Skull Base Surgery of the Posterior Fossa

166

Trigeminal neuralgia, hearing impairment, dizzi-
ness, headaches, diplopia, and facial paralysis are
common presenting symptoms of CPA epider-
moid cysts [ 7 , 8 , 17 ], whereas gait disturbance,
abducens and facial nerve palsy, and hydrocepha-
lus are common presentations for fourth ventric-
ular epidermoids [ 33 ]. Seizures and diplopia can
also be a presenting symptom for epidermoids of
the CPA extending to the middle fossa secondary
of mesial temporal structure compression [ 33 ].

Imaging and Differential Diagnosis

The signal characteristics of epidermoid tumor
on CT (Fig. 12.1) and standard MRI (Fig. 12.2)
are similar to those of cerebrospinal fluid (CSF).
Typically, these cysts appear as an extra-axial
hypodense lesion on CT scan, are hypointense on
T1-weighted MRI and hyperintense on
T2-weighted MRI, and display no contrast
enhancement on MRI. However, a rare CT scan
characteristic of some epidermoid cysts has been
referred to as “dense epidermoid cysts” or “white
epidermoids” [ 5 , 26 ]. These lesions have atypical
CT scan hyperdensity that has been variously
described as being a result of liquefaction of the
cyst content, representing high protein content or
prior hemorrhage into the cyst, indicating the
presence of ferro-calcium or iron-containing pig-
ment or denoting the presence of polymorpho-
nuclear leukocytes based on intraoperative
observation and pathological correlation [ 22 , 26 ,
31 , 36 ]. Other radiological features including cal-

cifications have been reported in 10–25% of

cases, and capsular enhancement related to per-

ilesional inflammation in the MRI has been

described [ 7 , 31 ]. Fluid-attenuated inversion

recover (FLAIR) sequence may show heteroge-

neous signal with central hyperintensity in epi-

dermoid cysts and can be used to differentiate

epidermoids from other cystic lesion of the CPA

[ 16 , 31 ]. True restricted diffusion on diffusion-

weighted MRI (hyperintense signal) and appar-

ent diffusion coefficient MRI (hypointense

signal) is the most characteristic MRI finding for

typical epidermoid cysts [ 6 ]. The main differen-

tial diagnosis for epidermoid cysts in the CPA is

arachnoid cysts. Uncomplicated arachnoid cysts

are usually isointense in FLAIR with no restricted

diffusion. The differential diagnosis for atypical

epidermoid cysts should include atypical appear-

ance of meningioma, cystic schwannoma, endo-

lymphatic sac tumors, teratoma, pilocytic

astrocytoma, hemangioblastoma, and gangliogli-

oma [ 15 , 31 ].

Management

Epidermoid cysts are slow growing and benign

and do not respond to chemotherapy or radiother-

apy, and asymptomatic small epidermoid cysts of

the CPA can be monitored with serial imaging.

However, for symptomatic or large asymptomatic

cysts with significant mass effect on the brainstem,

surgical removal is indicated. Complete surgical

resection of the epidermoid tumor along with its

Fig. 12.1 Axial (a), coronal (b), and sagittal (c) CT scans
of the brain demonstrating a hypodense lesion located at
the left CPA angle. Note the supratentorial extension of

the lesion and the mass effect on the brainstem and mesio-

temporal structures

G. Alzhrani and W.T. Couldwell