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TITLE:   PETROUS APEX LESIONS
RESIDENT PHYSICIAN:     Robert D. Hoffman, M.D.
FACULTY:                Jeffrey T. Vrabec, M.D.
DATE:     April 5, 1995
DATABASE ADMINISTRATOR: Melinda McCracken, M.S.
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"This material was prepared by resident physicians in partial fulfillment 
of educational requirements established for the Postgraduate Training 
Program of the UTMB Department of Otolaryngology/Head and Neck Surgery 
and was not intended for clinical use in its present form.  It was 
prepared for the purpose of stimulating group discussion in a conference 
setting.  No warranties, either express or implied, are made with respect 
to its accuracy, completeness, or timeliness.  The material does not 
necessarily reflect the current or past opinions of members of the UTMB 
faculty and should not be used for purposes of diagnosis or treatment 
without consulting appropriate literature sources and informed professional 
opinion." 


        Petrous apex lesions may remain undetected for extended periods
because patients often complain of vague or indistinct symptoms that
delay diagnosis.  Headaches, atypical facial pain, mixed hearing
loss, vertigo, eustachian tube dysfunction, and middle ear effusion,
while common otoneurologic complaints, may be the initial
manifestations of an extensive petrous apex lesion.  Clinical
manifestations can be well explained by involvement of the structures
contained within or adjacent to the apex.  Compression or irritation
of the gasserian ganglion of the trigeminal nerve will cause deep
aural and retro-orbital pain.  Diplopia will result from similar
compromise of the abducens nerve.  Facial nerve dysfunction is less
common, except in advanced lesions.  Hearing loss and vestibular
complaints may be caused by  eighth nerve involvement in the internal
auditory canal or by direct extension of the process into the bony
labyrinth.  

        Although the carotid artery is often compressed, vascular
symptoms are uncommon, except in the presence of an aneurysm.  These
lesions, especially benign cystic ones, frequently achieve
significant size with marked bony erosion before diagnosis is made.
Petrous apex lesions pose several diagnostic and therapeutic
dilemmas.  The relative inaccessibility of this location associated
with surrounding bony, neural, and vascular structures often
precludes traditional diagnostic techniques such as a simple biopsy.
Since vital structures here are encased in bone, thin section CT has
been the method of choice in imaging this complex area.  MRI plays an
instrumental role in further evaluating the apex.  The combination of
these two radiologic modalities along with the history and physical
examination are the primary methods used to base the differential
diagnosis.

        Once recognized, the differential diagnosis of petrous apex
lesions includes congenital or primary cholesteatoma, cholesterol
granuloma, primary mucocele, schwannoma, meningioma, chondroma,
chondrosarcoma, or metastatic neoplasms.  The most frequent cause of
petrous apex destruction is a secondary process from either
contiguous lesions or metastases.  Primary lesions intrinsic to the
petrous apex are less common, but are not rare.  Discernment of
pathologic processes in this area and evaluation of their extent is
critical in diagnosis and preoperative planning.  Surgical approaches
to the petrous apex evolved from purulence drainage procedures.
Multiple clinical and radiologic findings such as residual hearing,
facial nerve function, pathology, and extent of disease must be used
in combination to formulate a surgical plan.  The following
approaches can be employed: translabyrinthine-transcochlear, middle
cranial fossa, transcanal infracochlear, subtotal petrosectomy,
transsphenoidal, and suboccipital.  Some represent a compromise
between optimal access and preservation of cochlear function, and all
attempt to preserve associated brain stem, cranial nerve, cerebellar,
and vascular structures found in close proximity to the petrous apex.

PETROUS APEX ANATOMY

     The petrous apex is a truncated pyramid which makes up the
medial part of the temporal bone.  Its base is the bony labyrinth
including the semicircular canals and cochlea.  Anteriorly, the base
of this pyramid is partly the semicanal of the tensor tympani muscle
and the internal carotid artery.  The superior surface makes up the
major part of the middle cranial fossa floor from the arcuate
eminence of the superior semicircular canal to the beginning of the
ascending portion of the internal carotid artery and the gasserian
ganglion fossa (Meckle's cave).  This surface extends from the
superior petrosal sinus posteriorly to the petrosphenoid suture line
anteriorly.  

