Evaluation of the thoraco-laryngeal reflex (‘slap test’) as an indicator of laryngeal adductor myopathy in the horse.

The research conducted sought to evaluate the effectiveness of the ‘slap test’ in diagnosing laryngeal adductor myopathy in horses. Unfortunately, the ‘slap test’ proved to be ineffective at distinguishing between healthy horses and those with moderate to severe muscular changes.

Methodology

• The study ran over a span of 12 months and involved 15 horses with no clinical signs of idiopathic laryngeal hemiplegia (ILH).
• The horses’ thoraco-laryngeal reflexes were recorded through a video-endoscope, with the resultant ‘slap test’ responses being evaluated separately by 3 different assessors.
• All of the horses were ultimately euthanized, with muscle samples from the cricoarytenoideus lateralis (CAL) extracted for histopathological examination and for evaluation of denervation atrophy.

Results

• Even though the ‘slap test’ demonstrated normal adductory responses, moderate to severe atrophy of the left cricoarytenoideus lateralis (CAL) muscles was evident in five of the horses.
• The rest of the horses presented varied degrees of adductor myopathy, which were characteristically worse on the left side of the larynx.

Findings

• The study found that the ‘slap test’ as administered in the research was incapable of differentiating between horses with moderate-to-severe muscle changes and those without such changes.
• This conclusion renders the ‘slap test’ ineffective as a diagnostic tool for adductor myopathy.
• The research also suggested that the preservation of adductor function, even with advanced histological atrophy of the muscle, may be due to the degree of reinnervation discovered in the muscles.