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.