Tumors originating from the facial nerve sheath membrane are called nerve sheath membrane tumors or Schwannomas (Schwannoma). They can occur along any segment of the facial nerve fibers but are more commonly found around the geniculate ganglion. Schmidt first reported this condition in 1930, and to date, fewer than 300 cases have been documented worldwide, with only slightly over 20 cases reported domestically. According to Saito (1972), an examination of 600 temporal bone specimens revealed 5 cases, indicating an incidence rate of 0.8%. The clinical rarity of this condition can be attributed to the following reasons: ① The tumor grows slowly and is asymptomatic in its early stages. ② When deviation of the mouth occurs, it is often misdiagnosed as Bell's palsy or chronic otitis media. ③ Granulation tumors discovered during surgery are seldom sent for pathological examination. Most patients are adults, with the youngest case reported in the literature being a 4-year-old child.

bubble_chart Clinical Manifestations

Tumor biology is very slow, often asymptomatic for long periods. Tumors originating in the horizontal segment, due to the narrow bony canal, experience compression earlier and more severely than those in the vertical segment, leading to earlier and more pronounced deviation of the mouth, which can recur repeatedly. Additionally, 2/5 of patients initially present with facial muscle spasms, which later progress to deviation of the mouth. The author treated one case where deviation of the mouth recurred four times over 14 years and was misdiagnosed as Bell's deviation of the mouth. Therefore, in cases of Bell's deviation of the mouth that do not recover over time, the possibility of this disease should be considered. Jongkees found facial nerve tumors in 4 out of 250 cases (1.6%) during surgery for Bell's deviation of the mouth. Tumors originating in the tympanic segment, besides causing deviation of the mouth, may also lead to tinnitus and deafness. If the tumor originates in the internal auditory canal, it can easily be confused with acoustic neuroma. Pulec estimated that about 5% of patients clinically diagnosed with acoustic neuroma may actually have facial nerve sheath tumors.

A comprehensive examination of facial nerve function should be conducted, including tests for lacrimal gland and submandibular gland secretion, stapedius reflex, and tongue taste sensation. X-rays of the skull base and mastoid process may reveal bone destruction in the facial nerve canal. CT scans of the mastoid and skull base provide greater diagnostic value.

bubble_chart Treatment Measures

Depending on the size and location of the tumor, smaller ones can be removed via tympanotomy, while larger ones may require excision through a postauricular mastoid approach. For tumors occurring at the skull base, a middle cranial fossa approach via the temporal region can be used. Although facial nerve tumors may cause deviation of the mouth for many years, facial muscle atrophy is rare. If facial nerve anastomosis or nerve grafting is performed postoperatively, the outcomes are generally better than those for traumatic deviation of the mouth. According to Fisch's statistics, 60% of patients can recover 75% of their original nerve function after surgery.