ( serine deficiency) 610992 PSAT1 (AR) CSF amino acids (& PAA)

PSPH deficiency (serine deficiency) 614023 PSPH (AR) CSF amino acids (& PAA)

PTS deficiency ( biopterin deficiency) 261640 PTS (AR) CSF neurotransmitters & biopterin loading test

Pyridoxine dependent epilepsy 266100 ALDH7A1 (AR) Urine a-aminoadipic semialdehyde & plasma pipecolic acid

Sjogren Larsson syndrome 270200 ALDH3A2 (AR) Fatty aldehyde dehydrogenase enzyme activity

Smith Lemli Opitz syndrome 270400 DHCR7 (AR) Plasma 7-dehydrocholesterol:cholesterol ratio

SPR deficiency ( biopterin deficiency) 612716 SPR (AR) CSF neurotransmitters, biopterin & Phe loading test

(enzyme activity, gene analysis)

Thiamine responsive encephalopathy 606152 SLC19A3 (AR) Gene analysis

Tyrosine hydroxylase deficiency 605407 TH (AR) CSF neurotransmitters, gene analysis

VMAT2 deficiency 193001 SLC18A2 (AR) Urine mono-amine metabolites

ACP = acylcarnitine profile; CSF = cerebrospinal fluid; l.o. = late onset form; PAA = plasma amino-acids; Phe: phenylalanine; tHcy = total homocysteine; UOA = urine organic acids. Mode of inheritance for each gene denoted as: AD = autosomal dominant, AR = autosomal recessive, mt = mitochondrial, X-linked = X-linked.

with small numbers, clinical heterogeneity and outcomes requiring long-term follow-up, which impede clinical trials and generation of solid evidence [23].

3. Diagnostic recommendation for the identification of treatable causes of IDD

Despite the limited evidence for treatment effects of the majority of these rare IEMs, the authors agreed it worthwhile to develop the current recommendation to aid clinicians in the identification of treatable IEMs in children presenting with IDD of unknown cause. In addition to the systematic review of the literature described above, we used the consensus expert opinion generated via meetings with 6 investigators, including experts in the field of treatable IEMs and neurometabolic diseases (SS); evidence-based medicine in rare diseases and IDD (CvK); medical genetics and IEMs (JZ); medical genetics and IDD (JM); and pediatric neurology and global developmental delay/IDD (MS).

3.1. The algorithm

Our recommendation is depicted in Fig. 1, and comprises 2 tiers; it is based on a combination of evidence and clinical expertise and aims to support pediatric specialists by providing a structured approach to the identification of treatable IEMs in IDD of unknown cause.

The categorization of investigations required for reliable diagnosis of the 89 IEMs into the first tier (biochemical only) or second tier (biochemical and/or molecular) was based on availability, affordability,

yield and invasiveness. All tests in the first tier are provided by most biochemical genetics laboratories in the academic setting for reasonable prices, and have the potential to identify 3 or more IEMs. Although cerebrospinal (CSF) analyses meet these criteria, these were not included into first tier given the invasiveness of the lumbar puncture (+/- sedation). The second tier is mostly a 'single test per single disease' approach, directed by signs and symptoms and based on a clinical hypothesis.

3.2. First tier investigations

First tier investigations for treatable IDD (Table 2a) include the following, which can be collected at one time to reduce the burden of repeated (invasive) investigations on the patient: serum lactate; serum ammonia; serum copper; serum ceruloplasmin; plasma total homocysteine; plasma amino-acids, and bloodspot quantitative acylcarnitine profile (in blood); and creatine metabolites; purines and pyrimidines; organic acids; oligosaccharides; and glycosaminogly-cans (in urine).

