To the Editor: Delusional misidentification syndromes (DMS) are unusual in Parkinson's disease (PD). We describe a patient with advanced PD and combined Capgras and intermetamorphosis syndromes for whom known predisposing factors appear to be insufficient to explain DMS in PD. Rather, delayed habituation induced by abnormalities of dopaminergic or cholinergic systems may represent a novel mechanism to account for DMS. Moreover, this may represent a unifying feature of DMS across neuropsychiatric disorders.

DMS consist of a group of disorders characterized by the belief that familiar people, places, objects, or events have been reduplicated. Few reports of DMS in Parkinson's disease (PD) exist. We describe such a patient, and propose a novel mechanism to account for DMS across a range of neuropsychiatric disorders.

A 68-year-old Caucasian man with an 11-year history of PD (Hoehn & Yahr Stage 4, on L-dopa 50/carbidopa 200 mg QID and midrodine 2.5 mg TID) and no previous psychiatric history developed visual hallucinations (VH) 18 months before presentation. Six months before presentation, he began claiming his wife had been replaced by an identical, eponymous imposter. Less frequently, he called her by one of his daughters' names, and denied this was due to mispeaking. His wife successfully disrupted those phenomena by leaving his presence and immediately returning. He denied mood disturbance. Quetiapine was titrated to 50 mg HS PRN with no exacerbation of extrapyramidal symptoms. VH decreased in frequency but misidentification continued unabated. Donepezil 5 mg was started 6 months before presentation. Non-contrast head CT showed diffuse atrophy and mild microvascular disease. His medical history was otherwise signficant for type II diabetes and hypothyroidism.

Examination revealed bilateral resting tremor and facial masking. Mini-Mental State Exam score was 24/30. Neuropsychological evaluation elicited impairments of visuospatial processing and executive functions, and milder deficits in visual memory and language. Verbal memory was unremarkable. Fourteen months later, he developed aspiration pneumonia and died.

This patient manifested combined Capgras and intermetamorphosis syndromes. DMS are known to occur frequently in a wide range of neuropsychiatric disorder,¹ but in PD appear to be unusual, typically emerging in advanced disease; cognitive dysfunction and VH are frequent associated features.² Of DMS subtypes reported in PD, Capgras is the most frequent,² in contrast, no previous reports of inter-metamorphosis exist. One case of PD with Fregoli syndrome was precipitated by dopamine agonist augmentation combined with an anticholinergic medication, and resolved with adjustment of the offending agents.³

In our case, and more generally among patients with PD, L-dopa administration and cognitive impairment may represent predisposing factors, but we argue that they are insufficient to explain DMS. Based on the alteration of observed behavior in response to his spouse's intervention, we propose a novel mechanism as a critical feature underlying DMS phenomena: altered habituation. Responses to stimulus novelty are mediated by dopaminergic⁴ and cholinergic systems.⁵ In advanced PD, excessive dopamine associated with L-dopa administration, diminished acetylcholine, or both, may cause delayed habituation, effectively rendering familiar stimuli to be experienced as persistently or repeatedly novel, which, via interaction with severely disordered visuospatial processing, may lead to misinterpretation of visual features, manifesting as DMS. Excessive dopamine or diminished acetylcholine occur in a range of neuropsychiatric disorder; thus, we argue that this may represent a common mechanism and unifying feature of DMS in those conditions. Altered habituation further suggests a potential mode of intervention to reduce DMS-related symptoms.

This study was not funded. The authors have no financial conflicts of interest to report.