What is the differential diagnosis and management for an adult patient with no significant medical history presenting with trismus?

Differential Diagnosis for Trismus in Adults

In an adult presenting with trismus, immediately rule out life-threatening tetanus first, then systematically evaluate for temporomandibular joint disorders, infection, trauma, malignancy, and neuromuscular conditions including myasthenia gravis.

Immediate Life-Threatening Considerations

Tetanus (Cephalic or Generalized)

• Tetanus must be excluded urgently as it presents with trismus and dysphagia as the most common initial symptoms and is rapidly fatal without prompt treatment 1, 2
• Look for recent wounds, inadequate vaccination history, and progression to generalized muscle rigidity 1
• Cephalic tetanus can present with trismus followed by cranial nerve palsies (ptosis, ophthalmoparesis) and seizures 1
• Critical pitfall: Delayed recognition is fatal—successful treatment depends on timely administration of tetanus immune globulin within days of symptom onset 1
• If suspected, immediately administer tetanus immune globulin, metronidazole, and tetanus toxoid vaccine 1

Temporomandibular Joint (TMJ) Disorders

Intra-articular Causes

• TMJ dysfunction, internal derangement, or arthritis can cause mechanical limitation of jaw opening 3, 4
• Perform forced duction testing equivalent for the jaw to distinguish mechanical restriction from muscle-based limitation 5
• Imaging with CT or MRI of TMJ may be needed if structural pathology is suspected 1

Extra-articular Causes

• Myofascial pain dysfunction syndrome affecting masticatory muscles 3, 4
• Trauma causing articulation difficulty or triggering myositis ossificans (rare heterotopic bone formation) 6

Infectious Etiologies

Local Infections

• Odontogenic infections (dental abscesses, pericoronitis) are common causes 3, 4
• Parapharyngeal or retropharyngeal abscess can cause reflex trismus 4
• Preseptal or orbital cellulitis in severe cases 7
• Evaluate for fever, leukocytosis, and localized swelling or erythema 4

Systemic Infections

• Tetanus (discussed above as priority) 1, 2
• Consider Lyme disease and syphilis in appropriate clinical context 8
• Viral etiologies including COVID-19, Epstein-Barr virus 8

Trauma-Related Causes

Direct Trauma

• Mandibular or zygomatic fractures causing mechanical obstruction 5, 4
• CT orbit and maxillofacial bones without contrast with fine cuts and multiplanar reconstructions is the imaging study of choice 5
• Temporomandibular joint dislocation or hemarthrosis 4
• Muscle injury or hematoma in masticatory muscles 4

Iatrogenic Trauma

• Post-dental procedure complications (inferior alveolar nerve block, prolonged mouth opening) 3, 6
• Post-surgical scarring or fibrosis 3, 4
• Radiation-induced fibrosis following head and neck radiation therapy 6, 4

Malignancy

Primary or Metastatic Disease

• Oral cavity, oropharyngeal, or nasopharyngeal carcinoma invading pterygoid muscles or TMJ 6, 4
• Metastatic disease to mandible or skull base 8
• Meningioma or schwannoma of cavernous sinus or orbital apex affecting cranial nerves 8
• Oral submucous fibrosis (precancerous condition) can cause progressive trismus and undergo malignant transformation 6

Neuromuscular Disorders

Myasthenia Gravis

• Variable weakness that worsens with activity and improves with rest is pathognomonic 9
• Trismus may be part of bulbar involvement with difficulty chewing, swallowing, and slurred speech 7, 9
• Perform ice pack test: apply ice over jaw/face for 5 minutes—improvement suggests myasthenia 7, 8
• Look for associated ptosis, diplopia, or variable strabismus that worsens with fatigue 7, 9
• 50-80% of patients with initial ocular symptoms develop generalized myasthenia within a few years, which can progress to life-threatening respiratory failure 7, 9
• Order acetylcholine receptor antibodies (80-88% sensitive for generalized disease) 8
• If seronegative, test for anti-MuSK antibodies and LRP4 antibodies 7, 8
• Single-fiber EMG has >90% sensitivity even in seronegative cases 7, 8
• Obtain chest CT to screen for thymoma (present in 10-15% of myasthenia patients) 8
• Monitor respiratory function immediately—myasthenic crisis is life-threatening 9, 8

Other Neuromuscular Conditions

• Dystonia or extrapyramidal disorders affecting masticatory muscles 4
• Multiple sclerosis (demyelinating disease) in younger patients 8

Drug-Induced Causes

Medication-Related Trismus

• Antipsychotics causing dystonic reactions (olanzapine, other neuroleptics) 1, 4
• Critical warning for myasthenia patients: beta-blockers, IV magnesium, fluoroquinolones, aminoglycosides, and macrolide antibiotics can precipitate myasthenic crisis 9, 8
• Phenothiazines and other dopamine antagonists 4

Inflammatory and Autoimmune Conditions

Systemic Inflammatory Disease

• Thyroid eye disease (TED) can have co-existent myasthenia gravis 7
• Myositis affecting masticatory muscles—check CPK, aldolase if suspected 9, 8
• Scleroderma or other connective tissue diseases 4

Diagnostic Algorithm

Initial Assessment

1. Rule out tetanus immediately: Check vaccination history, recent wounds, presence of generalized rigidity or seizures 1, 2
2. Assess for life-threatening complications: Airway compromise, inability to swallow, respiratory distress 9, 2, 6
3. Perform targeted physical examination:
   • Measure maximum interincisal opening (normal >40mm) 3, 4
   • Palpate masticatory muscles and TMJ for tenderness, swelling 4
   • Examine oral cavity for dental pathology, masses, ulcers 1, 3
   • Check for ptosis, diplopia, variable strabismus suggesting myasthenia 7, 9
   • Perform ice pack test if neuromuscular disorder suspected 7, 8

Imaging Studies

• CT maxillofacial bones without contrast for trauma or structural abnormalities 5
• MRI brain with and without gadolinium plus MRA/CTA if cranial nerve involvement or cavernous sinus pathology suspected 8
• Chest CT if myasthenia gravis suspected (to screen for thymoma) 8

Laboratory Testing

• Complete blood count, inflammatory markers if infection suspected 4
• Acetylcholine receptor antibodies, anti-MuSK, LRP4 if myasthenia suspected 7, 8
• Thyroid function tests (increased risk of myasthenia with autoimmune thyroid disease) 7, 8
• CPK, aldolase, troponin if myositis or myocarditis suspected 9, 8
• Serologic testing for syphilis and Lyme disease in appropriate context 8

Specialized Testing

• Single-fiber EMG if myasthenia suspected and antibodies negative (>90% sensitivity) 7, 8
• Lumbar puncture if infectious meningitis or basilar subarachnoid pathology suspected 8

Management Priorities

Treat life-threatening conditions first: tetanus, airway compromise, myasthenic crisis, or respiratory failure take absolute precedence over symptomatic trismus management 9, 5, 1, 2. Address underlying cause definitively rather than symptomatic treatment alone 3, 4.