Biology Reference
In-Depth Information
ter predated audiology as it is practiced today and was the cutting edge of
auditory assessment in humans.
The pure tone air conduction audiogram is at best only a rudimentary
tool for measuring hearing loss. Today's physiologically based audiological
battery should include tympanometry, middle ear muscle reflexes, oto-
acoustic emissions, speech audiometry, and Auditory Brainstem Response/
Electrocochleography (ABR/EcochG). Auditory Brainstem Response and
Electrocochleography are computer-controlled averaged evoked responses
that delineate synchronous neural discharges, usually following a brief audi-
tory pulse. They correlate extremely well with hearing, although they are
not themselves hearing tests (Hood and Berlin 1986). Understanding why
these additional tests are essential for fully accurate phenotyping requires
review of the following basic principles of auditory physiology.
The five major electroacoustic events in the cochlea are:
1. The endocochlear potential
2. The cochlear microphonic
3. The compound action potential
4. The summating potential
5. Otoacoustic emissions
Selective abnormalities in one or a number of these elements can lead to
120 different permutations of these five events, although current level of
knowledge can rule out a number of impossible sequences. For example,
absence of endocochlear potential leads to an absence of all the other elec-
trical events, including the otoacoustic emissions. It is therefore impossible,
as far as we can tell today, to have an absent endocochlear potential and
recordable otoacoustic emissions, or any of the above events. Removing
such impossible sequences reduces the number of possible permutations to
about 70.
Nowadays, parts of the organ of Corti can be assessed separately. For
example, normal otoacoustic emissions indicate that the endocochlear
potential, the middle ear, and the (primarily) outer hair cells are working
well. ABR and EcochG provide information about the synchrony and
integrity of the inner hair cells and eighth nerve, as well as confirming neural
synchrony after the discharge of the primary neurons. Thus, these tests and
their successors should be used not just in identifying the presence or
absence of tumors, but also in assessing the integrity of the auditory nervous
system and helping develop methods to categorize various genetic hearing
losses audiologically. The same pure-tone audiogram might be observed in
one patient with a genetically based loss of outer hair cells, and another
with normal outer hair cells, but disturbed neural synchrony (Berlin et al.
1993; Starr et al. 1996). However, etiology and genotypes are quite differ-
ent, and classifying both patients according to their pure-tone audiograms
alone would not be adequate for genetic analysis. A more precise catego-
rization of the hearing loss can be made by testing otoacoustic emissions,