         The posterior surface of the petrous apex is the
anterolateral wall of the posterior cranial fossa and extends
medially from the posterior semicircular canal and the endolymphatic
sac to the petroclinoid ligament and the canal for the abducens nerve
(Dorello's canal).  This surface extends from the petro-occipital
suture line inferiorly to the superior petrosal sinus superiorly.
Inferiorly, the petrous pyramid is bounded by the jugular bulb and
the inferior petrosal sinus.  The inferior surface also has a foramen
for the entry of the internal carotid artery.  Medial to the jugular
fossa is a depression which is associated with the cochlear aqueduct
(perilymphatic duct).  The petrous bone articulates with the greater
wing of the sphenoid anteriorly.  The foramen lacerum is found
between the apex of the petrous bone and the sphenoid bone and
contains but does not transmit the internal carotid artery.

         The posteroinferior margin of the temporal bone articulates
with the occipital bone.  Laterally, the bone is fused with the
squamous portion of the temporal bone at the petrosquamosal fissure.
The internal carotid artery (transverse portion) and the internal
auditory canal traverse the petrous pyramid.  The petrous apex can be
conveniently divided into anterior and posterior areas by a vertical
plane through the modiolus of the cochlea and the internal auditory
canal.  The posterior portion, located between the internal auditory
canal and the semicircular canals, is usually composed of compact
bone and is rarely involved by disease processes.  The petrous bone
may be pneumatic, diploic (marrow-filled), or sclerotic.  About 10%
of the population has pneumatization of the petrous.  Primary petrous
apex lesions are those that arise from the anatomic boundaries of the
region.  Secondary lesions are those that impinge on the petrous from
an outside source.  This may be from invasion from an bordering
region or from a metastatic lesion.  Primary lesions account for
approximately 36% of petrous apex lesions.


Cholesterol Granuloma

     Cholesterol granuloma of the petrous apex forms as a result of
foreign body giant cell reaction to cholesterol crystals.  The poor
ventilation, interference with drainage and hemorrhage in a usually
pneumatized space are predisposing factors leading to the formation
of the cyst.  Negative pressure from air resorption lead to the
degradation of the blood and formation of cholesterol crystals.
These crystals initiate a foreign body reaction that results in
granuloma or cyst formation.  The cyst wall is made up of fibrous
connective tissue and lacks the keratinizing squamous epithelium seen
cholesteatoma.  Foreign body giant cells,  hemosiderin laden
macrophages and cholesterol crystals are seen on pathologic
evaluation.

         A characteristic finding on CT of a cholesterol granuloma  is a
sharply marginated expansile lesion.  The lesions are avascular and
therefore do not enhance with contrast.  They are isodense with brain
tissue.  A thin, peripheral calcified rim may be noted as well as
pneumatization of the contralateral petrous apex.  The MRI findings
of cholesterol granuloma are unique.  The lesions demonstrate high
signal intensity on both the T1 and T2 weighted images.  The unique
increased signal intensity on the T1 weighted image may be due to the
combination of cholesterol crystals, chronic hemorrhage and
proteinaceous crystals.  Occasionally it may be difficult to
distinguish a small cholesterol granuloma from normal marrow because
both may demonstrate increased signal intensity on T1 weighted
images.  They may be differentiated by examining the T2 weighted
images.  Marrow fat will exhibit a progressive decrease in signal
intensity with increase T2 weighting.  Cholesterol granulomas
maintain their high signal intensity on T2 weighted images.
        
        Specialized MR pulse sequences may sometimes be needed to
evaluate these lesions.  Partial saturation gradient recalled echo
(GRE) sequences may be read out instead the usual spin-echo
sequences.  GRE imaging demonstrates an enlarging peripheral ring of
decreased signal intensity as the TE is lengthened.  This indicates a
peripheral magnetic susceptibility effect suggesting that hemosiderin
laden macrophages are present in the wall of the lesion.  While this
pattern may be possible in a thrombosed aneurysm it is unlikely in
acoustic neuromas, cholesteatomas or mucoceles.  Protein chemical
shift imaging may also be used to evaluate these lesion.  The mixed
population of aliphatic (CH2) and water protons produce a cyclic
pattern of signal intensity in the center of the lesion.  A
cholesteatoma may produce this pattern but a thrombosed aneurysm,
acoustic or mucocele would not.  Because of this unique ability to
evaluate cholesterol granulomas, MRI is now used both for
preoperative evaluation and to follow the patient postoperatively for
recurrence.  

