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Vestibular Schwannoma (VS) which is also known as acoustic neuroma is a benign neoplasm of the eight cranial nerve, the vestibulocochlear nerve (CN VIII). CN VIII is a special sensory (special afferent) nerve and provides auditory information from the cochlea (auditory function) and balance information from the semicircular canals (vestibular function). It is divided into 2 major divisions anatomically as well. VS generally arises from the superior vestibular nerve.
Schwannomas account for 7% of intracranial tumors. The clnical incidence rate is 1-1.5 cases per 100,000. There is no gender predominance, and mean age of onset ranging from 46 to 58 years. Patients with Neurofibromatosis 2 have an earlier onset by approximately 20 years.
Presenting symptoms include (in ascending order): unsteadiness, vertigo, tinnitus, and hearing loss (the most common symptom). VS may also cause hydrocephalus, either from large size which may cause obstructive hydrocephalus, or from protein secretion into the cerebrospinal fluid, resulting in a communicating hydrocephalus. Around 20% of patients with VS will have concomitant hydrocephalus.
Hearing loss is the most common symptom and is usually gradual in onset. Acute onset occurs in 12-22% of patients. VS is rarely the cause of sudden deafness. An audiology evaluation is imperative for all patients with a VS because it will impact treatment decisions. In general, speech discrimination and high frequency pure-tone average audiometry are assessed. Serviceable hearing is defined as PTA <or= 50dB and a speech discrimination score of >or= 50%. However, many authors are using PTA <or= to 30dB and SDS >or= to 70% more recently.
The imaging study of choice is MRI with contrast. Many centers now offer a field of view which focuses on the IAC providing more detail. Additionally, a thin cut temporal bone CT scan may show the bony anatomy in greater detail than MRI.
Treatment of VS is somewhat controversial. Observation of incidentlly found VS may be warranted. Definitive treatment includes microsurgical resection or stereotactic radiosurgery. Complications of any treatment (surgical or radiation) include hearing loss, facial nerve weakness, facial numbness, and tumor recurrence. Some complications such as anesthesia risks and CSF leaks apply only to surgical treatment.
The natural history of VS suggests that a certain percentage of these tumors may not increase in size over a prolonged period of time. The natural history is quite variable. The percentage of tumors showing progressive growth ranges from 15% to 85% (mean 46%). The reported mean growth rate is 1.2 mm/year. Predictors of growth include previous tumor growth, younger age, and extracanalicular lesions. Failure from an observational posture is reported in 15-50% of patients. Any progression of symptoms or radiographic enlargement should be considered a failure of observational treatment.
See Vestibular schwannoma technique for more detailed descriptions.
Three approaches described for surgical resection of VS include:
- Suboccipital Craniotomy
This technique is considered the mainstay of VS resection. It is useful for tumors >2 cm in patients with serviceable hearing.
- Translabyrinthine Approach
This approach allows for a large exposure. Since the hearing structures are sacrificed, this approach is used for patients with non-serviceable hearing.
- Middle fossa Craniotomy
This approach has gained popularity for lesions which are <2cm, in patients with serviceable hearing, and have intracanalicular tumors. Long term preservation of serviceable hearing ranges from 50-70% with this technique.
SRS is a commonly used treatment for VS. It is a non-invasive outpatient procedure with significantly fewer risks than open surgery. While resection should be used for patients with larger tumors that are causing neurological deficits, SRS may be used for small or medium sized (up to 3 cm) tumors. The most common marginal dose administered currently is 13Gy. Tumor response is evident in up to 70% of tumors. Hearing preservation rates are roughly 50%.
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