IN THE NAME OF GOD

Chapter 165  EXTRA-AXIAL NEOPLASM OF THE POSTERIOR FOSSA

Department Of Oto-rhino-Laryngology Of Isfahan Medical Science

DDX OF SKULL BASE NEOPLASMS

POSTERIOR FOSSA:

1) Common Cerebellopontine Lesions: Acoustic neuroma-  Meningioma - Epidermoid- Nonacoustic

neuroma  -Paraganglioma  -Arachnoid cyst-  Hemangiom

2) Petrous Apex Lesions :  Cholesterol granuloma  -Epidermoid - Asymmetric

pneumatization  -Retained mucus or mucocele - Petrous carotid artery aneurysm

3) Uncommon Cerebellopontine Lesions : Metastatic  tumor -Lipoma –Dermoid-

Teratoma - Chordoma - Chondrosarcoma-  Giant cell tumor

4) Intra-Axial Tumors :

Hemangioblastoma- Medulloblastoma - Astrocytoma - Glioma - Fourth ventricle tumor

ANs most common tumors more than 90%

Secondary tumors paragangliomas 10% of CPA neoplasms

Acoustic Neuroma :benign schwannoma vestibular eighth nerve. 8% -10% of all intracranial tumors.

slowly growing- circumscribed-grossly displacing neural structures-firm and dense to soft with large cystic spaces.

pressure neurovascular structures,-auditory and vestibular symptoms earlier in IAC than CPA.

>95% unilateral -nonhereditary lesions

(NF) type 1 (von Recklinghausen's disease) :

AD - gene : 17, arising both intracranially -extracranially Schwann cells of any nerve; <5% ANs

bilateral AN is not a part of the syndrome.

NF type 2 :bilateral AN in > 96% of patients. Schwannomas of the other cranial nerves,

meningiomas and epandymomas are more in NF 2. gene : 22

ANs in NF 2 are onset early in life(often < 21 y)

AN s < 30y close evaluation of the contralateral ear.

technically more challenging to remove because of tendency to adhere to nearby structures.

Malignant schwannomas rarely occur,often in:

1) NF2) solitary schwannomas3) pigmented schwannoma growth rate : variable, slow growing(0.2

cm/y) ANs that are not treated are potentially

lethalGradual enlargement leading to indentation

of brainstem, ICP ,death during of 5-15y

Growth of ANs in three phases:1) IAC growth : 7-8 nerve compression and

Displacement 7,8 nerve and (AICA)

2) cisternal portion : Fourth ventricle shift often (2 to 3 cm ) -total ventricle obstruction - hydrocephalus - Trigeminal compression occurs at about the 3-cm

3) brainstem compression : cerebellar

tonsil herniation -death

Signs and Symptoms: Most: (SNHL progressive unilateral)-

tinnitus, dysequilibrium, facial hypesthesia loss SDSlarge tumors : facial sensation or corneal

reflex (compression 5 (CN)) -compress the fourth ventricle and brainstem, ataxia, Increased ICP (headaches, nausea)

Rotatory vertigo is far less common

>20% of AN with SSNHL may totally recoveronly 1% SSNHL have AN> 5% of AN have normal hearingany patient with asymmetric or sudden

hearing loss (even after total recovery) should be R/O retrocochlear lesion.

Diagnostic Studies 1) Audiometry 2) (MRI) (MRI) – audiometry are not sensitive to

detect small ANs3) ABR: most sensitive even small tumors

AN (95% -100%)4) Stacked ABR :modified ABR -improved

sensitivity for intracanalicular ANs

IMAGING DDX OF CPA LESIONS: 1) Extra-axial : Most common (AN) -

Common (Meningioma- Epidermoid)

2) Extradural: Common (Paraganglioma : glomus jugulare, vagale)

3) Intra-axial: (Rare) Astrocytoma, ependymoma, papilloma, hemangioblastoma, metastasis

Acoustic Neuroma:*Centered *on IAC- enlarge IAC -Spherical

or ovoid- acute bone-tumor angle-in CT scan : Mostly isodense- often

inhomogeneous – in MRI T1,T2 : Isointense or hypointense

 

MRI : lesions as small as 3 mmcontrast CT can fail to detect ANs smaller

than 1.5 cm

CT primary imaging modality in patients ;

