Inborn Errors of Metabolism: PKU, MSUD, Organic Acidaemias, Urea Cycle Defects - Newborn Screening, Dietary Management, and Acute Decompensation

Overview

Inborn errors of metabolism (IEMs) are a heterogeneous group of inherited biochemical disorders caused by enzyme or cofactor deficiencies that disrupt metabolic pathways. While individually rare, collectively they represent a significant cause of neonatal morbidity and mortality. The disorders covered here - phenylketonuria (PKU), maple syrup urine disease (MSUD), organic acidaemias, and urea cycle defects (UCDs) - share key characteristics: predominantly autosomal recessive inheritance (with notable X-linked exceptions), normal appearance at birth, non-specific clinical presentation mimicking sepsis, and outcome directly determined by speed of diagnosis and treatment. The affected neonate is typically well at birth because the placenta and maternal metabolism clear toxic metabolites in utero; symptoms emerge only after postnatal substrate accumulation begins.

Key principle: Outcome is directly related to the speed of diagnosis in treatable IEMs. Treatment before symptom onset gives the best neurological prognosis.

Epidemiology and Genetics

OTC deficiency is the most common UCD and is X-linked recessive: hemizygous males are severely affected; heterozygous females have variable expression (partial enzyme deficiency, protein aversion, episodic hyperammonaemia). CPS1 deficiency is autosomal recessive. The E1α-subunit of pyruvate dehydrogenase deficiency is X-linked dominant - nearly all cases (male and female) arise from new mutations.

Most IEMs are autosomal recessive; a positive family history of parental consanguinity or unexplained neonatal deaths should heighten suspicion.

Pathophysiology

General Principles

Deficient enzyme activity causes: (1) accumulation of toxic precursors upstream, (2) depletion of downstream products essential for normal metabolism, or (3) both. Clinical consequences depend on the substrate involved, residual enzyme activity, and metabolic load (protein intake, catabolism during illness). Some IEMs manifest only after the relevant dietary substrate becomes available in quantity - for example, galactosaemia and hereditary fructose intolerance present after initiation of the relevant feeds.

Disorder-Specific Mechanisms

The anion gap is the key bedside calculation in suspected organic acidaemia:

$$\text{Anion Gap} = [\text{Na}^+] - ([\text{Cl}^-] + [\text{HCO}_3^-])$$

Normal range: 8-16 mmol/L (or ~4 mmol/L higher if potassium is included). Elevation indicates unmeasured organic anions (ketoacids, complex organic acids in IEMs, lactate).

Clinical Presentation

Neonatal Period

Symptoms emerge hours to days after protein feeding begins. Two predominant patterns exist:

1. Encephalopathic pattern - lethargy, poor feeding, hypotonia, seizures, coma, apnoea; characteristic of UCDs, MSUD, non-ketotic hyperglycinaemia (glycine encephalopathy)
2. Metabolic-acidotic pattern - vomiting, tachypnoea (Kussmaul breathing), circulatory disturbance followed by depressed consciousness; characteristic of organic acidaemias

A dramatic improvement during IV fluid administration, followed by relapse when milk feeding resumes, is strongly suggestive of an IEM. Septicaemia is a frequent secondary event (especially in galactosaemia) and must not distract from the metabolic diagnosis.

Key Clinical Clues

Red Flags Warranting Urgent IEM Investigation

Beyond the Neonatal Period

• PKU (unscreened/untreated): progressive intellectual disability, behavioural disturbance, microcephaly, seizures, fair complexion (↓ melanin synthesis), musty/mousy odour (phenylacetic acid)
• MSUD (mild/intermittent forms): episodic encephalopathy and ataxia during intercurrent illness
• Organic acidaemias: recurrent ketoacidotic crises triggered by illness, fasting, or excess protein; chronic complications include cardiomyopathy (propionic acidaemia) and nephropathy (methylmalonic acidaemia)
• OTC deficiency (heterozygous females): episodic encephalopathy, cyclical vomiting, protein aversion, developmental delay
• Maternal PKU: if poorly controlled during pregnancy, causes fetal microcephaly, congenital heart disease, intellectual disability, and IUGR regardless of fetal genotype

Investigations

First-Line (All Neonatal Units)

Second-Line (Regional Metabolic Laboratory)

Specialised Investigations (Supraregional)

• Specific enzyme assays on leucocytes or cultured skin fibroblasts (e.g., branched-chain 2-ketoacid dehydrogenase in MSUD)
• DNA mutation analysis / gene panel sequencing
• Very long-chain fatty acids (VLCFAs), DHAP-AT (peroxisomal disorders)
• Bile acid analysis; lysosomal enzyme studies; plasma transferrin isoforms (CDG syndromes)

Before sending urgent metabolic samples: phone the laboratory to indicate urgency; provide details of drugs, diet, and prior blood transfusions; discuss with the metabolic consultant which tests are indicated.