        Treatment of cholesterol granulomas is based on surgical
drainage and aeration.  Complete surgical removal is rarely indicated
because the lesion lacks an epithelial lining.  Multiple surgical
approaches have been described for treating these lesions including:
infralabyrinthine, transcanal infracochlear, transsphenoidal, middle
cranial fossa, and retrosigmoid.  The approach taken depends on the
patients hearing and the site and extent of the lesion.

        The retrosigmoid and middle cranial fossa approaches are
designed for hearing preservation but do not provide for permanent
drainage or aeration.  Chemical meningitis from subarachnoid space
contamination by the contents of the cholesterol granuloma is also a
risk.  Drainage, aeration and hearing preservation are the goals of
the infralabyrinthine, transcanal infracochlear, and transsphenoidal
approaches.  Detailed analysis of the pre-operative CT scan to define
the jugular bulb, facial nerve, sigmoid sinus, bony labyrinth and
posterior wall of the sphenoid is mandatory before surgery.  If the
lesion does make a significant impression on the sphenoid, a
transsphenoidal approach may be used.  The approach has limited
exposure and risks the optic nerve and internal carotid artery.
Another draw back is the high rate of recurrence for this lesion.
Thedinger reported six patients who underwent this procedure for
cholesterol granuloma.  Five patients needed revision surgery for
recurrence.  Of those five patients, two needed multiple procedures.
Hearing results for this procedure have been described by Gianoli.
He reported on six patients, two showed hearing improvement and four
remained the same.  He did not report on recurrence.  

        The infralabyrinthine approach shares the same goals as the
transsphenoid approach.  Thedinger reported on three patients who had
this approach for cholesterol granuloma.  Two required surgery for
recurrence.  In contrast, Goldofsky reported on nine patients
followed from one to ten years who had this approach and only one
recurred.  Hearing results from fourteen patients showed seven
improved and seven remained the same.  The infralabyrinthine approach
involves a simple mastoidectomy then removing the air cells in
Trautmann's triangle.  The sigmoid sinus is followed until the
jugular bulb is identified.  This represents the inferior margin of
the approach.  The semicircular canals are skeletonized and the
infralabyrinthine air cell tract is developed anteriorly.  Once the
lesion is identified it is evacuated and the opening is enlarged.  A
stent may be place to prevent stenosis of the opening.  This approach
is limited if the patient has a high jugular bulb limiting access to
the infralabyrinthine air cell tract.  

        The transcanal infracochlear approach involves a
postauricular incision and reflecting the ear anteriorly.
Typanomeatal flaps are developed and the external auditory canal is
enlarged anteriorly and inferiorly to expose the hypotympanum.  The
chorda tympani is followed inferoposteriorly to define the extent of
posterior dissection possible without injury to the facial nerve.
The air cell tract below the cochlea is developed in the hypotympanum
to expose the course of the carotid artery and the jugular bulb.  The
round window provides the superior line of dissection, and Jacobson's
nerve leads to the "crutch" of the carotid and jugular bulb.  In a
study of ten cadaveric temporal bones, the average cross sectional
area of this region was 25.6 mm2.  The area ranged from 67.2 mm2 to
7.4 mm2.  If the plane of dissection remains below the round window,
the internal auditory canal structures will not be at risk.  Once the
lesion is entered is drained and a catheter may be placed.  A high
jugular bulb does not block access via this route as it does with the
infralabyrinthine approach and it provides more dependent drainage
for the cyst.

        The transsphenoidal approach is useful only when a cyst forms
a large surface area against the posterior wall of the sphenoid
sinus.  Multiple approaches may be used to approach the sphenoid
including: external ethmoidectomy sphenoidotomy, intranasal
sphenoethmoidectomy, intranasal sphenoidotomy, trans-septal
sphenoidotomy, or transpalatal approaches.  The lateral and superior
walls of the sphenoid sinus must be closely examined to locate
indentations of the pituitary gland, optic nerve, maxillary nerve,
carotid artery, and cavernous sinus.  Once the wall of the cyst is
identified it can be opened and drained.  A large opening should be
created to decrease the chance of postoperative stenosis.  Once the
landmarks are identified the roof of the petrous apex is opened and
the lesion is drained or removed.

