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Auditory Field Intensity Range Changes – a Sign of Early Noise Induced Hearing Impairment

R. Simeonov, D. Vicheva

Clinic of Ear-Nose-Throat Diseases, Medical University, Plovdiv, Bulgaria

ABSTRACT

Objective: 1. To study the auditory field intensity changes in patients who work in an intensive noise environment
2. To study the potential of the auditory examination to detect hearing impairment by a shortened method
Study design: Prospective study.
Setting: The measurements are performed in the ambulatory.
Patients: 250 intensive environment workers aged from 18 to 50 years. Female to male ratio was 1:1.
Interventions: Pure tone audiometry measurement of hearing and discomfort thresholds at 4000 Hz.
Main outcome measures: Hearing and discomfort thresholds; intensity field range and it’s changes.
Results: 120 (48%) of 250 patients had lowered threshold of discomfort. 20 patients had normal pure tone audiometry hearing threshold, 68 (28,3%) patients - lowered discomfort threshold and normal pure tone audiometry hearing threshold. In 112 cases threshold was lowered by 20 dB and pure tone audiometry hearing threshold raised by 60 dB. In 25% we found lowered discomfort threshold (approximately 10 dB) and heavily raised hearing threshold. In 8% we found significantly raised pure tone audiometry hearing threshold (more than 70 dB) and normal discomfort threshold.
Conclusions: Pure tone audiometry hearing threshold depends not only on the ears pent in noisy environment, but also on neurodynamics of auditory cortex the auditory field intensity range (between the hearing and discomfort thresholds) measured at 4000 Hz gives us the best idea of the neurosensorial auditory system in the conditions of noisy environment in the shortest terms and in numerical values.
Key words: Noisy environment, Pure tone audiometry, Hearing thresholds, Discomfort thresholds.
Pro Otology 3: 113-114, 2002

INTRODUCTION

Two totally controversial statements exist, considering the auditory field intensity changes in noise induced hearing impairment. According to the first one the narrowing of the intensity range in cases of early hearing impairment presents only by lowering of the discomfort threshold (1). Sagalovich states the opposite – mainly by elevation of hearing threshold and negligibly by changes of discomfort threshold (2). We settled down to investigate this issue.

Aims

1. To study the auditory field intensity changes in workers who have worked for different long time in an intensive noise environment

2. To study the potential of the auditory examination (pure tone audiometry threshold of discomfort and intensity range changes at 4000Hz in cases of noise induced hearing impairment) to detect hearing impairment by a shortened method.

MATERIALS

We examined 250 patients who worked in noisy environment, aged from 180 to 50 years. The male:female ratio was 1:1.

METHODS

We considered the pure tone audiometry threshold at 4000 Hz to be the most sensitive (1,2). At the same frequency we measured the threshold of discomfort. The so defined intensity range was 10dB. According to Simeonov K., Hood J.D. and Poole J., and our own control group we defined the discomfort threshold as follows: slightly lowered under 100dB; moderately lowered 80-90dB: heavily lowered under 80dB (3,4).

RESULTS

In 120 (48%) of 250 patients we found lowered threshold of discomfort (Table 1).

From the data in Table 1 it is obvious that:

20 patients had normal pure tone audiometry hearing threshold, although they have worked for more than 10 years in a noisy environment.

68 (28,3%) patients had lowered discomfort threshold and normal pure tone audiometry hearing threshold. The auditory field intensity range was narrowed.

In 112 cases we found discomfort threshold lowered by 20dB and pure tone audiometry hearing threshold raised by 60 dB. The auditory field intensity range was narrowed mainly because of the hearing threshold elevation.


Table 1. Audiometric results.

In 25% we found lowered discomfort threshold (approximately 10dB) and heavily raised hearing threshold. The changes in auditory field intensity range accounted mostly for the change of hearing threshold.

In 8% we found heavily raised pure tone audiometry hearing threshold (more than 70 dB) and normal discomfort threshold. The narrowed intensity range is due to changes of hearing threshold.


FIG 1. Auditory field intensity range before and after the patients had worked in intensive noise environment at 4000Hz: 1 - Normal auditory field; 2 - Early hearing impairment (lowering only of the discomfort threshold); 3 - Auditory field in the stage of adaptation; 4 - Auditory field in organic cochlear impairment combined with functional changes; 5 - Auditory field in organic cochlear impairment without functional changes.

DISCUSSION

The results allow us to disagree with those who state that hearing threshold elevates with 6 dB every year in these patients who have worked in noisy environment for more than 10 years. The narrowing of auditory field intensity range that accounts only for the changes of discomfort threshold we relate to disturbance of cortical neurodynamics. Vitanova states that discomfort threshold is a characteristic of the cortex (5). Our results give us the reason to support the thesis of Simeonov K., who states that the earliest intensive noise impairment of the neurosensorial system is characterized by lowered discomfort threshold and normal hearing threshold (1). This is also the characteristic of the neurotic stage of the noise disease.

According to Koichev and Karapanchev hearing threshold at 55-60dB should be considered critical (6,7). At higher values there are organic changes of the receptor of the neurosensorial hearing system. We assume that such a hearing threshold is representative of adaptation to intensive noise (8). The receptor lowers the environmental stimulus with as much dB as are needed to protect the central, more easily damaged structures.

Audiometric finding of hearing threshold more than 60 dB and discomfort threshold close to the normal we consider a representative for cochlear, organic damage combined with functional changes (FIG. 1).

Auditory field with heavily raised hearing threshold and normal discomfort threshold speaks of organic damage to the receptor and normalized cortical neurodynamics.

CONCLUSIONS

1. Pure tone audiometry hearing threshold depends not only on the ears pent in noisy environment, but also on neurodynamics of auditory cortex

2. The auditory field intensity range (between the hearing and discomfort thresholds) measured at 4000 Hz gives us the best idea of the neurosensorial auditory system in the conditions of noisy environment in the shortest terms and in numerical values.

 

REFERENCES

  1. Simeonov K. Die Tonintesitäsunterschiedschwelle des Ohres bei Neurosen. Monatschriff fûr Ohrenheilkunde und Laryngo-Rhinologie 1971;5:192-5.

  2. Sagalovich BM. Deafness. Moscow: Medicine, 1978.

  3. Simeonov K. Early impairment of auditory system by intensive noise. Doctorial thesis. Plovdiv, 1976.

  4. Hood JD, Poole J. Tolerable limit of loudness: its clinical and physiological significance. J Acoust Soc Am 1966;40:47-53.

  5. Vitanova L. Hearing system. In: Pirova B, Nachev N. Human Physiology. Sofia: ARSO, 2000:370.

  6. Koichev K. Ultrastructural and cytochemical organization of sensory structures of auditoryvestibular system. Doctorial Thesis. Pleven, 1978.

  7. Krapanchev A. The noise as a risk factor in the occupational environment with an experimental model of the process. Otorhinolaryngology 1999;4:16.

    8. Simeonov R. Structure and pathophysiological nature of the Karhart’s tooth. Proceedings of the 7th National Congress of Otorhinolaryngology, Plovdiv 2001.

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