An acoustic neuroma, more accurately called a vestibular schwannoma, is a benign, slow-growing tumor that develops on the vestibulocochlear nerve (cranial nerve VIII) — the nerve responsible for hearing and balance. Although non-cancerous, it can cause progressive hearing loss, balance problems, and in larger cases, compress the brainstem and cerebellum.
Acoustic neuromas are an important cause of unilateral hearing loss and tinnitus that should be evaluated with MRI to exclude a structural cause before attributing symptoms to other conditions. They arise near the brainstem in the cerebellopontine angle, a region where other tumors including meningiomas can also occur. In patients with bilateral acoustic neuromas, the diagnosis of Neurofibromatosis Type 2 should be considered, which is also associated with other nervous system tumors including cavernous malformations.
Causes
Sporadic tumor development
The majority of acoustic neuromas occur sporadically with no identifiable cause. They arise from an overproduction of Schwann cells — the cells that form the myelin sheath around the vestibulocochlear nerve. A somatic mutation in the NF2 tumor suppressor gene on chromosome 22 has been identified as a key molecular event in most cases, even in the absence of inherited NF2 syndrome. Prior radiation exposure to the head and neck is the only clearly established environmental risk factor.
Neurofibromatosis Type 2 (NF2)
Bilateral vestibular schwannomas are the defining feature of Neurofibromatosis Type 2 (NF2), a rare autosomal dominant genetic disorder caused by germline mutations in the NF2 gene. NF2-related tumors develop at a younger age — often in the teens or twenties — and present unique management challenges due to their bilateral nature, as treatment of one side risks hearing loss while the other side tumor continues to grow. NF2 is also associated with spinal schwannomas, meningiomas, and ependymomas.
Symptoms
Symptoms develop gradually due to the slow growth of the tumor and the brain's ability to compensate over time:
- Unilateral sensorineural hearing loss: The most common presenting symptom — typically gradual and progressive, affecting high frequencies first. Sudden hearing loss occurs in a small percentage of patients.
- Tinnitus: A persistent ringing, buzzing, or hissing in the affected ear, often accompanying hearing loss.
- Balance disturbance: Unsteadiness, dizziness, or a sense of imbalance — though frank vertigo is less common than with other inner ear conditions because the slow tumor growth allows central compensation.
- Facial numbness: Compression of the adjacent trigeminal nerve (cranial nerve V) can cause numbness or tingling in the face.
- Facial weakness: The facial nerve (cranial nerve VII) runs in close proximity and can be affected by larger tumors, causing facial weakness.
- Brainstem and cerebellar symptoms: Very large tumors compressing the brainstem cause headaches, difficulty swallowing, gait ataxia, and in severe cases, hydrocephalus from obstruction of CSF flow.
Diagnosis
Audiological evaluation — including pure-tone audiometry and auditory brainstem response (ABR) testing — documents the pattern and degree of hearing loss and cochlear nerve dysfunction. Asymmetric sensorineural hearing loss on audiometry is the key trigger for MRI evaluation.
MRI of the internal auditory canals with gadolinium contrast is the definitive imaging study and the gold standard for acoustic neuroma detection. Thin-slice sequences through the internal auditory canals (IACs) and cerebellopontine angle cisterns identify even small intracanalicular tumors (under 5mm) that are invisible on CT. The characteristic MRI appearance is an enhancing mass centered on the IAC, often with an "ice cream cone" shape extending into the cerebellopontine angle cistern. MRI also delineates tumor relationship to the brainstem, cerebellum, and facial nerve, which is critical for surgical and radiosurgical planning. CT scan of the temporal bone is used as an adjunct for assessing bony canal anatomy when surgery is planned.
Tumor Size Classification
Acoustic neuromas are classified by size, which directly guides treatment selection:
- Small (less than 15mm): Confined to the internal auditory canal or with minimal cerebellopontine angle extension. Observation or radiosurgery are appropriate options.
- Medium (15–30mm): Extending into the cerebellopontine angle with potential brainstem contact. Radiosurgery or surgery depending on growth rate, symptoms, and patient factors.
- Large (greater than 30mm): Significant brainstem compression requiring surgical treatment in most cases. Risk of facial nerve injury and hearing loss is higher at this size.
Treatments
Management is individualized based on tumor size, growth rate, patient age, hearing status, and overall health. All three options — observation, radiosurgery, and surgery — are legitimate choices in the right clinical context, and many patients benefit from multidisciplinary discussion at a dedicated skull base tumor program.
Observation (active surveillance):
Small, slow-growing tumors in older patients or those with minimal symptoms may be safely monitored with serial MRI every 6–12 months. Studies show that approximately 50% of acoustic neuromas show no growth over years of observation, and intervention can be initiated if growth is detected. This approach preserves hearing and avoids treatment risks for tumors that may never require intervention.
Stereotactic radiosurgery (Gamma Knife or CyberKnife):
A single high-dose, precisely targeted radiation treatment effectively halts growth in small to medium tumors in the majority of patients. It does not remove the tumor but arrests its growth, with long-term tumor control rates exceeding 90%. Facial nerve preservation rates are excellent, and hearing preservation is possible for patients with serviceable hearing at the time of treatment. Radiosurgery is the preferred treatment for most small to medium tumors in patients who are not surgical candidates or who prefer to avoid open surgery.
Microsurgical resection:
Open surgical removal is recommended for large tumors causing brainstem compression, rapidly growing tumors, cystic tumors, or cases unsuitable for radiosurgery. Three approaches are used — retrosigmoid (hearing preservation possible), translabyrinthine (sacrifices hearing but provides excellent facial nerve access), and middle fossa (best hearing preservation for small intracanalicular tumors). Intraoperative facial nerve monitoring is mandatory. Complete tumor removal is curative, though facial nerve function and hearing outcomes depend heavily on tumor size and preoperative nerve anatomy.
Get an MRI to Confirm Your Diagnosis
Before surgical planning or starting treatment, a clear MRI diagnosis ensures the right path forward. First Look MRI offers self-pay Brain MRI scans — no doctor's order or insurance required — at our locations in Georgia, Texas, and Colorado.