Proceedings of a Workshop on - The Havemeyer Foundation

Havemeyer Foundation Monograph Series No. 11
NON-RLN UPPER RESPIRATORY TRACT DISORDERS
FOUND IN A SURVEY OF 3,497 THOROUGHBRED
YEARLINGS
J. G. Lane
Department <strong>of</strong> Clinical Veterinary Science, University <strong>of</strong> Bristol, Langford House, Langford, Bristol,
BS40 5DU, UK
Surveys <strong>of</strong> structural and functional abnormalities
<strong>of</strong> the upper respiratory tract <strong>of</strong> the horse have
previously been based upon selected populations,
for example, those submitted to public sales (Lane
et al. 1987) or in training (Pascoe et al. 1981;
Raphael 1982; Baker 1983; Sweeney et al. 1991),
where unsaleable or untrainable individuals will
have been excluded earlier. In addition, no survey
<strong>of</strong> dynamic dysfunctional conditions, occurring
only under exercise, has been attempted to date.
Thus, it is generally held that dorsal displacement
<strong>of</strong> the s<strong>of</strong>t palate (DDSP) and recurrent laryngeal
neuropathy (RLN) are the 2 most frequently
encountered disorders <strong>of</strong> the region. The
prevalence <strong>of</strong> non-RLN upper respiratory tract
(URT) disorders found during the videoendoscopic
study <strong>of</strong> 3,497 yearlings was as
follows:
Fourth branchial
arch defects (4-BAD) 7 cases 0.20%
Sub-epiglottal cyst (SEC) 5 cases 0.14%
Epiglottal entrapment (EE) 2 cases 0.06%
Right laryngeal
malfunction 5 cases 0.14%
Hast (1972) has described the development <strong>of</strong> the
larynx in the early human embryo and showed that
the extrinsic structures are derived from the fourth
branchial arch and the intrinsic structures from the
sixth arch. 4-BAD is a syndrome <strong>of</strong> irreparable
congenital defects resulting from a failure <strong>of</strong>
development <strong>of</strong> some or all <strong>of</strong> the derivatives <strong>of</strong> the
fourth branchial arch (Lane 1993). The structures
involved are the wings <strong>of</strong> the thyroid cartilage, the
cricothyroid articulation, the cricothyroideus
muscles and the cricopharyngeal sphincter
muscles. Any permutation <strong>of</strong> aplasia or hypoplasia
<strong>of</strong> these structures may arise uni- or bilaterally.
The condition has been identified in other
breeds such as the Hanovarian, warmbloods,
Welsh Section A ponies and the Haflinger (Cook
1974; Goulden et al. 1976; Wilson et al. 1986;
Deegan and Klein et al. 1987; Klein et al. 1989;
Dixon et al. 1993). Generally, has been reported
under titles such as rostral displacement <strong>of</strong> the
palato-pharyngeal arch (RDPA) or
cricopharyngeal-laryngeal dysplasia.
The author has reviewed the findings in 60 4-
BAD-afflicted Thoroughbred horses and the
results are summarised here. The presenting signs
<strong>of</strong> horses with 4-BAD are variable and reflect the
severity <strong>of</strong> the absence <strong>of</strong> the structures involved.
However, in order <strong>of</strong> frequency, the signs are
abnormal respiratory sounds at exercise (50 out <strong>of</strong>
the 60 horses), belching (13), nasal discharge (10),
coughing (10) and recurrent colic (5). The
involuntary aerophagia and eructation sometimes
may be confused with the noises produced by
‘wind-suckers’.
There is no current evidence that the syndrome
is genetically transmitted. A complete evaluation
<strong>of</strong> the extent <strong>of</strong> 4-BAD can only be made at
exploratory surgery or autopsy but the combined
findings <strong>of</strong> palpation, endoscopy and radiography
are generally sufficient to justify a diagnosis.
When the cartilage components are defective, an
unusually wide gap can be palpated between the
caudal margin <strong>of</strong> the thyroid and the rostral edge
<strong>of</strong> the cricoid, whereas in the normal larynx the 2
structures overlap. The 2 endoscopic features to
alert the clinician to the possibility <strong>of</strong> 4-BAD are
RDPA (33 cases) where the caudal pillars <strong>of</strong> the
s<strong>of</strong>t palate form a cowl, which partly obscures the
corniculate processes dorsally and defective
arytenoid motility (45 cases). 4-BAD is the most
common explanation for apparent right sided RLN
(Tulleners et al. 1996) and there is a marked over-
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