Arachnoid cysts confined to the internal auditory canal or facial nerve canal

Introduction
Arachnoid cysts arising from the internal auditory canal
(IAC) or facial nerve canal are rare.1?3 They constitute
only 0.5 per cent of all neoplasms found in these locations.4,5
Because of their location, such arachnoid cysts usually manifest
with clinical symptoms similar to those of acoustic西安交通大学医学院第二附属医院耳鼻喉科张青
neuroma.5,6 Previously, intracanalicular arachnoid cysts
have been reported only in a small number of patients.
Most of these reported lesions were large and presented
with obvious clinical symptoms of nerve dysfunction.6?11
Arachnoid cysts confined to nerve canals have rarely been
reported as asymptomatic.
We herein present four cases of intracanalicular arachnoid
cyst, recently discovered during temporal bone examinations.
Materials and methods
All temporal bone samples were obtained from the temporal
bone collection of the department of otolaryngology, Teikyo
University, Japan.
These temporal bones had been removed during autopsy,
and were prepared according to routine pathology procedures.
The temporal bones were cut horizontally into 20-μm sections.
Every 10th slice was selected, stained with haematoxylin
and eosin (H&E) and observed under a light microscope.
The clinical records of the corresponding patients were
reviewed and compared with the results of histological
observation.
Results
Case one
A 74-year-old woman with breast cancer and acoustic
neuroma died of cancer metastasis and sepsis.
At the age of 71 years, the patient had complained of leftsided
hearing loss and tinnitus, two weeks prior to admission
to hospital. She denied otalgia or otorrhoea in her left ear. No
facial nerve palsy or vestibular problems could be detected
on physical examination. Otoscopic examination demonstrated
an intact tympanic membrane. The patient had previously
undergone a mastectomy for breast cancer, aged 58
years, and radiotherapy (6000 rads) due to recurrence at 70
years. No relevant family history was recorded.
Pure tone audiography revealed moderate sensorineural
hearing loss, with an average hearing threshold of
68.75 dB HL, in the patient’s left ear. Click-evoked auditory
brainstem response (ABR) disclosed no response in the same
ear at 100 dB nHL. No response could be induced on caloric
testing. Electronystagmography of the left ear produced no
spontaneous, positional or gaze nystagmus.
Computed tomography (CT) and magnetic response
imaging (MRI) revealed a space-occupying, tumorous
mass in the cerebellopontine angle and in the left IAC
(Figure 1). A diagnosis of acoustic neuroma was made.
The patient was recommended to have the tumour
removed, but she refused. She died of breast cancer metastasis
and sepsis at the age of 74 years. Both temporal bones

were removed at autopsy. Routine horizontal serial sections
were prepared for observation.
Histopathological findings. Two arachnoid cysts were
detected in the left temporal bone. One was found in the
facial nerve canal, the other in the IAC.
Light microscopy of serial temporal bone sections showed
a cystic lesion in the left facial nerve canal which had compressed
the facial nerve downward. The surrounding nerve
fibres and ganglion cells had atrophied. The facial nerve
canal was slightly enlarged at that position. The wall of the
cyst was thin, comprising a single layer of epithelium in
some places, and was collapsed in some areas. A small
amount of acidophilic material and a few mononuclear
inflammatory cells were seen inside the cystic cavity; this
may have been due to rupture of the cyst during histological
preparation (Figure 2).
The other arachnoid cyst was seen in the middle part of the
left IAC. The cyst very firmly compressed the facial nerve
downward and forward within the IAC. The wall of this
cyst was a little thicker than the cyst in the facial nerve
canal, and contained several layers of cells and some
matrix. The cyst was bi-lobed, and it was unclear whether
the two parts communicated with each other. A great many
deformed erythrocytes and inflamed cells were found
around the cyst (Figure 3).
An acoustic neuroma could be seen in the enlarged left
auditory canal, which took up most of the meatal space.
This schwannoma was composed of mixed Antoni A and B
type cells. The main part of the cochlear nerve and the inferior
vestibular nerve trunks had been replaced by the tumour. In
the cochlea, the number of spiral ganglion cells and cochlear
nerve fibres decreased in each turn. However, the outer and
inner hair cells werewell preserved. Some fluid and thickened
mucosa could be defined in the ipsilateral mastoid cells.
Case two
A 43-year-old man with haemophilia A, acquired immune
deficiency syndrome and long-term encephalitis died of
intracranial haemorrhage.
The patient had been diagnosed with haemophilia A at 16
years of age, after suffering continuous bleeding from a thigh
wound. In the following years, he had received several blood
transfusions for blood coagulation factor VIII.
In August 1990, he suffered from recurrent oral cavity
inflammation and chest wall folliculitis, of uncertain microbiological
origin. He was later investigated and found to be
positive for serous human immunodeficiency virus antibody.
