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Clinical Features

The differential diagnosis of cerebellopontine angle lesions includes, in order of occurrence, acoustic neurinomas, meningiomas, epidermoid tumors and arachnoid cysts. Significantly less common lesions include neurinomas of other cranial nerves, lipomas, glomus tumors and vascular lesions.

Acoustic neurinomas arise from the Schwann cells of the vestibular nerve. The vestibular nerve is ensheathed in oligodendrocytes for much of its course through the cerebellopontine angle. However, as the nerve enters the internal auditory meatus the oligodendrocytes are replaced by Schwann cells in a region known as the zone of Obersteiner-Redlich. This transitional zone usually lies at the mouth of the internal auditory meatus and thus Schwann cells invest the vestibular nerve along virtually all of its length within the canal. It is these cells within the canal which are thought to give rise to the acoustic neurinoma.

A history of progressive unilateral hearing loss, usually over many months and sometimes years, is the hallmark of an acoustic neurinoma. In most cases it is associated with tinnitus. As the tumor enlarges, the patient complains of unsteadiness and loss of balance. True rotational vertigo is rare. The facial nerve usually functions normally until the tumor reaches a large size. When nerve function is compromised, it is usually mild. Total facial paralysis is rare. Involvement of the trigeminal nerve likewise occurs late and is seen primarily in tumors more than 3 cm in diameter. As the tumor grows upward into the superior aspect of the cerebellopontine angle, it encroaches upon the trigeminal nerve, producing a gradual decrease of the corneal reflex and facial analgesia and anaesthesia. Tic douloureux occurs rarely.

It is unusual for patients with an acoustic neurinoma to present with complaints of swallowing dysfunction or hoarseness and lower cranial nerve involvement is unlikely unless the tumor is large. Cerebellar symptoms and signs also occur late in the clinical course of these tumors and are often found in association with compromised function of cranial nerves. Papilledema and symptoms of hydrocephalus can also be present and are usually secondary to compression of the brain stem and the fourth ventricle by a large tumor.

Meningiomas are the second most frequent tumor of the cerebellopontine angle. They constitute 3 to 13 percent of cerebello­pontine angle tumors. These tumors produce the same general symptoms and signs as do acoustic tumors, with several exceptions. Often these lesions originate from the superior-anterior lip of the porus acousticus, and are associated with early involvement of the seventh nerve. Hearing loss, however, occurs later. Thus, in terms of facial and auditory function, meningiomas are the exact opposite of acoustic tumors. Involvement of the posterior root of the fifth cranial nerve may lead to numbness of the face and tic­like symptoms. These symptoms, preceding hearing loss, suggest that a meningioma may be present or, less likely, a trigeminal neurinoma. Meningiomas also cause a higher incidence of lower cranial nerve abnormalities compared to acoustic tumors. The growth downward of these lesions results in hoarseness, numbness of the throat or complaints of difficulty swallowing. As with acoustic tumors, large meningiomas can produce cerebellar symptoms and signs or hydrocephalus with increased intracranial pressure.

Epidermoid tumors and arachnoid cysts are both rare lesions of the cerebellopontine angle, accounting for 2 to 6 percent and 1 to 3 percent of all lesions, respectively. Epidermoid tumors are benign and grow slowly. They can present with multiple cranial nerve abnormalities or cerebellar symptoms and signs which develop over a number of years. Patients with arachnoid cysts can present with a complaint of unilateral hearing loss, headache or imbalance. Facial or trigeminal nerve dysfunction can occasionally be observed.

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The cerebellopontine angle is an inverted triangular cistern in which the fifth, seventh and eighth cranial nerves, along with the anterior inferior cerebellar artery (AICA) and the superior petrosal vein are located. From a surgeon' s viewpoint, the cistern is bounded laterally by the back wall of the petrous bone, medially by the pons and cephalad by the tentorium. which forms the base of the triangle. This cistern communicates freely with the other cerebrospinal fluid (CSF) spaces within the posterior fossa, including a small diverticulum which extends into the porus acusticus.

At the upper aspect of the cistern, the fifth cranial appears as a broad white band, extending from the lateral aspect of the pons into Meckel's cave. The superior petrosal vein lies at the upper posterior edge of this nerve, and drains from the superior aspect of the cerebellum to the superior petrosal sinus. This vein is usually 1 to 2 mm in diameter and at times may be made up of a cluster of veins.

The seventh and eighth nerves course laterally from the ponto­medullary junction to the internal auditory canal. They cross the cistern as an apparent single nerve, which is composed of four discrete nerves: the superior and inferior vestibular nerves, the cochlear nerve and the facial nerve. When viewed from the suboccipital approach. the vestibular nerves form the posterior aspect, or the portion closest to the surgeon. The facial nerve makes up the anterior superior portion within this bundle and the cochlear division of the eighth nerve makes up the anterior inferior portion. When one looks into the posterior fossa from the extreme lateral aspect of a suboccipital approach, the sixth nerve is occasionally seen, coursing from its origin at the pontomedullary junction to its entrance into the dura of the clivus (Dorello's canal). In situations where the tumor has rotated and displaced the brain stem, this nerve may be confused with the seventh nerve, inasmuch as it exits on the same plane as the seventh nerve and enters the dura at the same level as the internal auditory canal.

The ninth, tenth and eleventh nerves, although not specifically within the cerebellopontine angle cistern, are found immediately below its inferior margin. The most superior of these nerves, the ninth, appears round and shiny and is made up of a single filament. The tenth nerve consists of multiple filaments that are flat, whereas the eleventh nerve is unique in having a spinal root traversing the foramen magnum.

The anterior inferior cerebellar artery has a variable location within the cistern. In acoustic tumors, this vessel is usually located in the arachnoid over the cleft between the cerebellum and the dome of the tumor.

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Bilateral Acoustic Tumors

Bilateral acoustic tumors are pathognomonic of central neurofibromatosis. In general, the goals for surgery are preservation of brain stem function followed by preservation of facial nerve function and hearing. It is not wise to remove both tumors at one operation. In general, the larger tumor is operated on first. Removal of the tumor is carried out using the technique outlined above. The patient only returns for surgery on the second side after completely recovering from the first procedure. This includes wound healing as well as recovery of facial nerve function. In the event of facial nerve paralysis following the first operation, the second one is delayed until the nerve recovers or a facial reanimation procedure can be performed. In general, tumor removal should be carried out as soon as the tumors are found because removal of smaller tumors is associated with better results for hearing preservation.

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