     The retrosigmoid approach is performed through a craniotomy
posterior to the sigmoid sinus.  The cerebellum is retracted
posteriorly, allowing for access to the cerebellopontine angle.  This
procedure has been of limited usefulness because of the location of
most petrous apex cholesteatomas anterior of the internal auditory
canal.  The interposition of the cerebellum, as well as brainstem and
cranial nerves, makes total extirpation of there tumors extremely
difficult.  This approach may be useful for removal of petrous apex
cholesteatomas with significant intracranial extension or for those
epidermoid lesions arising primarily within the cerebellopontine
angle.


Cholesteatoma

     Cholesteatomas may involve the petrous apex as a congenital
cholesteatoma or secondary to an acquired cholesteatoma.  Congenital
cholesteatomas are rare and can be found in a number of locations in
the temporal bone.  They differ from an acquired cholesteatoma in
that they develop behind an intact tympanic membrane.  Both
cholesteatomas are characterized pathologically by keratin filled
epithelial lined cysts.  Acquired cholesteatomas which involve the
petrous apex are aggressive and may extensively erode the petrous
bone.  Primary cholesteatomas of the petrous apex present with
symptoms produced from the mass effect of the lesion.  This includes
headaches, and aural fullness.  Most patients will present with
hearing loss with poor speech discrimination out of proportion to the
hearing loss and elevated or absent acoustic reflexes consistent with
a retrocochlear lesion.  ABR results will also be in line with a
retrocochlear lesion.  

        Large lesions may cause dysfunction of cranial nerves X, XI,
and XII if the lesion extends posterior or cranial nerves III, IV and
V if it extends anteromedially.  Acquired cholesteatoma usually
presents with headache, mixed conductive hearing loss, cranial nerve
VII paralysis and otorrhea.  Persistent otorrhea following previous
mastoid surgery is a strong indication persistent disease may exist
in the petrous apex.  Electronystagmagram abnormalities and
sensorineural hearing loss may also be seen if the labyrinth is
invaded by the cholesteatoma.  Facial nerve dysfunction is present in
approximately half of patients with petrous apex involvement.

        Cholesteatomas appear on CT scans as low density lesion that
does not enhance with intravenous contrast.  Preoperative CT is
important in evaluating the degree of temporal bone erosion and the
relationship of the lesion to structures such as the vestibular
labyrinth, cochlea, internal auditory canal, carotid artery and
jugular bulb.  MRI will not provide as much information on bone
erosion as a high resolution CT but will assist in diagnosis.
Cholesteatoma appears as an area of low signal intensity on T1 weight
images and high intensity on the T2 weighted images.  In general,
cholesteatomas have the same intensity on T1 and T2 weighted scans as
CSF.  MRI may also be used to evaluate for intracranial extension.

	Treatment for these lesions differs from the treatment of
cholesterol granulomas in that more than drainage is required.
Preoperative surgical planning should consider hearing, extent of
disease, prevention of recurrence and prevention of a CSF leak.  The
most commonly used approaches include the middle cranial fossa,
translabyrinthine-transcochlear and transmastoid approaches.  The
middle cranial fossa approach should be used only when hearing
preservation is attempted.  Because of limited exposure medially,
complete extraction of the disease may be difficult.  It can be used
to fully remove a cholesteatoma that invades the apex superiorly over
the semicircular canals.  It does not provide good visualization of
lesions that extend to the apex or involve the hypotympanum.  

        The transmastoid approach also preserves hearing and may be
used for lesions which have limited petrous apex involvement.
Following a standard tympanomastoidectomy, access to a cholesteatoma
involving the posterior petrous can be performed by using the
following air cell tracts: the tract in the sinodural angle
paralleling the petrous ridge, through the subarcuate tract beneath
the superior semicanal or below the posterior semicircular canal.
The third route can be followed anteriorly if the jugular bulb is low
to reach the anterior petrous apex.  Anterior petrous apex access
usually requires taking the external auditory canal down.  As
mentioned, following the tracts below the posterior semicircular
canal can lead anteriorly.  Hypotympanic air cell tracts below the
cochlea will also lead to the anterior petrous apex.  Lowering the
inferior portion of the external auditory canal may ben needed to
improve visualization.  