Four months later, the patient reported sensory disturbance
in his left hand (fourth and fifth fingers), and it was noted
that his pupils differed in size. Encephalitis was suspected,
and this diagnosis was later confirmed at autopsy.
Two weeks later, he was hospitalised with severe nasal
bleeding. At the end of a long period of hospitalisation, he
suffered a sudden and fatal intracranial haemorrhage (on
24 July 1991). Computed tomography revealed a huge haemorrhage
in the left basal ganglion area, which extended into
the left cerebral ventricle. Cerebral herniation advanced
very rapidly, and the patient died within hours.
No hearing data, hearing problems or facial palsy manifestations
were documented in the patient’s medical record.
Histopathological findings. Many red blood cells and mononuclear
inflammatory cells were found in the subarachnoid
space. A cystic, space-occupying lesion was present in the
left IAC. This cyst was located exactly in the middle of the
nerve trunks, and displaced the surrounding nerves in
various directions. Both the cochlear nerve and the inferior
vestibular nerve were compressed into an arch shape.
Slight atrophy of the posterior part of the left cochlear
nerve trunk was detected. The wall of the cyst was composed
of several epithelial layers and small amounts of matrix. No
obvious enlargement of the left IAC was detected (Figure 4).
A moderate amount of acidophilic material had accumulated
in the scala tympani, scala vestibule, cochlear duct,
and perilymphatic and endolymphatic spaces of the semicircular
canals, on both sides. The organs of Corti (at each turn),
neurosensory epithelia of the utricle and saccule, semicircular
canal cristae, and ganglion cells of various nerves were all
well preserved.
Case three
A 22-year-old man died of sudden, progressive tuberculous
meningitis. The disease had progressed rapidly, with only
25 days elapsing between onset and death.
The patient had initially experienced the sudden onset of a
severe headache. He visited his physician but returned home,
as no urgent signs could be found. Five days later, he experienced
nausea and began vomiting. The suspected diagnosis
at the time was vascular headache; however, the next day
he suddenly lost consciousness. An emergency lumbar puncture
was performed, and the intracranial pressure was found
to be 290 mmH2O.
An emergency cerebral ventricular drainage procedure
was performed. The patient regained consciousness a few
days later.
Unfortunately, his condition reversed on the fifth postoperative
day: he experienced neck stiffness and was positive
for Kernig’s sign. On the seventh post-operative day, deafness
and peripheral facial nerve palsy occurred on the right
but not on the left. No hearing examination was conducted.
The patient died three days later.
His temporal bones were removed 80 minutes after death,
and were prepared for observation using routine procedures.
FIG. 1
Magnetic resonance imaging scan for case one, demonstrating the
enlarged acoustic tumour (arrow) in the left cerebellopontine angle.

Post-operative histopathological examination disclosed
tuberculous meningitis as the final diagnosis.
Histopathological findings. A clear, space-occupying cyst
was located in the left IAC, among the nerve trunks. The
facial nerve and the superior vestibular nerve were both displaced
upward. The body of the cyst compressed the cochlear
nerve and the inferior vestibular nerve forward and backward,
respectively, with atrophy of the latter. The cystic
wall was folded at the top of the cystic cavity; this may be
because the arachnoid cyst had ruptured and collapsed
during the autopsy procedure, or during histological preparation
(Figure 5).
A few mononuclear inflammatory cells had accumulated
in the subarachnoid space of the IAC. Nerve fibres were
infiltrated with mononuclear inflammatory cells, and some
were seen to be swollen and degenerate. The neurosensory
epithelia of the apical and middle turns of the cochlea was
absent. The cristae of the semicircular canals and the
maculae of the saccules and utricles were severely
damaged on both sides.
Discussion
Theoretically, arachnoid cysts can occur at any site where
arachnoid membrane exists.4 In practice, these cysts commonly
occur on the surface of the brain at the level of the
main brain fissures, i.e. the sella turcica, the anterior
cranial fossa and the middle cranial fossa.12,13
Neither the IAC nor the facial nerve canal is a common site
for arachnoid cysts. To our knowledge, arachnoid cysts confined
to the facial nerve canal have not previously been
reported in the temporal bone.
Aetiology
Classification of arachnoid cysts into primary and secondary
types is widely accepted. Although the aetiopathogenesis of
primary arachnoid cysts is unknown, the secondary type
occurs in association with trauma, inflammation and neoplasms
of the meninges.14?17
According to this classification, all four arachnoid cysts in
the presented series should be classified as the secondary
type. Case one suffered from intracanalicular acoustic

neuroma; case two had long-term intracranial inflammation;
and case three had tuberculous meningitis. Histologically,
each of these arachnoid cysts had inflammatory cells and
red blood cells in spaces surrounding the cyst.
