Proceedings of a Workshop on - The Havemeyer Foundation

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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 79
Equine Recurrent Laryngeal Neuropathy significantly elevated. Thus, bilateral ventriculocordectomy effectively reduces inspiratory noise associated with laryngeal hemiplegia. The effects <strong>of</strong> prosthetic laryngoplasty on upper airway noise in laryngeal hemiplegia affected horses are already evident 30 days after surgery. However, key indices <strong>of</strong> sound intensity, including the sound intensity <strong>of</strong> formant 2, remain elevated relative to baseline levels. This indicates that prosthetic laryngoplasty is less effective than ventriculo-cordectomy in reducing laryngeal hemiplegia associated noise. The efficacy <strong>of</strong> prosthetic laryngoplasty in reducing airway obstruction and noise is variable in individual horses. The degree <strong>of</strong> remaining airway obstruction following surgery and the amount <strong>of</strong> residual respiratory noise are not correlated. Therefore residual respiratory noise following prosthetic laryngoplasty cannot be used to determine the degree <strong>of</strong> remaining airway obstruction in individual horses. Interestingly, there is a significant positive correlation between the degree <strong>of</strong> arytenoid abduction and inspiratory noise following surgery. That is, the more arytenoid abduction achieved, the greater the residual noise. The degree <strong>of</strong> arytenoid abduction following surgery does not correlate with residual airway obstruction. REFERENCES Cable, C.S., Ducharme, N.G., Hackett, R.P., Erb, H.N., Mitchell, L.M. and Soderholm, L.V. (2002) Sound signature for identification and quantification <strong>of</strong> upper airway disease in horses. Am. J. vet. Res. 63, 1707-1713. Derksen, F.J. (2003) Practice is alive with the sounds <strong>of</strong> horses. Equine vet. J. 35, 224-225. Derksen, F.J., Holcombe, S.J., Hartmann, W., Robinson, N.E. and Stick, J.A. (2001) Spectrum analysis <strong>of</strong> respiratory sounds in exercising horses with experimentally induced laryngeal hemiplegia or dorsal displacement <strong>of</strong> the s<strong>of</strong>t palate. Am. J. vet. Res. 62, 659-664. Derksen, F.J., Stick, J.A., Scott, E.A., Robinson, N.E. and Slocombe, R.F. (1986) Effect <strong>of</strong> laryngeal hemiplegia and laryngoplasty on airway flow mechanics in exercising horses. Am. J. vet. Res. 47, 16-20. Franklin, S.H., Usmar, S.G., Lane, J.G., Shuttleworth, J. and Burn, J.F. (2003) Spectral analysis <strong>of</strong> respiratory noise in horses with upper airway disorders. Equine vet. J. 35, 264-268. Hawkins, J.F., Tulleners, E.P., Ross, M.W., Evans, L.H. and Raker, C.W. (1997) Laryngoplasty with or without ventriculectomy for treatment <strong>of</strong> left laryngeal hemiplegia in 230 racehorses. Vet. Surg. 26, 484-491. Kidd, J.A. and Slone, D.E. (2002) Treatment <strong>of</strong> laryngeal hemiplegia in horses by prosthetic laryngoplasty, ventriculectomy and vocal cordectomy. Vet. Rec. 150, 481-484. Russell, A.P. and Slone, D.E. (1994) Performance analysis after prosthetic laryngoplasty and bilateral ventriculectomy for laryngeal hemiplegia in horses: 70 cases (1986-1991). J. Am. vet. med. Ass. 204, 1235-1241. Shappell, K.K., Derksen, F.J., Stick, J.A. and Robinson, N.E. (1988) Effects <strong>of</strong> ventriculectomy, prosthetic laryngoplasty, and exercise on upper airway function in horses with induced left laryngeal hemiplegia. Am. J. vet. Res. 49, 1760-1765. Tetens, J., Derksen, F.J., Stick, J.A., Lloyd, J.W. and Robinson, N.E. (1996) Efficacy <strong>of</strong> prosthetic laryngoplasty with and without bilateral ventriculocordectomy as treatments for laryngeal hemiplegia in horses. Am. J. vet. Res. 57, 1668-1673. 80
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