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Sound Analysis of Upper Airway Abnormalities Determines My Choice of Surgery for Laryngeal Hemiplegia
J.A. Stick
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Why Are We Interested in Recording and Evaluating Respiratory Sounds in Exercising Horses?
Soundness of wind has been a requirement of athletic horses for centuries. Approximately 14% of horses with poor performance have upper airway disorders. Initially, 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. How well can one hear respiratory noises in exercising horses? Not very well! Even the jockey or driver, who is nearest to the source of the noise most of the time, does not hear respiratory sounds well because the noise is obscured by other sounds associated with exercise such as wind noise and foot fall. A listener standing near the track gets a snapshot at best. Therefore, we and others have developed techniques to record and analyze respiratory sounds in exercising horses.1,2
How Does Endoscopy Compare to Respiratory Sound Analysis in Diagnosing Upper Airway Conditions?
The fiber-optic endoscope has given veterinarians the ability to clearly view upper airway lesions. However, because of the dynamic nature of these lesions, upper airway endoscopy in the standing horse is often insufficient, and videoendoscopy during exercise is necessary to make a diagnosis. Videoendoscopy during exercise requires a high-speed treadmill or a remote videoendoscope in ridden horses. However, neither of these techniques perfectly reproduce the conditions under which a horse competes. Additionally, videoendoscopy during exercise is not only cumbersome and expensive; it also may not yield an accurate diagnosis because of the artificial conditions under which the horse is exercised. In contrast, respiratory sounds can be recorded under field conditions with the horse exercising in its natural environment. In summary, endoscopy and respiratory sound analysis are often complementary, each helping to better interpret findings of the other.
Are Respiratory Sounds Characteristic for Specific Upper Airway Conditions?
If each specific upper airway condition is associated with a characteristic sound, sound analysis could yield a definitive diagnosis. Equine clinicians and horse handlers have observed that conditions such as Recurrent Laryngeal Neuropathy (RLN) and Dorsal Displacement of the Soft Palate are each associated with a characteristic noise. It has been suggested that these sounds differ from those associated with other upper airway conditions, but there are few studies in the literature to quantitatively evaluate characteristics of these sounds. Recently, we have conducted studies to begin to answer this question.
How Effective are Surgical Procedures in Reducing Respiratory Noise Caused by Upper Airway Conditions?
For sport horses, respiratory noise caused by conditions such as RLN can be more important than the obstruction itself. There is information in the literature describing the efficacy of surgical procedures in improving upper airway flow mechanics in exercising horses with RLN, but until recently, there was little information about noise reduction. This information is also important for race horses as residual respiratory noise after surgery is often interpreted as failure to improve upper airway flow mechanics. Presently the relationship between airway obstruction and noise following surgery is ambiguous.
Surgical Procedures and Noise Reduction in Horses with RLN
When selecting from the many surgical options for treating RLN, veterinary surgeons must weigh the ability of the surgical procedure to improve upper airway function and/or reduce the associated respiratory noise, while minimizing the likelihood and severity of postoperative complications. Thus, the surgical procedures selected will vary depending on the intended use of the horse after surgery, the expectations of its owners regarding noise reduction, and the risk tolerance of the owner for postoperative complications.
Surgical procedures used to treat RLN include prosthetic laryngoplasty, and various combinations of laryngeal ventricle and vocal cord excision, often combined with laryngoplasty, and nerve-muscle pedicle grafting. The source of the RLN-associated noise is not clearly understood. Therefore, the efficacy of surgical procedures to reduce the noise is difficult to predict. Several recent studies have addressed this issue.
In one study, bilateral ventriculocordectomy effectively reduced respiratory noise in horses with experimentally induced laryngeal hemiplegia.3 Respiratory noises associated with the condition were almost completely eliminated, although spectrum analysis revealed some residual abnormal respiratory sounds. Up to 90 days was required before the effect of surgery on noise reduction was maximized.
In recent years, endoscopically guided laser surgery has become popular in the upper airway. The advantage of this technique is that general anesthesia is not required, thus reducing the risk to the horse and the cost to the owner. In a recent study, we evaluated unilateral laser cordectomy, and concluded that the procedure does not effectively reduce upper airway noise in horses with experimentally induced laryngeal hemiplegia.4 Unilateral laser cordectomy results in removal of the left vocal cord, but not the laryngeal ventricle. This study suggests therefore that the laryngeal ventricle rather then the vocal cord may be the source of LH-associated noise.
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1. Wagner PC, Bagby GW, Grant BD, et al. Surgical stabilization of the equine cervical spine. Vet Surg 1979;8:7-12.
2. Papageorges M, Gavin PR, Sande RD, et al. Radiographic and myelographic examination of the cervical vertebral column in 306 ataxic horses. Vet Radiol 1987;28:53-59.
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