As a group, the first tier screening tests can identify 60% of all potentially treatable IEMs. First tier tests are generally accessible and offered by most biochemical genetics laboratories around the world with reasonable turn-around times and affordable prices. Each of these screening tests has the potential to specifically identify treatable IEMs, which is then often confirmed via molecular and/or enzymatic analysis. Some IEMs are diagnosed by a combination of first tier tests, e.g. inborn

1st Tier: Non-Targeted screening to identify 54 (60%) treatable IEMs

Blood: Urine:

► ammonia, lactate ► organic acids

► plasma amino acids ► purines & pyrimidines

► total homocysteine ► creatine metabolites

► acylcarnitine profile ► oligosaccharides

► copper, ceruloplasmin ► glycosaminoglycans

2nd Tier: Targeted lestirg to identify 35 (40%) treatable lEMs requiring 'specific testing'

> according to patient's symptomatology patient (Table -4) & clinician's expertise

> utilization of textbooks & digital resources

(WebApp: www.treatable-ID.arefi

v consider the following biochemical / molecular analyses:

> whole Wood manganese

> plasma cholestanol

> plasma 7-dehydroxy-cho[esterol:cholesterol ratio

> plasma pipecolic acid & urine AASA

> plasma very Eong chain fatty acids

> plasma vitamin B12 & folate

> serum & CSF lactatspyruvate ratio

> enzyme activities (leucocytes): arylsulphatase A, biotinldase, glucocerebrasidase, fatty aldehyde dehydrogenase

> urine deoxypyridanoline * CSF amino acids

> CSF neurotransmitters

> CSF: plasma glucose ratio

> CoQ measurement fibroblasts

> molecular: CASA, NPC1, NPC2, SC4MOL, SLC1BA2, SLC19A3, SLC3OAT0, SLC52A2, SLC52A3, POHA1, OLAT, PDHX, SPR. TH

Fig. 1. Two-tiered algorithm for diagnosis of treatable IEMs in IDD. The first tier testing comprises group metabolic tests in urine and blood which should be performed in every patient with IDD of unknown cause. Based on the differential diagnosis generated by the patient's signs and symptoms, the second tier test is ordered individually at a low threshold.

errors of Cobalamin metabolism (urine organic acids, plasma total homocysteine).

Results of the acylcarnitine profile are often supportive rather than primarily indicative of the 89 treatable IEMs on our list. For example, the organic acid profile is the primary indicator for proprionic acidemia (3-OH-proprionic acid), while the acylcarnitine profile further supports this diagnosis showing elevations of C2 and C3, which is also the case in methylmalonic acidemia. In the BVVL/FL syndrome, some but not all patients showed abnormal acylcarnitine profiles (and/or plasma flavin level), thus, on its own, this test is not sufficiently reliable, requiring molecular analysis for diagnosis [27].

3.3. Practical considerations

The set of the first tier tests exceeds usual practice in the clinical setting [19,33]; if the physician wishes to cast as broad a net as possible it is recommended that all these tests are done. Performing the urinary tests requires particular attention. Whereas usual screening for IEMs comprise organic acid analysis only, screening for treatable IEMs requires additional tests for urine creatine metabolites (urine creatine: creatinine ratio and guanidine-acetate:creatinine ratio), purines and pyrimidines.

A more focused approach can also be taken with the caveat that milder forms of IEMs can present with unspecific phenotypes and potentially be missed. The tiers can be adapted to local circumstances and/or practice. For example in British Columbia, our hospital laboratory's capacity for the urine oligo- and mucopolysaccharides tests are limited; therefore these are not ordered as first screening tier tests, but rather based on suggestive signs and symptoms only, thus as second tier only.

Samples for plasma amino acids and total homocysteine should not be obtained in the post-prandial period but rather after 4-8 h fasting (e.g. in the morning), and should be processed without delay. When ordering these tests, the clinician should take into account the risks of fasting in the particular patient, especially in the infant and young child. Results should be compared with appropriate age-related control values, taking nutritional status into consideration.

