Proceedings of a Workshop on - The Havemeyer Foundation

Equine Recurrent Laryngeal Neuropathy
abnormal (Grade 3 and 4) laryngeal movement
grades was inaccurate; the inter-relationship
between pathological changes in the left arytenoid
muscles and the arytenoid movements considered
to be indicative <strong>of</strong> these changes is unreliable or
erroneous; in young Thoroughbred horses, changes
in grade <strong>of</strong> arytenoid movements frequently occur.
In the present study, 8 <strong>of</strong> 11 <strong>of</strong> the horses changing
from what was considered normal laryngeal
movements to abnormal, changed from Grade 1 to
Grade 3. Similarly, <strong>of</strong> those horses changing
laryngeal grade from what was considered
abnormal to normal, 10 <strong>of</strong> 14 changed from Grade
3 to Grade 1. Although the results <strong>of</strong> this study will
be affected by the repeatability <strong>of</strong> the endoscopic
procedure used it is unlikely that errors were made
in differentiating Grade 3 laryngeal movements
from Grade 1. Not withstanding the other factors
mentioned, in this age group it appears that: a)
asymmetrical laryngeal movements are common;
b) laryngeal movements may interchange between
what is considered normal and abnormal; c)
development <strong>of</strong> more obvious degrees <strong>of</strong>
asynchrony is low (12%); and d) development <strong>of</strong>
laryngeal hemiplegia in horses that have
endoscopic evidence <strong>of</strong> deficient left abductor
muscle function is also low (5%).
RECENT INVESTIGATIONS
More recently, Dixon et al. (2002) reported on
endoscopic and/or clinical progression <strong>of</strong>
recurrent laryngeal neuropathy (RLN) in older
national hunt and sport horses (predominantly
Thoroughbred). Fifty-two <strong>of</strong> the 351 horses
examined (15%) showed evidence <strong>of</strong> progression
<strong>of</strong> the degree <strong>of</strong> laryngeal dysfunction over a
median period <strong>of</strong> 12 months (range 1.5–48
months) with the onset <strong>of</strong> progression occurring at
median age <strong>of</strong> 7 years.
The results <strong>of</strong> this study and the one reported
above (involving predominantly Thoroughbreds)
indicate that the progression <strong>of</strong> RLN or
alternatively, clinically significant arytenoid
abductor dysfunction, could vary between 5 and
15%. The age <strong>of</strong> onset <strong>of</strong> the deterioration in
arytenoid function is, however, markedly different.
The reason for this is unknown. In addition, Dixon
et al. (2002) have reported that the time or rate at
which progression can develop may be as short as
6 weeks. In other cases deterioration can take
months to years. This has important implications
for examination <strong>of</strong> horses for sale and supports the
clinical impressions <strong>of</strong> veterinarians involved in
sales endoscopy.
In contrast to the above study, Dixon et al.
(2002) found no evidence <strong>of</strong> improvement in
laryngeal function in the clinical cases examined.
In another endoscopic study <strong>of</strong> the laryngeal
movements <strong>of</strong> Thoroughbreds (Lane 2000)
reported on the results <strong>of</strong> 2 examinations
conducted 12 months apart on 197 foals. At the
time <strong>of</strong> the second examination 187 yearlings
were available. Video-endoscopic records were
reviewed ‘blindly’ on 3 occasions. The results
showed marked inconsistencies in the 2 series.
The laryngeal function <strong>of</strong> some foals appeared to
be within normal limits, but appeared to have
deteriorated over the following 12 months.
Conversely, 9 foals examined initially were
observed with marked abductor deficiency (Grade
4 <strong>of</strong> 5) but when examined 12 months later one
was considered normal (Grade 2 <strong>of</strong> 5), 5 were
considered equivocal (Grade 3 <strong>of</strong> 5) and 3
remained Grade 4. While it was concluded that
endoscopy <strong>of</strong> foals is not reliable and decisions on
whether to buy or to reject horses should not be
based upon this technique, it is possible that some
<strong>of</strong> the variation in laryngeal function could be the
result <strong>of</strong> successful re-innervation <strong>of</strong> deinnervated
intrinsic laryngeal musculature.
REFERENCES
Anderson, B.H., Kannegieter, N.J., Goulden, B.E. (1997)
Endoscopic observations on laryngeal symmetry
and movements in young racing horses. N. Z. vet. J.
45, 188-192.
Baker, G.J. (1983) Laryngeal asynchrony in the horse:
Definition and significance. In: Equine Exercise
Physiology. Eds: D.H. Snow, S.G.B. Persson and
R.J. Rose RJ., pp 46-50.
Dixon, P.M., McGorum, B.C., Railton, D.I., Hawe, C.,
Tremaine, W.H., Pickles, K. and McCann, J. (2002)
Clinical and endoscopic evidence <strong>of</strong> progression <strong>of</strong>
152 cases <strong>of</strong> equine recurrent laryngeal neuropathy
(RLN). Equine vet. J. 34, 29-34.
Lane, J.G. (2000) Limitations <strong>of</strong> assessment <strong>of</strong> laryngeal
function in the young Thoroughbred. Proc.
Racehorse Pract. Conf. Newmarket, England 22nd-
24th June.
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