Acoustic neuroma, also called a schwannoma, is a benign tumor that arises from Schwann cells covering the eighth cranial nerve (vestibulocochlear nerve) which is responsible for
transmitting sensory information of hearing and balance to the brain. Schwann cells provide a protective sheath to the vestibulocochlear nerve. Most of the schwannomas are sporadic and only 5% are genetic. Acoustic neuroma grows very slowly or not at all and can remain confined to the bony ear canal for decades. The symptoms appear to be due to direct damage to the cranial nerve or increased intracranial pressure due to the growing tumor. The main complication arises when the tumor compresses the surrounding soft tissue parts of the brain. This limits the flow of fluid to nearby areas such as surrounding other cranial nerves, cerebellum, and brainstem, and can result in hydrocephalus (buildup of fluid in the cavities of the brain).
Acoustic neuroma can be divided into 2 types; unilateral acoustic neuroma and bilateral acoustic neuroma based on whether the hearing is affected in one or both the ears.
Unilateral acoustic neuroma, also called sporadic type, affects hearing in only one ear. The sporadic cases are usually diagnosed during the 5th or 6th decade of life.
Bilateral acoustic neuroma also called genetic type, affects hearing in both ears. Genetic schwannomas are closely related to neurofibromatosis type II (NF2) and are caused by a mutation on chromosome 22. It manifests, earlier, during the 3rd decade of life.
Schwannoma constitutes about 8% of all brain tumors that develop inside the brain. In rare cases, acoustic neuromas can develop outside the brain parenchyma. Unlike other brain tumors, it does not spread to other parts of the brain and affects women more often than men. Most cases of acoustic neuroma develop in individuals between the ages of 30 and 60 years. Although quite rare, they can develop in children as well. Every year, around 2 in 100,000 people are diagnosed with an acoustic neuroma.
The symptoms are dependent on the location, size, and growth of the tumor. The most common symptoms of acoustic neuroma are hearing loss, tinnitus, and imbalance. Sporadic schwannomas have unilateral hearing loss whereas genetic schwannomas have bilateral hearing loss. Tinnitus is described as hissing, ringing, buzzing, or roaring in the ear. It occurs in almost 80% of acoustic neuroma patients. Other symptoms include decreased word understanding, vertigo, headaches, and facial numbness. Initially, when the tumor is limited in size and is located on the bony canal, it only causes unilateral hearing loss, tinnitus, and dizziness. But if the tumor grows and interacts with nearby parts, balance problems and involuntary eye movements are encountered. Various other symptoms appear depending upon the part of the brain with which the tumor is interfering (Table 1).
Part of the brain compressed
The facial nerve (7th cranial nerve)
Twitching of facial muscles increased lacrimation and facial weakness. In severe cases, secretions from the eyes, palate, and nose, and loss of ability to taste on some parts of the tongue.
Trigeminal nerve (5th cranial nerve)
Sensation problems, numbness, or facial pain. These symptoms are rare due to the distance of the trigeminal nerve from the tumor.