4.2 Sound localisation
The results of this study showed evident declines in directional hearing
in both children with UMA and
stimulated
UHL; however, the group of UMA children performed better than those with
stimulated UHL. Consistent with previous reports, good performers among
children with congenital UCHL may have learned to localize sound sources
using monaural cues and residual binaural difference cues after a long
period of adaption. Vogt et al. 11 confirmed that
patients with congenital UCHL rely on monaural spectral cues to detect
high-frequency sound sources by comparing localisation accuracy with and
without moulding the normal hearing ear pinna. Van et al.10 evaluated nine listeners with chronic unilateral
hearing loss through a group of broadband sound stimuli fixed at 60 dB;
the results indicated a strong reliance on the ambiguous HSE in familiar
acoustic environments.
Our study revealed no significant improvement in sound localisation
accuracy between the BCD-unaided and aided conditions. Similar results
have also been obtained in previous studies 8, 9regarding the application of bone-anchored hearing aids and Bonebridge
(Med-EL, Innsbruck, Austria) in congenital UCHL. The inability to
achieve binaural hearing may be a consequence of two factors. First,
hearing symmetry still exists, as the BCD is not able to provide the
same hearing threshold as that of the normal hearing ear, and the
insufficient intensity input also disrupts binaural hearing. Second, the
processing time delay and inconsistent stimulation are inherent
characteristics of the BCD signals; Additionally, the bone conduction
signals with less reliable and constant cues may also prevent children
with congenital UCHL from having a restored binaural hearing17. However, other studies showed improvement in sound
localisation accuracy when BCDs were used in patients with congenital
UCHL 6, 7, 18. Potential explanations for these
conflicting observations may be the age gap of the enrolled patients and
methodological differences in the procedure. Besides, it is favourable
that the sound localisation abilities of the intact ear did not
deteriorate owing to cross-hearing induced by the BCD, likely due to
insufficient high-frequency sound transmission of the BCD19, which did not interfere with the use of spectral
cues by the healthy ear. Furthermore, the original sound localisation
performance was a good predictor of sound localisation accuracy under
the BCD-aided conditions. Moreover, there is a high need for early
hearing intervention in poor performers who cannot make good use of
asymmetric binaural cues to localize sound sources.
CONCLUSIONS
Some children with UMA were able to compensate using the remaining
distorted binaural cues to detect sound sources, unlike the children
with acutely stimulated UHL; however, this compensating ability was
still far worse than children with NH and varied across individuals.
As the application of BCD
provided a definite benefit on speech recognition abilities and high
participant satisfaction,
it
is recommended that children, particularly those with poor sound
localisation performance, should be fitted with non-surgical BCDs at an
early age.