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Trismus

Trismus is an inability to open the jaw due to tonic spasm or contracture of the masticatory muscles, principally masseter and temporalis, effecting forced jaw closure ("lockjaw").

Recognized causes and associations of trismus include:
Dystonia of the jaw muscles (e.g., drug-induced dystonic reaction) Generalized tonic-clonic epileptic seizure
Neuromuscular diseases: polymyositis, tetanus, nemaline myopathy, trauma to the muscles of mastication, rabies, strychnine poisoning
Infection in the pterygomandibular space Metabolic disorders: Gaucher’s disease (type II)
Central disorders: brainstem encephalopathy, multiple sclerosis, pseudobulbar palsy.