Carnitine palmitoyltransferase II deficiency

Disease Description (Causes and Symptoms)

• Definition and causes
  • Phenylketonuria (PKU) is a congenital amino acid metabolic disorder caused by a genetic deficiency of phenylalanine hydroxylase (PAH), the enzyme that mediates the conversion of phenylalanine (Phe) → tyrosine (autosomal recessive), or by an abnormality in the biosynthesis of the PAH cofactor (BH4). Due to reduced PAH function, Phe accumulates in the blood and tyrosine levels relatively decrease.
• Summary of pathophysiology
  • Increased blood Phe → toxic effects on the central nervous system (impaired brain development, neurotransmitter imbalance) and accumulation of metabolites (ketone-like substances) → developmental delay, intellectual disability, seizures, etc.
• Main symptoms
  • Neonatal period: vomiting, poor appetite, skin eczema (atopic-like), unusual body odor (“mouse urine” or “hair” odor)
  • Development: delayed motor, language, and cognitive development; intellectual disability (severe if treatment is delayed)
  • Skin and appearance: pale or light-brown hair, reduced skin pigmentation
  • Nervous system: seizures, hyperactivity and behavioral disorders, etc.
• Diagnostic indicators (summary)
  • Measurement of blood Phe during newborn screening (heel prick): persistently elevated Phe ≥4 mg/dL (≈240 µmol/L) is considered abnormal. Phe ≥20 mg/dL (≈1,210 µmol/L) corresponds to classical PKU. Additional testing for BH4 responsiveness and genetic testing are used for confirmation.

Treatment Methods (Standard Treatment · Medications · Lifestyle Management · Precautions)

• Basic principles
  • If diagnosed and treated as early as possible, during the neonatal period, most complications, including intellectual disability, can be prevented.
• Dietary therapy (primary treatment)
  • Low-phenylalanine (low-Phe) diet: Limit protein intake, particularly phenylalanine, and supplement deficient essential amino acids and nutrients with special formula (or formulations).
  • Target blood Phe concentration (based on the information provided): During infancy and childhood, an example of the recommended blood Phe level is 3–8 mg/dL (≈180–480 µmol/L) (age-specific targets vary depending on the source). In adults and after puberty, lifelong maintenance within the range of 3–15 mg/dL (≈180–900 µmol/L) is recommended (the recommended range varies by study and guideline).
  • Nutritional caution: Excessive Phe restriction may cause a risk of protein deficiency symptoms such as growth delay and anemia → periodic nutritional assessment with a metabolic dietitian is necessary.
• Pharmacological treatment
  • BH4 (sapropterin): As a cofactor for PAH, it induces a reduction in Phe in patients who are BH4-responsive (example of the stated dosage: ≤20 mg/kg/day). BH4 responsiveness should be confirmed through testing.
  • Phenylalanine ammonia-lyase replacement (enzyme therapy): In adults, pegvaliase (approved in some countries) is used for patients who have difficulty achieving target levels with dietary management alone. (Indications, adverse effects, and monitoring are required.)
  • Other supportive treatments: Large neutral amino acid (LNAA) supplements (to inhibit Phe entry into the brain through competition), tyrosine supplementation (to correct deficiency as part of standard treatment)
  • In atypical cases (abnormality in BH4 synthesis rather than PAH deficiency): BH4 administration and, if necessary, supplementation with neurotransmitter precursors (e.g., L-DOPA, 5-HTP)
• Monitoring and follow-up
  • Regular measurement of blood Phe (and Tyr), growth and nutritional status, developmental assessment (infants and young children), and neurological and psychiatric evaluation are recommended. Dietary and medication adjustments are made according to blood Phe results.
• Pregnancy (maternal PKU) precautions
  • A low-Phe diet should be started before pregnancy, with intensive blood Phe management during pregnancy. The provided recommendation is to maintain serum Phe at ≤10 mg/dL (≈605 µmol/L) throughout pregnancy (to reduce the risks of congenital abnormalities, low birth weight, microcephaly, and developmental disorders). Preconception consultation and genetic counseling are essential.
• Risk of complications and precautions
  • If treatment is delayed, permanent complications such as intellectual disability, behavioral disorders, seizures, and microcephaly may occur.
  • Discontinuation of the diet may lead to a decline in neurocognitive function → lifelong management is recommended.
  • When medications such as BH4 and pegvaliase are administered, allergic reactions and adverse effects may occur → monitoring by specialists and the nursing team is necessary.
• Self-management tips
  • Regular consultations with a metabolic dietitian, recording blood Phe test results at home, keeping a food diary, and preconception consultation when planning a pregnancy.

Checklist Before an Outpatient Visit (Including Items to Bring · Points to Check · Example Questions)

• Items to bring (recommended)
  • List of currently used medications and supplements (including dosage and administration times)
  • Recent blood Phe and Tyr test results (if possible, the most recent 3–6 results) and test dates
  • Food diary (the most recent 1–2 weeks) and brand/dosage information for special formula and supplements
  • Previous diagnostic and genetic test results (if available) and newborn screening results (if possible)
  • Growth records (weight and height charts for children) and developmental history
• Record of symptoms and changes
  • Frequency of seizures or convulsions, changes in sleep or behavior, appetite changes, gastrointestinal symptoms (such as vomiting), skin changes, etc.
• Examples of items that may be tested or assessed during the outpatient visit
  • Blood Phe and Tyr, liver function and nutritional status (blood cell counts, iron, vitamins, etc.), developmental assessment (children)
• Examples of convenient questions for an outpatient consultation (for patient preparation)
  • What is my recent target blood Phe level? How often should it be tested?
  • Are there any nutrients lacking in the current diet? Do I need supplementation?
  • How do you assess whether BH4 or enzyme therapy is appropriate?

Examples of General Questions to Ask the Doctor

• What are the main adverse effects and monitoring requirements of this medication (or supplement)?
• What is my (or my child’s) target blood phenylalanine concentration, and how often should it be tested?
• What are the risks of discontinuing or deviating from the diet, and how should they be managed?
• If planning a pregnancy, what preparation is needed, and how will management be carried out during pregnancy?

Examples of Disease-Specific Questions

• If the BH4 (sapropterin) test indicates a treatment response, should medication be started? What are the criteria for determining responsiveness?
• Blood Phe is not well controlled with the current diet and medication. Could you assess whether pegvaliase (enzyme therapy) or other pharmacological options are appropriate for me (or my child)?
• For female patients, please explain specifically when a low-Phe diet should be started when planning a pregnancy and what the target Phe level is during pregnancy.

(Note) Unit conversion: 1 mg/dL of phenylalanine ≈ 60 µmol/L. Check the units of the test results (mg/dL or µmol/L) and coordinate the target range with your doctor.