The definition of atrophy is a decrease in the size of the tissue brought on by cellular shrinkage. The loss of organelles, cytoplasm, and protein makes cell size smaller [1]. Muscle atrophy, which manifests as muscle wasting and weakness, is classified into physiologic, pathologic, and neurogenic subtypes. It can be brought on by aging, denervation, and physical inactivity [2,3]. Compared to other subtypes, neurogenic atrophy occurs more rapidly and has significantly worse recovery prospects. It is mainly caused by trauma or diseases of the nerve that innervates the muscle [4].

The mandibular nerve, the third branch of the trigeminal nerve, pass through the foramen ovale. Its motor fibers innervate to the masticatory muscles including masseter, temporalis, medial pterygoid, and lateral pterygoid muscles. Several cases of trigeminal neuropathy and atrophy of masticatory muscles caused by brain tumors have been reported, but only one case evaluated the occlusal change [5].

In this paper, we report a case of unilateral trigeminal neuropathy and neurogenic muscular atrophy of the masticatory muscle caused by a tumor in the foramen ovale through which the mandibular nerve passes. This study was approved by the Institutional Review Board (IRB) of Seoul National University Dental Hospital (ERI22032), and the need for obtaining written informed consent was waived by the board.

A 59-year-old man visited the clinic, complaining of difficulty in right-sided mastication. He stated that four years ago, paralysis in the right mandible developed, and he could not give strength to his right side when eating. A clinical examination was performed based on the diagnostic criteria for temporomandibular disorders (DC/TMD). Both the maximal mouth opening and the comfortable mouth opening were normal at 53 mm. The temporomandibular joint (TMJ) capsule and masticatory muscles, could be palpated without causing any pain, and there was no sign of joint noise. The right maxillary first molar and all four second molars were missing and only the left first and second premolars and first molars were occluded. The right masseter and temporalis muscles showed considerable atrophy when the patient’s face was checked from the front.

An imaging analysis was performed. There were no significant findings on standard panoramic radiography, TMJ panoramic radiography, and transcranial view radiography (Fig. 1). Magnetic resonance imaging (MRI) of the TMJ was performed. As a result, anterior disk displacement with reduction of both sides of the TMJ was observed. Also, a mass suspected to be a neurogenic tumor of T2 and proton density weighted signals was observed in the right foramen ovale and cavernous sinus (Fig. 2), and severe atrophy of all right masticatory muscles was observed (Fig. 3). Ultrasonography was used to measure the thickness of both the masseter and temporalis musclesat resting and clenching. At resting, the average thickness of the right side masseter and temporalis is 0.34 cm and 0.75 cm, respectively, thinner than the left side masseter and temporalis muscles, which are 0.97 cm and 0.95 cm. During clenching, the thickness of the left side masseter and temporalis increased to 1.70 cm and 1.05 cm, respectively, whereas the right side muscles did not change (Fig. 4).

To evaluate the sensory function of the trigeminal nerve, qualitative sensory testing and current perception threshold (CPT) tests using Neurometer (Neurotron Inc., Baltimore, MD, USA) were performed in the infraorbital and chin regions. Qualitative sensory testing included contact threshold, direction, two-point discrimination, pin prick, and thermal discrimination. The detailed procedure and method for each sensory test were carried out in the same way as in the previous article [6]. As a result of the sensory test, all tests in the bilateral infraorbital area were normal, whereas, in the chin area, a functional decrease on the right side was observed in all tests. In the CPT test, results of both sides of the maxillary branch were normal, but A-β, A-δ, and C fibers of the right inferior alveolar nerve showed higher thresholds.

The electromyography (EMG) test of the bilateral masseter and temporalis muscles using a portable polysomnography device (Embletta MPR PG and ST+ Proxy; Embla Systems Inc., Kanata, ON, Canada) was performed to evaluate the motor function of the trigeminal nerve. Clenching and resting were repeated three times at 3-second intervals alternately. It was confirmed that the signal change during clenching of the left masseter and temporalis muscles was much more apparent than on the right side, even considering the noise in the right masseter channel (Fig. 5).

The patient was referred to the Department of Neurosurgery for additional assessment and treatment of the brain tumor. As a result of the contrast brain MRI, it was clinically diagnosed as trigeminal schwannoma. The treatment plan was established that considers stereotactic radiosurgery or an endoscopic approach when tumor size or symptoms increase with follow-up at 1-year intervals.