1) who cannot undergo MRI because of medical reasons (e.g., cardiac pacemaker or cochlear implant)

2) unmanageable phobic reactionsOxygen cisternography : used to diagnose

small, mainly intracanalicular lesions

MENINGIOMAS18% of intracranial tumors and 3% of CPA

tumorsarachnoid villae are the cells of origin benign but locally aggressive tumors globular mass firmly adherent to dura materpsammoma bodiesthin investing capsulebone without destructionhyperostotic in 25%

The signs and symptoms ; similar ANsSmall tumors: HL , tinnitus, imbalanceLarger tumors : other cranial nerve

involvement and hydrocephalusDiagnostic Studies Audiovestibular Testing75% abnormal ABRImaging: Unlike ANs, meningiomas are usually

eccentric to the porous. 60% extend to the middle fossa

CT, 2/3 meningiomas ; hyperintense - homogeneous and calcified

Hyperostosis (characteristic of meningiomas )

Eccentric IAC -Hemispherical,rarely plaquelike

obtuse bone tumor anglein MRI T1 : Isointense or hypointense in

T2 :Variable

PRIMARY CHOLESTEATOMAS: originates from epithelial rests within the

temporal bone or CPA.slow growing, and symptoms often do not

become apparent until the second to fourth decade of life.

expand, compression of surrounding structures variable shapes with irregular surfaces. They may burrow into crevices on the surface

of the brain or dumbbell into the middle fossa.

Diagnostic Studies :Auditory Testing,SDS; is poorer than degree

of pure-tone loss. ABR abnormalCT and MRIEpidermoids CPA should be DDX from arachnoid

cysts-arachnoid cysts have smoother surfaces than primary cholesteatomas.

epidermoids petrous apex should be DDX from the much more common cholesterol granulomas.

primary cholesteatoma requires excision, whereas drainage is sufficient for cholesterol granulomas and arachnoid cysts

CT: less dense than brain + no enhancement,

irregular margins-Anterolateral or posterolateral to brainstem-erosion- dumbbell into middle fossa or contralateral CPA –hypodense-peripheral calcium

MRI: inhomogeneous and hypointense (on T1) homogeneous and isointense or hyperintense on

T2.

special MRI (diffusion-weighted images) (DWI) may differentiate

Facial nerve neuroma: Signs and Symptoms depend portion of the

nerve affected. Peripheral involvement; parotid mass, ME

involvement (CHL )- IAC or CPA involvement (SNHL)

Unlike hemangiomas of the facial nerve, schwannomas do not produce facial weakness until the tumors are very large

Diagnostic Studies Auditory Testing : Impedance testing may

reflect motor fiber impairment on ipsilateral reflex testing or CN VIII involvement on contralateral reflex testing.

ABR : tumors arising in the IAC shows abnormalities

Electroneurography (ENOG) may be reduced in facial nerve

neuromas even when no facial weakness or tic is present,whereas ENOG remains normal in AN until the tumor becomes very large.

Imaging: Intratemporal facial nerve lesions( bone destruction )

facial nerve neuromas and Ans usually impossible to distinguish with CT.

Anterosuperior erosion of the IAC or erosion of the labyrinthine facial nerve canal if present may be the only diagnostic clue

More distal tumors enlarge the geniculate ganglion and fallopian canal.

Patients CPA neuromas should be warned preoperatively of a 1% risk of facial nerve neuroma,if preoperative ENOG is abnormal on the tumor side.

OTHER CRANIAL NERVE NEUROMAS ANs 95% of intracranial schwannomas

1) trigeminal neuromas are the next most common;

arise intradurally, from the nerve root in the CPA and Meckel's cave, and extradurally, from the gasserian ganglion in the middle cranial fossa.

Typically these lesions enlarge Meckel's cave and produce hypesthesia of the face

The tumor is nearly of CSF intensity because of the predominance of the intratumoral cystic components

2) Neuromas of CNs IX, X, and XI : smooth enlargement jugular foramen and

hypesthesia and weakness of the palate, vocal cord,shoulder

3) Hypoglossal neuromas ; motor hemiatrophy of the tongue and

enlargement hypoglossal canal on radiography.

