Laryngeal Hemiplegia
(“Roaring)
Dr. Britta
S. Leise
Dr. Rustin
M. Moore
Laryngeal
hemiplegia, also known as roaring, is a condition of the upper respiratory
tract that causes decreased performance and respiratory noise in the
horse. This condition is most commonly
observed in the racing Thoroughbred, but may also be seen in other large breed
performance horses including Warmbloods, Draft Horses and Standardbreds. It is estimated that about 8% of large sport
horses are affected.
The term laryngeal hemiplegia can be defined as
paralysis of one side of the larynx.
The most common side affected is the left (up to 95% of the time). The majority of the problem revolves around
the arytenoid cartilage. The arytenoid
cartilage (commonly referred to as the “flapper”) is a paired structure (having
a left and right side) that allows for a maximal amount of air to enter the
trachea during abduction (seen as an outward motion of the arytenoids creating
a larger entrance to the trachea).
Abduction of the arytenoid cartilage is controlled by the recurrent
laryngeal nerve, which innervates the cricoarytenoideus dorsalis (CAD)
muscle. It is believed that the left
recurrent laryngeal nerve, which is longer than the right and encircles the
aorta, undergoes degeneration (known as distal axonopathy). Once the nerve degenerates, the CAD muscle
has decreased or no innervation. This
results in muscle atrophy and the inability of the arytenoid to abduct. Although the exact cause of this
degeneration is unknown (hence the name idiopathic laryngeal hemiplegia),
several factors have been implicated.
These include trauma to the cervical (neck) region, prior neck surgery,
perivascular (around, but not in the vein) injection of an irritating drug,
esophageal rupture, guttural pouch mycosis, organophosphate intoxication, lead
poisoning, ingestion of toxic plants, Streptococcus
equi (strangles), some diseases of the central nervous system, and thiamine
deficiency. Involvement of the right
arytenoid is usually associated with direct trauma to the nerve including
perivascular injection on the right side of the neck.
Laryngeal hemiplegia often occurs in tall (usually
greater than 15 hands) male horses, generally between the ages of 3 to 7
years. Thoroughbreds usually are
affected at younger ages (3-5 yrs) compared with draft horses that are slightly
older at presentation (4-7 yrs). Owners
or trainers often complain of exercise intolerance that has progressed over a
few weeks to months and may notice a classic whistling or roaring noise
produced during exercise. As the
condition progresses, the horse may have impaired vocal ability, altering the
sound of the whinny, and may even appear to gasp for breath after light
exercise. Veterinarians may palpate the
throat latch to feel for atrophy of the left or right CAD muscle indicated by a
more prominent muscular process on the arytenoid cartilage.
Laryngeal hemiplegia is diagnosed and graded (I-IV)
by endoscopy. Standing endoscopy will
generally provide sufficient information for diagnosis; however, dynamic
endoscopy on a high-speed treadmill may be necessary to further evaluate horses
with grade III laryngeal hemiplegia. During standing endoscopy, the ability of
the arytenoids to abduct is evaluated by nasal occlusion and by inducing swallowing. Both should result in full, even abduction
of the right and left arytenoids. If
abduction is not symmetrical then the diagnosis of laryngeal hemiparesis
(partial paralysis)/hemiplegia can be made.
Severe cases (grade IV) generally show no movement of the affected
arytenoid during inspiration or expiration. With grade III laryngeal
hemiplegia, only a slight hesitation or flutter may be noticed just prior to
abduction during inspiration. Again,
these horses may need to be evaluated with treadmill endoscopy. During the standing endoscopic exam, the
veterinarian may also perform a slap test.
The slap test evaluates the adductor (the inward motion toward the
opening of the trachea) function of the arytenoids. The test is conducted by gently slapping the upper right thorax
(near the withers) with an open hand.
This should result in a flickering inward movement of the left
arytenoid. The test is repeated on the
opposite side. If this motion does not
occur or is reduced compared with the other side a diagnosis of laryngeal
hemiplegia is suggested.
Treatment plans
vary with the severity/grade, the breed, the age and the use of the individual
affected with laryngeal hemiplegia.
