Keywords
labyrinthine artery - internal auditory artery - aneurysm - internal auditory canal
- facial paralysis - acoustic neuroma - vestibular schwannoma - mass
Presentation
We present the first case report of a labyrinthine artery aneurysm masquerading as
an internal auditory canal tumor. A 72-year-old woman presented to the clinic with
a 1-week history of sudden onset right facial paralysis, right facial pain, hearing
loss, and vertigo. On examination, she was found to have a dense right-sided facial
paralysis involving all branches of the facial nerve, right facial hypoesthesia involving
all branches of the trigeminal nerve, no response to whispered voice in the right
ear, and left beating horizontal nystagmus. The Weber tuning fork test demonstrated
sound lateralization to the left, and the Rinne tuning fork test demonstrated no ability
to discern sound over the right ear and right mastoid. Behavioral audiometry revealed
a complete anacusis of the right ear.
Radiographic Findings
Magnetic resonance imaging of the brain demonstrated mild expansion of the right internal
auditory canal (IAC) by a mass of homogeneous signal intensity, slightly hyperintense
relative to cortex on T2 images. The mass, measuring 6 mm × 7 mm, filled the IAC but
did not extend beyond the porus acusticus. The mass demonstrated peripheral enhancement
following contrast administration ([Fig. 1]). Given its location, imaging characteristics, and gadolinium enhancement, coupled
with the patient's clinical presentation, the mass was thought to most likely represent
a vestibular schwannoma. Peripheral enhancement with central nonenhancement also suggested
a perineural inflammatory or infectious process, or less likely a metastatic or malignant
process with central cyst or necrosis.
Fig. 1 (A) Magnetic resonance imaging (MRI) three-dimensional coherent oscillatory state
acquisition for manipulation of imaging contrast (3D COSMIC), axial, mid-internal
auditory canal (IAC), demonstrating homogeneous appearance of mass filling the IAC,
with T2-weighted signal relatively isointense to that of the cerebral cortex. (B)
MRI T1 with gadolinium contrast, axial, mid-IAC, demonstrating peripherally enhancing
mass filling and widening the right IAC. (C) MRI T1 with gadolinium contrast, coronal,
mid-IAC, demonstrating peripherally enhancing mass within the right IAC. (D) MRI T1
with gadolinium contrast, sagittal, mid-IAC, demonstrating peripherally enhancing
mass within the right IAC with a distinctive “ring”-type structure verifying circumferential
enhancement.
Treatment Options
All treatment options were discussed in complete detail with the patient and her husband.
These included (1) observation and serial imaging of the IAC mass with primary medical
treatment of facial paralysis; (2) fractionated or hypofractionated radiotherapy treatment
of the IAC mass; (3) stereotactic radiotherapy treatment of the IAC mass; (4) middle
fossa approach for primary surgical decompression of the IAC; and (5) surgical resection
of the IAC mass for primary treatment of the mass, histopathologic diagnosis, and
decompression of neural structures within the IAC. The surgical approaches were further
characterized as middle fossa, retrosigmoid, and translabyrinthine approaches, with
the usual advantages and admonitions specific to each approach described in detail.
After exhaustive consideration of all treatment options, and given the acute onset
of symptoms, acute and complete paralysis of facial function, imaging characteristics
of the IAC mass with potential to represent a malignant or metastatic process, and
lack of histopathologic diagnosis of the IAC mass, the patient elected to proceed
with resection of the IAC mass via a translabyrinthine approach. This approach would
provide complete visualization of the IAC from fundus to porus, allow resection of
mass and decompression of the IAC with no displacement of the facial nerve, and allow
maximal opportunity for recovery of neural function of the facial nerve.
Intraoperative Findings
The translabyrinthine approach to the internal auditory canal was accomplished without
difficulty. The IAC dura was incised and IAC contents exposed in the usual fashion,
with the gross appearance of a solid nonpulsatile mass filling the IAC volume and
compressing the facial nerve anterosuperiorly ([Fig. 2]). The IAC tumor was dissected off of the effaced facial nerve with minimal difficulty.
Intraoperative dissection was notable for brisk bleeding at the medial base of the
tumor just as the tumor was dissected off its medial fibrous attachments. A posteriori
note was made of the lack of bleeding during dissection of the lateral end of the tumor out of the fundus.