Glomus Tumors first symptom : pulsatile tinnitus, after CHL

develops. Involvement of the nerves of the jugular foramen and the hypoglossal nerve (progressive neurologic deficits)

CT with bone review: irregular destruction of the jugular foramen.

biopsy is not indicated in these lesions.If surgical resection requires manipulation of the

artery, preoperative assessment of the adequacy of collateral flow via the circle of Willis is necessary-preoperative embolization when surgical resection is planned.

MRI : salt and pepper mixturelimitations of MRI in evaluating

paragangliomas:1) bone changes not visualized 2) distinguishing tumor intensity from bone

marrow is difficultMRI : information about infralabyrinthine

and intracranial tumor extensions

Arachnoid Cysts : thin-walled sacs: contain yellow, entrapped CSF.theory : congenital developmental anomaliesSymptoms : mass effect on surrounding

structures similar ANs(retrocochlear pattern) similar to epidermoids (Enlargement of IAC )

CT : smooth-surface lesion, approximates CSFMRI : nonenhancing +isointensity or

hypointensity T1 and hyperintensity T2

management of these lesions:1) not total resection.surgical drainage via

retrosigmoid 2) diuretic therapy

Hemangiomas; symptoms : compression of adjacent

structures. **Capillary hemangiomas: arise of the

geniculate ganglion. progressive facial weakness . pulsatile tinnitus.

CT : smooth enlargement of the geniculate ganglion and enlargement of the labyrinthine portion of the fallopian canal by a soft-tissue mass.

Enhancing, honeycomb bone, irregular -intratumoral bone spicules

Intratemporal vascular tumor in the region of the geniculate ganglion

***Cavernous hemangiomas: in the IAC symptoms typical of an AN. symptoms more rapidly than an AN

CT and MRI; slightly more hyperintense than AN.

PETROUS APEX LESIONS ; Cholesterol Granulomas 1) occlusion + 2) Hemorrhage into the air cells

results 3) foreign body reaction+ 4) granuloma

formation.expansile lesion with extension into CPA and

signs and symptoms of CN VIII dysfunction.

CT: punched-out lesion - isodense mass -does not enhance-rim enhancement

MRI, T1 and T2 : hyperintense Cholesterol granulomas ; more common than

epidermoids.

CT, epidermoids ( rim enhancement) MRI: hyperintense on T1 and T2; whereas epidermoids are hyperintense only on T2. Total excision of cholesterol granulomas: unnecessary. Drainage

Asymmetric Petrous Apex Pneumatization

should be distinguished from true neoplasms.lack of bone destruction or expansion on CT,

absence of contrast enhancement with gadolinium

hypointensity on T2-weighted distinguish this finding from a neoplasm

Petrous Carotid Artery Aneurysms rare, appear as expansile, well-defined

masses preoperative identification is critical Carotid aneurysms may be confused with the

radiologic appearance of chondrosarcomas.

Giant Cell Tumors extremely rare. originate from

undifferentiated cells of the supporting connective tissue

consist of multinucleated giant cells in a background of spindle-shaped stromal cells.

Patients with this unusual condition have retrocochlear signs and symptoms.

CT : diffuse lesion of the temporal bone compressing the contents of the IAC.

Metastatic Tumors ; metastasize to the CPA from ; lung, breast,

prostate, oropharynx, and cutaneous melanomas

suggest especially in a patient with a history of another malignancy

rapid progression of symptoms and associated neurologic signs in addition to hearing loss and dizziness.

lytic lesions in the petrous apex.other cranial neuropathiesbrainstem dysfunction

Chordomas arise in remnants of the embryonic notochord.1/2 arise in the sacrococcygeal regions, 1/3 at the

skull base in the region of the clivus or less commonly, the upper cervical vertebrae

extensive bone destruction and progressive cranial nerve palsies.

not unusual. frontoorbital headache and vision complaints

(e.g., limited visual fields, diplopia, loss of acuity) are more common, occasionally the initial symptoms (extension into the CPA)

CT: bone destruction, masses are homogeneous with moderate enhancement and a greater density than bone

MRI : isointense T1 image and hyperintense T2 image

Chondrosarcomas ;clinically indistinguishable from chordomas

except that they are centered more laterally.

