Jaw Muscle Weakness

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