There Are so many Reported Abnormalities of the Larynx, Please Explain!

The larynx acts as a conduit between the pharynx and the trachea and has three main functions: breathing, deglutition and phonation. Abnormalities of the larynx are common and differentiating between diseases and interpreting their clinical significance can be difficult.

The most commonly used diagnostic tests for the differentiation of laryngeal abnormalities are resting and exercising endoscopy. More recently, transcutaneous and transoesophageal ultrasound have been used, and occasionally, computed tomography.

The variability, interaction and pitfalls of these diagnostic tests will be discussed.

Recurrent laryngeal neuropathy (RLN) is the most common laryngeal abnormality in the horse, and is a major cause of poor athletic performance. RLN is a distal axonopathy that predominantly affects the left recurrent laryngeal nerve [1]. RLN causes axonal loss and demyelination and subsequent loss of cricoarytenoideus dorsalis (CAD) muscle volume. This results in a loss of arytenoid abduction particularly during fast exercise. Several studies have identified RLN-associated neuropathological changes in both recurrent laryngeal nerves in Thoroughbred horses that are clinically unaffected, indicating that most horses are subclinical cases [2-6].

Resting endoscopy remains the most used diagnostic modality for horses with RLN. It is sensitive and highly specific for predicting laryngeal function at exercise [7] but has several limitations. Daily variability in resting endoscopic grading of laryngeal function has been reported, suggesting that results of single-day resting endoscopic examinations should be interpreted with caution. In addition to this, resting endoscopic examination does not perfectly predict laryngeal function at exercise. The discrepancy occurs most commonly in horses assigned a resting grade 2 or 3. A small proportion of horses (11.9%) with resting grade 2 laryngeal function and most horses (84%) with resting grade 3 laryngeal function have abnormal laryngeal function at exercise [7]. Worsening subgrades within a resting grade 3 are associated with an increased risk of partial or complete collapse at exercise. In these horses, and those where exercising endoscopic examination is not feasible, laryngeal ultrasound should be performed. Transcutaneous and transoesophageal laryngeal ultrasound have been shown to be useful predictors for RLN [8] and can be used to assess CAD muscle depth, although in the author’s experience the latter is difficult to consistently perform in breeds other than Thoroughbreds. Histological examination has demonstrated that by the time there is a 20% loss of left CAD muscle volume, 85% of recurrent laryngeal nerve fibres are lost. Identification of horses prior to significant loss of muscle volume is difficult as clinical signs may be variable. Combining laryngeal ultrasound with resting endoscopy allows the practitioner to identify and treat horses early in the disease process and improve outcomes. Other less common laryngeal abnormalities such as fourth branchial arch deformities and arytenoid chondritis will be discussed.

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