Cerebellopontine Angle Lesions:
1. Acoustic schwannoma
* most common mass in the CPA, up to 75% of cases
* usually arises from the superior vestibular nerve
* usually a solid space-occupying mass with a tail in the internal acoustic meatus/ canal that uniformally enhances with contrast; can cause compression of the pons and cerebellar peduncles
* surgical approaches- A. Suboccipital retrosigmoid (Figure 1)
Figure 1: T1 -weighted coronal view (with contrast) of a left cerebellopontine angle lesion showing enhancement. The lesion is predominately in the posterior fossa with a tail in the internal acoutic canal. This lesion was excised through a suboccipital/ retrosigmoid approach.
B. Trans-labyrinthine presigmoid (Figure 2)
Figure 2: A: T1-weighted coronal view (with contrast)
of a right cerebellopontine angle lesion showing uniform enhancement. This
lesion is mostly in the internal acoustic canal and was excised through
a translabyrinthine approach. The patient had no useful hearing preoperatively
and had preservation of the facial nerve postoperatively.
B: Intraoperative of the tumor resection. The tumor is carefully dissected away from the cranial nerves in the porus acousticus.
C: Note the preservation of the cranial nerves after complete resection of the tumor.
C. Middle Fossa (Figure 3)
Figure 3: T1-weighted axial view (with contrast) of a small intracanalicular left acoustic tumor (arrow). The patient had useful hearing preoperatively and thus this lesion was resected through a middle fossa approach. The patient had preserved hearing and facial function postoperatively.
* second most common lesion, up to 10% of cases
* uniformally enhancing mass; dural tail
3. Ectodermal inclusion tumors: Epidermoid (Figure 4)- also known as 'congenital cholesteatoma"; 5-7% of cases in the CPA; cystic space-occupying, non-enhancing lesion
Figure 4: T2-weighted axial view showing a hyperintense lesion in the left cerebellopontine angle. This is a typical appearance for an epidermoid lesion.
5. Paraganglioma- "glomus jugulare tumor" arising in the jugular foramen and extending into the CPA; incidence: 2-10%
6. Other schwannomas: 2-5% incidence; trigeminal and facial nerves are probably the most common sites of nonacoustic schwannomas. Other cranial nerves involved are: VI, IX, X, XI and rarely XII.
7. Vascular lesions (2-5% incidence)
* dolichobasilar ectasia: 3-5%
* aneurysm: 1-2%
* vascular malformation: 1%
8. Choroid plexus papilloma: 1%; primary in the CPA or extension via the lateral foramina of Luschka
9. Ependymoma: 1%; extension from the 4th ventricle
10. Rare lesions: incidence <1%
* arachnoid cyst
* lipoma (CPA is the 2nd most common site in brain)
* exophytic brain stem or cerebellar astrocytoma
* osteocartilaginous tumors
Cranial Nerves at the Internal Acoustic Meatus (Figure 5):
Figure 5: A diagrammatic view of the cranial nerves at the internal acoustic meatus. Taken from Surg Neurology 8:388,1977.
* there are five nerves in the meatus: nervus intermedius (sensory component of the VIIth nerve), facial motor root, cochlear nerve, inferior and superior vestibular nerves.
* position of the 5 nerves is most constant in the lateral portion of the meatus, which is divided into a superior and an inferior portion by a horizontal ridge (transverse or falciform crest): facial and superior vestbular nerves (SVN) are superior to the crest; facial nerve is anterior to the SVN and is separated from it at the lateral end of the meatus by a vertical ridge of bone (Bill's bar); nervus intermedius (NI) is between the facial motor root and the SVN (it may be adherent to the SVN); cochlear nerve and the inferior vestibular nerve (IVN) run below the transverse crest with the cochlear nerve located anteriorly.
FACIAL NERVE: Anterior-superior
COCHLEAR NERVE: Anterior-inferior
*because acoustic neurinomas most frequently arise in posteriorly placed vestibular nerves, they usually displace the VIIth nerve anteriorly (facial nerve is stretched around the anterior half of the tumor capsule).
*because the facial nerve enters the facial canal at the anterior-superior quadrant of the lateral margin of the meatus, it is usually easiest to locate it here after the posterior lip of the meatus has been removed, rather than at a more medial location where the degree of displacement of the nerve is more variable, depending on the site of origin and growth characteristics of the tumor.
*while the posterior meatal lip is removed, mastoid air cells that extend into the lip may be opened, and must then be sealed carefully to prevent CSF leak or meningitis.
*during removal of the posterior meatal wall, care is taken to avoid the posterior semicircular canal, which is lateral to the posterior wall of the meatus- to avoid this semicircular canal, bone lateral to the tranverse crest should not be removed.
*labyrinthine arteries and their branches typically lie below the nerves- are the sole supply to the membranous labyrinth.
The LSUHSC Skull Base Team (Neurosurgery, Otolaryngiology, Plastic Surgery, Neuroradiology, Neurophysiology departments) routinely deals with CPA lesions, especially acoustic tumors. In 1999, 10 acoustic tumors were removed at LSU: 4 suboccipital; 3 translab; 3 middle fossa.