        Another route may be found over the area of the tensor
tympanic muscle and anterior to the geniculate ganglion.  The tensor
tympani can be removed from its semicanal to improve visualization.
Dissection can then be continued in the triangle bounded by the
carotid artery, the cochlea and the middle cranial fossa dura.  The
complete apicectomy, without labyrinth sacrifice, may be performed by
displacing the contents of the glenoid fossa anteriorly.  The
anterior wall of the external auditory canal may be removed or
allowed to remain intact.  The medial wall of the fossa can then be
dissected to expose the internal carotid artery which can then be
used as a landmark to access the medial extent of the apex.
Following this approach a wide cavity with a wide meatus is formed
exteriorizing the cholesteatoma.  The translabyrinthine transcochlear
approach should be used when hearing is not preserved.

        In 1978, House, De La Cruz, and Hitselberg described the
translabyrinthine-transcochlear approach.  A postauricular incision
is made and a simple mastoidectomy is done and carried at least 2 cm
behind the sigmoid sinus, exposing the dura of the posterior fossa.
A labyrinthectomy is performed and the internal auditory canal is
skeletonized, identifying the transverse crest and the vertical bar.
The incus is removed, the facial nerve is completely decompressed,
and opened widely into the hypotympanum (extended facial recess
approach).  The external auditory canal and tympanic membrane are
left intact to decrease the risk of postoperative CSF leak.  The
greater superficial petrosal nerve is transected and the facial nerve
is completely removed from the stylomastoid foramen to the internal
auditory canal and rerouted posteriorly.  The fallopian canal,
stapedius muscle, and the turns of the cochlea are completely
exenterated, carrying the dissection forward until the internal
carotid artery wall is identified.  

        If the lesion extends intracranially, the dura is opened over
the posterior fossa and internal auditory canal.  The facial nerve is
identified both laterally and medially and may be found coursing
laterally over the lesion, in contrast to acoustic neuroma where the
nerve is usually seen medially.  Atlas reported on facial nerve
function following this procedure for cholesteatoma in 1992.  He
reported on fourteen patients and seven had normal facial nerve
function preoperatively.  Following this approach, 50% of patient had
a House grade I or II function and another 21% had grade III
function.


Petrous Apicitis

	Gradenigo described a complex of symptoms consisting of
diplopia, retro-orbital pain and otorrhea associated with supprative
disease of the petrous apex.  Since the wide spread use of
antibiotics, the classic presentation of this triad is uncommon.
Petrositis may be classified as acute, chronic or complicated.  Acute
petrous apicitis usually represents an extension of acute otitis
media and mastoiditis and may occur in a healthy patient.  Abscess
formation in the apex may result.  More commonly, petrous apex
involvent comes from chronic otitis media and is usually caused by
Pseudomonas aeruginosa.  Typically, the patient has had previous
mastoid surgery, persistent purulent otorrhea and complains of deep
retrobulbar pain.  In a series of eight patients, four patients had
deep facial pain, four had eighth nerve involvement, five had hearing
loss, only two had abducens paralysis, and two had meningitis on
presentaion.  The mastoid and petrous apex are commonly poorly
pneumatized.  Osteomyelitis may also result from long standing
infection.  

        Complicated petrositis is reflected by the development of
meningitis, extradural abscess, cavernous sinus thrombosis,
extradural abscess or labyrinthitis from either acute or chronic
infections.  Acute petrous apicitis is seen as an expansile lesions
that does not enhance on CT scan.  Chronic petrous apicitis has an
irregular bony destruction and decreased pneumatizations of the
mastoid and apex.  MR scans of an acute infection demonstrates low
intensity T1 images and high intensity T2 images with a rim of
gadolinium enhancement.  Chronic infections have an irregular
appearance apain with a high T2 signal.  Inflammation of the mucousa
may cause some gadolinium enhancement.  If osteomyelitis is
suspected, nuclear medicine studies are important in evaluating the
process.  Technetium scans will show increased uptake in involved
bone before significant bony demineralization can be detected with CT
scans.  Gallium, which labels inflammatory cells, will also enhance
the involved areas and can be used scanning provides a map of the
area involved by inflammation.  Since technetium remains positive
long after the infection has resolved, sequential gallium scanning is
used to guide the length of antibiotic treatment.  

        Acute petrositis may be treated with appropriate intravenous
antibiotics.  Surgery is indicated if an abscess has formed or if the
patients shows no response or their symptoms progress on antibiotics.
Chronic petrositis is usually treated with both intravenous
antibiotics and surgery.  The goal of surgery is to eradicatie the
focus of infection.  The surgical routes typically used are those
described in the cholesteatoma section.


