Section: Diagnosis In-depth
The diagnosis of laryngeal hemiplegia is suspected based on the characteristic respiratory noise. Palpation of the musculature surrounding the right and left sides of the larynx can also lend to a presumptive diagnosis of laryngeal hemiplegia. Atrophy of the left CAD muscle facilitates palpation of the laryngeal cartilages in comparison to the normal side. An endoscopic (fiber optic) examination of the larynx is necessary, however, to confirm the diagnosis.
The degree to which the arytenoid cartilages abduct during the exam is assessed. In quietly standing horses there will be little movement from either the right or left arytenoid cartilage. If the horse is stimulated to swallow both arytenoids can be seen to close to prevent aspiration and then open rapidly again to allow breathing. If the horse is stimulated to take a deep breath by partially holding off the airflow through the nostrils, both arytenoids should fully abduct (open to their maximum diameter). Horses will also fully abduct when they are slapped on either side of their withers (known as the slap test). Likewise during trotting, cantering, galloping, or pacing both arytenoids should fully abduct.
A grading system, consisting of four grades, exists for documenting arytenoid movement in the non – exercising horse:
Grade I – Arytenoids are in normal resting position and when stimulated to open have simultaneous complete abduction.
Grade II – Arytenoids are in normal resting position but when stimulated to open the left responds more slowly than the right but will completely abduct.
Grade III – The left arytenoid is in a more closed position than normal, opens more slowly than the right side, and does not abduct fully.
Grade IV – The left arytenoid is in a closed (adducted) position and fails to show any movement when stimulated to abduct.
Horses with Grade III function should be further evaluated either immediately following maximal exercise or during an exercise treadmill test. In the latter, the horse is asked to gallop (trot or pace if a Standardbred) on a treadmill, with the endoscope held in place to allow real-time assessment of the arytenoid function. About 80% of the horses with resting Grade III function will have normal function during exercise. In the remaining 20%, the CAD muscle is too weak to overcome the strong negative pressures created in the airway to draw in air during inspiration.
Horses with Grade IV function have complete loss of the left recurrent laryngeal nerve and CAD muscle. These horses require no further diagnostic tests.
