Acoustic Neuroma Cp Angle

[Eustacio Gadit]

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This case report first reviews the intracranial tumors associated with symptoms of trigeminal neuralgia (TN). Among patients with TN-like symptoms, 6 to 16% are variously reported to have intracranial tumors. The most common cerebellopontine angle (CPA) tumor to cause TN-like symptoms is a benign tumor called an acoustic neuroma. The reported clinical symptoms of the acoustic neuroma are hearing deficits (60 to 97%), tinnitus (50 to 66%), vestibular disturbances (46 to 59%), numbness or tingling in the face (33%), headache (19 to 29%), dizziness (23%), facial paresis (17%), and trigeminal nerve disturbances (hypesthesia, paresthesia, and neuralgia) (12 to 45%). Magnetic resonance imaging with gadolinium enhancement or computed tomography with contrast media are each reported to have excellent abilities to detect intracranial tumors (92 to 93%). This article then reports a rare case of a young female patient who was mistakenly diagnosed and treated for a temporomandibular disorder but was subsequently found to have an acoustic neuroma located in the CPA.

Though acoustic neuromas are generally slow-growing tumors and their associated hearing loss is usually progressive, they may also present with sudden sensorineural hearing loss (SNHL). All patients with sudden SNHL should be imaged for work up of acoustic neuroma, even if they respond to steroids or their hearing spontaneously recovers.

A study by Foley et al of 945 persons with acoustic neuroma reported unilateral hearing loss to be the most common presenting system (80% of patients). Unilateral tinnitus was the next most frequent presenting symptom, occurring in 6.3% of patients, with ataxia, vertigo, and headache being the presenting symptoms in 3.8%, 3.4%, and 2% of cases, respectively. Patients with larger tumors were more likely to suffer from headaches, facial weakness, and abnormalities in tandem gait and facial sensation. [1]

The definitive diagnostic test for patients with acoustic tumors is gadolinium-enhanced MRI. Gadolinium contrast is critical because nonenhanced MRI can miss small tumors. If suspicion is high and MRI is contraindicated, high-resolution computed tomography (CT) scanning with contrast can be used, although this may also miss smaller tumors.

Acoustic neuromas are managed in one of the following 3 ways: (1) microsurgical excision of the tumor, (2) arresting tumor growth using stereotactic radiation therapy, or (3) careful serial observation.

Stereotactic radiotherapy uses radiation delivered to a precise point or series of points to maximize the amount of radiation delivered to target tissues while minimizing the exposure of adjacent normal tissues. It is commonly delivered as a single dose or, less commonly, as multiple, fractionated doses.

Microsurgical removal remains the treatment of choice for tumor eradication. Various surgical approaches can be used to remove acoustic tumors, including the translabyrinthine approach, the retrosigmoid approach, and the middle cranial fossa approach. Decision on type of access depends on tumor location, tumor size, hearing status, facial nerve status, and surgeon preference.

The operative mortality rate has dropped dramatically, from 40% in the early 20th century to less than 1%. With current microsurgery, postoperative facial paralysis, once the rule, is now an uncommon permanent sequelae. Attempts at hearing conservation, unimaginable at the beginning of the 20th century, are increasingly successful.

Clinically diagnosed acoustic neuromas occur in 0.7-1.0 people per 100,000 population. The incidence may be rising, a reflection of the increasing frequency with which small tumors are being diagnosed with the more widespread use of MRI. A 2005 study by Lin et al suggested the prevalence of incidental acoustic neuromas to be 2 in 10,000 people. [4] Careful autopsy studies can detect small vestibular schwannomas in a higher percentage of elderly patients, which suggests that many acoustic neuromas never become clinically apparent.

Most patients diagnosed with an acoustic neuroma have no apparent risk factors. Exposure to high-dose ionizing radiation is the only definite environmental risk factor associated with an increased risk of developing an acoustic neuroma. Multiple studies have determined cell phone use is not associated with an increased risk of developing an acoustic neuroma, although data on the effects of long-term cell phone use are still pending. [5]

Neurofibromatosis type II occurs in individuals who have defective tumor suppressor gene located on chromosome 22q12.2. The defective protein produced by the gene is called merlin or schwannomin. Bilateral acoustic tumors are a principle clinical feature of neurofibromatosis type II, although other manifestations, including peripheral neurofibromata, meningioma, glioma, and juvenile posterior subcapsular lenticular opacities, are often present as well. Many patients with neurofibromatosis type II present in late adolescence or early adulthood but occasionally may present later in the fifth to seventh decade with slowly growing tumors.