Careful interpretation of results is crucial. Findings might be subtle in attenuated disease variants. For example, females with OTC deficiency do not always have high ammonia; low citrulline and elevated glutamine in plasma along with orotic aciduria would be more consistent findings [34]. Although in the interpretation of laboratory results the focus is mostly on values above the normal range, care should be taken not to miss pathognomonic decreases, e.g. in plasma serine concentrations which may indicate one of the serine deficiency disorders. Low urinary creatinine levels are usually considered to be due to diluted urine, but could also result from inborn errors of creatine synthesis (GAMT and AGAT deficiency).

It must be acknowledged that for the 89 IEMs the sensitivity and specificity of each of the listed first and second tier tests varies; for further information we refer to the "Laboratory Guide to methods in biochemical genetics" [35] and other similar resources.

3.4. Second tier investigations

For the remaining 29 IEMs that are not ruled out with the first tier testing as indicated above, specific investigations are necessary. Mostly this is a 'single test per disease' approach, but some tests such as neurotransmitter analysis in cerebrospinal fluid (CSF) have the potential to identify multiple (up to 7) treatable IDDs. An overview of these specific investigations is provided in Table 2b.

Those conditions for which mutation analysis might serve as the primary specific test, because a biochemical marker is unavailable or unreliable and/or the test requires an invasive procedure, are listed in Table 3.

In practice, the clinician might combine the first tier tests with investigations for those genetic conditions, which do not classify as IEMs (e.g. chromosomal micro-array to detect copy number variants) and one or more specific tests of the second tier, as appropriate, based on the observed clinical phenotype of the patient. The clinical judgment of the medical or biochemical geneticist is particularly important regarding the differential diagnosis and the design of the patient-specific testing strategy. Table 4 provides an overview of symptomatology that could indicate a specific (set of) investigation(s) to test for particular treatable IDD(s) with fitting phenotype(s).

4. Resources

4.1. New online resource

Van Karnebeek et al, have developed a resource App (freely accessible as a digital App via www.treatable-id.org as well as a native App via the App store), accepted by the rare disease community with a publication in a peer-reviewed journal [40]. The App targets the clinical specialist and supports both tiers of the algorithm and also functions as an information portal on these rare diseases and their treatments. It does not replace a careful clinical evaluation but supports the clinician in defining a differential diagnosis focused on treatable conditions.

Treatable IEMs are presented according to biochemical categories; neurologic and non-neurologic signs and symptoms; diagnostic investigations; therapies and effects on primary (IQ/developmental quotient) and secondary outcomes; and available evidence. A 'Disease Page' is provided for each condition with an information portal comprising an overview of all signs and symptoms; a figure showing the effected biochemical pathway; information on available diagnostic tests; and causal therapies. In addition, each page contains numerous linked online

Table 3

IEMs for which molecular analysis is the primary specific test.

Gene(s)

AGAT deficiency

Biotin responsive basal ganglia disease Brown-Vialetto-Van Laere/Fazio Londe syndromes Carbonic anhydrase VA deficiency Cerebral glucose transporter deficiency

Co-enzyme Q10 deficiency

l.o. CPS deficiency Creatine transporter deficiency Hyperinsulinism-hyperammonia syndrome MELAS

l.o. NAGS deficiency Niemann-Pick disease type C

Pyruvate dehydrogenase complex deficiency SC4MOL deficiency Serine biosynthesis defects Sjögren-Larssen disease Thiamine-responsive encephalopathy

612718 607483 211500, 114761 606777 612126 607426

614651

614652 612016 614654 614650 608307 300352 606762 540000

237310

257220 (NPC1 ) 607625 (NPC2) 312170 607545

601815,610992,614032 270200

607483 (same as biotin responsive basal ganglia disease)

(same as thiamine responsive encephalopathy) 211530

(GLUT1 deficiency syndrome 1)

(GLUT1 deficiency syndrome 2)

(primary 1)

(primary 2)

(primary 3)

(primary 4)

(primary 5)

(primary 6)

(CPS1)

