As I Hear It: Fitting the Patient's Physiology Instead of the Audiogram

Charles I. Berlin, PhD

If the only tool you use is a hammer, the whole world looks like a nail.

Hearing aids do an excellent job at compensating for outer (and some inner) hair cell losses and dysfunction. They also do an excellent job at compensating for middle or even outer ear obstruction. But as audiologists, we are in a unique position to look past basic audiological tests and better manage those patients who need our help. If the only tool you use is a hammer, the whole world looks like a nail; if the primary way you assess and estimate hearing function is with a pure tone audiogram, you will operate with only a limited understanding of the various physiological states that can generate identical audiograms. In fact, it is often posed as a paradox… "How come people with the same audiograms have such different performances with hearing aids?" The real paradox is why we would believe that the audiogram is always the gold standard test of auditory function.

Imagine if you went to a health practitioner whose only tools were a thermometer, a scale and a tongue depressor. The likelihood that the practice would see lots of overweight people with fevers and sore throats becomes quite high; but they would also have other disease processes about which the practitioner would likely be unaware. Similarly, those of us who "measure hearing with audiometers" will see the world of our patients in terms of hearing loss and deviations from normal sensitivity. Keep in mind that only a few years ago patients (and physicians) were being told there are two kinds of hearing loss…conductive loss and nerve loss…and the latter is rarely helped by hearing aids which "amplify all sounds".

We would all agree that some major changes are in order for a colleague who thinks that way. In response to such conductive vs. nerve-deaf thinking, let me briefly review what I think is a better way for evaluating new patients than focusing on "Air, bone and speech".

As we take a history on a new patient (child or adult) , we ask about familial hearing loss, birth and balance difficulties, tinnitus and feelings of fullness, noise exposure, etc., and then look in the patients' ears. I ask my colleagues to begin their testing in the following order before they go to behavioral audiometry or the pure tone audiogram:

Tympanometry

Middle Ear Muscle Reflexes both ipsi and contra.

Otoacoustic emissions.

Speech Reception threshold.

Let's see why.

As you recall, there are five electrophysiologic events in the cochlea which are recordable from mammals, four of which are also recordable in living humans. Different combinations of dysfunction in these events can lead to similar-looking pure tone audiograms but require vastly different management.

The five events are:

The Endocochlear potential (EP), or the battery which drives all the other events in the ear. It is an 80/1000ths of a volt force without which none of the events listed below can occur. Notice that if the battery is gone, the patient can have a corner audiogram BUT ALL THE NERVE FIBERS AND HAIR CELLS WILL REMAIN! EP losses sometimes show up as sudden hearing losses or as part of a fluctuant hearing loss.

The cochlear microphonic, or hair cell potential. This electrical event is part of the ABR or the cochleogram, and it inverts its polarity when the stimulus is inverted. It comes from hair cells, both outer and inner, but in human surface recordings reflects mostly basal turn outer hair cells.

The Compound Action Potential. This is the Wave I of the ABR and represents synchronous neural discharge from onset-sensitive units at the base of the cochlea.

The summating potential which is a dc shift of the baseline electrical activity around the cochlear microphonic.

Otoacoustic Emissions, or sounds which come primarily from the outer hair cells. There is some evidence that Distortion Product emissions have a different generator than Transient Evoked Emissions (Shera and Guinan, JASA, 1999).

Now, why the special test order? Tympanometry must be done first to rule out middle ear obstructions which cloud the interpretation of emission and reflex abnormalities. If tympanometry is normal, we look for middle ear muscle reflexes and emissions both to be normal.

If there are both reflexes and emissions present we know that the EP is present, that there should be cochlear microphonics and compound action potentials present and we expect enough synchrony to generate a middle ear muscle reflex. This usually suggests an intact auditory nerve. Then, if the audiogram suggests a hearing loss, hearing aids can be considered as a rational treatment since we now know the auditory nerve is synchronously discharging and will respond to low level amplified signals.

However, if tympanometry is normal, reflexes are absent and emissions are present, then we are likely to have someone with normal outer hair cells and normal EP but poor neural synchrony. These patients surprisingly can give audiograms ranging from normal to "anacusis" but still have otoacoustic emissions. Hearing aids may increase sensitivity (and "improve the audiogram") but they will not improve the impaired neural synchrony. They are currently called Auditory Neuropathy (AN) patients but are better described as patients with Auditory Dys-synchrony.

Most of us who practice diagnostic audiology see between 10 and 12 sensorineural patients per year per thousand who have vestibular schwannomas (the proper nomenclature for most VIIIth N. Tumors). In contrast, we have seen and/or consulted on over 200 AN patients in the past two years with absent ABRs and normal emissions. This co-incidence of events…normal emissions with absent ABRs and absent middle ear muscle reflexes…has occurred many times more frequently in our practice than the diagnosis of VIIIth N. tumors.

So now, back to the recommended order to properly categorize patients before they even take a pure tone audiogram or before we even accept a pure tone audiogram as having any diagnostic or clinical value.

Tympanometry first…because as long as it is normal we should see emissions and reflexes. Reflexes second because if they are present we have ruled out AN and the presence or absence of emissions can now be properly interpreted. Then, if the emissions are present we now know that the EP, the outer hair cells and the middle ear should all be normal and the patient's voluntary audiogram should be at or near normal.

Conversely, if the reflexes are absent and the emissions normal we have an auditory dys-synchrony and know to interpret the audiogram with great care and ignore it as an index of hearing aid utility.

Finally, if the tympanometry is abnormal, and we do not see reflexes or emissions, we know that the true status of the inner ear is still unclear and other tests and or medical attention will be needed to unravel this problem.

If you start with Pure Tone Audiometry and then have to deal with the "mysteries" that follow, you may either neglect to go any further than a normal pure tone and speech result, thus missing a powerful set of diagnostic options, or you may erroneously pronounce a patient "normal" who has serious auditory dys-synchrony that is invisible to a pure tone audiogram.