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Cricothyroid Muscle Function and Vocal Fold Stability in Horses
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Vocal fold instability and collapse sometimes occurs in horses and can cause mild to severe problems when the animals are exercised. This interesting study determines (1) if the cricothyroid muscle had respiratory-related electromyographic (EMG) activity that increased with respiratory effort and (2) if bilateral cricothyroid myotomy resulted in vocal fold instability and collapse in exercising horses.
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Seven (3 EMG; 4 cricothyroid myotomy) standardbred horses were included in this experimental study.
Three horses exercised on a treadmill at speeds corresponding to the speed that produced maximum heart rate (HRmax), 75% of maximum heart rate (HR75%max), and 50% of maximum heart rate (HR50%max) for 60 seconds at each speed while EMG activity of the cricothyroid muscle and nasopharyngeal pressures were measured.
Another 4 normal horses were exercised on the treadmill at HRmax and HR75%max for 60 seconds at each speed before and after bilateral cricothyroid myotomy. Upper airway pressures were measured and videoendoscopic examinations were performed and videotaped at each speed.
Results: Peak phasic EMG activity of the cricothyroid muscle was coincident with inspiration and increased with treadmill speed. Bilateral cricothyroid myotomy resulted in vocal fold collapse in all horses.
Mean peak inspiratory pressures were significantly more negative compared with control values at both HRmax and HR75%max.
Conclusions: Cricothyroid muscle dysfunction may be implicated in vocal fold collapse and likely causes inspiratory airway obstruction in exercising horses. Conditions compromising cricothyroid muscle function or motor innervation could result in vocal fold collapse.
Source: HOLCOMBE, SUSAN J., RODRIGUEZ, KATIE, LANE, JENNIFER & CARON, JOHN P. (2006): Cricothyroid Muscle Function and Vocal Fold Stability in Exercising Horses. In: Veterinary Surgery 35 (6), 495-500.
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