Tandem Mass Spectrometry - Key Metabolite Patterns

Tandem MS is the most efficient initial test for diagnosing most fatty acid oxidation disorders and many organic acidaemias, as well as amino acid disorders, from a capillary DBS sample.

Newborn Screening

Australia and New Zealand

The Australian National Newborn Bloodspot Screening Programme collects DBS at 48-72 hours of age (or before discharge if earlier), using tandem MS as the primary platform. Conditions screened include:

• PKU, MSUD
• Organic acidaemias (propionic, methylmalonic, isovaleric, glutaric aciduria type I, 3-methylcrotonyl-CoA carboxylase deficiency, 3-MCC)
• Fatty acid oxidation defects (MCADD, LCHAD, VLCAD, and others - panel varies by state)
• UCDs: citrullinaemia type I and argininosuccinic aciduria included in most panels; OTC deficiency is not reliably detected by current screens
• Congenital hypothyroidism, congenital adrenal hyperplasia, cystic fibrosis, galactosaemia, biotinidase deficiency
• Severe combined immunodeficiency (SCID) - added in some states

New Zealand operates a comparable National Newborn Metabolic Screening Programme with DBS at 48-72 hours.

In the UK, universal newborn screening is currently offered for PKU and MCADD (historically), with expanded MS-based screening including additional conditions; this remains more limited than North American and Australasian programmes.

Critical limitations of newborn screening: - A normal result does not exclude all IEMs - UCDs (especially OTC deficiency) and MSUD can cause life-threatening decompensation before the screen result returns (day 4-7 of life) - Clinical suspicion must always override a normal screening result - A positive screen requires same-day urgent contact with the metabolic team

Diagnosis

Confirmed by integration of: 1. Clinical presentation + family history 2. Biochemical phenotype (metabolic profile on plasma and urine) 3. Specific enzyme assay (leucocytes or fibroblasts) 4. Molecular genetics (gene sequencing; multi-gene panel increasingly first-line)

Management

Acute Decompensation - General Principles

The overarching goal is to stop catabolism and reduce toxic substrate load while providing anabolic support. Early specialist metabolic physician involvement is mandatory - correct diagnosis and management require highly specialised expertise, not simply laboratory testing.

Step-by-Step Acute Protocol

Condition-Specific Acute Interventions

Mechanism of nitrogen scavengers in UCDs: - Sodium benzoate conjugates glycine → hippurate (renally excreted): each mole removes 1 mole of nitrogen - Sodium phenylbutyrate → phenylacetate conjugates glutamine → phenylacetylglutamine (renally excreted): each mole removes 2 moles of nitrogen

Long-Term Dietary Management

Maternal PKU: Women with PKU must achieve strict metabolic control (plasma Phe <360 µmol/L, ideally 120-240 µmol/L) before conception and throughout pregnancy to prevent maternal PKU syndrome in offspring (microcephaly, congenital heart disease, intellectual disability, IUGR - independent of fetal genotype).

Liver Transplantation

Liver transplantation corrects the primary hepatic enzyme defect in selected IEMs, including some organic acidaemias (propionic, methylmalonic), UCDs, tyrosinaemia, and certain glycogen storage diseases. It accounts for approximately 10-15% of paediatric liver transplant indications. Important caveats: - Transplantation may not fully prevent neurological complications in conditions where extrahepatic enzyme expression is relevant (e.g., methylmalonic acidaemia - renal and neurological involvement persists) - Organ allocation in children uses the PELD score (Pediatric End-stage Liver Disease, for children ≤12 years), based on INR, total bilirubin, serum albumin, age <1 year, and height <2 SD; the MELD score applies from 13 years

Complications

Prognosis and Follow-up

Prognosis correlates with: 1. Speed of diagnosis - treatment before symptomatic decompensation dramatically improves neurological outcome; this is particularly true for MSUD, organic acidaemias, and UCDs 2. Degree of lifelong metabolic control 3. Severity of underlying mutation (residual enzyme activity)

Follow-up Framework

• Multidisciplinary team: metabolic physician, metabolic dietitian, neuropsychologist, genetic counsellor, social worker
• Routine plasma amino acids and acylcarnitine monitoring (typically quarterly in children; frequency adjusted to condition and clinical stability)
• Annual neurodevelopmental assessment and growth monitoring
• Brain MRI in MSUD and UCDs if neurological concerns
• Renal function (eGFR, urinalysis) annually in methylmalonic acidaemia
• Cardiac surveillance (ECG, echocardiogram) in propionic acidaemia
• Ophthalmological review as indicated

When to Refer and Admit

Criteria for Urgent Admission

• Plasma ammonia >100 µmol/L in neonates (or >80 µmol/L in older children) with clinical symptoms
• High-AG metabolic acidosis (AG >20 mmol/L) without clear cause
• Encephalopathy, seizures, or coma in known or suspected IEM
• Plasma leucine >400 µmol/L in MSUD
• Intercurrent illness with inability to maintain oral intake in any known IEM
• Any febrile