AGAT (AR) SLC19A3

SLC52A2, SLC52A3

SLC6A19

COQ2, APTX, PDSS1, PDSS2, CABC1, COQ9

SLC6A8 GDH

MTTL1, MTTQ, MTTH, MTTK, MTTC, MTTS1, MTND1, MTND5, MTND6, MTTS2 NAGS

NPC1 & NPC2

PDHA, DLAT, PDHX SC4MOL (AR) PHGDH, PSAT, PSPH FALDH SLC19A3

Direct molecular or gene(s) analysis was deemed the most appropriate diagnostic approach for an IEM if: the biochemical marker is unavailable or unreliable and/or the test requires an invasive procedure and/or the test is difficult to access. This table lists a total of 14 such IEMs with 33 encoding genes.

resources, listed below as well as online abstracts of journal articles, clinical trials, and patient resource websites.

The App provides search capabilities using specific combinations of signs and symptoms, enabling the specialist to refine the differential diagnosis. The App displays a dichotomy: those identifiable by first tier tests (white background) versus those requiring a specific test (green background; second tier). Thus the clinician can immediately discard the IEM with the white background from the differential diagnosis, provided first tier testing was negative. The data on clinical disease features were synthesized based on data extracted from diverse textbooks and online resources [36-39] and combined with the authors' personal experience and expertise, if required. The App lists the main clinical presentation of each disease, i.e. the most characteristic, specific and consistent neurologic signs and symptoms. Cautioned use is required, as the absence or presence of signs and/or symptoms in the App does not in any way rule out the specific disorder in a patient. Further, these data are subject to change as new diagnostic techniques provide novel insights into the spectrum of phenotypic presentations and the natural history of metabolic diseases.

4.2. Other resources

Diverse textbooks and online resources provide valuable information on IEMs, including but not limited to: Scriver et al. [36], Valle et al. [37], Zschocke et al. [38], Fernandes et al. [39], Orphanet (www.orpha.net), OMIM (http://www.ncbi.nlm.nih.gov/omim), Gene Reviews http:// www.ncbi.nlm.nih.gov/sites/GeneTests), Online Metabolic and Molecular Bases of Inherited Disease (www.ommbid.org), Gene Cards (www. genecards.org), and Pubmed (www.ncbi.nlm.nih.gov/pubmed).

5. Using this recommendation for metabolic diagnostic evaluation in practice

5.1. General considerations

These recommendations are based on the current best evidence and offer the expert clinician a set of tests to consider when evaluating the child or adolescent with IDD for a treatable IEM. The literature does

not address the clinical utility of each individual test discussed in this paper, nor does it address the entire "set" of tests contained in Tier 1. The papers of Engbers et al. and Papavasiliou et al. are cited as two efforts to address the complex problem of identifying treatable IEMs in a series of patients with IDD: however, a larger study systematically implementing the recommended tests for IEMs is necessary to answer the essential questions of clinical utility, costs and treatment outcomes. Such a study is currently in progress in BC Children's Hospital, Vancouver Canada. It might also be noted that not all expert clinicians consulting for children with IDD are trained in biochemical genetics nor consider themselves expert in detecting all clinical signs and symptoms of potential IEMs. This paper offers recommendations for such clinicians, particularly those who might be remotely located. For the approach to treatable IEMs in adults presenting with neurologic symptoms, the reader is referred to the publication by Sirrs et al. [41].

These recommendations are not meant to function in a stand-alone capacity, but rather can be superimposed on the 2011 American Academy of Neurology practice parameters [22], which advises diagnostic tests according to their yield: chromosome micro-array; Fragile X (FMR1 gene) testing; Rett syndrome; MRI/spectroscopy brain; electroencephalography (EEG) and other neurophysiologic tests; and thyroid function (TSH). Thorough concurrent audiologic and ophthalmologic screening should be used to rule out remediable primary sensory deficits in every patient presenting with IDD.

