I. Pathogenesis: Most tumors originate from the vestibular part of the auditory nerve, 3/4 originate from the superior vestibular nerve, and a few from the cochlear part. Tumors originate from Schwann cells, about 3/4 of tumors occur in the lateral part of the internal auditory canal, and only 1/4 occur in the medial part. As the tumor grows, it can cause enlargement of the internal auditory canal, protrude into the pontocerebellar horn and fill the pontocerebellar horn. Most of the tumors are unilateral and a few are bilateral; the opposite is true in the presence of neurofibromatosis. The incidence of bilateral occurrence varies, but in general, the incidence of left and right side is almost equal. The pathogenesis: Auditory nerve sheath tumor causes cerebellopontocerebellar syndrome, including: dysfunction of the vestibular and cochlear parts of the nerve. The manifestations include dizziness and vertigo. The symptoms include dizziness, vertigo, tinnitus, deafness, irritation or paralysis of adjacent cranial nerves, balance disorders, unstable walking, and symptoms of increased intracranial pressure. However, the actual symptoms are not the same and can be mild or severe, which are mainly related to the starting site, growth rate, development direction, tumor size, blood supply and whether there is cystic change. At the beginning of the tumor, the vestibular part of the patient is the first to be damaged, therefore, at the early stage, there are thirty or diminished vestibular functions and irritation or partial paralysis of the cochlear nerve (early symptoms are dizziness, vertigo, tinnitus, deafness, etc.). With the growth of tumor, the anterior pole of tumor can touch the sensory root of trigeminal nerve and cause ipsilateral facial pain, facial hypesthesia, dull or loss of corneal reflex, hypesthesia of tongue tip and tongue side, if the motor root of trigeminal nerve is also affected, the patient can have weakness of ipsilateral masticatory muscles, open mouth and jaw to the affected side, atrophy of masticatory muscles and temporal muscles. Pathology: Auditory nerve sheath tumor has a complete envelope, and the surface is mostly smooth. The shape and size of the tumor depends on the growth of the tumor. Generally, after the clinical diagnosis is established, the volume of the tumor is more than 2.5cm in diameter. The parenchymal part of the tumor is grayish yellow to grayish red in color, firm and brittle in texture. The tumor tissue often contains cystic cavities of varying sizes, containing yellowish transparent cystic fluid and sometimes fibrin clots. The tumor is tightly adherent to the cerebellar adventitia, but generally does not invade the cerebellar parenchyma and is clearly demarcated. One corner of the tumor penetrates deeply into the internal auditory canal, enlarging its opening, where the meninges are often closely adherent to the tumor. The facial nerve is closely attached to the medial side of the tumor, and because of the large number of adhesions, it is often impossible to distinguish it with the naked eye, making surgical preservation of the facial nerve a challenge. The histological features of auditory nerve sheath tumors can be summarized into four types: 1) small fence-like arrangement of tumor cells; 2) intertwined fiber bundles; 3) foci of degeneration and small pigmented areas; 4) fence-like arrangement of foam cells and nuclei, and parallel protofibers of cells, with intertwined cell bundles and protofibers. This protofibrillar quasi-polar arrangement of tumor cells is called Antioni A tissue, while Antioni B tissue has a loose reticulated non-polar arrangement, which is also called mixed type. Regardless of the composition of the tumor, the interstitium is composed of fine reticular fibrous tissue with few collagen fibrils, and may be accompanied by various degenerative changes such as fatty changes, chromatophores and small areas of hemorrhage.