The portion of the vestibular pathway that is primarily affected in this disease is thought to be the superior part of the vestibular nerve trunk, which was observed to show degenerative changes by Schuknecht and Kitamura. Earlier, Dix and Hallpike had reasoned that the lesion was located central to the labyrinth, as hearing is spared and vestibular function usually returns to normal. They used the term vestibular neuritis because of the uncertainty of more precise localization within the peripheral vestibular pathway. The cause of vestibular neuritis is still uncertain, but many authorities have attributed it to a viral infection of the vestibular nerve, analogous to Bell palsy, and from time to time, enhancement of the eighth nerve or the membranous labyrinth is seen after gadolinium administration on MRI. For want of more specific etiologic or pathologic data, many neurologists prefer the term vestibular neuropathy or neuritis or acute unilateral peripheral vestibulopathy. It is likely that the conditions described under the terms epidemic vertigo, epidemic labyrinthitis, and acute labyrinthitis or neurolabyrinthitis refer to the same process. Certainly, herpes zoster oticus causes this syndrome (as well as affecting the seventh nerve); this characterizes the Ramsay Hunt syndrome described in Chaps. 9 and 44.
During the acute stage, antihistamine drugs, promethazine, clonazepam, and scopolamine may be helpful in reducing the symptoms. Vestibular exercises are recommended by Baloh (2003) in his review of the subject. One clinical trial has demonstrated a more rapid recovery with the use of methylprednisolone, 100 mg orally, tapered over 3 weeks; valacyclovir did not have this effect (Strupp et al).
Other Causes of Vertigo of Vestibular
Vertigo may occur with diseases that involve the eighth nerve in the petrous bone or at the cerebellopontine angle. Aside from vestibular neuritis, discussed previously, the two most common causes of vertigo of eighth nerve origin are probably an acoustic or vestibular schwannoma and vascular irritation or compression by a small branch of the basilar artery. The frequency of the vascular compression syndrome as a cause of otherwise undifferentiated vertigo is not known (see earlier).
Regarding vestibular schwannoma, vertigo is rarely the initial symptom; the usual sequence is deafness affecting the high-frequency tones initially, followed some months or years later by mild chronic imbalance rather than vertigo and by impaired caloric responses, and then, if untreated, by additional cranial nerve palsies (the seventh, fifth, and tenth nerves), ipsilateral ataxia of limbs, and headache. Variations in the sequence of development of symptoms are frequent, and probably many vestibular schwannomas discovered in the process of an evaluation for vertigo are incidental; that is, almost 1 percent of the general population harbors small tumors. In the diagnosis of vestibular and acoustic schwannoma, MRI and BAEP are the most important ancillary examinations. Bilateral vestibular/acoustic Schwannomas are almost always a manifestation of neurofibromatosis type 2.