Background:
After receiving cochlear implants (CI), speech discrimination of the CI recipients
improves gradually. The knowledge of a normal range for this rate may be relevant
for quality control as part of a clinical CI program.
Methods:
We selected 35 functionally deaf adult patients who were fitted with CI, 29 of them
with postlingually and 6 with prelingually acquired hearing loss. Speech discrimination
was tested with the Freiburger monosyllabic and two-digit numbers speech intelligibility
test at timepoints of 1, 3 and 6 months after implantation. The test was performed
at 50, 65 and 80 dB speech level. The performance levels were approximated by a function
y = a*(1 – exp(-t/τ)). The time constant τ is closely related to the initial slope
of the function at t = 0 (i.e. first fitting).
Results:
In postlingually deaf adults, the speech recognition of two-digit numbers at 65 dB
increased from 62.5% at 1 month to 84.3% at 3 months and 94.5% at 6 months after implantation.
The fit of the function y = a*(1 – exp(-t/τ)) to the data yielded a recovery time
τ= 1.2 months for postlingually deaf patients. From the same treatment of the data
corresponding to the recognition of monosyllables at 65 dB, the result τ= 1.9 months
was derived. The patient cohort of prelingually deaf adults was too small to develop
a mathematical function.
Conclusion:
We have shown that for postlingually functionally deaf patients post implantation,
the development of speech recognition can be described by the formula y = a*(1 – exp(-t/τ))
and it could therefore be possible to foresee an individual patients improvement in
speech recognition. The recovery time τ amounts to 1.2 months for the discrimination
numbers and 1.9 months for monosyllables.
