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The abnormal pelvic limb gait was first seen about 5 months prior to this video. The signs have waxed and waned during this period but did not disappear.
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Horse - Warmblood
Neurons located in the L4-S2 spinal cord segments, spinal nerves, spinal ganglia, femoral, sciatic, fibular, and tibial nerves, all on the right side.
A disorder here could explain the hyperactive flexor muscles of the hip, stifle, tarsus and fetlock that is observed on protraction of the limb.
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These clinical signs are typical of what is referred to as stringhalt. Although mild forms of this are seen occasionally in horses with equine motor neuron disease or equine protozoal myelitis, the stringhalt action is much less obvious than the other clinical signs typical of these disorders.
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Stringhalt.
The origin of this term is a subject of speculation. One consideration was provided by Dr. Brian Farrow of the University of Sydney, School of Veterinary Medicine. His source was from Shakespeare's Henry the VIIIth, Act one, Scene three: "They have all new legs and lame ones; one would take it, that never saw 'em pace before, the spavin or springhalt reigned among 'em". Over the years springhalt may have been converted to stringhalt.
Stringhalt is described as occurring in two forms: sporadic and epidemic.
The sporadic form occurs with a sudden onset in an individual horse, in one or both pelvic limbs unassociated with the ingestion of any known toxin. It is seen worldwide. No lesions are seen in the CNS, PNS or pelvic limb muscles at necropsy. Some horses improve after removal of the tendon of insertion of the lateral digital extensor muscle. Some improve on treatment with phenytoin. The lack of lesions and the improvement on drug therapy suggests that this form of stringhalt may be a functional disorder. This is supported anecdotally by the observation of Dr. Ryland Edwards of the University of Wisconsin. He observed a 22-year-old horse that had exhibited stringhalt for many years that disappeared when the horse developed extreme discomfort due to a digestive system disorder. When the discomfort was relieved, the stringhalt returned.
The epidemic form of stringhalt occurs in Australia and on the west coast of the United States in dry seasons when horses graze on pastures that contain an abundance of various species of dandelions (flatweed, catsear). The bilateral stringhalt occurs suddenly after many weeks of grazing. The stringhalt disappears if the affected horse is immediately removed from the pasture. Unlike what occurs in the sporatic form, those chronically affected with the epidemic form will exhibit extensive denervation atrophy of their crural muscles and a significant neuropathy of pelvic limb nerves at necropsy. The neuropathy, specifically an axonopathy, consists of selective involvement of the large diameter axons resulting in reduced nerve conduction velocities. Return of nerve conduction velocity may occur coincident with improved clinical signs.
The horse in video 1 was considered more likely to be an example of the sporadic form of stringhalt despite the presence of “hairy cat’s ear” (Hypocheris radicata) in the pasture where she grazed. This was based on the asymmetry and waxing and waning of her clinical signs, the absence of any crural muscle atrophy and the lack of recovery after being removed from the pasture.
We thank Dr. April Mitchel of the Monroe Veterinary Clinic in Monroe GA and Dr. Marc Kent of the University of GA for the use of this case and Video 1.
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