Of equal concern to neurologists is the onset in an adult of sudden and permanent unilateral hearing loss without vertigo and lacking all the other features of Ménière disease. The clinical syndrome has been described by Fetterman and colleagues. Little is known about the pathogenesis of this (idiopathic) syndrome. A vascular causation (occlusion of the cochlear artery or arterial spasm) has been postulated, on uncertain grounds. We do not know how to interpret the findings of DeFelice and colleagues as well as others, who report that the posterior communicating arteries are absent in a disproportionate number of patients with sudden hearing loss. A few cases are due to complicated herpes zoster and mumps parotitis, but aside from these there is no proven relationship to the usual viral respiratory infections. An immune-mediated cause may also be operative in some patients, a hypothesis that has led some neurologists and otologists to treat such patients with a brief course of corticosteroids. In a prospective report of the natural history of 88 cases of acute sensorineural hearing loss, two-thirds recovered their hearing completely within a few days or a week or two (Mattox and Simmons). In the remaining patients, recovery was much slower and often incomplete; in this latter group, the hearing loss was predominantly for high tones and in some cases was associated with varying degrees of vertigo and hypoactive caloric responses.
The same problem has been reported to follow cardiopulmonary bypass surgery and has been ascribed, without confirmation, to microemboli. Less often, such an event follows general anesthesia for nonotologic surgery (Evan et al); the pathogenesis is obscure. None of the currently popular therapeutic agents—such as histamine, calcium channel blockers, anticoagulants, inhalation of carbogen (30 percent carbon dioxide), and corticosteroids—seems to clearly affect the outcome of sudden unilateral or bilateral deafness without vertigo. Nonetheless, as mentioned, corticosteroids are often prescribed, based on the uncertain theory that this illness is analogous to an immune form of vestibular neuritis.
A large number of genetically determined syndromes that feature a neural or conductive type of deafness—some congenital and others having their onset in childhood or early adult life—have come to light (see articles by Tekin et al and Gorlin et al). The majority of cases of congenital deafness are inherited as an autosomal recessive trait with no other syndromic features. In most of the remainder, inheritance is autosomal dominant in type and in a small number, it is sex linked.
The singular genetic advance in this field has been the identification in recessive nonsyndromic deafness of a mutation of the connexin-26 gene on chromosome 13 (designated GJB2). This mutation is found in half of recessive familial cases of pure deafness; what is more striking is that the same gene abnormality occurs in 37 percent of cases of sporadic congenital deafness, almost certainly from a spontaneous mutation (Estivill et al and Morell et al). The connexin protein is a component of gap junctions and the mutation is theorized to interfere with the recycling of potassium from the cochlear hair cells to the endolymph. As a result of the human genome project, more than 20 other gene loci have been detected that may be related to congenital deafness syndromes; these have been summarized by Tekin and colleagues, but none, except the one for connexin, accounts for more than a very small proportion of cases. These unattached, nonsyndromic types of congenital deafness are denominated by genes in a family called DFN (for DeaFNess); for example, the connexin mutation is in DFNB1. The mutations in this gene can be recessive, dominant or X-linked. The genetic errors involve either cytoskeletal or structural proteins of the organ of Corti or the ion channel apparatus.