CT ; bone destruction and invasiveness.

MRI ; hyperintense on T2 in the area of bone destruction in the skull base

The mass is markedly but inhomogeneously enhancing, with components containing small, poorly enhancing or nonenhancing foci in Meckel's cave (arrow in A) and in petrous apex (arrows in B), as well as involvement of the

longus colli muscle and jugular fossa (arrows in C).

Lipomas ;thinly encapsulated -soft, multilobular masses of

typical adult adipose tissue.Lipomas within the IAC : symptoms typical of an

AN. on CT : less dense than neuromas MRI is diagnostic; hyperintense on T1,

nonenhancing with gadolinium, and hypointense on T2 .

Dermoid Tumors ;skin-lined cystic tumor containing dermal and

adnexal structures. The lining of the cyst : mature, stratified squamous epithelium.

slowly expanding (symptoms similar to a primary cholesteatoma)

CT ; non homogeneous cystic mass that

contains calcium but less dense than brain

Teratomas ; arise from multipotential cells various tissues,

representing more than one germ layer. Contain: ectodermal, mesodermal,

endodermalCarcinomatous or sarcomatous : 10% -35% When malignant degeneration occurs,

symptoms progress rapidlyotherwise symptoms progress slowly like

benign CPA tumors.

CT : non homogeneous lesion of less density than brain without enhancing

INTRA-AXIAL TUMORS 1) arise from the brainstem (gliomas)2) cerebellum :(medulloblastomas from the

vermis or astrocytomas from the peduncles)3) the fourth ventricle (choroid plexus

papillomas and ependymomas). highly unusualIn children, brainstem gliomas ; most

common source of CPA neoplasms.Intra-axial tumors are usually isointense on

T1 MRI and hyperintense on T2 images

Hemangioblastomas ;tumors of blood vessel origin in the

cerebellum. They may also occur in the cerebral

hemispheres and in association with a similar retinal tumor and may be multicentric.

Histologically: benign, may produce major neurologic

dysfunction by compression of the brainstem.Rapidly progressing signs and symptoms of

cerebellar dysfunction Hearing and balance are likely to remain

normal.

Imaging studies : intrinsic to the cerebellum that may extend into the CPA.

Medulloblastomas ;arise from the cells of the external granular

layer of the cerebellar folia. exophytic masses on the cerebellum with

extension into the CPA. Symptoms : destruction of cerebellar tissue

and mass effect of the tumor on the CPA's adjacent structures.

rapid development of symptoms + hearing loss and dizziness

neurologic findings include facial weakness, dysmetria of speech and hand motion, perverted nystagmus ,abnormal peripheral reflexes.

Temporal bone imaging : normal, CT and MRI show a lesion intrinsic to the cerebellum.

Brainstem Gliomas; Exophytic gliomas may arise on the surface of

the pons and grow into the CPA. intrinsic to the brainstem.

signs and symptoms similar to those of ANs, predominantly long tract signs.

Patients with exophytic lesions may be diagnosed preoperatively to have ANs.

Long tract signs in association with characteristic brainstem distortion on imaging make preoperative diagnosis possible in the patients with intrinsic lesions.

TUMORS OF THE FOURTH VENTRICLE Malignant Choroid Plexus Papillomas

and Ependymomas arise from the fourth ventricle cause CPA symptoms by growing through the

foramen of Luschka. early signs of CN VIII dysfunction.

Ependymomas may calcifyMRI: isointense with brain on T1 -mildly

hyperintense to brain on T2 images

CPA ependymoma. A and B, Pre- and post-gadolinium images. The mass protruding into the right CPA from the foramen of Luschka shows clear demarcation and strong enhancement. C, T2-weighted image. The tumor is hyperintense and does not extend into the IAC

In malignant ; CT shows a mass with enhancing characteristics of a schwannoma

Malignant ependymomas require multimodality therapy:

1) role of surgery is biopsy 2) brainstem decompression3) management of hydrocephalus.

SELECTION OF SURGICAL APPROACH The principal goal : tumor removal with minimal

postoperative morbidity. surgical approach should be tailored to patient's

pathology and functional status. In cases without serviceable hearing, the

translabyrinthine; 1) provides wide exposure 2) maximal facial nerve safety3) minimal cerebellar retraction4) low incidence of severe postoperative

headache.

