Understanding Occupational Hearing Loss - 4

Saturday, August 16, 2003 | 0

The following article is the fourth installment of a 4 part series for physicians and non-physicians on occupational hearing loss by Dr. Stuart Gherini, a qualified medical examiner and expert in the field of otology. In this installment, Dr. Gherini reviews the various audiometric testing procedures.

Stuart Gherini, M.D.
Otology

After a complete history and physical examination, the hearing is tested by an audiologist. An audiologist is a professional with at least a Master's Degree in Audiology who has obtained a Certificate of Clinical Competence in Audiology (CCC-A) by serving a year's internship under a more senior audiologist. All hearing testing is performed in a sound-insulated booth meeting American National Standards Institute (ANSI) 23.1-1977 specifications using testing equipment calibrated to ANSI S3.6-I969 standards. It is important to document that the subject has had no significant noise exposure during the 24 hours prior to the examination to eliminate the possibility of a temporary threshold shift affecting the test results.

Impedance Testing

The hearing test begins with impedance audiometry. Impedance testing is a useful tool in identifying different types of middle ear and retrocochlear (along the auditory nerve) disorders. In the first part of impedance testing, an airtight probe or plug containing three tiny tubules is placed in the ear canal. The first tube is connected to a speaker which delivers a low frequency tone to the ear canal. The second tube is connected to a small pump which allows the pressure in the ear canal to be varied. The third tube is connected to a microphone to record the amount of sound in the ear canal. If the middle ear is working properly, the maximum amount of sound should be absorbed by the ear drum and three ossicles at atmospheric pressure. The results are plotted on a graph called a tympanogram. If the ear is normal, there is a "Type A" tympanogram. If there is fluid trapped in the middle ear space, the ear drum will not be able to move and capture sound. Most of the sound delivered to the canal will be bounced off the drum and back into the canal. This results in a "Type B" (bad) tympanogram. If the eustachian tube is not delivering enough air to the middle ear space, the ear drum will be pulled inward, stiffening the ossicular chain and resulting in less sound absorption by the ear drum and ossicles. This results in a "Type C" (contracted) tympanogram.

The second part of impedance testing is Acoustic Reflex Testing. When a loud enough tone is presented to the ear there are two muscles (one attached to the malleus and the other attached to the stapes) which contract and stiffen the ossicular chain. This reflex is automatically recorded by the impedance audiometer. (This acoustic reflex is just like when the doctor taps with a rubber hammer below the knee to elicit a reflex.) To have an intact acoustic reflex the ear drum, three ossicles, inner ear, and auditory nerve must all be functioning well. If the acoustic reflex is absent, it could be due to a hole in the ear drum, stiffening of the ossicles, decreased inner ear function, or a tumor along the auditory nerve.

Impedance audiometry is part of a constellation of different audiometric tests which enable an otologist to make a correct diagnosis. By comparing the results of impedance audiometry with the microscopic examination of the ear drum, the tuning fork tests, and the remainder of the audiometric examination, a diagnosis of the type of hearing loss may be made.

Pure Tone Testing

The results of pure tone air conduction testing are those most often used for disability ratings. (See "How to Read an Audiogram" above.) In a sound-treated room, wearing calibrated earphones, the patient is asked to push a button when he hears a tone. This so-called "air conduction" hearing is tested for both ears at 250, 500, 1000, 1500, 2000, 3000, 4000, 6000, and 8000 Hz. The intensity is tested in five-decibel steps from 0 dB to as high as 100 dB if needed. The hearing threshold at each frequency is that intensity of sound that the patient can just barely hear. The results are usually presented in both table form and graphic form (an audiogram). Bone conduction hearing is tested using an oscillator placed over the mastoid bone behind the ear.

Speech Testing

Speech testing examines the patient's ability to discriminate different speech sounds. A speech reception threshold (SRT) is first determined. This is done by presenting a standardized list of two syllable "spondee" words at varying intensities. That intensity at which half the words are correctly identified is called the "speech reception threshold" or SRT. Next, the intensity is increased to a comfortable listening level and the patient is asked to identify words from a standardized list of one-syllable words. The percentage of words correctly identified is the speech discrimination score or SDS (sometimes called the speech recognition score). In medical-legal evaluations, to eliminate variations caused by different voice qualities of different audiologists, both the SRT and SDS are determined utilizing a standard tape recording. The SRT and SDS scores are typically shown below the audiogram.

The above comprise the basic audiometric examination. There are numerous ingenious tests which have been devised for identifying the malingering patient or patient with an exaggerated hearing loss. With a competent audiologist well-versed in industrial evaluations, working closely as a team with an otologist, these other tests are rarely needed as the malingerer will usually be discovered by this stage of the evaluation. We look for consistency in the history, physical examination, and audiometric studies.

Dr. Gherini is represented by MEDLink in the Sacramento, CA. area. Appointments can be made through MEDLink at: www.camedlink.com>.

Part 1 provided a brief history on occupational hearing loss and a comprehensive analysis on causes, specific types and costs associated with the condition. In Part 2 Dr. Gherini discusses identifying and evaluating occupational hearing loss and examines long-term implications. Part 3 reviewed how repetitive noise exposure damages hearing.

(c) 2001 MEDLink -- Reprinted by Permission. Additional articles may be read at www.camedlink.com . MEDLink affiliated QME physicians may be reached by e-mail at: camedlink@camedlink.com .