5.2. Yield

While the incidence of the individual 89 conditions is low in the general population, ranging from 1:10,000 to less than 1:200,000 [42], their collective frequency in the population at risk (i.e. those with IDD) may be higher. Current evidence in the literature suggests low yield of metabolic testing (0.8-2.5%). However in the year 2013 a comprehensive metabolic evaluation as suggested here has not been reported for larger groups of IDD patients [12]. There are several studies which do suggest a higher yield of metabolic testing in the tertiary care setting. For example, Engbers et al. [43], using a multidisciplinary approach, identified IEMs in nearly 3% (n = 12) of433 individuals with IDD despite 'normal initial metabolic studies before referral'; IEMs comprised 20% of all

Table 4

List of specific biochemical tests and the symptoms which could indicate their use.

Specific test

Disease(s) (OMIM#)

Neurologic symptoms

Non-neurologic symptoms

(Whole) Blood manganese (riboflavin?)

Plasma cholestanol

Plasma 7-dehydroxycholesterol: cholesterol ratio

Plasma pipecolic acid & urine a-amino

adipic semi-aldehyde Plasma very long chain fatty acids

Plasma vitamin B12 & folate

Serum & CSF lactate:pyruvate ratio

Enzyme activity (leucocytes): arylsulphatase A

Enzyme activity (leucocytes): biotinidase

Enzyme activity (leucocytes): fatty

aldehyde dehydrogenase Enzyme activity (leucocytes): glucocerebrosidase

CSF amino-acids

CSF neurotransmitters (incl. tetrahydrofolate)

CSF:plasma glucose ratio Urine deoxypyridonoline

Urine mono-amine metabolites

Co-enzyme Q fibroblasts (+/- molecular analyses COQ2, APTX, PDSS1, PDSS2, CABC1, COQgenes) Molecular analysis CA5A gene

Molecular analysis NPC1 & NPC2 genes

Molecular analysis PDHA1, DLAT, PDHX genes

Molecular analysis SC4MOL gene Molecular analysis SLC18AA2 gene Molecular analysis SLC19AA3 gene

Molecular analyses SLC52A2, SLC52A3 genes

Molecular analysis SPR, TH genes

Hypermanganesemia with dystonia, polycythemia, and cirrhosis (#673280) Cerebrotendinous xanthomatosis (#213700)

Smith Lemli Opitz syndrome (#270400)

Pyridoxine dependent epilepsy (#266100)

X-linked Adrenoleukodystrophy (#300100)

Congenital intrinsic factor deficiency (#261000), Immerslund Grasbeck syndrome (#261100) PDH complex deficiency (#312170; 245348; 245349)

l.o. Metachromatic leukodystrophy (#250100)

Biotinidase deficiency (#253260)

Sjögren-Larsson syndrome (#270200)

Gaucher disease type III (#231000)

PHGDH deficiency (#601815), PSAT deficiency (#610992), PSPH deficiency (#614023), l.o. Non-ketotic hyperglycinemia (#605899)

DHFR deficiency (#613893), DHPR deficiency (#261630), GTPCH deficiency (#233910), PTS deficiency (#261640), SPR deficiency (#612716), tyrosine hydroxylase deficiency (#605407) GLUT1 deficiency (#606777) Menkes disease, Wilson disease

AADC deficiency, VMAT2 deficiency

Co-enzyme Q10 deficiency (#607426)

Carbonic anhydrase VA deficiency (#114761)

Niemann-Pick disease type C (#257220)

PDH complex deficiency (#312170; 245348; 245349)

SC4MOL deficiency (#607545)

VMAT2 deficiency (#193001)

Biotin responsive basal ganglia disease (#607483), thiamine responsive encephalopathy (#606152)

Brown-Vialetto-Van Laere (#211500)/Fazio Londe syndromes (#211530)

SPR deficiency (#612716), tyrosine hydroxylase deficiency (605407)

Dystonia

Ataxia, dementia, neuropathy, behavioral/psychiatric disturbances

Hypotonia/myopathy

Epilepsy, brain cysts/dysgenesis, white matter abnormalities Dementia, brain white matter abnormalities, behavioral/psychiatric disturbances, sensorineural hearing loss, vision loss Ataxia, dystonia, epilepsy

