Neurodegenerative disorder masquerading as psychosis in a forensic psychiatry setting

A man presenting in his 50s, following conviction for a non-violent crime, to forensic psychiatric services, and then to a neuropsychiatry service with an unusual presentation of psychosis: second person auditory hallucinations, grandiose delusions and somatic delusions. Detailed collateral and family history revealed a background of progressive cognitive deficit and a family history of motor neuron disease. MRI of the brain revealed asymmetrical parieto-occipital volume loss and genetic testing demonstrated a pathogenic expansion of the chromosome 9 open reading frame 72 (C9ORF72) gene consistent with familial frontotemporal dementia caused by a hexanucleotide repeat expansion at C9ORF72, a recently discovered cause of familial frontotemporal dementia/motor neuron disease. This form of frontotemporal dementia should be considered as an important potential differential diagnosis for patients presenting with psychotic symptoms in later life, in whom a detailed family history and thorough cognitive assessment is essential.