Comprehensive management of VS is multifaceted and nuanced. Thoughtful conversations discussing the benefits and risks of each modality as well as patient and tumor characteristics are critical when counseling patients and their families.
Sarah Mowry, MD, and Daniel E. Killeen, MD
Vestibular schwannomas (VS), also referred to as acoustic neuromas, are benign neoplastic growths that arise on the vestibular nerve. Although usually indolent in nature, they occur in “high-priced real estate” within the internal auditory canal and posterior fossa of the intracranial cavity. The majority of these growths are sporadic in nature (85%-90%) and are not known to have a causative agent. A smaller fraction of patients suffers from a germ line mutation in the gene that encodes for the Merlin protein on chromosome 22 and suffers from neurofibromatosis type 2, resulting in multiple schwannomas of the central nervous system.
The exact incidence of these growths is not entirely clear. Although historically a rate of 1:100,000 has been described, a recent study suggested the true incidence of VS ranges from 3.0 to 5.2 per 100,000 person-years but is 20.6 per 100,000 person-years in patients aged 70 years and older, which may be attributable to the advent of magnetic.1
Those patients who are symptomatic typically present with unilateral otologic symptoms such as sensorineural hearing loss (SNHL) and subjective tinnitus as well as balance complaints. This triad is common to both large and small VS. VS patients can present with sudden unilateral SNHL, which in turn may respond to oral steroid medication even with the presence of VS. In fact, as many as 10.2% of patients with sudden SNHL were found to have VS, and therefore, sudden SNHL patients should be referred for magnetic resonance imaging (MRI) imaging.2 Larger VS, with significant extension into the cerebellopontine angle, may have facial hypoesthesia when the fifth cranial nerve is compressed. Only 2.8% of VS patients were found to experience facial motor symptoms from these tumors.3 In fact, facial motor symptoms are a possible indicator of a rare facial nerve schwannoma and should prompt further imaging for other tumor niduses that are commonly seen in facial schwannomas. Giant VS can cause severe brainstem compression with resultant obstruction of the fourth ventricle and acute hydrocephalus.
Once identified, a thorough discussion of treatment options is warranted. Patients may be offered a “wait and scan” approach, stereotactic radiotherapy, or open microsurgery. In general, the factors that will affect the initial treatment recommendation are tumor size, hearing status, and patient age, in addition to patient preference.
Vestibular Schwannoma Algorithm
Reprinted with permissions from Daniel E. Killeen, MD.
The management of VS has dramatically changed as the understanding of the natural history of these tumors has become better. When Walter Dandy described his series of patients who survived VS surgery in the early twentieth century, nearly all had complete facial paralysis, and many suffered severe vascular injuries and resultant anterior inferior cerebellar artery strokes. For many decades thereafter, VS was considered primarily a surgical disease. The advent of accurate and reliable imaging has allowed for the option of observation with serial MRIs over years. This “watchful waiting” may be a good choice for patients with small tumors and excellent hearing in the ipsilateral ear; patients over age 65 or medically frail patients may also benefit from this approach.
Hunter et al. found that 43% of tumors between 1 mm and 20 mm in greatest diameter demonstrated growth over the following five years of serial imaging, and 31% of patients required further intervention (thus ending the period of serial imaging). In that series, small tumors (<5 mm) at diagnosis were more likely to remain in the serial imaging group. Furthermore, the larger the tumor at diagnosis, the more likely a patient would need intervention; in tumors >20 mm at diagnosis, the hazard ratio for intervention was 7.6.4
Preservation of hearing function is another commonly expressed reason for opting for serial imaging. It should be noted that those who present with asymmetric SNHL at diagnosis will likely continue to experience progressive hearing loss during this observation period. The exception to this may be patients who present with 100% word understanding at diagnosis. When followed over time, this group does have less risk of hearing loss.5 However, even in this group, 50% will lose serviceable hearing after 10 years of observation.6
Although there is technically no upper size limit for observation with serial imaging, significant posterior fossa symptoms including brain stem impaction and certainly compression would push most patients toward intervention. Within nine months, 50% of tumors >20 mm in diameter demonstrated significant growth and moved out of observation.4 Also, the growth patterns for these tumors are erratic. Patients may have several years of quiescence and then demonstrate growth. Consequently, serial imaging is lifelong commitment unless and until the patient needs an intervention.