Langerhans' Cell Histiocystosis

	Gianoli reported on the hearing results for primary petrous
apex lesions in 1994.  Of his patients, 12% were classified as
eosinophilic granulomas.  These cases were not discussed in detail.
The only other report of an eosinophilic granuloma of the petrous
apex was reported in 1993 by Goldsmith.  Langerhans's cell
histiocytosis or histiocystosis X is a spectrum of diseases
characterized by varying degrees of body tissue involvement.
Letterer-Siwe disease presents in infants and young children and
involves multiple organs.  It is the most severe form of the
spectrum.  Hand-Schuller-Christian syndrome affects children one to
five years of age and involves histiocytic invasion of the bones,
shin and bran.  The classic triad is exopthalmos, radiolucent skull
lesions and diabetes insipidus from involvement of the hypothalamic
pituitary axis.  Eosinophilic granulomas affect children and young
adults.  It usually presents as solitary radiolucent lesions of the
skull, long bones ribs or pelvis.  

        Pathologic evaluation requires Langerhans' cells
characterized on electron microscopy by the presence of Birbeck
granules (trilaminar rod shaped organellespresent within the nuclear
cytoplasm).  CT evaluation reveals a destructive lesion of the
petrous.  MR showed a marked enhancement with gadolinium.  Treatment
of the disease is based on the extent of involvement.  For easily
accessible lesions, curettage alone may be curative.  Temporal bone
lesions however are usually treated with low dose radiation therapy.


Chondroma and Chondrosarcoma

        Chondromas of the petrous apex are rare and are believed to be
derived from neoplastic transformation of cartilaginous rests in the
region of the foramen lacerum.  Radiographically, they appear as
lytic lesions on CT that do not enhance and therefore resemble
cholesteatomas and cholesterol granulomas.  Chondrosarcomas
histologically lie between the benign chondroma and malignant
sarcomas and its prognosis can be correlated with histologic grading.
Grade I lesions show changes similar to benign cartilaginous diseases
and normal hyaline cartilage.  Grade II and III exhibit electron
microscopic changes similar to spindle cell sarcomas.  Survival is
related to histologic grade with five year survival rates being:
grade I - 90%, grade II - 81%, grade III - 43%.  CT scans are used to
evaluate the extent of bony destruction.  These lesions will enhance
on MRI scan and these scans along with the CT should be used evaluate
extent of disease and resectability.  An arteriogram may be required
to evaluate the patency of the carotid artery.

	In 1986, Kveton et al reviewed five patients treated for
chondrosarcoma of the skull base.  Four of these five patients
presented with hearing loss as an initial complaint.  Other
complaints included facial nerve dysfunctions (4 of 5),  vertigo or
unsteadiness (3 of 5), and pulsatile tinnitus (2 of 5).  Diagnostic
workup included an audiogram, ENG, ABR, CT scan and angiography.
Surgical approaches included three translabyrinthine/suboccipital
(TL/SO), two infratemporal fossa/translabyrinthine/transcochlear
(IFT/TL/TC), one transmastoid/suboccipital (TM/SO), and one
infratemporal fossa (IFT).  Initial resections were done using the
TM/SO and TL/SO approaches on the first two patients of the series.
Two procedures were required to eradicate disease.  Difficulty was
encountered with tumor that extended anteriorly into the petrous
apex, specifically that disease that extended medial to the internal
carotid artery.  The three remaining patients had procedures based on
the infratemporal fossa approach.  This procedure allowed for better
exposure and more complete resection.  The patients who had this
procedure were NED at 2 years.  

        The infratemporal fossa approach begins by isolating the
facial nerve and its branches, the internal jugular vein, the carotid
aretery and cranial nerves IX, X, XI and XII in the neck.  A radical
mastoidectomy is then performed.  The facial nerve is exposed from
the stylomastoid foramen to the geniculate ganglion and it is
transposed anteriorly.  The anterior canal wallis taken down and the
mandibular condyle is displaced anteriorly.  Disection antrior to the
cochlear reveals the intratemporal carotid artery.  The vessel is
skeletonized medially requiring exenteration of the eustachian tube.
Labyrinthectomy, resection of the mandibualr condyle and out fracture
of the zygomatic arch may all provice improved visualization of the
med aspect of the tumor.  After resection, the cavity can be
obliterated with abdominal fat.

