Vestibular Schwannoma (VS), are benign tumours of Schwann cell origin, that are located within close proximity to cranial nerve VIII (vestibulocochlear nerve). The current management of VS is by microsurgery or radiosurgery and they can lead to post-operative hearing loss (POHL), a very common complication. There are many factors that neurosurgeons can assess, which can indicate the likelihood of developing POHL. Pre-operatively, tumour size, PTA range, MRI findings, pre-operative tinnitus, hearing loss duration >1-year, abnormal auditory brainstem response and unhealthy internal auditory canal fundus are all associated with increased likelihood of POHL. Post-operatively, only speech discrimination score (SDS) and PTA is associated with POHL. Surgical approach also dictates the probability of hearing loss and in general, microsurgery, especially middle fossa approach, has lower incidence of POHL.
Vestibular Schwannoma (VS), also known as Acoustic Neuroma, Acoustic Neurinoma and Acoustic Schwannoma, are benign tumours that are located within close proximity to cranial nerve VIII (vestibulocochlear nerve). These tumours originate from Schwann cells, which are cells that form a protective layer around nerves. VS has an annual incidence rate of 1 per 100,000 people per year and 8% of brain tumours diagnosed in adults are VS. Overall, this makes VS a relatively common brain tumour and one that every Neurosurgeon will encounter at some point in their practice.
There are a myriad of clinical symptoms associated with VS, with the most common being, unilateral hearing, unilateral tinnitus, vertigo, ataxia and headaches . The diagnosis of VS is through neuroimaging via MRI scans. Due to the current ease of access to MRI scans, it is used extensively in medical practices and as a result, the incidence of VS has been increasing .
Currently there three main modes of treatment for VS and they are, microsurgery (operative), radiosurgery and observation. The 3 main approaches used in microsurgery for VS resection are, middle fossa approach, translabyrinthine approach and retrosigmoid approach. Whereas, in radiosurgery, Stereotactic radiosurgery and Gamma-knife radiosurgery are the main approaches employed.
VS and its close proximity to cranial nerve VIII, makes VS treatment very challenging. As a result it is common to see
complications such as, damage to, facial nerve, vestibulocochlear
nerve and blood vessels intra-operatively . There are also many post-operative complications that can arise as a consequence of VS surgery, such as headaches, hearing loss, tinnitus, just to name a few . The aim of this review is to collate some of the predictive factors for post-operative hearing loss (POHL), following VS surgery.
There are multiple pre-operative factors that have been associated with POHL, after VS surgery. Tumour size is one of the strongest predictors of POHL, with studies indicating that the incidence of hearing loss being, low in small tumours (<1.5cm), but, high in large tumours (>3cm) [5-9]. Studies conducted by Rohit et al.  and Ferber et al. , found statistically significant correlation (p <0.05) between pre-operative Pure Tone Audiometry (PTA) and POHL. Rohit et al.  found, the group with preserved hearing had a pre-operative PTA of 23.44dB and the hearing loss group had pre-operative PTA of 29.17 .
Findings on neuroimaging has also been studied to determine their ability to predict POHL. Cochlear enhancement on T1-weighted spin-echo image has shown to be well correlated with total hearing impairment post-operatively . In a study by Dubrulle et al. , 13 participants had cochlear enhancement pre-operatively, and following VS surgery, all of the participants had total hearing impairment.
The correlation between age and POHL is a highly debated topic,
with strong evidence, both for and against the prognostic capability
of patient’s age [5,8,12]. For example, Rohit et al.  found, no
statistically significant correlation between age and POHL, while,
Abboud et al.  did find significant correlation [5,8]. Such variation
in results are primarily due to the age cut-offs used by each study
There are many other pre-operative factors that have been
correlated with POHL. This include pre-operative tinnitus, hearing
loss duration >1-year, abnormal auditory brainstem response and
unhealthy internal auditory canal fundus [10,13,14].
The choice of surgery has a great impact on the likelihood of
patients experiencing POHL. Microsurgery and radiosurgery are
the 2 main modes of intervention for patients with VS. They both
have similar level of disease control, with no statistically significant
(p >0.05) difference between them [15,16]. However, POHL is
reduced in microsurgery compared to radiosurgery and a study by
Karpinos et al.  found hearing loss to be 42.5% in microsurgery
and 85.6% in radiosurgery . More detailed studies comparing
microsurgery and radiosurgery found that, incidence of POHL
depends not only on surgical mode, but also, the tumour size [17-
19]. VS with size <1.5cm are classified as small and microsurgical
resection of this tumours are associated with lower incidence of
hearing loss [17,18]. On the other hand, VS with size >3cm (classified
as large) and volume >10cm3, with tolerable/asymptomatic mass
effect, microsurgery and radiosurgery, both had the same incidence
of POHL .
As discussed above, there are 3 different microsurgical
approaches for VS resection and they are, middle fossa,
translabyrinthine and retrosigmoid approach. Middle fossa
approach is the safest approach and in the case of small tumours,
there is only minimal hearing loss observed . Retrosigmoid
approach is considered the 2nd best approach for preventing POHL.
Ansari et al.  found that, when the VS size was <1.5cm (small
size), middle fossa approach was always superior to retrosigmoid
approach in minimising POHL, but, when VS size was >1.5cm
(medium – large size), both middle fossa and retrosigmoid approach
had the same incidence of hearing loss . Unlike retrosigmoid
approach, where hearing is sometimes preserved, translabyrinthine
approach is associated with complete hearing loss.
Radiosurgery is generally associated with higher incidence of
POHL [15,17-19]. In radiosurgery, a low radiation dose has been
correlated with lower incidence of POHL . When the cochlea
is exposed to <5.3Gy radiation dose, the incidence of POHL is low,
but significant increase in incidence is observed when the radiation
dose is >5.3Gy . However, a dose of 5.3Gy is too low to control
and reduce tumour growth. In a study by Franzin et al. , radiation
dose of approximately 13Gy was suggested for treating VS, since it
maintained a good balance between minimising post-operative
hearing loss and disease control . This is further supported
by Kim et al.  who showed that a radiation dose >13Gy was
associated with a significantly higher probability of hearing loss.
Unlike pre-operative factors, there are only two postoperative
factors that are associated with predicting POHL. Speech
discrimination test is method of analysing auditory function. Based
on the performance of the patient, they are assigned a speech
discrimination score (SDS). In a study by Woodson et al. , 90%
of the patients who had a SDS >70%, immediately after operation,
remained in this category in a 2 year follow up and 88% remained
in this category in a 5 year follow-up. While this does not suggest
that the patients didn’t have any POHL, it does indicate that, postoperatively,
if the SDS is high, the patient will most likely remain in
this category without (further) deterioration in hearing, in the long
Deterioration in PTA is another post-operative indicator for
POHL . In a study, patients who had ≥5dB deterioration in their
PTA within 6 months post-surgery, had ≥15dB deterioration in a 5
year follow-up. Similarly, patients who had <5dB deterioration in
their PTA within 6 months post-surgery, had <15dB deterioration
in a 5 year follow-up .
While VS not associated with mortality, due to the post-operative
complications, it can be a burden to the patient, both physically and
emotionally. For this reason, it is important to treat the VS, while
minimizing the risk of complications that can hinder the quality
of life. The indicators of POHL above can be used to assess the
likelihood of developing hearing loss and use this knowledge to not
only communicate the risks and benefits to the patient, but also, to
determine the best treatment option for the patient.