Brain cysts/dysgenesis, white matter abnormalities, neuropathy Dementia, brain white matter abnormalities, neuropathy, psychiatric/behavioral disturbances, spasticity

Encephalopathic crisis, epilepsy, sensorineural hearing loss Spasticity

Dementia, encephalopathic crisis, epilepsy, ocular movement abnormality

Ataxia, dystonia, epilepsy, cerebral/ cerebellar atrophy, brain cysts/ dysgenesis, neuropathy, psychiatric/ behavioral disturbances

Dystonia, epilepsy, ocular movement abnormality

Epilepsy, cerebral/cerebellar atrophy

Dystonia, epilepsy, ocular movement

abnormality

Hypotonia/myopathy

Episodic encephalopathy

Ataxia, dementia, dystonia, epilepsy, ocular movement abnormality, behavioral/psychiatric disturbance Ataxia, dystonia, brain cysts/ dysgenesis, white matter abnormalities, neuropathy Microcephaly

Parkinsonism, non-ambulation, mood disorder, autonomic instability Ataxia, dystonia, encephalopathic crisis, cerebral/cerebellar atrophy, brain cysts/dysgenesis, ocular movement abnormality Bulbar palsy, sensorineural hearing loss, facial weakness

Dystonia, ocular movement abnormality

Polycythemia, liver cirrhosis

Bones (osteoporosis), skin (xanthomata), eye (cataract), gastro-intestinal (diarrhea), heart (myocardial infarct), liver (cholestatic icterus) Bones (congenital anomalies), skin (photosensitivity), facial dysmorphisms, gastro-intestinal (feeding problems), growth/stature, liver (cholestatic icterus)

Endocrinology (adrenal insufficiency)

Hematology (macrocytic anemia), immunology, kidney (proteinuria, atypical HUS)

Facial dysmorphisms

Skin (rash/alopecia)

Skin (ichthyosis) & eye (macular dystrophy, retinitis pigmentosa) Hepato-/splenomegaly

Abnormal growth/stature

Megaloblastic anemia

Bones (osteoporosis), skin (pili torti, skin laxicity), facial dysmorphism, gastrointestinal (feeding problems & diarrhea)

Heart (cardiomyopathy), kidney (renal dysfunction)

Liver (hepato-splenomegaly, cholestatic icterus)

Arthritis, congenital cataracts, psioriasiform dermatitis

Respiratory failure

established causes diagnosed, and 4 of these 12 were treatable (a.o. 2 cases with creatine transporter deficiency). Papavasiliou et al. [33] reported the results of individualized investigations based on clinical and neuro-radiologic findings in a highly selected group of children with IDD; IEMs were identified in 13.6% (n = 16).

Several reviews have been published concerning metabolic causes of IDD, most of which reflect expert opinions and individual expertise in the field of IEMs [14,17,18]. The need for multiple tests to exclude a few rare to ultra-rare conditions and the limited availability of laboratories offering comprehensive diagnostic testing, explain why outside highly specialized centers, metabolic work-up of patients with IDD is time-consuming, expensive and often remains incomplete. Because of all these limitations, the diagnostic yield of metabolic testing has been incomplete, varies per publication and is reportedly low in patients presenting with IDD. However, diagnostic standardization through use of the current recommendation could foster greater awareness of otherwise unrecognized causally treatable conditions. While the rate of positive findings may remain relatively lower than, for example CMA testing, the treatment effect is greater with inherent improvement in outcomes.

5.3. Newborn screening

Normal newborn screening results in a patient with IDD of unknown origin should not be interpreted such that all treatable IEMs have been ruled out. Newborn screening panels target only a minority of treatable IEMs even in countries with a very high number of targets, such as the USA, and some children may have been born in countries that do not perform newborn screening. Even for those IEMs included in many newborn screening programs, such as classic organic acidemias and urea cycle defects, 'late-onset' phenotypic variants constituting treatable IDDs can be missed, as newborn screening may not be sensitive and specific enough to safely detect such disease-variants [44]. Therefore, it is prudent to verify the results whenever possible. The disorders included in the specific regional newborn screening panels are often available online (http://genes-r-us.uthscsa.edu). Thus, although newborn screening is not a substitute for the first tier testing, and completion of all the listed investigations is advised, locally available resources and practices may prioritize certain first tier tests above others. For example, urine creatine metabolites is almost always indicated as these IEMs are not included in panels except for a few scattered pilot studies screening for GAMT deficiency (e.g. ongoing in BC Children's Hospital) [45].

