【Overview】 Intracranial nerve sheath tumors account for about 10% of all brain tumors, while acoustic neuromas account for more than 90% of intracranial nerve sheath tumors, most of which occur in the vestibular portion of the auditory nerve, and a few in the cochlear portion of the nerve, and they are most prevalent in the middle-aged people, with a peak age of 30-50 years old. Most of the acoustic neuromas are unilateral, and a few are bilateral. Auditory neuroma has a complete envelope, the surface is mostly smooth, sometimes it can be nodular, and its shape and size depend on the growth time of the tumor. The parenchymal part of the tumor is grayish-yellow to grayish-red in color, and the texture is firm and brittle, which is easy to clamp, but some of them are tougher. The tumor tissue often contains cysts of varying sizes, ranging from several millimeters to several centimeters in diameter, containing yellowish transparent cystic fluid. The tumor adheres tightly to the cerebellum adjacent to the cerebellum, but generally does not invade the cerebellar parenchyma and is well demarcated. As the tumor grows outside the arachnoid membrane, with the gradual increase of the tumor, the surface of the tumor is covered with a layer of arachnoid membrane, sometimes wrapped with a certain amount of cerebrospinal fluid, which seems to be like an arachnoid cyst at first glance. In the process of growth, the cochlear and facial nerves accompanying the vestibular branch of the auditory nerve travel in the interlayer of the tumor peritoneum and arachnoid membrane. The blood supply to the tumor is primarily from the anterior inferior cerebellar artery. This artery divides laterally from the lower 1/3 of the basilar artery, splits into a branch close to the tumor to enter the tumor envelope, and then divides into several smaller branches to enter the tumor tissue. In addition, the superior cerebellar artery, the pontine artery, the internal auditory artery, and the posterior inferior cerebellar artery, which branches off from the basilar artery, may also have branches that supply the tumor. The surface in contact with the cerebellum also receives blood supply from the superficial cerebellar arteries. The main venous return of the tumor enters the superior petrosal sinus through the petrosal vein. Clinical manifestations The course of the disease of acoustic neuroma is very long, and the period from onset to hospitalization is 3.6-4.9 years, and the duration of the symptoms may vary from a few months to more than ten years. The first symptoms of the disease are almost always those of the auditory nerve itself, including dizziness, vertigo, unilateral tinnitus and deafness. The tinnitus is a high-pitched, cicada-like or siren-like sound that is continuous and often accompanied by hearing loss. Decreased ability to recognize speech in noisy environments is typical of early hearing loss in patients with acoustic neuroma. As the dizziness and vertigo of this disease are not intense and not accompanied by nausea and vomiting, they are often neglected by patients. Auditory neuroma mainly caused cerebro-brachio-cerebellar triangle signs, including dysfunction of vestibular and cochlear branches of the auditory nerve, irritation or paralysis of adjacent cerebral nerves, cerebellar symptoms, brainstem symptoms (including dysfunction of long conduction tracts), and symptoms of increased intracranial pressure in the late stage, etc. Cushing described the symptoms of acoustic neuroma in detail, and the procedure of the symptoms was as follows: (1) the symptoms of cochlea and vestibular nerves; (2) the cerebellar symptoms of the cerebellum; (3) the cerebellar symptoms of the cerebellum; (4) the symptoms of the cerebellum. Symptoms of cochlear and vestibular nerves; ② cerebellar ataxia; ③ symptoms of damage to neighboring cerebral nerves, such as facial muscle twitching, facial hypesthesia, peripheral facial paralysis, etc.; ④ symptoms of increased intracranial pressure; ⑤ symptoms of late stage, such as difficulty in swallowing, choking on food and coughing, etc., and finally, the emergence of cerebellar crises, respiratory distress, etc. However, it only applies to some typical cases. However, it is only applicable to certain typical patients with acoustic neuroma, and atypical patients often do not meet it. According to the size of the tumor and its corresponding clinical manifestations, the development process of the tumor can be divided into 4 stages: Stage I: intracanalicular type, diameter <1cm, located in the inner ear canal, with only the manifestation of damage to the auditory nerve. Stage 2: small tumor, 1-2 cm in diameter, in addition to auditory nerve symptoms, there may be adjacent cerebral nerve symptoms such as trigeminal nerve and cerebellar