In meningiomas and tumors that do not affect the IAC and have not affected hearing:

extended middle fossa approach is used if posterior and middle fossa exposure is necessary.

retrolabyrinthine approach is used for limited lesions of the CPA only

retrosigmoid approach : for more extensive lesions of the posterior fossa.

Extensive anteromedial exposure is needed for lesions of the clivus.

Translabyrinthine: 1) Large, medium, or small cerebellopontine

angle tumor2) Wide exposure3) facial nerve identified, immediate repair4) limited cerebellar retraction4) Total hearing loss

Retrosigmoid (suboccipital): 1) Cerebellopontine angle tumors without

extensive IAC 2) Hearing preservation possible3) wide exposure4) cerebellar retraction : hydrocephalus,

intradural drilling may result in severe headaches

Retrolabyrinthine: 1) CPA lesions without IAC 2) biopsy of cerebellopontine angle lesions3) Hearing preservation possible4) bone removal is extradural, no cerebellar

retraction5) Limited exposure

Transcochlear: 1) Extensive lesions of petrous apex and clivus2) Wide exposure of skull base with access to

clivus, vertebral, and basilar arteries and full exposure of petrous carotid artery

3) Temporary facial nerve paralysis4) total hearing loss

Transotic: Same as transcochlear

Middle fossa: 1) Intracanalicular tumors with minimal CPA

involvement 2) good hearing Hearing preservation possible3) Small tumors only4) temporal lobe retraction

Extended middle fossa :1) Petroclival lesions involving posterior and

middle fossa with good hearing2) Hearing preservation possible3) Extensive temporal lobe retraction

small tumors and good hearing have three options for surgical :

1) translabyrinthine approach that destroys hearing

2) the middle fossa approach in young patients with small, mainly intracanalicular tumors

3) the retrosigmoid approach in small tumors of the CPA that do not extend to the fundus of the IAC.

Hearing preservation is usually not feasible in

such lesions and the transcochlear or transotic approach may be used.

PATIENT MANAGEMENT AND SURGICAL COMPLICATIONS

Preoperative Management 1) Preoperative antibiotics are not routinely

used in uncomplicated cases.2) Osmotic agents and diuretics for brain

relaxation are usually not used in the translabyrinthine approach; however, the retrolabyrinthine, retrosigmoid, middle fossa, and extended middle fossa approaches require such techniques to minimize the need for retraction.

Intraoperative Monitoring 1) Facial nerve monitoring is used routinely

in all cases of posterior fossa

2) ABR monitoring is not routinely used.

Postoperative Care 1) The compression mastoid dressing until po

4th2) neurologic intensive care unit for 1 to 2

days and then transferred to the general ward.

3) Limited activity is begun on the first postoperative morning and ambulation usually begins po 3-4th

Complications1) Anteroinferior Cerebellar Artery changes in vital signs Atkinson's syndromeinfarction of the lateral tegmental pons2) Meningitis The mean time of onset of meningitis in

patients with AN is 8 days postoperatively.Aggressive medical management

3) Cerebrospinal Fluid Leaks

In translabyrinthine ; through the wound is rare(<10%)-usually

responds :mastoid dressing. obliteration of the eustachian tube In rare case of persistent leakage, lumbar CSF

drainage for 3 daysIn refractory cases, abdominal fat Hydroxyapatite cement

………..csf leakIn the retrosigmoid : mastoidectomy is also obliterated with fat. In

some institutions,fluid leakage through the eustachian tube.

Aggressive bone wax over mastoid air cells is necessary to avoid leaks

persistent leaks: fat obliteration of the mastoid and waxing of air cells are usually successful

4) Facial Nerve ideally managed ; immediate repair -

interposition graft

direct repair is impossible or intact nerve does not resume function within 1 year; facial hypoglossal anastomosis

Temporalis muscle transfer

5) Ophthalmologic facial paralysis and corneal insensitivity, in

large tumors. managed prolonged facial paralysis (6

months) ; a brow lift and eyelid spring. management partial facial weakness: 1- lubricating drops2- ointments3-contact lenses4- moisture chamber protection, and nightly

taping of the affected eye is undertaken5- gold weight implantation to the upper lid