The effect of noise on the vestibular system has remained a neglected aspect of occupational acoustic trauma research. Patients with noise-induced hearing loss (NIHL) sometimes complain of balance disorders; but clinical reports are highly controversial, giving contradictory results regarding vestibular disturbances in such patients.

Recent research involving military personnel has provided objective evidence of vestibular involvement in combination with cochlear damage resulting from acoustic trauma in individuals with asymmetrical NIHL ["The Effects of Noise on the Vestibular System," American Journal of Otolaryngology, 22 (3): 190-196]. This has medical and legal implications with regard to compensation claims on the grounds of noise exposure for individuals with NIHL in whom one ear is more affected than the other.

American and Israeli researchers evaluated the effect of impulse and impact noise on the vestibular labyrinth in subjects with symmetrical NIHL and in another group with asymmetrical NIHL, in which the frequencies below 3000 Hz also were affected. Using members of the Israeli Defense Force as test subjects, the investigators examined the correlation between a subject's complaint of dizziness and vertigo and the results of vestibular function tests, and the correlation between the severity of the hearing loss and the vestibular symptomatology and pathology.

A total of 258 men, ages 20-35, were studied. They experienced heavy exposure to loud noises during their military service, such as impulse and impact noises of various firearms and large caliber weapons, as well as helicopters, tanks and other military vehicle engines. Before the study began, several audiograms confirmed that each subject had incurred NIHL.

The subjects were divided into two groups according to the findings in their previous audiograms: One group was comprised of 134 subjects with a symmetrical, high-tone hearing loss in both ears who had 3000-4000 Hz notches of 35 dB HL or worse. The remaining 124 subjects had asymmetrical threshold shifts with one ear affected in the 1000 and/or 2000 Hz and with a difference of 15 dB or more between the two ears in the affected frequencies.

Each group then was divided into two subgroups based on the presence or absence of vestibular complaints. In the symmetrical hearing loss group, 11 percent had complained frequently of vertigo or dizziness; and 21 percent of those in the asymmetrical hearing loss group reported similar symptoms.

Thirty-five healthy civilian male volunteers who were never exposed to gun shooting or any other hazardous noise served as a control group.

Each subject underwent a thorough otolaryngological, audiological and neurological examination to exclude any possible diseases affecting the balance system; and an electronystagmography (ENG) test was administered. Each subject with asymmetrical hearing impairment underwent either a computed tomography (CT) scan or a magnetic resonance imaging (MRI) test to exclude retrocochlear pathology.

The biggest difference was found regarding the results of the ENG tests between the two groups. In the group with symmetrical hearing loss, only 5.2 percent of the subjects had abnormal findings, while 46.8 percent of the subjects in the asymmetrical hearing loss group had pathological findings.

There also was a strong correlation between the subjects' symptoms in the group with asymmetrical hearing loss and the abnormal ENG findings. More than 80 percent of the subjects who complained of dizziness or vertigo had abnormal ENG findings. No correlation was found between the severity of the NIHL and the vestibular symptoms and abnormal findings.

In 82 percent of the subjects with asymmetrical hearing loss, the left ear was more affected. Based on the results of the audiological tests, hearing loss was classified as cochlear in all exposed individuals. No retrocochlear pathology was found in any individual with asymmetrical hearing impairment.

In the group with asymmetrical hearing loss, spontaneous nystagmus was seen in eight subjects, with no statistically significant difference between the two subgroups. Gaze nystagmus, positional nystagmus, reduced caloric response and directional preponderance were significantly more common in the symptomatic subgroup.

Less significant was the finding that 11 percent of the subjects with symmetrical NIHL had symptoms of imbalance, dizziness or vertigo, while 21 percent in the asymmetrical group had such complaints.

In addition, seven subjects (5.2 percent) in the group with symmetrical hearing loss had one or two abnormal ENG findings. Three were asymptomatic, and four were symptomatic. Although there seemed to be a difference between these two subgroups with regard to the low number of cases that had abnormal ENG findings, the difference was not statistically significant.

Avishay Golz, MD; David Goldenberg, MD; Aviram Netzer, MD; Henry Z. Joachims, MD; and Milo Fradis, MD, all from Haifa, Israel, and Liane Westerman; S. Thomas Westerman, MD; and Tracey Wiedmyer, from Shrewsbury, NJ, authored the study.