The coexistence of arachnoid cysts and neoplasms has
been reported in several papers. Francis et al. reported the
coexistence of heterotopic neuroglial tissue with an intracanalicular
arachnoid cyst.7 Parnes et al. and Thijssen et al.
both described a close relationship between arachnoid cysts
and intracanalicular acoustic neuroma.8,9 Surprisingly, in
our first case we found one arachnoid cyst in the IAC and
another in the facial nerve canal, together with an acoustic
neuroma in the same temporal bone.
Morphology
The morphology of intracanalicular arachnoid cysts is
unique. They develop within narrow spaces, forming distinct
spherical or oval-shaped, space-occupying cysts with a sharp
boundary with the surrounding tissue.4,18 In our series, we
found largely transparent cyst contents. Each of our cases
showed compression-induced nerve trunk atrophy, meeting
the diagnostic criteria recommended by Schuknecht et al.4
Each of our cysts had a very thin wall, histologically consisting
of a single layer or countable layers of epithelial cells.
Surprisingly, intracanalicular arachnoid cysts can heavily
compress surrounding nerve trunks, even if their cyst wall
is very thin. For example, in our first case, both cysts compressed
the facial nerve trunk, leading to atrophy of nerve
fibres and ganglion cells. This compression may have been
due to the location of these intracanalicular arachnoid
cysts, within hard, bony meatal walls, with extremely
limited space.
• Arachnoid cysts confined to the internal auditory
canal or facial nerve canal are rare
• We report three arachnoid cysts occurring in the
internal auditory canal and one in the facial nerve
canal
• The intracanalicular arachnoid cysts identified in
this series, via temporal bone histology, were
asymptomatic
In contrast to arachnoid cysts lying on the surface of the
brain, the arachnoid cyst in case one, occurring within the
IAC, was discovered to be lobulated from within. It has previously
been assumed that arachnoid cysts are usually unilocular;
however, within the IAC the situation may be
different.18 In case one, the intracanalicular space was
deep, narrow and relatively static (being slightly distant
from the cerebrospinal fluid circulation, and with an acoustic
neuroma blocking the outer part of the IAC). Therefore, it is
possible that the intracanalicular cystic wall on one side may
have extended such that it touched another part of the wall,
then adhered to it from within, thus displaying a lobulated
appearance on histological examination.
Clinical manifestations
Many reports have described intracanalicular arachnoid cysts
as aggressive lesions. These lesions are capable of destroying
the surrounding bony canal walls and the cochlea, leading to
florid symptoms of nerve dysfunction such as facial palsy,
hearing loss, tinnitus and loss of balance.6?11 In contrast,
the presented cases did not exhibit any detectable cranial
nerve dysfunction, although severe nerve trunk compression
was revealed on histological analysis. (Our first patient
reported left ear hearing loss and tinnitus two weeks before
hospitalisation; however, it is more likely that these symptoms
were induced by this patient’s acoustic neuroma
rather than her arachnoid cysts.) This distinct clinical difference
may have occurred because these patients’ arachnoid
cysts were at an early stage, in contrast to the relatively
advanced-stage lesions which generally present to the
specialist clinic. On the other hand, it might also indicate
that meatal nerves have a powerful self-adjustment capacity
when compressed within nerve canals.
Arachnoid cysts are not invasive in every case. Previous
reports have described spontaneous resolution and longterm
stability without growth.19?21 In the present study,
each of the four identified cysts differed from the others.
At present, it is not possible to distinguish between those arachnoid
cysts that are strong enough to destroy surrounding
structures and to induce severe complications, and those
that are not. It is possible that an unknown mechanism
may prevent some intracanalicular arachnoid cysts from
growing large enough to induce severe symptoms.
Additionally, it may be the case that some occult intracanalicular
arachnoid cysts do not grow large, but do induce unremarkable
meatal nerve dysfunction. Since so many patients
present to specialist clinics suffering from hearing loss, tinnitus,
vertigo, facial palsy or spasm of unknown cause, we
propose the hypothesis that some of these symptoms may
be due to occult arachnoid cysts within the IAC or facial
nerve canal. In light of the present pathological study, we
suggest that dynamic observation, together with high resolution,
cross-sectional imaging, may be helpful in detecting
such occult lesions at an early stage.
Conclusion
Arachnoid cysts can develop within the IAC and facial nerve
canal, and may compress adjacent nerves. In the present,
post-mortem study, the identified arachnoid cysts were
asymptomatic, in contrast to most of the intracanalicular arachnoid
cysts encountered in clinical practice.
Acknowledgements
The authors are grateful to Ms Kayoko Sekiguchi for her help
in observational work. We also thank Ms Dongtao Lin for
copy-editing this manuscript. This work was supported in
part by a research grant from the Society for the Promotion
of International Otorhinolaryngology (Japan), by a scholarship
from the Takeda Science Foundation (Japan), and by a
school president grant from Xi’an Jiaotong University (xjj
2008019, China).
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