Summary

Klinefelter’s syndrome (KS; karyotype 47,XXY) is associated with specific neurocognitive impairments, especially delayed language development and impaired socioemotional evolution. There is an increased risk for psychiatric disturbances, particularly schizophrenia and affective spectrum disorders.

A 51-year-old monozygotic male twin with KS is described of whom one was referred for long-lasting paranoid psychotic symptoms. Both were treated with testosterone and had an average level of intelligence. Detailed psychiatric and neuropsychological assessment in the referred patient demonstrated quasi-psychotic symptoms with illusions, delusion-like ideas, paranoid ideation, magical thinking, circumstantial speech and thinking and eccentric behaviours. In addition, attentional deficits and executive dysfunctions could be demonstrated. A diagnosis of schizotypal personality disorder was made. A less pronounced identical clinical picture was found in his brother. The psychopathological phenotype of KS is characterised by a schizotypal personality which originates from its specific cognitive defects and that, with increasing age, may develop into a schizophrenia-like psychosis.