“Before high-speed treadmill (HSTM) endoscopy, we did not recognize the complexity of maintaining a open airway under enormous pressure swings during inspiration and expiration (breathing in and out),” said Eric Parente, DVM, associate professor of surgery at the University of Pennsylvania’s New Bolton Center, during the In-Depth Respiratory session at the 2005 American Association of Equine Practitioners convention, held Dec. 3-7 in Seattle, Wash. “The pharynx and larynx have limited structural strength and are highly dependent on muscular strength to maintain an adequate lumen (open space or cavity in the airway). The limits of muscular strength and the potential for muscular fatigue can often result in airway collapse. HSTM endoscopy is the best method to determine the specific source of collapse.”

Parente discussed protocols for conducting an HSTM examination, which he said is best used in three cases, all of which might feature abnormal respiratory sounds:

• For horses that are suspected to have upper airway problems but don’t have visible abnormalities on endoscopic examination at rest;
• When the clinical history and resting endoscopic examination don’t agree for a diagnosis, and
• To evaluate patients who still have some problems after respiratory surgery.
Parente also reported that the following problems can be diagnosed with HSTM endoscopy. “Some of these diagnoses can often be made without HSTM endoscopy, whereas others are strictly HSTM diagnoses,” he noted.

Laryngeal hemiplegia (LH)–In this disorder, the muscles of one side of the larynx are partially or fully paralyzed because of nerve dysfunction (called recurrent laryngeal neuropathy), so they can’t fully pull the arytenoid cartilages (to which vocal cords attach) out of the airway. “While originally it was thought the majority of horses with grade III hemiplegia were able to support an adequate airway, more recent evidence with clinical patients yields evidence that most horses with a grade III hemiplegia experience dynamic arytenoid collapse under strenuous exercise,” Parente reported