Natural History and Treatment Options
Acoustic neuroma (a.k.a. vestibular schwannoma) are non-cancerous (benign) tumours that grow in the canal connecting the brain to the inner ear. More than 300 Australians are diagnosed with acoustic neuroma every year. Nonetheless, it is not known why some people develop these types of tumours.
Acoustic neuroma develop from the vestibular (balance) part of the 8th cranial nerve (vestibulocochlear nerve). Pressure from the tumour can cause hearing loss (~95% of cases), ringing in the ear (~71% of cases) and unsteadiness (~77 of cases). Other symptoms may include a sense of fullness or blocking in the affected ear, headache (varies from 20 to 85% of cases), earache (~28% of cases) and rarely double vision (~7% of cases). When the tumour grows to be very large, numbness of the face (~7% of cases) or incoordination due to pressure on the cerebellum may occur.
The exact natural history of acoustic neuromas is not clear as most studies on tumour growth lack long-term prospective data. Available scientific evidence estimates a typical growth rate of <2mm/year, with an average of 1.5mm/year. Reported growth rates in the literature are highly variable. However, around 1 in 10 patients with acoustic neuroma will harbour tumours that are destined to grow much faster than the rest, at a rate of >9mm/year. The growth pattern is not always constant with up to 40% showing exponential growth compared to steady gradual enlargement.
Risk factors for growth include:
Age: Younger patients are more likely to grow at an increased rate.
Tumor size: larger tumours demonstrated greater tendency for growth
Tumor location: tumours that extends towards the brainstem are more likely to grow at a faster rate compared to tumours purely located purely inside the inner ear canal.
Tumor features: Cystic tumours, and those that have previously bled show increased rate and tendency to grow.
Previous growth: Tumours that grow within the first year following diagnosis tend to continue growing. Similarly, the longer a tumor remains stable, the likelihood of subsequent growth reduces.
Optimal way to preserve hearing
Based on the available clinical evidence, the most consistent predictor of hearing preservation is good preoperative hearing with intact word recognition and/or pure tone thresholds.
Most patients’ hearing will eventually decline over time. For patients who have been conservatively managed with serial surveillance scans, it is expected that 75-100% will maintain hearing at 2 years. This number will then decline to 50-75% at 5 years and 25-50% at 10 years. Tumour size at the time of diagnosis, age, and sex do not predict future deterioration of hearing during observation.
When patients undergo treatment with radiosurgery, hearing will decline relatively more quickly, with 50-75% of patients able to hear at 2 and 5 years, and 25-50% at 10 years.
Surgical patients have the highest rate of hearing decline, with 25-50% of patients able to hear at 2, 5 and 10 years.
However, these outcomes may reflect biases of treatment selection, such that larger tumours (presumably poorer preoperative hearing status) are subjected to surgery, while smaller tumours (likely better baseline hearing) may be managed conservatively.
There are several ways to manage patients with acoustic neuromas.
Watchful waiting with regular and frequent scans
Or a combination of the above
Recommendation for a particular treatment modality take into consideration patient factors, tumour factors and surgeon factors.
Patient factors include consideration for the patient’s chronological, and more importantly, their physiological age (which takes into account general health and any other medical problems they may have). A poor physiological age may negatively impact on postoperative recovery and increase the risk of postoperative complications, even if the surgery is performed satisfactorily. It is also important to establish patient’s baseline hearing for both the affected and non-affected side as this may influence the decision making as to which treatment modality best provide satisfactory long-term outcomes. At the same time, addressing patient’s expectations and psychological wellbeing in the preoperative period is crucial to mitigate potential postoperative confusion. Establishing treatment goals such as whether to achieve tumour control, complete tumour removal, hearing preservation or improve balance.
Tumour factors include tumour size and the degree of brainstem compression. Small tumours may be safely observed or might be suitable for radiation treatment. Larger tumours with brainstem compression may require surgical removal upon diagnosis. Understanding the natural behaviour of these tumours (what happens to the tumour without treatment) and their growth rate is important to determine which cases may be appropriate for observation and which may require early intervention.
Surgeon factors take into account personal experience and preference. It is important that the surgeon knows all the available approaches, so that he or she is able to choose the best approach for the individual patient, given the clinical context.
Radiation therapy was initially used as an adjunct to surgery in patients with an incompletely removed tumour but can now also be used as the main treatment method for certain acoustic neuromas. Its goal is to stop the tumour growing. In general, there are three types of radiation techniques: stereotactic radiosurgery (Gamma Knife), Linear accelerator (LINAC) and fractionated stereotactic radiation therapy. All three types of radiation technique provide an acceptable tumour control rate with satisfactory hearing preservation and low rates of injury to nearby nerves.
Surgery offers the most direct and complete tumour control but carries a higher risk of procedure-related complications. In general, there are three ways to access the tumour: 1) middle cranial fossa (MCF); 2) retrosigmoid; and 3) translabyrinthe approaches. Each has its own risks and benefits. The choice of surgical approach depends on patient’s hearing status, tumour size, patient’s physiological age and health status, degree of brainstem compression, and the surgical team’s expertise and preferences.
Some useful references
Paldor, I., et al. Growth rate of vestibular schwannoma. J Clin Neurosci 2016; 32: 1-8
Stangerup, S. E., et al The natural history of vestibular schwannoma. Otol Neurotol 2006;27(4): 547-552.
Hunter, J. B., et al. Single Institutional Experience with Observing 564 Vestibular Schwannomas: Factors Associated With Tumor Growth. Otol Neurotol 2016;37(10): 1630-1636.
Moffat, D. A., et al. Growth characteristics of vestibular schwannomas. Otol Neurotol 2012;33(6): 1053-1058.
Goldbrunner, R., et al. EANO guideline on the diagnosis and treatment of vestibular schwannoma. Neuro-Oncology 2020;22(1): 31-45
Ben Ammar, M., et al. Surgical results and technical refinements in translabyrinthine excision of vestibular schwannomas: the Gruppo Otologico experience. Neurosurgery 2012;70(6): 1481-1491
Johnson, S., et al. Long-term results of primary radiosurgery for vestibular schwannomas. J Neurooncol 2019;145(2): 247-255
Golfinos, J. G., et al. A matched cohort comparison of clinical outcomes following microsurgical resection or stereotactic radiosurgery for patients with small- and medium-sized vestibular schwannomas. J Neurosurg 2016 125(6): 1472-1482
Chung L, Nguyen T, Sheppard J, Lagman C. A Systematic review of Radiosurgery Versus Surgery for Neurofibromatosis Type 2 Vestibular Schwannomas. World Neurosurgery 2018;109: 47-58