show severely distorted or absent auditory brainstem response (explained in Chapter 3) with recordings showing prominent cochlear microphonic (CM) components, and normal otoacoustic emissions. The otoacoustic emissions and CM findings in patients with auditory neuropathy usually indicate that the cochlear hair cells, at least the outer hair cells, are functioning normally while the abnormal auditory brainstem response is indicative of disease in the inner hair cells, auditory nerve, or brainstem. Some theorize that the disorder is a specific neuropathy of the auditory nerve; thus the name of the disorder (Starr et al., 1996; Starr, Picton, and Kim, 2001). Starr et al. (2001) have found indirect evidence of peripheral nerve involvement based on sural nerve biopsy or nerve conduction velocity measures. More recently, they have documented specific neuropathy of the auditory nerve in a patient with well-documented clinical signs of auditory neuropathy on audiological tests (Starr et al., 2003). Such histological findings (fair to good hair cell populations along with poor ganglion cell and nerve fiber survival) have been reported previously (Hallpike, Harriman, and Wells, 1980; Merchant et al., 2001; Spoendlin, 1974; see Nadol, 2001, for a review of these pathologies in humans). However, others suggest that there is a “general lack of anatomic foundation for the label” (Rapin and Gravel, 2003, p. 707) because of difficulty in documenting specific peripheral neuropathy in patients, especially at the level of the auditory nerve.
When auditory neuropathy exists, neither the auditory brainstem response nor the otoacoustic emissions can be used to determine the degree of hearing loss. The degree of hearing loss in patients with this condition can be anywhere from none to profound (Sininger and Oba, 2001). The hearing loss of patients with auditory neuropathy can fluctuate dramatically and rapidly, sometimes within a single day (Sininger and Oba, 2001). In rare cases, increases in core temperature from fever can bring on severe to profound hearing loss that will return to prefever levels as the condition resolves (Starr et al., 1998). Recent publications report that cochlear implants have been effective in individuals displaying signs of auditory neuropathy (Peterson et al., 2003; Shallop, Peterson, Facer, Fabry, and Driscoll, 2001). This finding could be due to electrical synchronization of the neural response, or it may suggest that the etiology of profound deafness could be located in the inner hair cells.
An excellent review of sensorineural hearing loss in children can be found in a chapter by Brookhouser (1993).
Although many diseases and disorders can induce hearing loss, relatively few result in profound sensorineural deafness. Infections, immune-