Bilateral neurogenic atrophy of the masticatory muscles

A 61-year-old woman with marked limitation in opening her mouth to 1 cm, along with strength loss in her jaws since the age of 1, was referred to our hospital for MRI. There were no previous surgeries or significant medical history. Blood tests were unremarkable.

MRI revealed a severe, bilateral and symmetric atrophy of muscles of mastication (masseter, temporal, anterior digastric, medial and lateral pterygoid muscles), along with fatty infiltration, suggesting bilateral neurogenic (trigeminal) atrophy.
The muscles floor of the mouth, intrinsic muscles of the tongue, posterior digastric, levator veli palatini, styloglossus and stylohyoideus muscles were unaffected and slightly hypertrophic.
There were no significant alterations of the buccinator muscle and orbicular muscle of the lip. The rest of the examination was unremarkable.

In the acute phase of denervation, the signal intensity of the muscles may be normal on MRI [1]. The earliest detectable change in denervated muscles is a diffuse increased T2-weighted signal (due to oedema), which may occur without a T1-weighted signal change [1, 2, 3]. Atrophic changes in muscle are reflected by decreased muscular mass and increased intramuscular, linear, T1-weighted signal due to fatty infiltration.
The muscles of mastication are innervated by the trigeminal nerve which is the fifth cranial nerve.
The extracranial trigeminal nerve has three major divisions: V1, V2, and V3, the mandibular nerve's major branches are the auriculotemporal, inferior alveolar and lingual nerves. A separate motor division innervates the temporal and masseteric muscles and the tensor tympani, pterygoid, mylohyoid, and tensor veli palatine muscles. The mandibular nerve also has meningeal branches. Lesions of the V3 may result from dentistry, implantation, mandible resection, haematoma of the lower lip, or bites [4].
The symptoms of trigeminal nerve lesions are predominately sensory and rarely motor.
Trigeminal motor lesions are rarely symptomatic and could cause a mono or diplegia masticatoria [4]. When the patient's mouth is opened widely, the jaw will deviate to the affected side. In our case, there was no significant deviation with the mouth opened; suggesting a bilateral, symmetrical neurogenic atrophy.
There are several factors conditioning neurogenic atrophy, however, the great majority of them are unilateral: traumatic, iatrogenic, congenital, exposition to head and neck radiation, trigeminal nucleus lesions (tumoral/vascular) or at any point along the trajectory of the nerve (mainly if it affects V3), compressive, toxic, cavernous sinus lesions (V1 e V2) and temporomandibular joint dysfunction.
All the above causes can be excluded in this case because no previous history of surgery/trauma was known and imaging findings did not reveal any additional alteration.
An inflammatory/immune mediated aetiology can also be questionable (sensory trigeminal neuropathy, Sjorgen, scleroderma, demyelinating disorders, among others), however, these pathologies usually affect one or two nerve branches unilaterally, causing numbness and pain, which were not observed in our case.
Consequently, all these entities were clinical (the patient did not refer any symptoms and no systemic disease was found) and CT/MR excluded and also laboratory work up was normal.
Subsequently, the most probable cause of this case of neurogenic atrophy might be an infectious (viral) aetiology, which is extremely rare, because, in this case, it caused a bilateral and symmetric trigeminal involvement. Idiopathic aetiology can also be considered as an exclusion diagnosis.