Acoustic neuroma, also known as vestibular schwannoma, is a benign tumor that develops on the vestibulocochlear nerve, which is the nerve responsible for transmitting sound and balance information from the inner ear to the brain. The tumor arises specifically from Schwann cells, which are the cells that form the protective myelin sheath around nerves. Despite the name “acoustic neuroma,” these tumors most commonly originate from the vestibular portion of the nerve, which is involved in balance rather than hearing.
The exact cause of acoustic neuroma is not fully understood, but it is primarily linked to genetic factors. Most cases occur sporadically, meaning they happen by chance without a clear inherited pattern. However, a significant cause of bilateral acoustic neuromas—tumors on both sides—is associated with a genetic disorder called neurofibromatosis type 2 (NF2). NF2 is a rare inherited condition that causes tumors to grow on nerves throughout the body, including the vestibulocochlear nerve. In people with NF2, the tumors tend to develop earlier in life, often in the third decade, whereas sporadic cases usually appear between the ages of 40 and 60.
At the cellular level, acoustic neuromas develop when Schwann cells begin to grow uncontrollably. This abnormal growth forms a mass that can press on surrounding structures, including the cochlear nerve (responsible for hearing) and other nearby cranial nerves, as well as the brainstem and cerebellum. The pressure from the tumor can disrupt normal nerve function, leading to symptoms such as hearing loss, tinnitus (ringing in the ears), and balance problems.
The cause of hearing loss in acoustic neuroma is complex and not solely due to the physical compression of the cochlear nerve by the tumor. Several mechanisms have been proposed to explain this hearing impairment:
– Disruption of the blood supply to the inner ear or nerve, leading to nerve damage.
– Altered permeability of the blood-labyrinth barrier, which normally protects the inner ear fluids.
– Dysfunction in the flow of cerebrospinal fluid around the inner ear structures.
These factors may contribute to nerve damage and hearing loss even when the tumor is relatively small.
Environmental or lifestyle factors have not been definitively linked to the development of acoustic neuroma. Unlike some other tumors, there is no strong evidence that exposure to loud noise, infections, or toxins directly causes these tumors. The primary driver remains genetic mutations affecting Schwann cells.
In summary, acoustic neuroma is caused by abnormal growth of Schwann cells on the vestibulocochlear nerve, mostly due to genetic mutations. Sporadic cases arise without a clear inherited cause, while bilateral tumors strongly suggest neurofibromatosis type 2. The tumor’s growth disrupts nerve function through compression and possibly other mechanisms affecting blood flow and fluid balance in the inner ear, leading to symptoms like hearing loss and balance disturbances.
