Keywords
translabyrinthine vestibular schwannoma resection - vestibule - hearing preservation
- translabyrinthine resection
Introduction
The translabyrinthine (TL) approach for resection of a vestibular schwannoma (VS)
is considered the most reliable method for complete tumor removal. In addition, it
is generally associated with better facial nerve function.[1]
[2] However, it is thought to lead to complete hearing loss. The other two common approaches,
middle cranial fossa (MCF) and retrosigmoid (RS), are more difficult with a worse
angle of dissection and with greater risk to the facial nerve due to its location
during the procedure (MCF) and a greater risk of incomplete tumor removal in the lateral
internal auditory canal (IAC) for the retrosigmoid approach. These two approaches
are considered hearing sparing with better hearing preservation reported in the MCF
approach for small < 1 cm tumors that do not extend to the fundus. With larger tumors,
the RS approach is advocated when hearing preservation is a possibility. However,
in medium to large tumors (Hanover stage T3 [2.0–3.5 cm] and T4 [> 3.5cm]), the hearing
preservation rate using the RS approach is 44% and 18%, respectively.[3]
In 1991, McElveen et al presented the first case of hearing preservation using a modified
TL approach.[4] Their modification involved the attempt to preserve the “pars inferior” portion
of the inner ear and filling the vestibule with saline. Since then, there have been
a few reports of hearing preservation after a classical TL acoustic neuroma resection.[5]
[6]
[7]
[8] To our knowledge, our article presents the fifth case of a patient with partial
hearing preservation after a classical TL resection of a VS. In addition, we discuss
the implication of partial hearing preservation and the possibility of cochlear implantation
in selected cases. Institutional review board approval was obtained before proceeding
with the article.
Case Report
A 42-year-old very obese woman presented for evaluation of right-side tinnitus and
hearing loss that had been present for 6 months. She denied any dizziness or vertigo
and had not had similar problems before. Audiogram showed a sloping moderate high-frequency
sensorineural hearing loss (SNHL). A subsequent magnetic resonance imaging (MRI) of
the brain revealed a right-side cerebellopontine angle (CPA) enhancing mass most likely
a VS. It measured 10.6 × 9.7 mm. The patient was presented with options of observation,
stereotactic radiotherapy, and microsurgical resection. She opted for observation
and was closely followed. In the following few months, she developed deep venous thrombosis
of her lower extremities and was placed on anticoagulation. Unfortunately, during
later follow-up, her hearing worsened. A repeat MRI showed growth of the tumor. It
increased in size from 10.6 × 9.7 mm to 16 × 11 mm. After her other medical problems
were controlled and she was weaned off the anticoagulant, the patient decided to have
the VS surgically removed. Her preoperative hearing test showed significant SNHL in
the right ear, the speech reception threshold (SRT) was 40 dB, and word recognition
score (WRS) was 40% ([Fig. 1A]). Preoperative auditory brainstem response (ABR) was abnormal, and videonystagmogram
indicated right-sided 24% reduced vestibular response. It was recommended due to the
size of the tumor, the degree of her hearing loss, and her body habitus that the TL
approach was her best option. She obtained a second opinion at another institution,
and a similar treatment was recommended. The patient underwent surgery in February
2012 where a classical TL approach was used. Intraoperatively, she had a very high
riding jugular bulb that was decompressed, retracted gently with a Freer elevator,
and bone wax was placed to keep the bulb depressed and out of the way. The view was
still quite narrow due to a very anterior sigmoid sinus. During the surgery, no attempt
was made to preserve hearing. However, we packed the area of the vestibule with fat,
and Tisseel as well as bone wax was placed over the open vestibule. Moreover, the
CPA vasculature was left intact when possible. Eighth nerve monitoring was not used
in this case. A subtotal removal of the tumor was performed due to the adherence of
the deep part of the tumor capsule to the facial nerve. It was believed that ∼ 90%
of the tumor was removed ([Fig. 2]). Care was taken in delivering the tumor out of the IAC and preserving the acoustic
nerve along with the facial nerve. The incus was removed, the middle ear and eustachian
tube was plugged with muscle, fat, and Tisseel, and the antrum was sealed with bone
wax.