It has been thought that tumors demonstrating significant growth (2 mm in any direction or growth from the internal auditory canal [IAC] into the cerebellopontine angle [CPA]) will continue to grow and thus intervention should be recommended. However, a large multicenter study showed in VS that demonstrated growth and continued to be observed, 77% and 72% of intracanalicular and CPA VS, respectively, did not demonstrate further growth after one year.7
In a large multicenter study examining VS volume, 65% of tumors demonstrated growth at a median time to growth of 1 mm/year, and those that continued observation demonstrated persistent growth at a similar rate.8 If the majority of VS grow, it may lead to the conclusion that upfront treatment is the best approach. On the other hand, this growth is typically slow and may be saltatory in nature. Further, it is unclear if further growth would impact treatment outcomes. Therefore, some clinicians may prefer to continue observing tumor growth until the tumor meets certain size thresholds for intervention as further growth may lead to worse outcomes. Although there is still incomplete information on the optimal threshold for intervention, in a study examining surgical outcomes based on VS diameter, a tumor diameter of 17 mm or less was found to be the optimal diameter for maintaining serviceable hearing with surgery, while 23 mm in diameter or less was found to be a significant threshold for improved facial nerve outcomes.9
Stereotactic radiosurgery (SRS) was popularized by Leksell in the 1960s with the advent of the Gamma Knife system. SRS refers to the delivery of ionizing radiation in a single fraction (or sometimes two). Stereotactic radiotherapy (SRT) refers to the delivery of radiation to the tumor in multiple fractions. Most centers deliver 12-13 Gy using SRS or SRT paradigms to the tumors with the goal of arresting tumor growth. The literature regarding this intervention references excellent tumor control for tumors <20 mm in diameter. However, many series are difficult to interpret as patients are often referred to SRS without demonstrable growth on serial MRIs—SRS is the primary treatment after diagnosis, and follow-up times in these series can be as short as two years. It is certainly likely that some patients have been radiated who would not have experienced tumor growth. However, in well-designed studies with appropriate length of follow-up, SRS/SRT has demonstrated a good control rate. Marston et al. showed that tumors with slow pretreatment growth rates (<2.5 mm/year) demonstrated 97% control, whereas those with >2.5 mm growth per year had 69% control.
Size of the lesion also impacted success of SRS/SRT with extension into the CPA decreasing control to 85%.10 SRS/SRT should not be considered a hearing-sparing treatment option for most patients. Greater than 60% of patients will lose serviceable hearing after SRS/SRT within 6 years even if they had serviceable hearing prior to treatment and had the cochlear dose limited to less than 3 Gy.11 The cochlea, in particular the stria vascularis, does not tolerate more than 3-4 Gy, which is difficult to achieve even with modern isodose planning software. SRS may be a good option for patients with tumors <20 mm, without significant brainstem impaction, and who are older (>65 years old). For larger tumors, SRS may be the best option in medical frail patients when the risk of open microsurgery is high.
Microsurgical tumor resection may be performed on any tumor, through translabyrinthine (TL), retrosigmoid (RS), or middle fossa approaches, depending on hearing status and tumor extent. Generally, all craniotomies risk postoperative cerebrospinal fluid (CSF) leak, meningitis, facial paralysis, hearing loss, wound infection, long-term postsurgical headache, bleeding, stroke, and very rarely death. While the goal of resection is for gross total resection (GTR), facial nerve function has been prioritized in recent years. Many centers elect to perform a near total (NRT) or subtotal resection (STR) to preserve facial nerve function. SRS may then be used to prevent growth of the remnant tumor.
Popularized by William House in the 1960s, the TL approach to the IAC and CPA can be used for resection of nearly any lesion regardless of size. This approach does not spare hearing and can be technically difficult when the jugular bulb is high within the temporal bone. However, TL affords access to the CPA from the tentorium to the lower cranial nerves. The facial nerve is visualized from the labyrinthine segment early in the resection. CSF management is critical as the fluid space is in continuity with the middle ear and eustachian tube.
The RS cranitotomy affords broad access to the CPA and brainstem. The IAC is exposed by removing the posterior lip of the porous acoustics. For tumors with limited extension to the distal IAC, the RS approach can be used in a hearing-sparing operation. This approach may be preferred in very large tumors to reduce operative time. One purported downside is that visualization of the facial nerve distally is not possible until the majority of the tumor is removed. Patients undergoing RS do seem to have increased risk of postsurgical headaches.
The middle fossa approach, also described by William House, is a hearing-sparing approach most commonly used for tumors limited to the IAC. In experienced centers, 70% of patients with tumors 10 mm or less will retain their preoperative hearing levels with excellent facial nerve preservation rates as well. Tumors >10 mm have lower hearing preservation rates.12 Specific to this approach is temporal lobe elevation that risks temporary aphasia (when operating on the dominant language side) and seizures; temporal lobe retraction is poorly tolerated in patients older than 65.
