The importance of longitudinal evaluation using DAT‐SPECT in organophosphate‐induced toxic parkinsonism

Background: Clinicians encounter considerable challenges in the diagnosis and treatment of organophosphate-induced toxic parkinsonism following acute poisoning. Here, we report the importance of longitudinal evaluation of a patient with organophosphate-induced toxic parkinsonism using dopamine transporter single-photon emission computed tomography (DAT-SPECT). This case may provide crucial insights into the pathophysiology of the syndrome and inform therapeutic decision-making. Case Presentation: We report a case of organophosphate-induced toxic parkinsonism with persistent parkinsonian symptoms evaluated longitudinally using DAT-SPECT. A 68-year-old woman with a history of bipolar disorder was found collapsed at home on Day 0 with impaired consciousness, miosis, hypersalivation, and bradycardia. Organophosphate poisoning was diagnosed based on markedly reduced cholinesterase levels and the presence of an organophosphate pesticide in a shed at her home. Parkinsonian symptoms gradually emerged from Day 18, including hypophonia and a gait disturbance. On Day 60, DAT-SPECT showed reduced radiotracer uptake in the bilateral striatum, more pronounced on the right. After toxic parkinsonism was diagnosed, l-dopa was titrated up to 600 mg/day without clinical benefit and discontinued due to adverse effects. Treatment with amantadine and trihexyphenidyl was then initiated, resulting in partial symptomatic improvement, although symptoms persisted. Despite clinical improvement, follow-up DAT-SPECT on Day 131 showed a further reduction in striatal tracer uptake, whereas quantitative improvement was observed on Day 200. Conclusion: Parkinsonism is an uncommon sequela of organophosphate poisoning and may be accompanied by abnormal DAT-SPECT findings. Moreover, amantadine and trihexyphenidyl may represent effective therapeutic options for this condition.