Stringhalt is a gait abnormality characterized by exaggerated upward flexion of the hindlimb that occurs at every stride at walk. The gait abnormality usually lessens at trot and is not evident at canter. It may occur unilaterally or bilaterally. All degrees of hyperflexion are seen, from mild, spasmodic lifting and grounding of the foot, to extreme cases in which the foot is drawn sharply up until it touches the belly and is then struck violently on the ground. In severe cases, there is atrophy of the lateral thigh muscles. In Australian stringhalt and lathyrism, the condition may be progressive and the gait abnormality may become so severe that euthanasia is warranted. Mild stringhalt may be intermittent. The signs are most obvious when the horse is sharply turned or backed. In some cases, the condition is seen only on the first few steps after moving the horse. The signs are often less intense or even absent during warmer weather. Although it is regarded as unsoundness, stringhalt may not materially hinder the horse’s ability to work, except in severe cases when the constant concussion gives rise to secondary complications. However, the condition may make the horse unsuitable for some equestrian disciplines (eg, dressage).
The etiology is unknown, but lesions of a peripheral neuropathy have been identified in the sciatic, peroneal, and tibial nerves. Severe forms of the condition have been attributed to lathyrism (sweet pea poisoning) in the USA and possibly to flat weed intoxication in Australia.
Diagnosis is based on clinical signs but can be confirmed by electromyography. If the diagnosis is in doubt, the horse should be observed as it is backed out of the stall after hard work for 1–2 days. False stringhalt sometimes appears as a result of some temporary irritation to the lower pastern area or even a painful lesion in the foot.
When intoxication is suspected, removal to another paddock may be all that is required. Many of these cases apparently recover spontaneously. In chronic cases, tenectomy of the lateral extensor of the digit, including removal of a portion of the muscle, has given best results. Improvement may not be evident until 2–3 wk after surgery, and not all cases respond. This is not surprising, because the condition is a distal axonopathy. Other methods of treatment include large doses of thiamine and phenytoin.