Perhaps one of the most interesting trends in VS literature in the past 5 years is the exploration of patient quality of life (QOL) related to management decisions. Carlson et al. prospectively followed patients’ QOL in each of the treatment modalities and found that there were no statistically significant differences between the groups using a disease-specific QOL tool. Only patients who had open microsurgery experienced improvement in the anxiety related to their diagnosis after treatment.13 All patients experienced a decline in their QOL compared with patients without a diagnosis of VS. In light of these data, patient preference for treatment modality is particularly important.
In summary, the contemporary comprehensive management of VS is multifaceted and nuanced. Thoughtful conversations discussing the benefits and risks of each modality as well as patient and tumor characteristics are critical when counseling patients and their families. Decisions around treatment modality should be made in collaboration with the patient and the multidisciplinary treatment team.
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References
Marinelli JP, Beeler CJ, Carlson ML, Caye-Thomasen P, Spear SA, Erbele ID. Global incidence of sporadic vestibular schwannoma: a systematic review. Otolaryngol Head Neck Surg. 2022;167(2):209-214. doi:10.1177/01945998211042006. Epub 2021 Aug 31. PMID: 34464224.
Chandrasekhar SS, Tsai Do BS, Schwartz SR, et al. Clinical practice guideline: sudden hearing loss (update). Otolaryngol Head Neck Surg. 2019 Aug;161(1_suppl):S1-S45. doi: 10.1177/0194599819859885. PMID: 31369359.
Matsushima, K, Kohno M, Ichimasu N, Nakajima N, Yoshino M. Preoperative facial nerve palsy in patients with vestibular schwannoma: clinical features and postoperative functional prognosis in a case series of 34 among 1228 consecutive patients. Oper Neurosurg (Hagerstown). 2022;22(1):14-19. doi: 10.1227/ONS.0000000. PMID 34982900
Hunter JB, Francis DO, O’Connell BP, et al. Single institutional experience with observing 564 vestibular schwannomas: factors associated with tumor growth. Otol Neurotol. 2016 Dec;37(10):1630-1636. doi: 10.1097/MAO.0000000000001219
Stangerup SE, Caye-Thomasen P. Epidemiology and natural history of vestibular schwannomas. Otolaryngol Clin North Am. 2012 Apr;45(2):257-268, vii. doi: 10.1016/j.otc.2011.12.008. Epub 2012 Feb 28.
Hunter JB, Dowling EM, Lohse CM et al. Hearing outcomes in conservatively managed vestibular schwannoma patients with serviceable hearing. Otol Neurotol. 2018;39(8):e704-e711.
Marinelli JP, Carlson ML, Hunter JB, et al. Natural history of growing sporadic vestibular schwannomas during observation: an international multi-institutional study. Otol Neurotol. 2021;42(8):e1118-e1124. doi: 10.1097/MAO.0000000000003224
Marinelli JP, Schnurman Z, Killeen DE, et al. Long-term natural history and patterns of sporadic vestibular schwannoma growth: a multi-institutional volumetric analysis of 952 patients. Neuro Oncol. 2022;24(8):1298-1306. doi: 10.1093/neuonc/noab303
Macielak RJ, Wallerius KP, Lawlor SK, et al. Defining clinically significant tumor size in vestibular schwannoma to inform timing of microsurgery during wait-and-scan management: moving beyond minimum detectable growth. J Neurosurg. 2021;1-9. doi: 10.3171/2021.4.JNS21465. Online ahead of print.
Marston AP, Jacob JT, Carlson ML, et al. Pretreatment growth rate as a predictor of tumor control following Gamma Knife radiosurgery for sporadic vestibular schwannoma. J Neurosurg. 2017 Aug;127(2):380-387. doi: 10.3171/2016.5.JNS153013. Epub 2016 Nov 25.
Berger A, Alzate JD, Bernstein K, et al. Modern hearing preservation outcomes after vestibular schwannoma stereotactic radiosurgery. Neurosurgery. 2022;91(4):648-657.
Woodson EA, Dempewolf RD, Gubbels SP, et al. Long-term hearing preservation after microsurgical excision of vestibular schwannoma. Otol Neurotol. 2010 Sep;31(7):1144-1152. doi: 10.1097/MAO.0b013e3181edb8b2
Carlson ML, Barnes JH, Nassiri A, et al. Prospective study of disease-specific quality-of-life in sporadic vestibular schwannoma comparing observation, radiosurgery, and microsurgery. Otol Neurotol. 2021 Feb 1;42(2):e199-e208. doi: 10.1097/MAO.0000000000002863
Acoustic Neuroma (Vestibular Schwannoma)
Vestibular Schwannoma Algorithm
