What is the pathophysiology of acoustic neuromas?

Updated: Apr 28, 2020
  • Author: Joe Walter Kutz, Jr, MD, FACS; Chief Editor: Arlen D Meyers, MD, MBA  more...
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Answer

The vast majority of acoustic neuromas develop from the Schwann cell investment of the vestibular portion of the vestibulocochlear nerve. Less than 5% arise from the cochlear nerve. The superior and inferior vestibular nerves appear to be the nerves of origin with about equal frequency. Overall, 3 separate growth patterns can be distinguished within acoustic tumors, as follows: (1) no growth or very slow growth, (2) slow growth (ie, 0.2 cm/y on imaging studies), and (3) fast growth (ie, ≥ 1.0 cm/y on imaging studies). Although most acoustic neuromas grow slowly, some grow quite quickly and can double in volume within 6 months to a year.

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.

Acoustic tumors, like other space-occupying lesions, produce symptoms by any of 4 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 relatively 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 2.0 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.


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