Abstract
Auditory neuropathy (AN) is a type of hearing loss that can be present from birth or acquired postnatally. AN results in a loss of sensitivity for pure tones and very poor speech perception abilities, disproportionate to the degree of pure tone sensitivity impairment. A deficit in the timing of neural signals transmitted by the auditory nerve appears to underlie the significant speech perception disability. AN is diagnosed by considering the results of electrodiagnostic tests of auditory function in conjunction with pure tone and speech perception tests. This article will review the methods used to assess AN, the audiometric and speech perception findings for infants and children with AN, and the habilitation methods that are recommended. The etiologies and pathophysiologic processes of AN will also be discussed.
HEARING IMPAIRMENT in infancy and early childhood has long been recognized as a potentially disabling condition. Speech perception and production can be seriously affected by hearing impairment. If a spoken or signed language is not introduced early in life, cognitive, emotional, and social development may also be delayed. Thus, early detection and early intervention for congenital or acquired hearing impairment in infancy has been an overriding goal for pediatric audiology for over 40 years. Successful early intervention for hearing impairment is dependent upon assessment methods that reveal the type of hearing loss (conventionally classified as conductive, sensorineural, or mixed conductive-sensorineural), the degree of hearing loss (defined by loss of sensitivity as mild, moderate, severe, or profound), and occasionally, the etiology of the hearing loss (genetic, such as from a mutation of a gene involved in cochlear function, or environmental, such as from infection). Conductive hearing impairments are those that are caused by a disease or abnormality of the external ear, eardrum, or middle ear. Some forms of conductive hearing impairment may be treated medically, some surgically, and some with amplification, or a combination of all 3 modalities. Sensorineural hearing impairments are those that are caused by disease or abnormality of the cochlea (inner ear) and the afferent portion of the auditory nerve (cranial nerve VIII). The precise determination of the degree of sensorineural and/or conductive impairment across the range of frequencies needed for speech perception will influence treatment decisions, and also the type of amplification recommended, and so audiologists consider it crucially important to perform thorough diagnostic tests to make these distinctions.
Improved technologies for assessment of hearing have led to more sensitive and specific definitions of impairment types. It is now possible to evaluate the integrity of the cochlear outer hair cells (OHCs), by recording otoacoustic emissions (OAEs), the by-product of sound transmission in the healthy cochlea. Loss or damage of the OHCs will cause a "sensory" hearing loss, and OAEs will be absent. Sensory impairment is characterized by a loss of hearing sensitivity, and difficulty with fine frequency discrimination. This, in turn, results in impairment of sound awareness and speech understanding. The disability for speech understanding in cases of sensory impairment is generally predictable from the loss of sensitivity for pure tones. Speech perception abilities will decrease as the severity of hearing impairment increases. Amplification is used to remediate the loss of sensitivity and this usually results in gains for speech perception.
In contrast, disorders or pathology of the auditory nerve may not result in a significant loss of sensitivity (as measured by pure tone tests), but speech understanding is generally poor. This is because the auditory nerve is the "main trunk" for the transmission of the neural code for speech. In general, making speech louder (through amplification) does not improve speech understanding when the auditory nerve is compromised.
Neural function can be tested by (noninvasive) recording of electrical potentials from the auditory nerve (compound nerve action potential, CAP) and/or from the auditory pathway of the brainstem, known as the auditory brainstem response (ABR). In cases of neural dysfunction, the CAP and ABR will be grossly abnormal or absent, even if there is relatively little loss of pure tone sensitivity. The auditory nerve may be compromised in the case of tumors (schwannomas) of the acoustic nerve, and also by disorders or degeneration of the nerve sheath, the myelin, which facilitates nerve transmission. Progressive loss of auditory nerve fibers is also seen when the cochlear inner or outer hair cells have been damaged. Conversely, damage to the auditory nerve fibers may cause degeneration of cochlear hair cells (Nadol, 2001). Because degeneration of cochlear sensory cells and auditory nerve fibers is seen when either the cochlea or nerve is compromised, it is appropriate to label these as sensory-neural, or sensorineural hearing losses. Tests of OHCs (OAEs) and tests of neural function (CAP and ABR) will both be abnormal.
It was not until electroacoustic (OAEs) and electrophysiologic (ABR) tests had widespread clinical use that sensory impairments could be distinguished from neural impairments. The growth of newborn hearing screening programs that utilize OAE and ABR testing methods, contributed to the recognition of an apparently neural type of hearing loss in newborns and young infants. It was those infants who had abnormal ABR tests who were then tested and found to have normal OAEs, that first were suspected of having a neural hearing disorder. The presence of OAEs implies normal cochlear, or at least, OHC function and the absence of ABRs indicates that the auditory nerve is dysfunctional. In one of the first reports of this type of hearing disorder, children and adults with impaired hearing for tones and speech, normal OAEs and absent ABRs also demonstrated neuropathy or dysfunction of other peripheral nerves (Starr, Picton, Sininger, Hood, & Berlin, 1996). Since the primary complaint for these patients was difficulty hearing and understanding speech, the disorder was labeled auditory neuropathy, AN. Subsequent research has shown that the presence of normal OAEs in conjunction with abnormal or absent ABR usually, but not always, signifies a significant functional hearing deficit. Although AN can be identified only based upon the findings from both OAE and ABR tests, it is important to measure the functional hearing abilities of those with AN's distinctive electrodiagnostic test results. For the purposes of this review, AN is a hearing disability characterized by abnormal pure tone thresholds and/or abnormally poor speech perception abilities, absent CAP and ABR,* and presence of OHC function evidenced by OAEs or other evoked responses of hair cell origin.