A 32-year-old man with schizophrenia has been on risperidone 6 mg/day for 8 months. He presents with complaints of progressive difficulty in initiating movements, muscle stiffness, and tremor at rest. His wife notes he has become emotionally withdrawn and apathetic over the past 3 months. On examination, he has cogwheel rigidity in both upper limbs, resting tremor, and bradykinesia. His temperature is 37.2°C, and creatine kinase is 245 U/L. What is the most likely diagnosis?

Explanation

## Diagnosis: Antipsychotic-Induced Parkinsonism ### Clinical Presentation This patient presents with the classic triad of Parkinson-like symptoms: 1. **Bradykinesia** — difficulty initiating movements 2. **Rigidity** — cogwheel rigidity on examination 3. **Tremor** — resting tremor Additionally, he exhibits **negative symptoms** (apathy, emotional withdrawal) which are common in antipsychotic-induced parkinsonism and reflect dopamine blockade in the mesolimbic pathway. ### Key Distinguishing Features | Feature | Parkinsonism | NMS | Tardive Dyskinesia | Akathisia | |---------|-------------|-----|-------------------|----------| | **Onset** | Days to weeks | Hours to days | Months to years | Hours to days | | **Temperature** | Normal | Elevated (>38.5°C) | Normal | Normal | | **CK level** | Normal | Markedly elevated (>1000) | Normal | Normal | | **Rigidity** | Yes (cogwheel) | Yes (lead pipe) | No | No | | **Tremor** | Resting | Absent | Choreiform | Absent | | **Consciousness** | Clear | Altered/confused | Clear | Clear | | **Reversibility** | Yes (anticholinergic) | Requires drug cessation | Difficult/irreversible | Yes (beta-blocker) | **Key Point:** Antipsychotic-induced parkinsonism occurs in 20–30% of patients on typical antipsychotics and 5–10% on atypical agents. It is **reversible** with dose reduction or anticholinergic agents (benztropine, trihexyphenidyl). **High-Yield:** The presence of **normal temperature** and **normal CK** rules out neuroleptic malignant syndrome. The **resting tremor and cogwheel rigidity** (not choreiform movements) rule out tardive dyskinesia. The **clear sensorium and lack of inner restlessness** rule out akathisia. **Clinical Pearl:** Risperidone, despite being an atypical antipsychotic, carries a higher risk of extrapyramidal side effects (EPS) than quetiapine or clozapine, particularly at doses >4 mg/day. ### Mechanism Dopamine D2 antagonism in the nigrostriatal pathway leads to loss of dopaminergic inhibition of cholinergic neurons in the basal ganglia, resulting in relative cholinergic excess. ### Management - **First-line:** Anticholinergic agents (benztropine 1–2 mg BD or trihexyphenidyl 2–5 mg TDS) - **Alternative:** Reduce antipsychotic dose or switch to an atypical with lower EPS risk (quetiapine, clozapine) - **Prognosis:** Symptoms resolve within days to weeks after intervention