dysfunction, but there is no increase in intracranial pressure, mild increase in protein content in cerebrospinal fluid, and enlargement of the internal auditory canal. Stage III: medium-sized tumor with a diameter of 2-3 cm, in addition to the above symptoms, there are effects on the posterior cranial nerves and brainstem function, cerebellar symptoms are more obvious, and there are different degrees of increased intracranial pressure, enlarged internal auditory canal and bone resorption. Stage 4: large tumor, the disease development has reached the advanced stage, the obstructive hydrocephalus manifests seriously, the damage to the brainstem is also very obvious, some even have consciousness disorder or even unconsciousness, and may have ankylosing-like tonic seizures. Differential diagnosis For adults with unexplained tinnitus accompanied by progressive hearing loss, full attention should be paid and the possibility of acoustic neuroma should not be easily excluded, and the following examinations should be carried out. 1, hearing test can distinguish whether the deafness is conductive or sensorineural. Air conduction < bone conduction in conductive deafness and air conduction > bone conduction in sensorineural deafness. Comparison of bone conduction test (Weber), conductive deafness sound favoring the affected side, sensorineural deafness sound favoring the healthy side. Electro audiometry is more accurate and has greater value in the early diagnosis of acoustic neuroma, and the following characteristics can be obtained: (1) the patient’s hearing loss is consistent with neural sensorineural deafness; (2) there is no resonance phenomenon in the binaural alternating loudness balance test, and the resonance phenomenon is an important method of identifying cochlear sensorineural deafness from neural sensorineural deafness; (3) the Bekesy audiometric test is classified into four types based on the intermittent sound and sustained sound interactions, and III and IV are classified into four types, and III and IV are classified into four types. Bekesy hearing test, according to the interrelationship between intermittent and sustained tones, is divided into 4 types, type Ⅲ and Ⅳ are mostly auditory neuropathy. 2.Vestibular function test Auditory neuroma mostly originates from the vestibular part of the auditory nerve, and the early hot and cold water (warming) test can almost always find that the vestibular function on the affected side is impaired (the response disappears completely or partially disappears). The vestibular direct current stimulation test is another method of identifying end organ lesions from neuropathy. When the vestibular system is stimulated with direct current, the fast phase of nystagmus is always to the cathode side. If the peripheral neurons and vestibular end organs are destroyed, this response remains, and if the peripheral neurons and vestibular nerve fibers are destroyed, the DC response disappears completely. Therefore, this method can be used for early diagnosis to distinguish the auditory neuroma from cochlear lesion. The main manifestations are enlargement of the internal auditory canal, bone erosion or bone resorption, which can be seen in most cases. The normal width of the internal auditory canal is 4~7mm, the average is 5.5mm, and there can be a difference of 1~2mm. Exceeding this difference is diagnostic. Larger tumors can also cause destruction of the rocky bone crest, or even form bone defects, with sharp and neat edges of destruction. If the acoustic neuroma causes increased intracranial pressure, it may also have the X-ray manifestation of increased intracranial pressure. 4.CT examination: There is no abnormal discovery for only the acoustic neuroma located in the inner ear canal or the small tumor with diameter less than 1cm invading into the cerebral bridge and cerebellar triangle, unless continuous CT scan of the rocky bone is made. If the tumor is large, it shows round or lobulated low-density foci with clear borders, which are closely connected with the posterior edge of the petrous bone, and in some cases, the tumor is isointense with unclear borders, and a few of them are slightly high-density. 