Late-life decompensation in longstanding obsessive compulsive disorder: a psychiatric prodrome of dementia with Lewy bodies? A case report.

Psychiatric-onset dementia with Lewy bodies (poDLB) is a recognised but poorly characterised prodromal phenotype. The existing literature focuses almost entirely on de novo psychiatric presentations; decompensation of pre-existing primary psychiatric illness as a manifestation of poDLB has not previously been described. We present a case of a patient with a near-lifelong history of severe obsessive compulsive disorder (OCD), decompensation of which in her early 70s was later recognised as a harbinger of dementia with Lewy bodies (DLB). Escalating OCD symptoms, emergent parkinsonism, and cognitive fluctuations culminated in a probable DLB diagnosis within one year of psychiatric presentation, supported by significantly abnormal dopamine transporter imaging. Recognising decompensation of longstanding primary psychiatric illness as a potential manifestation of poDLB has important implications for timely diagnosis, safe prescribing (particularly neuroleptics) and early biomarker-driven referral.