Although there may be many root causes for a hearing loss (such as disease, inherited disorders, birth defects, environment, etc), the most common proximate cause is the loss of hair cells. Such hearing loss is typical identified and measured by an audiogram, which measures the minimum sound pressure level (at a particular frequency) that can be detected. The figure below shows the frequency and level of various common sounds, along with the actual measurements of a tested child. Hearing loss is identified as mild, moderate, severe, or profound, depending on the detection thresholds for a particular subject.
|This audiogram records were taken on a two-year-old female, and shows levels of many typical sounds. The RED S are
the measured thresholds for the left ear, and the GREEN S are for the
right ear. Because this child had some residual hearing, the original
treatment strategy was to use hearing aids. The PURPLE A shows the audiogram
results for this child while she wears hearing aids. Note that the
hearing aids place her hearing at approximately the minimum required
to understand spoken language.
Typically, a cochlear
implant is considered for use only in the case of extreme hearing
loss. Even when the loss of hearing is profound, the auditory
nerve may be essentially undamaged. In particular, if the loss of hearing
is due to damage in either the middle or the inner ear, as long as the nerve
path remains intact, there is the possibility of sending signals to the
auditory processing portions of the brain. Since nerve cells may become
degraded if not used, this implies that the less time between loss of hearing
and implantation, the better chances of successful implantation, but implants
may be effective even after years of hearing loss.
The C markings on this cochlear
implant show actual results for the same child as the previous audiogram. The
subject suffered continued degradation of her right ear until (by age
three) she could not register any response on the audiogram testing. Her
right ear was implanted. The above test results were taken
at age six. Note that the CI has given her threshold levels
well above that required for speech. The left hear (X) remains
at the moderate-to-severe level.