There are four treatment options for this condition. The first and most common treatment is
prosthetic laryngoplasty (also known as a tieback). An incision is made in the throat latch area and two strands of
suture (the prostheses) are place from the muscular process of the arytenoid
cartilage to the cricoid cartilage (another structure located behind the
arytenoids that help make up the equine larynx). The suture is placed to mimic the pull of the CAD that has
atrophied, resulting in the affected arytenoid to be abducted at all
times. This procedure is the treatment
of choice in all grade IV and selected grade III cases. Complications of this surgery include
failure of the suture resulting in failure to maintain abduction, infection at
the surgery site, coughing, regurgitation, aspiration of food and or dirt particles
resulting in tracheitis or aspiration pneumonia, and arytenoid chondritis
(inflammation and resulting deformation of the arytenoids). Coughing is the most common complication and
is often seen immediately following surgery; however, only 5 to 10% will
continue to cough long term. The
success of the laryngoplasty ranges from 5-95%, with the average success
between 50-70%. Although debatable, success rates for laryngoplasty may improve
if a ventriculectomy (removal of laryngeal ventricle/saccule) is also
performed. If the suture fails and
another laryngoplasty may be performed, but with repeat surgeries the success
rate decreases and chance of subsequent failure increases.
The second treatment is a ventriculectomy/cordectomy. Paralysis of the arytenoid cartilage often
results in collapse of the vocal cords and interference of the airway by the
ventricles, (which is the pocket of excess tissue between the vocal cord and
aryepiglottic fold). The ventriculectomy/cordectomy
consists of removing the lining of the ventricle and the vocal cord on the
affected side. This removal can be done
surgically through an incision under the jaw, known as a laryngotomy, or it can
be performed using an endoscopic-guided laser.
Laryngotomy incisions require general anesthesia and are left open to
heal on their own. Laser techniques are
performed with the horse standing in the stock under sedation. The laser technique is the treatment of choice
for Draft horses. Because of their
large size, they are poor candidates for general anesthesia and may have
difficult recoveries. Furthermore, it
is not necessary for them to perform at high rates of speed and this procedure
alone may improve exercise intolerance in these horses. Success rates for ventriculectomy
/cordectomy range from 5%-100%. This
technique will not stabilize the arytenoid cartilage and concurrent
laryngoplasty is required for most performance horses with exercise
intolerance. Respiratory noise,
however, has been reported to improve (success rates ranging from 10-80%) with
this procedure alone.
The third treatment for laryngeal hemiplegia is arytenoidectomy. The purpose of this procedure is to remove
the affected arytenoid to increase the cross-sectional area of the tracheal
opening. This surgery is not the first
treatment of choice and is usually reserved for those horses that have had a
failed laryngoplasty. There are three
types of arytenoidectomies classified according to which part of the arytenoid
cartilage is removed. There is the
subtotal arytenoidectomy, which removes only the body of the arytenoid leaving
the muscular process and corniculate process intact. The second type is the
partial arytenoidectomy, which removes the corniculate process and body of the
arytenoid. The last type is the total
arytenoidectomy, which removes the whole entire cartilage. Reports using a
partial arytenoidectomy showed the least percentage of performance limitation
(9%) of the three types. Complications
are least common with the subtotal and most common with the total
arytenoidectomy. These complications
include dysphagia (difficulty swallowing) and aspiration pneumonia.
The last treatment is a neuromuscular pedicle graft. Young horses with
grade III laryngeal hemiplegia are good candidates for this procedure. This surgical treatment involves
reinervation of the CAD muscles by isolating a nerve (1st cervical
nerve) from one of the neck muscles (omohyoideus muscle) and placing a branch
of that nerve in the atrophied muscle.
Reinervation requires time with reports ranging from 6 to 12
months. Grade III horses will respond
faster to reinervation than grade IV horses.
Horses that have had a previous laryngoplasty are not candidates for
this procedure due to surgical damage to the 1st cervical
nerve. Although this technique is
relatively new, reports show 50% of horses improved their performance.
Laryngeal hemiplegia is a common upper respiratory disorder; however,
there is little known about its cause.
Diagnosis is not difficult, but treatment requires some type of surgical
procedure. Treatment is based on the
use, breed and age of the horse.
Success rates vary, but are generally around 70%. Complications to the surgical procedures are
not uncommon and should be discussed with your veterinarian in detail prior to
selection of treatment.
Right laryngeal ventricle Left vocal cord Epiglottis