5.4. Treatable IEMs as a cause ofunspecific IDD

The majority of the 89 treatable IEMs present with more multiple co-morbidities including epilepsy, neurologic symptoms and signs, and behavioral and psychiatric disturbances. Systemic manifestations occur in 70% of conditions [23]. However, the clinical spectrum of treatable IDD is variable and the absence of co-morbidities does not exclude the presence of a treatable IDD. Rather, the clinical picture is determined by the state of disease progression and by particular disease variants. For example, progressive neurologic decline is characteristic of advanced stages ofX-linked adrenoleukodystrophy; subtle loss of cognitive functions accompanied by behavioral disturbances is often the first manifestation. Recognition of the diagnosis at this early disease stage opens a unique window of opportunity for causal treatment with stem cell transplantation, which is not effective at a later stage in the disease course [46]. While clinical co-morbidities are traditionally considered characteristic of metabolic causes of IDD, the absence of such co-morbidities does not exclude them. The same is true for neuro-degeneration, as many of the 89 treatable IEMs present with 'stable IDD', i.e., without a history of regression or plateauing.

5.5. 'Late-onset' or atypical variants of conditions

'Late-onset' or atypical variants of conditions typically presenting as acute metabolic decompensation in the neonatal period deserve special attention. While patients with acute metabolic crisis are diagnosed before they are assessed for IDD, the clinical presentation of the attenuated or 'late-onset' forms of these conditions is often unspecific and chronic in nature. For example, OTC deficiency in males typically manifests with severe neonatal hyperammonemia and outcomes are extremely poor in affected males. However, females with heterozygous OTC deficiency often present with IDD and/or behavioral problems as the only manifestation(s) [34]. Timely recognition of the underlying metabolic defect that enables appropriate treatment to control blood ammonia levels not only helps to prevent acute hyperammonic crises at a later stage of life but also improves cognitive function and behavior.

5.6. Primary gene analysis

Primary gene analysis can enhance the diagnostic yield in conditions with unspecific clinical and biochemical presentation. For example, low urinary excretion of guanidinoacetate is characteristic of AGAT deficiency, a treatable disorder of creatine synthesis, but the detection of low levels continues to pose an analytical challenge in the laboratory, as currently available methods mainly detect extreme elevations of accumulating metabolites. The traditional diagnostic approach to Niemann-Pick Disease Type C (NPC) requires demonstration of free cholesterol via filipin staining in cultivated skin fibroblasts. This test is invasive, time- and cost-consuming, available in a limited number of laboratories worldwide, and is not always sensitive. Recent recommendations added primary NPCI and NPC2 gene sequencing as an alternative diagnostic strategy [47]. High-throughput sequencing technologies may be considered as an alternative diagnostic approach as they facilitate analysis of multiple genes in one sample in a cost-effective manner, although interpretation of previously unreported variants may be challenging. Determination of oxysterols in plasma is a promising, novel approach for low threshold screening for NPC [48].

6. Costs

An overview of the costs of the individual chemistry and metabolic/ biochemical tests, based on the costs at one Canadian academic center (Vancouver), is provided in Table 5. The total cost is $527.97 (CAD), which is reasonable in comparison with individual tests (molecular analysis of a single gene often exceeds $500) and future cost-savings, if a child can be treated in a timely way to reach his full potential and participate in society. These costs are likely comparable to those provided by other international academic centers, yet far exceeded by commercial companies.

Table 5

Costs and turn-around time of the individual tests in the first tier screening.

