Zonisamide as an Adjunctive Treatment to Balance Psychiatric and Motor Symptoms in a Patient With Parkinson’s Disease Dementia: A Case Report

Psychiatric symptoms such as hallucinations, delusions, and impulsive behaviors frequently complicate the clinical course of Parkinson's disease (PD), particularly in patients with PD dementia (PDD).Clinicians often face a therapeutic dilemma when treating these patients: reduction in antiparkinsonian medications may alleviate psychiatric symptoms but can significantly worsen motor function.Zonisamide, a sulfonamide antiepileptic drug, is approved in Japan as adjunctive treatment for motor symptoms in PD; however, its role in balancing motor and psychiatric symptoms in PDD has not been well established.We report the case of a 55-year-old man with PD who experienced progressive motor symptoms accompanied by hallucinations, irritability, impulsivity, and cognitive decline and was subsequently diagnosed with PDD.Gradual reduction and discontinuation of antiparkinsonian medications (levodopa/carbidopa and trihexyphenidyl) resulted in improvement of impulsive behaviors but caused marked worsening of tremor and rigidity.Treatment with rivastigmine led to an improvement in hallucinations; however, reintroduction of antiparkinsonian medication raised concern about recurrence of psychiatric symptoms.Levodopa/carbidopa was therefore reintroduced and titrated under adjunctive zonisamide (25 mg once daily).Motor symptoms gradually improved to a clinically acceptable level without recurrence of psychiatric symptoms, whereas cognitive impairment persisted.Adjunctive zonisamide may help balance motor and psychiatric symptom management in PDD when adjustment of antiparkinsonian medications alone is challenging.This observation does not indicate that zonisamide improves dementia itself.Further studies are warranted.