Fig. 1 (A) Preoperative audiogram (right ear: speech reception threshold [SRT]: 40; word
recognition score [WRS]: 40%). (B) Two-month postoperative audiogram (right ear: SRT:
95; WRS: 2%).
Fig. 2 Magnetic resonance imaging of the brain and internal auditory canal. (A) Before resection.
(B) After resection.
Postoperative facial nerve function was House-Brackmann (HB) 4/6 that worsened to
a HB 5/6 by the week 4 follow-up. Her postoperative imbalance gradually improved.
By the week 5, her facial nerve function started to improve, and the patient stated
that she could hear sounds out of her right ear. By postoperative month 3, facial
nerve function improved to HB 2/6, and her audiogram at 2 months revealed that she
had persistent hearing in the operative ear ([Fig. 1B]). The SRT decreased from 40 to 95 dB due to a significant air–bone gap as a result
of the removal of the incus. The bone conduction pure tone average (average of BC
HL at 500 Hz, 1 kHz, and 2 kHz) was 65 dB, a decrease of 7 dB from a preoperative
value of 58 dB. However, the WRS decreased to 2% at 105 dB presentation. For the patient,
it was usable in day-to-day routines such as driving and being able to discern the
location of sounds. She has subsequently been fitted with a Soundbite device. One
year later, her hearing was stable at the lowest frequencies ([Fig. 3]). She continues to use the Soundbite device. Her facial function is normal (HB 1/6).
Fig. 3 Postoperative audiogram at 1 year (right ear: speech reception threshold: 95; word
recognition score: 0%).
Discussion
The TL approach is one of the most common methods used in the removal of a VS or a
CPA tumor. In counseling patients, it is common to inform them that the chance of
hearing preservation is almost 0%. However, there have been a limited number of case
reports demonstrating some preservation of hearing after a classical TL approach to
acoustic neuroma resection.[5]
[6]
[7]
[8] The present report adds the fifth case to the literature of preservation of some
hearing after TL resection. The patients presented by Smith et al[5] and Springborg et al[7] eventually lost their hearing 12 months and 6 years later, respectively. The patients
in the other two reports (Rizvi and Goyal[6] and Tringali et al[8]) had stable hearing 2 years and 5 years, respectively, after surgery.
McElveen et al[4] demonstrated that hearing can be preserved after a modified TL excision. Using intraoperative
ABR, they maintained the vasculature and cochlear nerve integrity. They preserved
the saccule and cochlea, and used bone wax to seal the vestibule after opening it.
Later they filled the vestibule with Ringer lactate and sealed off the labyrinth with
bone wax. One of their two patients lost hearing at 1-month follow-up. The second
patient was successfully treated with diuretics for fluctuating hearing loss.
The common belief is that removal of the labyrinth and opening of the vestibule results
in deafness. However, our patient's result as well as that of the previously mentioned
reports suggests that, in these patients, deafness is not an absolute certainty. This
is in accordance with animal studies demonstrating that hearing can be preserved after
ablation of the semicircular canals or partial labyrinthectomy.[9]
[10]
[11] Several studies have also shown that hearing can be preserved in humans after injury
or destruction of the semicircular canals.[12]
[13]
[14] The challenge is we can routinely preserve some hearing in these patients undergoing
“destructive” surgery. Our patient maintained her bone conduction values to an appreciable
level with an air–bone gap occurring due to need for incus removal for packing the
eustachian tube and middle ear space. However, her speech discrimination decreased
significantly. It is however, conceivable that there are preserved spiral ganglion
cells. It has been shown that up to 75% of spiral ganglion cells can be damaged without
an increase in pure tone thresholds.[15] This suggests a mechanism for the success of concurrent cochlear implantation after
TL acoustic neuroma resection as first described by Ahsan et al.[16]
The factors that seem to be most important for hearing preservation are preservation
of the cochlear nerve, CPA vasculature, maintenance of a fluid-filled vestibule, and
preservation of endolymphatic fluids.[4]
[7]
[17] Smith et al suggested that the collapse of the saccule and/or the ductus reuniens
may have preserved the cochlear fluids.[5] Rizvi et al suggested that in their patient, while attempting to seal off the middle
ear with bone wax, they may have sealed off the ductus reuniens, which contributed
to the stability of the hearing in their patient.[6] Further study needs to be done to accurately define the critical steps needed in
preserving hearing in TL cases.
