CLINICAL DIAGNOSTIC MANUAL
Jaw muscle weakness, also known as masticatory weakness, is a condition that can significantly impact a person’s ability to chew and speak. The masticatory muscles are innervated by the motor branch of the trigeminal nerve (cranial nerve V), specifically its mandibular branch.
Weakness may result from lesions at different levels of the motor pathway, either at the lower motor neuron (peripheral nerve level) or the upper motor neuron (central level). Identifying the underlying cause is crucial to providing appropriate treatment and improving jaw function.
Condition | Clinical Symptoms and Signs | Suspected Diagnosis | Confirmatory Diagnosis |
|---|---|---|---|
Lower motor neuron lesion of the trigeminal motor branch | Weakness in the masticatory muscles (masseter, temporalis, pterygoids), muscle atrophy, difficulty chewing, jaw deviation toward the affected side on opening | Clinical history, focal neurological exam | EMG, nerve conduction studies, cranial MRI or CT |
Upper motor neuron lesion of the trigeminal motor branch | Weakness in masticatory muscles, spasticity, signs of corticobulbar pathway involvement (pseudobulbar palsy) | Clinical history, neurological exam showing upper motor neuron signs | Brain MRI, functional imaging studies |
Myasthenia gravis | Progressive muscle fatigue, weakness worsening with use, improving with rest, ptosis, dysarthria | Clinical history, ice pack test, edrophonium test | Antibody tests (AChR, MuSK), EMG, repetitive nerve stimulation test |
Amyotrophic lateral sclerosis (ALS) | Progressive weakness, fasciculations, muscle atrophy, dysarthria, dysphagia | Clinical history, neurological exam showing upper and lower motor neuron signs | EMG, brain and spinal MRI |
Tumours of the posterior fossa or skull base | Unilateral jaw weakness, facial pain, additional neurological symptoms (headache, vertigo) | Clinical history, focal neurological signs | Skull base and posterior fossa CT or MRI, biopsy if needed |
Stroke (CVA) | Sudden neurological deficit, jaw muscle weakness, dysarthria, facial paralysis | Clinical history, signs of acute neurological deficit | Brain CT or MRI, cerebral angiography |
Craniofacial trauma | Jaw muscle weakness, possible jaw fracture, pain, swelling | Trauma history, physical exam, signs of fracture | Cranial CT, evaluation by maxillofacial surgeon |
Poliomyelitis | Muscle weakness, atrophy, fever, malaise, signs of lower motor neuron involvement | History of poliovirus contact, physical exam | Serological tests for poliovirus, EMG |
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