77%~95% of the inner ear canals show conical or funnel-shaped enlargement, and the fourth ventricle is shifted to the contralateral side by the deformation of the compression or is completely occluded, and there are varying degrees of enlargement of the ventricle above the obstruction, and the pool of cerebral bridge and cerebellar triangle of the diseased side is more occluded, and the residual part of the enlargement is sometimes seen. The cerebellar triangle pool of the bridge on the diseased side is mostly occluded, and the residual part is occasionally seen to be expanded. On enhancement examination, the tumor has obvious enhancement, and there are often areas of unequal density in the enhanced area, representing cystic or necrotic parts. The low-density area sometimes expands and occupies almost the whole tumor, and only the periphery is strengthened; occasionally, intra-tumor hemorrhage occurs, and the tumor wall can be seen to be strengthened in a ring. MRI examination On MRI images, the tumor is compared with neighboring brain tissues. The tumor shows slightly low signal or equal signal on T1-weighted image, and obviously high signal on T2-weighted image. The tumor may be a substantial mass or a partial cystic degeneration. After enhancement with Gd-DTPA injection, its solid part shows enhancement with obvious increase in signal, and the cystic part has no enhancement.MRI is the most reliable method to diagnose acoustic neuroma.MRI enhanced thin-layer scanning can diagnose and detect small acoustic neuromas at an early stage, and it is recognized as the most reliable method at present. MRI manifestation of acoustic neuroma 【Treatment Overview】 Acoustic neuroma is a benign tumor, and the principle of treatment is total resection, which can be cured. However, sometimes, due to the patient’s physical condition and anatomical relationship of the tumor, total resection cannot be achieved, and the function of facial nerve and auditory nerve cannot be preserved, which may even be life-threatening. In recent years, due to the application of microscopic neurosurgical techniques and early diagnosis of tumors, the rate of total resection of tumors has been greatly improved, and the preservation of facial nerve anatomy and function has reached a high level, which reduces the mortality rate of surgery. The era of striving for the preservation of hearing function has been entered. Surgical treatment 【Decision of surgical approach】 There are 3 approaches for the surgery of acoustic neuroma: ① Unilateral suboccipital approach: It is the most commonly used traditional approach. The advantages are good visualization and preservation of hearing and facial nerve function. The disadvantage is that the surgical injury is large, the cerebellum must be exposed and stretched, and the operation time is long. ②Translabyrinthine approach: It is routinely used for patients with small tumors and complete hearing loss. The advantage is that the surgery is completely outside the dura mater, causing little disturbance to the cerebellum and brainstem, and is less dangerous. The main disadvantage is the permanent loss of hearing. (iii) Middle cranial fossa approach: the surgery is performed outside the dura mater above the ear and is suitable for small tumors. The three surgical approaches have their own indications for surgery, except for some tumors <3cm in diameter with relative or absolute contraindications to surgery, which can be treated by stereotactic radiosurgery such as γ-knife and X-ray knife, and can be treated according to the size and location of the tumor, the degree of hearing impairment, the age of the patient, and the habits and preferences of the neurosurgeon. There is no absolute standard for the choice of surgical approach. The views of some recent scholars are introduced here. 1. Medium-sized tumors (2.5-4.0 cm) with hearing loss are the most common. Although the tumor already has varying degrees of compression on the brainstem, clinical symptoms are rare. Up to now, it is rarely possible to restore the lost hearing by surgery, so suboccipital approach or transvaginal approach can be used. At present, most of the tumors can be treated radically and preserve the function of all cerebral nerves except the auditory nerve. Large tumor (>4.5cm) with hearing loss The most commonly used approach is the suboccipital approach, which can clearly identify the relationship between the cerebral nerves and separate them from the tumor peritoneum. Translabyrinthine approach is also used, but the percentage of total resection is significantly reduced. If these patients have obstructive hydrocephalus, it is safer to do shunt surgery or ventricular drainage before surgery. Small tumors (<2cm) with hearing loss can be easily resected by vagus or suboccipital approach, and it cannot be assumed that hearing can be preserved if the tumor is small. However, if hearing loss is not complete, the suboccipital approach should be preferred in an attempt to preserve hearing. In the elderly, the transvaginal approach may be preferred for safety reasons. In addition, stereotactic radiosurgery is also an option, but ultimately cannot preserve hearing. 