A Rare Presentation of Classical Homocystinuria: Cerebral Thrombophlebitis in an Adolescent

Homocystinuria is a rare constitutional aminoacidopathy characterized by elevated plasma and urinary homocysteine levels, most often due to cystathionine beta-synthase (CBS) deficiency, and represents the second most frequent metabolic encephalopathy after phenylketonuria. We report the case of a 13-year-old adolescent who presented with a painful, red left eye associated with intense photophobia and helmet-type headaches without signs of intracranial hypertension. Cerebral MR angiography revealed thrombophlebitis of the left sigmoid sinus, and plain foot radiographs showed mild bone demineralization. The diagnosis of homocystinuria was suspected based on clinical and radiological findings and confirmed by qualitative blood and urine amino acid chromatography, which demonstrated elevated methionine in blood and homocysteine in urine, along with markedly increased plasma homocysteine levels (155 µmol/L). Treatment with folic acid, pyridoxine, and vitamin B12 resulted in stabilization of clinical signs. Homocystinuria remains a rare but severe disease, with its prognosis largely determined by neurovascular complications and intellectual disability related to circulating homocysteine levels. Early recognition is crucial, particularly in cases of stroke associated with a marfanoid phenotype, to reduce complications and enable preventive measures in siblings.