State and federal regulations for hearing.

Mandated federal and state requirements for hearing function prescribed for military and pilot performance, schoolchildren, and commercial and school bus drivers are summarized in the following review. The material is directed toward involved otolaryngologists, industrial physicians, and appropriate review boards for reference in licensure examinations or student hearing assessments.

Vascular analysis of individuals with drop attacks.

The authors used magnetic resonance angiography to examine the intracranial and cervical vascular structures of individuals who suffer from drop attacks. Normal structural configurations of the circle of Willis and the vertebrobasilar arterial system were compared to the vascular patterns of 10 subjects with these episodic tonic or atonic attacks. Overall, multiple areas of arterial occlusion, stenosis, or hypoplasia were visualized in the images of 8 of the 10 subjects. Specific anomalies of the vertebral and basilar arteries were identified in 4 individuals, and 8 images depicted nonvisualization of the posterior communicating arteries. We suggest that the pathological aberrations in the regional circulation of the hindbrain support the hypothesis that a transient hypovolemic episode may have an impact upon the neural activity involved in maintenance of motor tone and postural stability.

Functional impairments due to unilateral deafness.

The authors investigated the impact of unilateral listening upon two central auditory functions: speech discrimination in a noisy environment, and the ability to process and clearly recognize modestly accelerated speech by 30% compression (shortened exposure). We determined that the negative impact of listening to speech in the milieu of noise was individually variable. The data revealed a wide range of impairment, from mild to relatively severe (ie, 0% to 60% [mean, 34%]; normative data, 4% to 36% [mean, 14%]). The performance scores for recognition of compressed sentences were not markedly influenced by unilateral deafness, but the functional results of the residual ear were correlated with a loss of high-frequency sensitivity (mean of 54% correct in severe unilateral deafness with an associated high-frequency loss versus 93% correct in normal controls). Reorganization of the central nervous system by "plasticity" or biological maturation over time failed to improve the performance of monaural listening. The currently recommended methods of remediation for this sensory deficit are discussed, and newer technologies under investigation are examined.

Incompetent circle of Willis and vertebrobasilar insufficiency.

An analysis of the vascular basis of vertebrobasilar insufficiency was derived from the examination of 135 subjects by magnetic resonance imaging and angiography. The data from this cohort were compared to those from normal, ie, asymptomatic, individuals. We concluded that there is a significant difference in the potential for intracerebral collateralization and regional perfusion due to structural defects such as stenosis or developmental absence of the posterior communicating arteries in this symptomatic group. In addition, an occlusion of one or more of the source vessels, the internal carotid and vertebral arteries, in conjunction with an incomplete circle of Willis increased the potential for hindbrain perfusion deficits. Clinical cases illustrate the relationship between failure of internal shunting and the myriad symptoms of hindbrain dysfunction.

Evaluation of vertigo by auditory brain stem response.

The authors examined the hypothesis that abnormal patterns of the auditory brain stem response (ABR) could supplement the neuro-otological evaluation and assist in localizing the site of vestibulocerebellar dysfunction. This project is based upon the fact that the sources of waves I through V have been regionally identified. Absent or delayed patterns can be referenced to the normal data, and the site of a lesion generating vertigo can be established. We found absence of waves or prolonged interpeak latencies in 25% of the vertiginous subjects with normal hearing and magnetic resonance images of the brain. We conclude that in selected cases, lesions affecting the vestibular system can influence the ABR, and the electrophysiological tests of audition may suggest regionalization of the dysfunction in the hindbrain and midbrain.