Page 663 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

638 CHAPTER 3

VetBooks.ir The precise cause of the axonopathy is not known pouch mycosis is often detected as part of an endo-
scopic examination for another presenting sign,
but a genetic pre-disposition seems highly likely.
The result is ‘die back’ of the nerve from the distal
(laryngeal) end. Large-diameter myelinated nerve notably dysphagia or epistaxis.
fibres are preferentially affected. This results in neu- Differential diagnosis
rogenic atrophy of the intrinsic musculature of the The differential diagnoses include most of the upper
larynx. The only significant laryngeal muscle not airway obstructions of the exercising horse, includ-
innervated by the recurrent laryngeal nerve is the ing axial deviation of the aryepiglotttic folds, pha-
cricothyroid muscle. ryngeal collapse, epiglottic retroflexion or arytenoid
Pathological changes are first observed in the chondritis. DDSP has some features of presentation
principal adductor muscles of the larynx such as that are similar, but has several that are distinctly
the cricoarytenoideus lateralis. These changes are different. Notably, DDSP will usually present with
seldom of clinical significance. Neurogenic atrophy a low-frequency expiratory noise, which only occurs
occurs next in the cricoarytenoideus dorsalis – the at extreme exercise.
principal abductor of the larynx. These changes are
clinically significant. Diagnosis
Listening to the horse, during exercise either on the
Clinical presentation lunge or ridden, on both reins, is essential prior to
The presentation is almost invariably of an athletic undertaking other diagnostic techniques. Adductor
horse that is performing poorly, usually associated function fails initially and therefore affected horses
with inspiratory respiratory stertor. The abnormal are often unable to close their glottis. This can be
sounds range from a soft musical whistle to a harsher detected as an audible expiration at a time when no
roaring sound. The sound usually gets louder and respiration would be expected. If the horse is threat-
longer with increased work but disappears very ened suddenly, or forcibly stimulated to contract the
quickly after the animal stops. At canter and gallop, abdominal musculature by any means, an afflicted
equine breathing is coupled to locomotion – as the horse may let air through the glottis resulting in a
back legs are driving forwards the colon and other grunt – the ‘grunt to a stick test’.
abdominal contents are driven into the diaphragm Palpation of the larynx is useful. The skin on
and the horse will exhale. As the legs are forced back the ventral aspect of the larynx should be felt and
(during the weight-bearing phase of the stride) the should be rolled gently between the fingers. A pre-
opposite occurs and inspiration results. It is possible vious laryngotomy scar can be noted as a thickened
for a horse to overcome this but the metabolic cost cord in the skin. Secondly, the lateral aspect of the
of breathing then outweighs any extra ventilation. larynx should be felt for the normal anatomical
Therefore, a rider or observer can listen and watch structures of the larynx. The lateral lamina of the
and conclude if a horse is making an inspiratory or thyroid cartilage should be palpable, overlapping
expiratory noise. Tradition has it that expiratory the cricoid and forming a complete lateral side
noise is normal and often vibrant – ‘high blowing’ – to the larynx, with the ventral portion of the ring
but that inspiration should be silent. Equally, tradi- of the cricoid palpable emerging from underneath
tion has it that an inspiratory ‘whistle’ is associated and slightly caudal to the thyroid cartilage. Finally,
with collapse of the vocal cord and a harsher ‘roar’ is with the operator facing forwards, fingers are slid
collapse of the arytenoid cartilage. Research using up underneath the sternomandibularis muscle and
sound analysis of treadmill cases has revealed that forwards to palpate the dorsal aspect of the larynx
a large number of airway obstructions give similar (Fig. 3.75). Both sides are palpated concurrently
inspiratory noises. More severe cases can present so they can be compared. Normally the muscular
with dyspnoea and even collapse. RLN following process of the arytenoid can be appreciated as a
perivascular injection often occurs in association smooth rounding of the dorsal larynx. If there is
with Horner syndrome. RLN caused by guttural extensive muscular atrophy, then the process can