Costs ($CAD) Turn-around time

Bloodspot acylcarnitine $41.28 3 days

Plasma amino-acids $78.42 1 week

Plasma total homocysteine $22.97 1 week

Serum ceruloplasmin $10.15 1 day

Serum copper $49.19 1 day

Urine creatine metabolites, purines & pyrimidines $65.01 4-6 weeks

Urine glycosaminoglycans $59.55 4-6 weeks

Urine oligosaccharides $32.65 4-6 weeks

Urine organic acids $105.41 1 week

Urine purines & pyrimidines $63.34 4-6 weeks

Total $567.97

7. Metabolic testing for "non-treatable" inborn errors of metabolism

This recommendation is focused on the identification of treatable IEMs, however each of the first tier screening tests also identifies IEMs for which no causal therapy is available or effective, according to the definitions provided here [49]. For example, organic acid analysis also reveals untreatable conditions with major CNS involvement, such as Canavan's disease and 2-hydroxy-glutaric aciduria. Similarly, urine glycosaminoglycans and oligosaccharides analyses can detect conditions such as Schindler disease, MPS IX (Natowicz disease) and sialidosis. Additionally, urine purines and pyrimidines will reveal causally untreatable conditions such as isolated sulfite oxidase deficiency and ureidopriopionase deficiency. Identification of non-treatable causes of IDD is still beneficial to the affected individual and his/her family as it ends the burdensome diagnostic odyssey and allows for reliable genetic counseling.

8. Limitations and future directions

The authors acknowledge the following limitations and suggest strategies to overcome them:

1) As the literature is silent in regard to the effectiveness of this set of recommendations, additional research is necessary to inform our practice in the tertiary care setting. One such effort is a 3-year funded project (www.tidebc.org) initiated in 2011 at B.C. Children's Hospital in Vancouver, Canada where 1200 patients with IDD are evaluated per year by various services including Neurology, Developmental Pediatrics, Psychiatry, Clinical Genetics and Biochemical Diseases. Our one-year pilot study identified treatable IEMs in > 5% of 210 IDD patients (personal communication). Of note, urine glycosaminoglycans and oligosaccharides were included only as second tier tests due to logistic limitations. Although these results are preliminary and should be interpreted with caution due to potential referral bias, this yield is encouraging and supports this recommendation. IEMs are all rare diseases, so much larger numbers of patients are required to provide reliable data.

2) Access to first and second tier testing may be challenging in practices located remotely from academic laboratories or dependent on commercial laboratories with inherent burden of costs, as well as in regions with significant financial restrictions. The clinician may then choose to select first and/or second tier tests according to feasibility, insights and patient presentation.

3) These recommendations focus on a moving target. As noted, during the period since the literature search was performed for our systematic review in 2011,8 new 'treatable IDDs' have emerged either through gene discovery and/or generation of evidence for new treatments; the latter is also true for conditions already on the list (e.g. lysine restricted diet as adjunct treatment for pyridoxine dependent epilepsy [50]). To incorporate exciting diagnostic and therapeutic advances such as these, as well as more reliable information on yield, the recommendations should be modified regularly to optimize utility in daily practice.

Funding

This work was supported by funding from the B.C. Children's Hospital Foundation as "1st Collaborative Area of Innovation".

Conflict of interest

CvK and SS developed the content of, and contributed to the design of the freely available App described here (www.treatable-id.org).

Acknowledgments

The authors would like to thank Drs. Graham Sinclair and Hilary Vallance (Biochemical Genetics Lab) for data on cost of first tier testing;

Ms. Claire Sowerbutt (medical writer) for text editing; Mrs. Ruth Giesbrecht. Mrs. Maria Boldut and Mrs. Mirafe Lafek for administrative assistance with the tables and manuscript; Dr. Melissa Crenshaw (All Children's Hospital, St. Petersburg, USA) and Wendy Smith (Barbara Bush Children's Hospital, Portland, USA) for their support; Mr. Roderick Houben and Mr. JeffJoa (Health2Media) for App design and updates.

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