The primary neurological sign of trigeminal neuropathy is hypesthesia in the trigeminal nerve region. Because the mandibular branch forms the fascicular portion where sensory and motor fibers merge before leaving the lower pontine level, the motor and sensory nerves are affected together more frequently than other branches [7]. It has been reported that trigeminal neuropathy of the mandibular branch, combined with motor and sensory symptoms, is caused by extradural tumors, trauma [8], and viral infection [9]. The patient in this report showed trigeminal neuropathy of the mandibular branch induced by the tumor of the foramen ovale and followed the general tendency of trigeminal neuropathy with decreased motor function and hypesthesia.

Among the reports on unilateral trigeminal muscle atrophy, only Bertot et al. [5] evaluated the occlusion. The patient in the previous report was a 16-year-old boy with intracranial trigeminal schwannoma with atrophy of the left masticator muscles, underdeveloped left mandible body and ramus, and underbite occlusion. Underbite usually represents class III malocclusion, but in the context of the previous article, it is thought to represent a posterior open bite due to reduced masticatory force. Unlike the previous article, which had a unilaterally underdeveloped mandible body and ramus,there was no underdevelopment in this report. However, the prior article did not specify whether the patient in our research had a bilateral unilateral posterior open bite. Additionally, additional potential occlusion-influencing variables (e.g., TMJ osteoarthritis, TMJ retrodiscitis) were not evaluated clinically and radiologically in the prior report. Through DC/TMD-based evaluation and imaging such as standard panoramic radiography, TMJ panoramic radiography, transcranial view radiography, and TMJ MRI, we were able to rule out factors other than trigeminal motor neuropathy in this patient that could result in an occlusal shift.

According to a recent study, the masseter muscle in young Korean male adults was 11.3±1.1 mm thick when relaxed and 14.7±1.5 mm when contracted [10]. In our case, the masseter on the left side was identical to the findings of the previous study at 9.7 mm and 17.0 mm during resting and clenching, but the right side was much thinner at 0.34 mm. Further, since there was almost no change in the thickness of the masseter muscle, even during clenching on the right side, it was possible to conclude that the right side’s motor fibers had been chronically denervated inthe same analysis that could be done through the EMG results.

Schwannomas rarely occur in the trigeminal nerve. It accounts for 0.07%-0.3% of intracranial tumors and 0.8%-5% of intracranial schwannomas [11]. Trigeminal schwannomas are rare in children and have the highest incidence in middle age between 38 and 40. The main symptoms include numbness, headaches, gait disturbance, hearing deterioration, diplopia, facial pain, visual deterioration, pathological laughter, proptosis, limb weakness, and seizures [12]. In addition, cranial nerve palsies originating from trigeminal nerve branches are common and may affect the cranial nerves III, IV, VI, and VII-X [12-14]. The clinical symptoms and signs of trigeminal schwannomas depend on the site of the tumor, the extent of its expansion to other brain regions, and the size of the tumor. The possibility of trigeminal schwannomas can be considered when there is the presentation of slow but progressive symptoms such as facial sensory change and atrophic changes of masticatory muscles [12]. In our case, the location of tumor was limited to the foramen ovale region. Therefore, there were no symptoms related to the other cranial nerves and the ophthalmic and maxillary branches of the trigeminal nerve; only numbness and masticatory muscle atrophy of the mandibular branch area were present.

No effective treatment for trigeminal motor neuropathy exists yet [15]. In a previous report, there was no improvement in clinical symptoms even though the tumor size near the foramen ovale was decreased by performing Gamma Knife radiosurgery [7]. Further studies are needed to advance diagnostic criteria and treatment protocols for trigeminal motor neuropathy.

In conclusion, this case shows trigeminal neuropathy caused by a tumor around the foramen ovale that causes the atrophy of the ipsilateral masticatory muscles. For an accurate diagnosis, it is essential to identify the cause of muscle atrophy with neurologic signs through a more detailed clinical examination, including sensory testing and brain MRI, and consider a referral to neurology or neurosurgery for the differential diagnosis of the intracranial disorder.

No potential conflict of interest relevant to this article was reported.

Conceptualization: JH, JWC. Data curation: JH. Formal analysis: JWC. Methodology: JWC. Visualization: JH. Writing original draft: JH. Writing review & editing: JWC.