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Assessment Tools: Introduction and Practical Use of Otoacoustic Emissions in Hearing Conservation Programs

By: Sarah E. Ervin, M.A., CCC-A

Within the past 25 years there has been an explosion of technology in the field of Audiology. One of the most important advances in this technology and knowledge about the functioning of the human ear has been through the identification of Otoacoustic Emissions (OAE).

Otoacoustic Emissions were first discovered by Dr. David Kemp in 1978 (Hall, J.W., 2000). The first commercial OAE system was not produced for sale in the United States until 1988. Since this time, OAE testing has developed into a significant addition in evaluating the auditory pathway. It can be used for diagnostic and/or screening purposes through the use of a hand-held device.

The following is a brief description of the anatomy and physiology which creates the OAE, the definition of an OAE, the importance of incorporating OAE evaluations in a hearing conservation program, and how an OAE is measured.

The Auditory Pathway

A simplified description is in order of how sound travels from creation to perception. A disturbance in the environment causes sound waves to be created which travel through the air. The sound is funneled into the ear canal by the ear flap and strikes the ear drum. The outer section of the ear including the canal is called the outer ear.

It then vibrates three small bones (ossicular chain) in the middle ear, the malleus, incus and stapes. The middle ear is the cavity containing the ossicular chain and is connected to the back of the throat by way of the Eustachian tube. Problems in the middle ear are associated with head colds, earaches and ear infections.

Once the ossicular chain is set into motion, the footplate of the last bone, the stapes, is vibrated and the sound enters the inner ear. The inner ear is divided into two parts, the sensory and the neural. The sensory portion begins in the cochlea. The cochlea is named after the conch shell because it circles around two and one half turns. Proceeding up the fluid filled cochlea from the base to the tip is the organ of Corti. It is housed in a middle section of the cochlea and has rows of small receptor cells called ‘hair cells’. They are called hair cells because their appearance resembles small hair follicles. A closer look at these cells shows that there are several rows. The outer hair cells have three or four rows and the inner hair cells have a single row. Studies have shown that when the sounds of different frequencies vibrate different sections of the organ of Corti, the higher frequencies vibrate the basal end and the lower frequencies vibrate the apex. When the organ of Corti vibrates, the outer hair cells create an electro-physical discharge (otoacoustic emission or OAE) that tunes and amplifies the stimulation for the benefit of the inner hair cells. When stimulated, the inner hair cells initiate the neural impulse that is carried up the auditory or eighth cranial nerve to the brain stem and on to the cortex of the brain where the signal is sent by way of association fibers to other parts of the brain for recognition, interpretation and understanding.

What are Otoacoustic Emissions?

Sound travels in a forward direction down the ear canal, vibrating the eardrum, ossicular chain, organ of Corti and outer hair cells generating the OAE. The OAE travels in a reverse direction back through the fluid of the cochlea, across the ossicular chain, vibrating the eardrum. Special sensitive equipment with advanced filters and a probe inserted into the ear canal can detect the minute OAEs.

OAEs occur naturally and spontaneously. Persons with normal hearing and a healthy middle ear usually show the presence of OAEs. Many hearing losses originate in the cochlear at the outer hair cell level. The presence and absence of OAEs can assist in identifying the disease or pathology that cause hearing loss. They can provide essential information for treatment and rehabilitation of the hearing loss.

Even though middle ear problems can greatly affect OAE testing, small eardrum perforations or tubes inserted by the physician usually do not interfere with detecting the presence of OAEs.

Why are they important?

  • An OAE evaluation is fast, objective and does not require employee participation.
  • Excessive noise exposure affects outer hair cell function and an OAE evaluation is dependent on outer hair cell function (Hall, J.W. & Mueller, H.G., 1997).
  • An OAE evaluation aids in the determination of work-relatedness on the OSHA 300 Log by specifying cochlear involvement and is seen as a legally binding tool.
  • An OAE evaluation can be used during the Audiologist plant visit to evaluate employees being held for plant visits and medical referrals.
  • Gives employees more validity for following-up with incomplete medical referrals.
  • An OAE evaluation can provide a "warning sign" of outer hair cell damage due to noise before it is evident on an audiogram.

How are they recorded?

To record the sounds that are made from the outer hair cells in the cochlea, a probe tip, which contains an earphone and microphone, is fitted into the outer ear canal. The probe which is connected to a hand-held device delivers an auditory stimulus and the response to the stimulus is measured by the microphone in the probe tip. In order for the equipment to detect the small vibrations from the cochlea to the eardrum, the employee must have normal middle ear function. Similar to the audiogram that is performed on each employee, the OAE evaluation will test a variety of frequencies. The responses obtained for each frequency are analyzed by the hand held device which indicates a pass/fail rating for each frequency measured.

What do the results indicate?

An OAE evaluation resulting in a pass typically indicates normal hearing and normal middle ear function. If OAEs are present and an employee reveals a mild to moderate hearing loss, this may be an indication of a retrocochlear (beyond the cochlea) hearing loss. Instances where OAEs are present but an employee’s audiogram suggests a significant hearing loss may be an indication of a malingering employee.

Results revealing absent OAEs are typically observed in employees who have congenital (present at birth) hearing loss, abnormal middle ear function, and/or dysfunction of the outer hair cells in the cochlea. As indicated previously, damage to outer hair cells can be created by exposure to occupational noise.

Before drawing conclusions from OAE test results in reference to hearing loss diagnoses, a variety of audiologic evaluations should be performed.

Summary

This OAE evaluation is not designed to replace the audiogram. It is to serve as a supplement to the hearing tests performed by the OHC in determining the cochlear function of the employee. No single test in the assessment of hearing will tell the audiologist all that is needed to identify and provide direction for treatment and rehabilitation of hearing loss. Different tests will provide information about the functioning of different sites of injury along the auditory pathway. Testing for OAEs in the hearing system is unique in identifying the presence or absence of problems in cochlear function and can aid in screening medical referrals and employees who show a non work-related OSHA recordable hearing loss in the plant.

References & Additional Resources


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