4. Tumors in the internal auditory canal: These tumors are found with basically intact hearing and only tinnitus or mild hearing loss. Suboccipital approach or middle cranial fossa approach can be chosen. Translabyrinthine approach is the safest way, but at the expense of hearing. Elderly and frail patients with large acoustic neuroma Elderly patients in good physical condition can withstand the total resection of large acoustic neuroma and retain the damaged cerebral nerves including the facial nerve, only that the postoperative recovery time is significantly prolonged. However, patients with other diseases have poor ability to tolerate total resection, and diabetes and hypertension can increase the mortality rate. In this case, intracapsular resection of acoustic neuroma via labyrinthine approach is the best choice, which can cause short recovery time and little risk of cerebral dysfunction, and can be operated again in case of recurrence of tumor. Elderly patients with small tumors can be treated with stereotactic radiotherapy if they have hearing loss. If the hearing is intact, the patient can continue to be followed up once every 3 months in the first year, once every 6 months in the second year, and once every year thereafter. Occasionally the tumor grows quickly and the procedure can be selected based on the systemic situation. As long as the tumor remains at a small size and there is no significant progression of symptoms, small tumors in the elderly can be left unoperated. 7. Young patients with small tumors or tumors in the internal auditory canal with hearing still present Theoretically, this is the type of surgery that currently has the best chance of preserving hearing. The median cranial recess approach is the best approach for resecting tumors in the internal auditory canal. Although the growth rate of an auditory neuroma cannot be determined, small tumors in young patients undoubtedly increase in size and therefore aggressive surgical treatment is advocated. So far, there is no data to show whether the hearing of these patients can be preserved or not, so it is still a problem that needs to be explored and solved at present. Auditory neuroma with contralateral hearing loss For large tumors, there is no choice but surgery. Every effort should be made to preserve the VIIIth cerebral nerve, and modern techniques such as cochlear implants have been used to increase hearing, but the results are not yet certain. If the symptoms are not too severe, surgery can be postponed until the patient has learned to mute and is fully prepared for surgery. 9. Bilateral acoustic neuroma The patients are all suffering from neurofibromatosis. The aim of treatment is to preserve hearing for as long as possible to buy time to learn mute language. Generally, the tumor on the side that has lost hearing is removed first, and then the other side is done when the symptoms on the opposite side are serious. However, if there is hearing on the side of the larger tumor and the symptoms of brain compression are severe, only the resection of the larger tumor can be performed first. In neurofibromatosis tumors of the pontine cerebellar angle, some tumors do not come from the auditory nerve, but from the facial nerve, and hearing preservation can be expected in these rare cases. Preservation and restoration of the facial nerve] At present, the facial nerve can be preserved regardless of the size of the tumor, and even if it is dissected intraoperatively, anastomosis can be performed in most cases. If there is no intraoperative facial nerve injury but there is functional deficit, the recovery time varies greatly and can be judged according to the following conditions. Patients with normal postoperative facial nerve function have mild facial muscle weakness, but usually recover within a few days. 2.Patients with normal postoperative facial nerve function, but gradually develop obvious facial paralysis within 24-72 h. If the paralysis is partial, it can be recovered in a few weeks to a few months, and if it is complete facial paralysis, it will take 3-6 months to recover. 3.The postoperative facial nerve function is normal but suddenly complete facial paralysis within a few days after the operation, the patient can tell exactly when it happened, it is considered to be caused by vascular factors, the recovery is very slow, 6~12 months or longer. 4.Partial facial paralysis after awakening from anesthesia, but not aggravated, mostly recovered in several weeks to months. 5.Partial facial paralysis gradually develops into complete paralysis after waking up, mostly recovered in 3~6 months. 