The facial nerve was maintained intact anatomically but never demonstrated electrical
function, either at the brainstem or at the meatal foramen, either before or after
dissection.
Fig. 2 (A) Intraoperative surgical view, internal auditory canal (IAC), prior to opening
of the IAC dura, demonstrating mass filling the IAC. (B) Intraoperative surgical view,
IAC, with tumor partially dissected, demonstrating exposed fundal end which appears
solid, not vascular.
Histopathology
Final histopathology of the resected mass demonstrated a blood vessel with mucinous
degeneration of the medial layer of the vessel wall. Immunohistochemical staining
demonstrated positive staining to human muscle actin throughout the vessel wall, confirming
the presence of smooth muscle cells within the wall of the mass, and negative staining
to S-100, militating against neoplasm of neural or nerve sheath origin as the composition
of the tumor specimen ([Fig. 3]).
Fig. 3 (A) Hematoxylin and eosin (H&E) stain, low power, demonstrating luminal structure
of vascular mass. (B) H&E stain, high power, demonstrating muscular wall of vascular
mass consistent with artery. (C) Human muscle actin immunohistochemistry, high power,
demonstrating positive staining of smooth muscle cells of the arterial muscle wall.
(D) S-100 immunohistochemistry, high power, demonstrating absence of staining, thereby
negating a diagnosis of schwannoma, which would stain avidly positive.
Long-Term Results
Despite the lack of electrical stimulability to the facial nerve before, during, and
at the conclusion of intracanalicular dissection, the patient ultimately had significant
success as a result of her intervention. Intraoperatively, the patient underwent gold
weight implantation of the ipsilateral upper eyelid at the conclusion of her tumor
dissection. She maintained a dense facial paralysis for approximately 1 year from
the date of her surgery. At ∼ 15 months, she began exhibiting minimal movement to
the upper branches of the facial nerve, which progressed further over the ensuing
6 to 9 months. At ∼ 2 years, her facial function recovery had progressed significantly
to a House-Brackmann grade 3/6, and her upper eyelid gold weight was explanted. At
3 years postoperatively, she has maintained a House-Brackmann grade 3/6 facial function,
with persistent mild to moderate synkinesis but excellent complete volitional eye
closure and symmetrical upper and lower branch volitional motor function and blink,
and she is pleased with her result.
The patient's initial presentation of unilateral facial pain and hypoesthesia were
puzzling, given the limited extension of the enhancing mass on preoperative imaging.
We did not have a straightforward explanation for these trigeminal symptoms, but the
out-of-the-ordinary presentation added additional motivation to obtain definitive
histopathologic diagnosis of this mass. Postoperatively, the patient maintained facial
pain and hypoesthesia acutely. Her facial pain resolved rapidly and had completely
resolved within 3 months. Her hypoesthesia resolved much more slowly but did resolve
completely by her 2-year postoperative anniversary. As her facial pain and hypoesthesia
appear to parallel the timeline of recovery of her facial function, our most compelling
explanation of these symptoms is of the patient subjectively interpreting her acute
facial paralysis not only as loss of motor function but also as a loss of facial sensation.
Discussion
The labyrinthine artery, also called the internal auditory artery, arises as a branch
of the anterior inferior cerebellar artery, in its loop section within the cerebellopontine
angle. The labyrinthine artery travels the length of the IAC and supplies the entirety
of the otic capsule via its anterior vestibular, vestibulocochlear, and cochlear artery
branches.[1]
[2] Because the labyrinthine artery is the only named artery that traverses the IAC,
it must by default be the arterial source of the aneurysmal mass described here.
Summary
This case represents the first reported presentation of a labyrinthine artery aneurysm
within the IAC masquerading as either a schwannoma, inflammatory process, or metastatic
process involving the intracanalicular vestibulocochlear or facial nerve. Recognition
of the existence of the diagnostic entity of a labyrinthine artery aneurysm will allow
surgeons and radiologists to include and consider such aneurysms of the labyrinthine
artery within the differential diagnosis of similar clinicopathologic presentations,
and it allows surgeons to counsel such patients and approach treatment decisions more
comprehensively. Early identification and intervention via decompression of the ICA
and corresponding facial nerve can lead to favorable long-term results, as demonstrated
in this seminal case report.
