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BALANCE DISORDERS MORE LIKELY IN PATIENTS WITH NOISE-INDUCED HEARING LOSS IN ONE EAR
Israeli soldiers are test subjects in a new study. Researchers believe hunters are at risk.
Denver, CO -- It is well known that loud tones can cause damage to the cochlea, with subsequent hearing impairment. Surprisingly, the effect of noise on the vestibular system has been given less attention, and remained a neglected aspect of occupational acoustic trauma. Patients with noise-induced hearing loss (NIHL) sometimes complain of balance disorders, but reports of clinical series give contradictory results with regard to vestibular disturbances in such patients and are highly controversial. Lately, workers, employers, lawyers, compensation companies and the governments of many countries are recognizing the effect of noise not only on hearing, but also on the balance system.
Now, Israeli researchers set out to evaluate the effect of impulse and impact noise on the vestibular labyrinth both in subjects with symmetrical NIHL and in subjects with asymmetrical NIHL in which the frequencies below 3000 Hz were also affected. Using members of the Israeli Defense Force as test subjects, they examined the correlation between the subjects' complaints of dizziness and vertigo and the results of the vestibular function tests, and the correlation between the severity of the hearing loss and the vestibular symptomatology and pathology.
The authors of "Noise-Induced Vestibular Dysfunction: Fact or Fiction," are Avishay Golz, MD, David Goldenberg, MD; Aviram Netzer, MD; Henry Z. Joachims, MD, all from, Haifa, Israel; and Liane M. Westerman and S. Thomas Westerman, MD from Shrewsbury, NJ. Their findings were presented on September 10, 2001, at the American Academy of Otolaryngology--Head and Neck Surgery Foundation Annual Meeting/OTO EXPO, being held September 9-12, 2001, at the Colorado Convention Center, Denver, CO.
Methodology: Two hundred fifty-eight male military personnel of the Israel Defense Forces, age 20-35 years, were enrolled in the study. The study was limited to subjects who were heavily exposed to loud noises during their military service. The exposure was to both impulse and impact noises of various firearms and large caliber weapons as well as helicopters, tanks and other military vehicle engines. The subjects were all healthy, with no history of ear infections, head trauma, hypertension, or diabetes. None of the personnel enrolled had a family history of hearing impairment, and no one was ever treated with known ototoxic (harmful to the ears) drugs. All participants entered military service with normal hearing, as confirmed by normal pure tone and speech audiograms performed at the time of their recruitment. Cases of acute acoustic trauma or blast injury were excluded.
Before the study began, it was confirmed by several audiograms that each subject had incurred noise-induced hearing loss (NIHL). NIHL was defined as a high tone sensorineural hearing loss on the audiogram, with a clear history of noise exposure for two years or more.
The subjects were divided into two groups according to the findings in their previous audiograms:
One hundred thirty-four subjects had a symmetrical high-tone hearing loss in both ears, with 3000-4000 Hz notches of 35 dB HL or worse. The remaining 124 subjects had asymmetrical threshold shifts with one ear affected in the 2000 and/or 1000 Hz, and with a difference of 15 dB or more between the two ears in the affected frequencies.
Each group was then divided into two subgroups according to the presence or absence of vestibular complaints. In the symmetrical hearing loss group, 11 percent had frequently complained of vertigo or dizziness, and in the asymmetrical hearing loss group, 21 percent had such symptoms.
Thirty-five healthy civilian volunteer males of the same age who were never exposed to gun shooting or any other hazardous noise, served as a control group. None had a history of ear infection, head trauma, or was under any known ototoxic medication.
All of the subjects underwent a thorough otolaryngological, audiological and neurological examination to exclude any possible diseases affecting the balance system. Each subject with asymmetrical hearing impairment underwent either a CT scan or a MRI to exclude retrocochlear pathology. Electronystagmography (ENG), a test of the balance system, was performed on all of the subjects.
Results: There was no significant difference in the ages of both the study group subjects and the controls. Eleven percent of the subjects with symmetrical NIHL had symptoms of imbalance, dizziness or vertigo whereas in the asymmetrical group, 21 percent had such complaints, but the difference between these groups is not of statistical significance.
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 symmetrical hearing loss, seven subjects (5.2 percent) had one or two abnormal ENG findings: three asymptomatic cases (2.5 percent) and four symptomatic cases (27 percent). Although there seems to be a difference between these two subgroups with regard to the low number of cases with abnormal ENG findings, this difference is not statistically significant.
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.
A statistically significant difference was found regarding the results of all the ENG tests between the two groups. In the symmetrical hearing loss group only 5.2 percent of the subjects had abnormal findings, whereas in the asymmetrical hearing loss group, 46.8 percent of the subjects had pathological findings. This difference is statistically significant.
There was a strong correlation between the subjects' symptoms in the asymmetrical hearing loss group and the abnormal findings on the ENG: almost 81 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.
Conclusion: The authors believe they have displayed objective evidence of vestibular involvement in combination with cochlear damage, in acoustic trauma only in those individuals with asymmetrical NIHL. This has medico-legal implications with regard to compensation claims on grounds of noise exposure, especially in those individuals with NIHL, in whom one ear is more affected than the other.
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