In a patient undergoing chemotherapy who develops new‑onset involuntary jaw movements (clenching, chewing‑like motions, or rhythmic twitching), what is the most likely cause and how should it be managed?

Involuntary Jaw Movements Associated with Chemotherapy

Most Likely Diagnosis: Acute Dystonia from Antiemetic Medications

The involuntary jaw movements are most likely acute dystonia caused by antiemetic medications (particularly metoclopramide or other dopamine antagonists) rather than the chemotherapy itself. 1, 2

Key Clinical Features to Identify

• Timing: Symptoms typically occur within the first 24-48 hours after antiemetic administration 1
• Movement patterns: Involuntary motor tics or spasms involving the face, jaw clenching, rhythmic tongue protrusion, chewing-like motions, trismus (jaw spasm), or grimacing 1, 2
• Associated symptoms: May include oculogyric crisis (eye deviation), torticollis (neck twisting), or bulbar-type speech 1, 2
• Risk factors: More common in patients under 30 years of age, occurs in approximately 1 in 500 patients receiving standard metoclopramide doses 2

Immediate Management Algorithm

Step 1: Confirm the diagnosis by reviewing recent antiemetic use 1, 2

• Check for metoclopramide, prochlorperazine, promethazine, or droperidol administration within the past 48 hours
• Higher chemotherapy doses (used for nausea prophylaxis) increase dystonia risk 2

Step 2: Administer immediate pharmacologic reversal 1, 2

• Diphenhydramine (Benadryl) 50 mg intramuscularly - first-line treatment, symptoms usually subside rapidly 1, 2
• Alternative: Benztropine (Cogentin) 1-2 mg intramuscularly can also reverse the reaction 1, 2

Step 3: Discontinue the offending antiemetic 2

• Switch to a different antiemetic class (5-HT3 antagonists like ondansetron or palonosetron are less likely to cause extrapyramidal symptoms) 1

Critical Pitfall: Life-Threatening Laryngeal Dystonia

Assess immediately for laryngeal involvement by asking about choking sensation, difficulty breathing, or listening for stridor 1. Laryngeal dystonia is rare but potentially fatal and requires emergent airway management 1.

Alternative Diagnoses to Exclude

While acute dystonia from antiemetics is most common, consider:

Tardive dyskinesia 1, 2

• Develops after prolonged antipsychotic or antiemetic use (typically months to years, not days)
• Characterized by rapid involuntary facial movements including blinking, grimacing, chewing, or tongue movements
• Risk increases with duration of treatment (5% per year in young patients) 1
• Key difference: TD develops gradually over time, not acutely after chemotherapy

Drug-induced Parkinsonism 1, 2

• Presents with bradykinesia, tremor, cogwheel rigidity, or mask-like facies
• Usually occurs within first 6 months of treatment 2
• Key difference: Parkinsonian symptoms are slower movements, not involuntary spasms

Chemotherapy-induced focal dystonia 3

• Extremely rare case reports with 5-FU and doxorubicin
• Onset ranges from 8 days to 34 months after chemotherapy completion 3
• Key difference: This is a delayed phenomenon, not acute

Prevention for Future Chemotherapy Cycles

• Avoid metoclopramide and other dopamine antagonists in patients who developed acute dystonia 2
• Use 5-HT3 receptor antagonists (ondansetron, granisetron, palonosetron) combined with dexamethasone and NK-1 antagonists (aprepitant) for antiemesis 1
• Document the dystonic reaction prominently in the medical record to prevent re-exposure 2

When to Suspect Other Causes

If symptoms do not resolve rapidly with diphenhydramine or benztropine, or if the patient has not received dopamine antagonist antiemetics, consider neurologic consultation to evaluate for:

• Neuroleptic malignant syndrome (if accompanied by fever, rigidity, altered consciousness, and autonomic instability) 1, 2
• Central nervous system metastases
• Metabolic derangements (hypocalcemia, hypomagnesemia)