Trismus

Trismus is restricted mouth opening from a disorder of the muscles, joints or soft tissues of mastication. It is defined as a maximum interincisal opening under 35 mm in adults (normal 40 to 55 mm), and under 20 mm is severe. It is a symptom, and the cause decides urgency.

Causes by frequency:

• Odontogenic infection and pericoronitis of the lower third molar, and deep space infection.
• Surgical trauma and haematoma after third molar surgery or an inferior alveolar nerve block.
• Temporomandibular joint disease (disc displacement without reduction, arthritis) and masticatory muscle pain.
• Trauma and fracture (zygomatic complex, arch, mandible, condyle).
• Radiotherapy to the head and neck, and tumours of the oropharynx, masticator space and infratemporal fossa.
• Fibrosis from oral submucous fibrosis and scleroderma.
• Tetanus, drug induced dystonia and, rarely, neuroleptic malignant syndrome.

Pathophysiology. Inflammation or irritation of the masseter, medial pterygoid and temporalis causes reflex spasm. Fibrosis from radiation, injury or chronic disease shortens muscles and capsule. Mechanical block arises from bone impingement, joint ankylosis or a space occupying mass.

Tetanus toxin blocks inhibitory neurotransmitters and causes sustained masseter contraction (lockjaw), often the first sign.

Focused history: onset (sudden or gradual), pain, fever, swelling, trauma, recent dental work or injection, joint noises and locking, previous radiotherapy, tobacco or areca nut use, drugs (antipsychotics, metoclopramide), wounds and tetanus immunisation status, and difficulty swallowing or breathing.

Examination: measure the interincisal opening in millimetres with a ruler. Note deviation, pain on opening, a hard or soft end feel, dental sources (pericoronitis, abscess), facial swelling, floor of mouth, oropharynx, neck nodes, joint tenderness and crepitus, mucosal banding, and cranial nerves.

Airway threat (drooling, stridor, floor of mouth or neck swelling), fever with spreading swelling, painful muscle spasms with a recent wound (tetanus), trauma with malocclusion, neurological signs, and progressive painless trismus with a mass need same day escalation.

• Step 1: exclude emergencies: airway, deep neck infection, tetanus and fracture. Check temperature, heart rate, wounds and vaccination history.
• Step 2: dental examination and panoramic radiograph for pericoronitis, abscess, fracture and joint changes.
• Step 3: contrast CT of the face and neck for suspected deep space infection, fracture, tumour or radiation change. MRI for joint disc and soft tissue masses.
• Step 4: bloods: full blood count, C-reactive protein, glucose and, for suspected tetanus, a clinical diagnosis without waiting for tests.
• Step 5: biopsy for any mass or mucosal change, with specialist assessment.
• Review drugs: dystonia responds to an anticholinergic within minutes.

Differential diagnosis.

• Pericoronitis and odontogenic space infection
• Post-surgical muscle spasm and haematoma
• Disc displacement without reduction and TMJ arthritis
• Zygomatic complex or condylar fracture
• Radiation fibrosis and tumour of the oropharynx or infratemporal fossa
• Oral submucous fibrosis and scleroderma
• Tetanus
• Drug induced dystonia
• TMJ ankylosis

Immediate:

• Secure the airway and treat sepsis if present. Urgent admission for deep space infection with drainage and antibiotics: benzylpenicillin 1.2 g intravenously every 6 hours (or amoxicillin 1 g every 8 hours) with metronidazole 500 mg intravenously every 8 hours. Penicillin allergy: clindamycin 600 mg intravenously every 6 to 8 hours.
• Tetanus: admit to intensive care, secure the airway, give human tetanus immunoglobulin 500 IU intramuscularly, metronidazole 500 mg intravenously every 8 hours for 7 to 10 days, muscle relaxation with diazepam 10 mg intravenously titrated to effect, and magnesium sulphate 40 mg/kg intravenously over 30 minutes then 1 to 2 g each hour titrated to spasm control with monitoring of reflexes and respiration. Give tetanus toxoid after recovery.
• Drug induced dystonia: benzatropine 1 to 2 mg intravenously or intramuscularly, or procyclidine 5 mg intramuscularly, repeated once if needed, and stop the cause.

By cause:

• Pericoronitis: irrigate under the flap with chlorhexidine 0.2%, debride, and remove the cause. Amoxicillin 500 mg three times daily with metronidazole 400 mg three times daily for 5 days when there is spread or systemic upset.
• Post-surgical trismus: ibuprofen 400 mg every 8 hours with food and paracetamol 1 g every 6 hours (maximum 4 g daily), heat, and jaw stretching begun early. A single dexamethasone dose of 4 to 8 mg given before surgery reduces trismus and swelling.
• Muscle spasm and TMJ disease: splint, physiotherapy, short NSAID course and, in selected patients, cyclobenzaprine 5 to 10 mg at night for up to 2 weeks.
• Radiation trismus: daily jaw stretching with a device or stacked spatulas, started early and continued for 6 to 12 months, with physiotherapy. Pentoxifylline 400 mg orally two to three times daily with tocopherol 500 to 1000 IU daily for at least 6 months has been used for fibrosis, with limited evidence, and is avoided with bleeding risk and severe renal impairment.
• Oral submucous fibrosis: stop areca nut and tobacco, and intralesional triamcinolone or dexamethasone with hyaluronidase, with physiotherapy. Evidence is limited.
• Fracture, ankylosis, tumour: surgical or oncological management by the specialty concerned.

Monitoring. Record opening in millimetres at every visit. Review in 48 hours for infection and at 1 to 2 weeks otherwise. Persistent trismus beyond 2 to 4 weeks without a clear cause needs imaging and biopsy.

Referral and escalation. Same day referral to oral and maxillofacial surgery for infection, trauma or airway concern, and to critical care for tetanus. Refer to oncology for mass or radiation fibrosis, and to physiotherapy for exercise treatment.