Homocystinuria

Disease Description (Causes and Symptoms)

• Definition and cause: Homocystinuria is an autosomal recessive congenital metabolic disorder mainly caused by cystathionine beta-synthase (CBS) deficiency. Due to abnormalities in the methionine metabolic pathway, homocysteine and methionine accumulate. The CBS gene is located on the long arm of chromosome 21.
• Pathophysiology: When the CBS enzyme is deficient, homocysteine accumulates, causing vascular endothelial damage, a tendency toward thrombosis, abnormalities in connective tissue and skeletal development, and central nervous system damage.
• Main symptoms:
  • Neurodevelopmental: intellectual disability, seizures, motor and gait disturbances
  • Ophthalmic: lens dislocation (usually downward and inward), myopia, cataracts, reduced vision
  • Skeletal and body habitus: Marfan-like body habitus (a tendency toward long limbs and trunk), osteoporosis, increased risk of fractures
  • Vascular: increased risk of thrombosis (arterial and venous) — a major cause of death
  • Other: sparse hair (thin and scant hair), changes in facial features
• Diagnosis: Elevated blood methionine on newborn screening (heel-prick blood test) and increased urinary excretion of homocystine; genetic testing for CBS (or related gene) mutations for confirmation; monitoring of total blood homocysteine (tHcy) and methionine levels.

Treatment

• Goal: To lower total blood homocysteine and methionine to safe ranges and prevent or alleviate complications involving the brain, eyes, skeleton, and blood vessels.
• Medication treatment
  • Pyridoxine (vitamin B6): High-dose oral administration (generally in the range of 200–1,000 mg/day; individual response must be assessed). Some patients respond to B6, with improvement in symptoms and biochemical markers.
  • Betaine: Reduces homocysteine by methylating homocysteine to methionine (generally in the range of 6–9 g/day). Blood methionine levels should be monitored during use.
  • Supplementary vitamins: Vitamin B12, folic acid, vitamin C, and others may be used to support regulation of metabolism and homocysteine.
• Dietary therapy
  • Low-methionine diet (protein restriction and special formula/medical nutrition). Early dietary management beginning in infancy can improve intellectual and developmental outcomes.
• Procedures/surgery
  • Ophthalmic complications (severe lens dislocation) require ophthalmic intervention, such as surgery.
  • In some patients with severe B6-unresponsive disease, liver transplantation may correct the metabolic deficiency (consult a specialized center).
• Complication management and prevention
  • Thrombosis prevention: In patients with a history of thrombosis or in high-risk situations (surgery, dehydration, pregnancy and the postpartum period, prolonged inactivity), thrombosis-prevention measures (e.g., anticoagulant use and preventive strategies during periods of immobility) should be considered.
  • Osteoporosis: Bone density testing and preventive management (exercise and evaluation and supplementation of calcium and vitamin D).
  • Regular ophthalmic examinations (vision and lens status) and neurodevelopmental assessments.
• Monitoring
  • Regular blood tests: Total homocysteine (tHcy), methionine, vitamin levels (B6, B12, folic acid), liver function, and others.
  • Adjust medication doses and diet according to treatment response.
• Precautions (including side effects)
  • High-dose vitamin B6: Long-term high-dose use may cause sensory neuropathy (peripheral neuropathy) — adjust the dose if symptoms occur.
  • Blood methionine levels may increase with betaine administration, so monitoring is required.
  • The risk of thrombosis increases during pregnancy, surgery, dehydration, and prolonged inactivity — preventive measures are necessary.
  • Hormone replacement therapies (such as oral contraceptives) may increase the risk of thrombosis and should be considered cautiously.

Checklist Before an Outpatient Visit (Including Example Questions)

• Items and information to prepare
  • List of all currently used medications and supplements (including doses and dosing times)
  • Recent test results (blood tHcy, methionine, B12/folic acid, etc.) and newborn screening results, if applicable
  • Genetic test results, if available, or family history (similar diseases, history of thrombosis, etc.)
  • Record of symptom changes: seizure frequency, changes in vision, fractures or pain, limitations on physical activity, etc.
  • History of thrombosis and whether anticoagulants are being taken
  • Lifestyle information: dietary patterns (amount of protein intake), smoking and alcohol use, and pregnancy plans (for women)
• Materials recommended for submission to the doctor at the outpatient visit
  • Recent prescriptions and test reports, vision test results, and bone density test results, if available
• Points to note during the outpatient visit
  • Confirm whether fasting is required before testing and instructions for taking medications (follow instructions given before the blood test).
  • Worsening symptoms (severe headache, difficulty breathing, sudden weakness, numbness in one side of the body or limbs, etc.) require immediate emergency evaluation (suspected thrombosis).

Examples of General Questions to Ask the Doctor

• "What are the expected effects and common side effects of this medication (e.g., high-dose vitamin B6 or betaine)?"
• "What are the target levels of total blood homocysteine and methionine for me (or my child)? How should they be managed to achieve the targets?"
• "What precautions should I take in daily life to reduce the risk of thrombosis?"
• "Please tell me how often regular testing is needed and which tests should be performed and when."

Examples of Disease-Specific Questions

• "How can we determine whether I (or my child) respond to vitamin B6? How will the treatment plan differ depending on responsiveness?"
• "I had thrombosis in the past. Do I need to continue anticoagulant treatment? I am planning a pregnancy; what changes will be needed in my treatment or prevention strategy?"

Additional guidance: Treatment and monitoring vary greatly from patient to patient. Medication responsiveness and treatment goals differ according to blood levels, genotype, and clinical manifestations. Please consult a specialist (such as a pediatric endocrinologist/metabolic specialist, medical geneticist, or vascular/hematology specialist) to establish an individualized treatment plan.