Proceedings of a Workshop on - The Havemeyer Foundation

Havemeyer Foundation Monograph Series No. 11
TREATMENT OF RECURRENT LARYNGEAL
NEUROPATHY: EVALUATION BY RESPIRATORY
SOUND ANALYSIS
F. J. Derksen
College <strong>of</strong> Veterinary Medicine, Michigan State University, 200 Westborough Road, North Grafton,
Michigan 48824-1314, USA
Upper respiratory disease is suspected in an
exercising horse when an abnormal respiratory
noise is heard and when the horse’s performance is
reduced. While performance reduction can be
caused by dysfunction in many systems,
respiratory noise during exercise is specific for an
upper airway problem.
For sport horses, respiratory noise caused by
conditions such as recurrent laryngeal neuropathy
(RLN) can be more important than the obstruction
itself. There is information in the literature
describing the efficacy <strong>of</strong> surgical procedures in
improving upper airway flow mechanics in
exercising horses with RLN, (Derksen et al. 1986;
Shappell et al. 1988; Tetens et al. 1996) but there
is little information about noise reduction. This
information is also important for racehorses, as
residual respiratory noise after surgery is <strong>of</strong>ten
interpreted as failure to improve upper airway flow
mechanics (Russell and Slone 1994; Hawkins et
al. 1997; Kidd and Slone 2002). However, the
relationship between noise and upper airway
obstruction has not been evaluated critically
(Derksen 2003).
RECORDING AND ANALYSING RESPIRATORY
SOUNDS IN EXERCISING HORSES
The first challenge associated with the quantitative
evaluation <strong>of</strong> respiratory sound in exercising
horses is the squelching <strong>of</strong> extraneous noises. To
accomplish this, a dynamic unidirectional
microphone is placed in such a way that the
recording microphone is directed towards the
nostrils and rests approximately 4 cm from the
horse’s nose. The microphone is connected to a
cassette recorder containing an automatic gain
control and a compression circuit. The combined
features <strong>of</strong> this system reduce extraneous noises.
The recorded sounds are evaluated using
computer-based spectrum analysis (Derksen et al.
2001).
RLN AND RESPIRATORY NOISE
Exercising horses with RLN make a distinctive
inspiratory noise. In affected horses, highintensity
sounds are present throughout inhalation
(Cable et al. 2002; Franklin et al. 2003). This
sound is characterised by 3 frequency bands called
formants which are centred at approximately 400,
1700, and 3700 Hz. Quantitative indices
describing the loudness and character <strong>of</strong> the sound
include inspiratory sound level and the sound
intensity <strong>of</strong> the 3 inspiratory formants (Derksen et
al. 2001). The sound intensity <strong>of</strong> the formant 2,
centred at about 1700 Hz, is most important,
because this formant is in a frequency range where
human hearing is most acute.
EFFICACY OF SURGICAL PROCEDURES IN
REDUCING NOISE CAUSED BY RLN
Surgical techniques recommended for the
treatment <strong>of</strong> RLN include prosthetic
laryngoplasty, the nerve muscle pedicle graft
technique, ventriculectomy, ventriculocordectomy,
laser cordectomy, and total, partial,
and subtotal arytenoidectomy. Thus far, we have
only studied the effect <strong>of</strong> prosthetic laryngoplasty
and bilateral ventriculo-cordectomy on respiratory
noise caused by experimentally induced laryngeal
hemiplegia. Thirty days after surgery, bilateral
ventriculo-cordectomy has no effect on inspiratory
noise. However, at 90 and 120 days after surgery
most indices <strong>of</strong> inspiratory noise, including the
sound intensity <strong>of</strong> formant 2, return to baseline
levels. Inspiratory sound level remains slightly but
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