8-08: Diseases of the Central Auditory & Vestibular Systems

Lesions of the eighth cranial nerve and central audiovestibular pathways may produce hearing loss and dizziness (Table 8–3). One characteristic of neural hearing loss is deterioration of speech discrimination out of proportion to the decrease in pure tone thresholds. Another is auditory adaptation, wherein a steady tone appears to the listener to decay and eventually disappear. Auditory evoked responses are useful in distinguishing cochlear from neural losses and may give insight into the site of lesion within the central pathways.

The evaluation of central audiovestibular disorders usually requires imaging of the internal auditory canal, cerebellopontine angle, and brain with enhanced MRI.

1. VESTIBULAR SCHWANNOMA (ACOUSTIC NEUROMA)

Eighth cranial nerve schwannomas are among the most common intracranial tumors. Most are unilateral, but about 5% are associated with the hereditary syndrome neurofibromatosis type 2, in which bilateral eighth nerve tumors may be accompanied by meningiomas and other intracranial and spinal tumors. These benign lesions arise within the internal auditory canal and gradually grow to involve the cerebellopontine angle, eventually compressing the pons and resulting in hydrocephalus. Their typical auditory symptoms are unilateral hearing loss with a deterioration of speech discrimination exceeding that predicted by the degree of pure tone loss. Nonclassic presentations, such as sudden unilateral hearing loss, are fairly common. Any individual with a unilateral or asymmetric sensorineural hearing loss should be evaluated for an intracranial mass lesion. Vestibular dysfunction more often takes the form of continuous dysequilibrium than episodic vertigo. Other lesions of the cerebellopontine angle such as meningiomas and epidermoid tumors may have similar audiovestibular manifestations. Diagnosis is made by enhanced MRI. Treatment consists of observation, microsurgical excision, or stereotactic radiotherapy, depending on such factors as patient age, underlying health, and size of the tumor. Bevacizumab (vascular endothelial growth factor blocker) has shown promise for treatment of tumors in neurofibromatosis type 2.

2. VASCULAR COMPROMISE

Vertebrobasilar insufficiency is a common cause of vertigo in the elderly. It is often triggered by changes in posture or extension of the neck. Reduced flow in the vertebrobasilar system may be demonstrated noninvasively through MRA. Empiric treatment is with vasodilators and aspirin.

Vascular loops that impinge upon the brainstem root entry zone of cranial nerves have been shown to cause dysfunction. Widely recognized examples are hemifacial spasm and tic douloureux. It has been suggested that hearing loss, tinnitus, and disabling positioning vertigo may result from a vascular loop abutting the eighth cranial nerve, although this is controversial.