6. It is difficult to predict the percentage of recovery in those with total paralysis after waking up. Those who appear to have an intact facial nerve may take up to 1 year to recover, whereas those with significant nerve thinning may recover within a few months. In the case of intraoperative nerve dissection with tension-free end-to-end anastomosis, recovery of facial nerve function may take at least 1 year. If at the end of 1 year there is still clinical and electromyographic evidence of nerve recovery, nerve grafting is justified. Facial muscle stimulation should be used prior to facial nerve grafting to maintain healthy facial muscle tone. Nerve grafts may be used from the sublingual nerve or the sternocleidomastoid branch of the parasympathetic nerve. Factors affecting the preservation of facial nerve] 1. The nature of tumor is huge, hard, with thin peripheral membrane and rich blood supply, it is easy to cause facial nerve injury during surgery, especially the influence of blood supply is the biggest, the blood supply of the rich tumor bleeds more during surgery, the blood is all over the surgical field, the nerve's path is not easy to identify, and the nerve is easy to be mistakenly injured during the operation. The length of the nerve stretched by the tumor is not significantly elongated, and the length is less than 3cm, which is easy to be retained. The degree of paralysis of the nerve before operation. patients with obvious facial paralysis before operation, the facial nerve has been pushed into a thin slice by the tumor, and the appearance is like a fibrous band, and the operation will lead to nerve dissection or nerve axon damage. 20% of the patients can have delayed facial paralysis after operation, which is related to the oedema of the facial nerve at the inner auricle, and although most of the patients can completely recover the function of the facial nerve, a part of the patients can have mild paralysis or total paralysis. Therefore, intraoperative decompression of the labyrinthine segment of the facial nerve from the base of the internal auditory canal to the geniculate ganglion and placement of a dexamethasone-impregnated gelatin sponge near the oedematous nerve are often effective and avoid the side effects of systemic medication. Postoperatively, due to facial paralysis, incomplete eyelid closure, part of the cornea and sclera exposure and dryness, can cause eye infections and keratitis, the application of aureomycin ophthalmic ointment will be sealed in the affected eye, and add an eye mask for protection; such as postoperative trigeminal nerve dysfunction, loss of sensation in the cornea, trophic corneal ulcers are very easy to form, enough to cause intraocular infection in the affected eye and blindness, should be done as early as possible eyelid sutures, and to be partially recovered after the function of the trigeminal and facial nerve before removing the eyelid. The eyelid suture should be made as early as possible and should be removed only after the trigeminal nerve and facial nerve functions have partially recovered. Due to the influence of the function of the posterior cerebral nerves and the brainstem, the operation of acoustic neuroma may cause temporary dysphagia, loss of pharyngeal and coughing reflexes, which may easily lead to aspiration pneumonia and asphyxia, therefore, the patient should be fasted for 1~2d after the operation, and if the patient has not recovered from the operation after the operation, he/she should be given nasal feeding, and encourage the patient to cough in order to keep the respiratory tract open. If postoperative patients are weak, unconscious, or cannot cough up sputum by themselves, tracheotomy should be performed to prevent obstruction of the airway. Recently, the operative mortality rate reported in many bulk cases of acoustic neuroma has been extremely low, and some of them are even zero. However, a few patients still require staged surgery. Inappropriate separation of the adherent portion of the brainstem can cause limb paralysis, and excessive cerebellar traction and separation injury can cause cerebellar dysfunction. The most common complication is cerebrospinal fluid leakage. Treatment Techniques Stereotactic Radiation Therapy (STR) is based on the principle of applying a focused beam of radiation to kill tumor cells while maintaining adjacent normal tissue structures. The knife system, linear gas pedal, and cyclotron comprise the advantages of STR are that the treatment can be completed in 1 session and can be performed on an outpatient basis or with a maximum hospital stay of 1 to 2 d. The goal of STR is to inhibit the growth of the tumor, but the tumor does not disappear; the tumor cells are still present and there is a possibility of latent growth occurring.The complications of STR are the same as for microsurgical resection of the acoustic neuroma. They include facial palsy (17%-32%) and facial numbness (19%-34%). Cerebral neurologic deficits are delayed, most often appearing 5 to 6 months after STR, and long-term hearing is preserved in about 25% of cases. Radiation therapy is generally considered to be limited to those who are unable or unwilling to undergo surgery.