Effusion within a Pneumatized Apex

         The petrous apex air cell system may develop an effusion
associated with an upper respiratory tract infection.  This may be
apparent in an asymtomatic patient who has a MRI for an unrelated
condition.  MRI findings are low to intermediated signal intensity on
T1 and high signal intensity on T2 scans.  A rim of enhancement may
be seen with the use of gadolinium contrast.  A CT scan reveals a
soft tissue density filling the petrous.  However, the complete bony
architecture of the air cell system is intact and the contralateral
petrous is often pneumatized.  It is theorized that a simple effusion
may evolve into an expanding cholesterol granuloma or mucocele over
time.  Surgery is not indicated initially but close follow up with
serial radiographs should be performed.


Asymetric Pnuematization

	Asymetry of the petrous apex may be initially interpreted as
an abnormal finding because of increased signal intensity on T2
images of the nonaerated side.  In a review of 500 CT scans, Roland
found 34 patients with some asymetry of pneumatization of the petrous
apex.  Of these 34, four had magnetic resonance imaging studies.  As
discussed earlier with cholesterol granulomas, bone marrow tends to
have decreased signal intensity with prolonged T2 intervals.  This
was the finding of these four patients with four patients with
asymetric pneumatization.  It is felt therefore that normal bone
marrow composed the "abnormal" MR finding.  In a review of 1600
tomograms, 15 patients were found to asymetric petrous pneumatization
with air cells greater than 1.5 cm.

Chordoma

	Chordomas are rare, slow growing neoplasms which account for
approximately 1% of all intracranial tumors.  They arise from
notochordal remnants and most frequently originate from the clivius.
They may also arise from the petrous apex.  CT demonstrates a well
defined extra-axial soft tissue mass that is of increased density
relative to the neural axis.  Bone destruction and foci of
calcification are typical.  MR shows a well defined tumor with low
signal intensity on T1 and a very hyperintense image on T2.

Meningioma

        Although rarely originating from the petrous apex, meningiomas
may infiltrate the region.  On CT scan they typically are isodense to
slightly hyperdense to brain.  The density is generally homogeneous
and sharply marginated.  Calcification may be present in a number of
forms.  Small punctate (psammomatous) calcifications are common.
Occasionally, large nodular calcifications may be present.  Dense
calcification of the entire tumor that obscures contrast enhancement
is not uncommon.  Typically, contrast produces a homogeneous, intense
enhancement.  MRI shows T1 images to be isointense to mildly
hypointense when compared to adjacent brain. T2 images reveal lesions
that are isointense or mildly hyperintense to cortex.  Tumors may
also have a mottled appearance on MRI.
     
Schwannoma

	Schwannomas usually effect the petrous apex by extension from
their nerve of origin.  The typical schwannoma on CT scan is isodense
to the brain stem and prominent enhancement with contrast.  Erosion
of the petrous apex is smooth and with a distinct margin.
Schwannomas have a smooth margin and appear isointense or hypointense
on T1 images and typically enhance after contrast.  The appearance on
T2 weighted images depends on the size of the lesions.  Small and
medium sized tumors appear minimally to moderately hypointense.
Large tumors typically undergo myxoid degenerative changes that show
areas of hyperintensity intermixed with isointense or slightly
hyperintense cellular portions.  Destruction of the adjacent bony
landmarks, rather than smooth erosion, suggests malignant change.

Metastasis

	The petrous apex is the most common area in the temporal bone
for hematogenous metastases to be found.  In order of frequency,
metastatic lesions of the following tumors have been found: breast,
lung, kidney, prostate, and stomach.  Metastasis to the temporal bone
occurs late in the disease process and there is usually either
physical or radiologic evidence of other systemic lesions.
Radiologic appearance of these lesions is variable.  Most will
produce a bone eroding lesion.  However, a smooth rounded defect may
also be produced.

Intrapetrous Carotid Artery Aneurysm

         Aneurysms arising from the petrous pert of the internal
carotid artery are unusual but may present as a petrous apex lesion.
Angiogram is the preferred diagnostic procedure over surgery.  A
noncontrast CT scan shows uniform hyperdensity that may have adjacent
bone erosion.  With contrast, enhancement is homogeneous and intense.
MRI findings are dependent on the patency of the artery, thrombus
formation and the age of the lesion.

----------------------------------------------------------------------
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