Classical homocystinuria (cystathionine beta synthase deficiency) is a rare autosomal recessive disease of methionine metabolism that causes accumulation of homocysteine in the blood and cysteine deficiency. It is characterized by intellectual disability, ectopia lentis, skeleton abnormalities resembling Marfan syndrome and thromboembolic episodes. The majority of patients have psychiatric disturbances as depression, behavioral disorders, personality disorders, obsessive-compulsive disorder and, less commonly, bipolar disorder and psychosis.

To briefly review psychiatric disturbances in patients with homocystinuria and present a case report.

Literature research and analysis of patient's clinical data.

A 22-year-old male was diagnosed with classical homocystinuria at age 4 due to intellectual disability and renal alterations. With aging, other problems emerged: epilepsy; postural tremor; dysesthesia; ectopia lentis; orofacial myofunctional disorder; asthma; and patellar instability. He went to a special education program. At age sixteen, he initiated Child Psychiatry consultations due to anxiety and behavioral changes, as difficulty in controlling impulses, establishing relationships and in the perception of the self. Nowadays, the patient is followed in psychiatric consultations, where he has demonstrated high difficulty to empathize. He is being treated with vitamin supplements; betaine; levetiracetam; clobazam; and propranolol, combined with a special diet.

It is not practical to screen every psychiatric patient for Homocystinuria, but this disease should be considered when there is a family history, early and/or acute onset, intellectual disability, atypical symptoms, unusual response to treatment, progressive cognitive change and other organic disturbances present in this disorder.

The authors have not supplied their declaration of competing interest.