Page 779 - Adams and Stashak's Lameness in Horses, 7th Edition

Lameness of the Proximal Limb 745

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B C
A

Figure 5.151. Characteristic action of the hindlimb of a horse
affected with fibrotic myopathy. The limb jerks backward and
downward (dotted outline of the affected foot) during the last 3–5 in.
of the stride, resulting in slapping of the foot on the ground. If the
gracilis muscle is affected, the swing of the hindlimb is lateral to
medial instead.

immediately above it (Figure 5.150). Most often, the main
Figure 5.150. Drawing depicting the muscles in the fibrotic area area of fibrosis can be felt over the semitendinosus.
in the gaskin of a horse affected with fibrotic myopathy. (A) Occasionally, the lesion is deep and difficult to palpate, or
Semitendinosus. (B) Semimembranosus. (C) Biceps femoris. it may be in the medial gaskin affecting the gracilis muscle.
In the chronic stages, no pain is present on palpation.
Fibrotic myopathy demonstrates a gait abnormality
must be considered in cases such as western perfor- characterized by a slapping down of the foot at the end
mance horses or those that involve any other repetitive of the cranial phase of the stride (Figure 5.151). Usually
situation. As the injury heals, fibrosis of the injured mus- the lameness is most noticeable when the horse walks
cle as well as adhesions to surrounding musculature par- and is diminished when the horse trots. The character of
tially inhibits the normal action of the muscles, causing the gait of a semitendinosus‐affected horse has been
a shortened cranial phase of the stride (inability to described in detail. In the cranial phase of the stride,
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extend the limb forward due to lack of muscular com- the foot of the affected hindlimb is suddenly pulled
pliance). In some cases, the fibrosis will become ossified caudally 3–5 in. just before contacting the ground
and is believed to be a more severe progression. (Figure 5.151). The affected hindlimb approaches the
The etiology of the congenital form is unknown, ground more vertically than normal and strikes it toe
although parturient muscle trauma has been described to first, making more of a slap than the normal heel–toe
cause rupture of the gastrocnemius muscle. It is conceiv- manner of landing. Ultimately, the cranial phase of the
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able that similar trauma could also affect the semitendi- stride is shortened with the caudal phase lengthened. If
nosus under different stresses at birth or shortly after. the semitendinosus is more affected, the slap is cranial to
Three cases of fibrotic myopathy accompanied by caudal. If the gracilis is more affected, the slap is from
neurogenic atrophy of the affected muscles have been lateral to medial. Some have reported a semiflexed
reported. All three underwent necropsy, where periph- stance at rest with the toe touching the ground. 13,31
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eral nerve and muscle degeneration were discovered.
One horse had a fracture of the greater trochanter and
accompanying sciatic nerve impingement. No gross Diagnosis
lesions were found in the other 2 horses. In chronic cases, the diagnosis is straightforward as it
is based upon the altered gait and palpation of a hard-
Clinical Signs ened area on the caudal or caudomedial surface of the
limb proximal to the level of the stifle. In making the
In the acute stages, generalized swelling and sensitivity diagnosis, stringhalt should also be considered. In string-
to palpation of the gaskin area are present. As the injury halt, the limb is pulled sharply toward the abdomen dur-
starts to heal, an area of firmness with a variable pain ing the cranial phase of the stride, while in fibrotic
response can be palpated over the affected muscles on the myopathy the foot is pulled toward the ground in a cau-
caudal surface of the limb at the level of the stifle joint and dal direction just before the foot contacts the ground.