Although some tumors adhere to one or another of these growth patterns, others appear to alternate between periods of no or slow growth and rapid growth. Tumors that have undergone cystic degeneration (presumably because they have outgrown their blood supply) are sometimes capable of relatively rapid expansion because of enlargement of their cystic component. Because acoustic tumors arise from the investing Schwann cell, tumor growth generally compresses vestibular fibers on the surface. Destruction of vestibular fibers is slow; consequently, many patients experience little or no disequilibrium or vertigo. Once the tumor has grown sufficiently large to fill the internal auditory canal, it may continue growth either by expanding bone or by extending into the cerebellopontine angle. Growth within the cerebellopontine angle is generally spherical, which is different than the sessile growth pattern seen in a meningioma of the cerebellopontine angle.

Acoustic tumors, like other space-occupying lesions, produce symptoms by any of four recognizable mechanisms: (1) compression or distortion of the spinal fluid spaces, (2) displacement of the brain stem, (3) compression of vessels producing ischemia or infarction, or (4) compression and/or attenuation of nerves.

Because the cerebellopontine angle is relatively empty, tumors can continue to grow until they reach 3-4 cm in size before they contact important structures. Growth is often sufficiently slow that the facial nerve can accommodate to the stretching imposed by tumor growth without clinically apparent deterioration of function. Tumors that arise within the internal auditory canal may produce early symptoms in the form of hearing loss or vestibular disturbance by compressing the cochlear nerve, vestibular nerve, or labyrinthine artery against the bony walls of the internal auditory canal.

As the tumor approaches 2 cm in diameter, it begins to compress the lateral surface of the brain stem. Further growth can occur only by compressing or displacing the brain stem toward the contralateral side. Tumors greater than 4 cm often extend sufficiently far anteriorly to compress the trigeminal nerve and produce facial hypesthesia. As the tumor continues to grow beyond 4 cm, progressive effacement of the cerebral aqueduct and fourth ventricle occurs with eventual development of hydrocephalus.

Treatment depends on multiple factors including the age and medical status of the patient, tumor size and location, hearing status, and patient preference. In older patients with small tumors, careful observation may be elected consisting of serial MRIs. In older patients with a growing tumor, radiosurgery may be an appropriate option. Young patients, large tumors (greater than 2.5 to 3 cm) and patients with small tumors and intact hearing may choose surgery. See Surgical therapy.

The cerebellopontine angle is a space filled with spinal fluid. It has the brain stem as its medial boundary, the cerebellum as its roof and posterior boundary, and the posterior surface of the temporal bone as its lateral boundary. The floor of the cerebellopontine angle is formed by the lower cranial nerves (IX-XI) and their surrounding arachnoid investments. The flocculus of the cerebellum may lie within the cerebellopontine angle and may be closely associated with cranial nerves VIII and VII as they cross the cerebellopontine angle to enter the internal auditory canal.

The facial nerve arises 2-3 mm anterior to the root entry zone of the vestibulocochlear nerve. The foramen of Luschka (ie, the opening of the lateral recess of the fourth ventricle) is located just inferior and posterior to the root entry zones of the facial and vestibulocochlear nerve. A tuft of choroid plexus can frequently be observed extruding from it. Inferior and a bit anterior to the foramen of Luschka is the olive, and just posterior to the olive lie the rootlets of origin for cranial nerves IX, X, and XI. The hypoglossal nerve exits the brain stem through a series of small rootlets anterior to the olive.

The most important vascular structure within the cerebellopontine angle is the anterior inferior cerebellar artery (AICA). It arises most commonly as a single trunk from the basilar artery but can arise as two separate branches. In rare cases, it originates as a branch of the posterior inferior cerebellar artery (PICA). As the AICA moves from anterior to posterior, it first follows the ventral surface of the brain stem, but within the cerebellopontine angle it takes a long loop laterally to the porus acusticus. In 15-20% of cases, the AICA actually passes into the lumen of the internal auditory canal before turning back on itself toward the posterior surface of the brain stem. (These AICA loops are not symptomatic.) The AICA can thus be divided into the premeatal, meatal, and postmeatal segments.

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