Stringhalt (Equine Reflex Hypertonia)

What is stringhalt?

Stringhalt, or equine reflex hypertonia, is a neuromuscular condition that causes a gait abnormality characterized by involuntary, exaggerated upward movement of one or both of the hindlimbs. It looks like a jerk or hop, with the affected hindlimb(s) snapped up towards the abdomen. This generally occurs with every stride at the walk, but can lessen at the trot and is usually absent at the canter. The degree of hyperflexion varies from mild to severe and is most obvious when the horse is turning sharply, backing, going down a slope, in the first few walking steps after standing still, or during gait transitions. A hopping gait may be exhibited in severe cases. Technically considered an unsoundness, some affected horses successfully remain in work without impairment, although they may not be suited for certain disciplines like dressage. Stringhalt is not a reaction to pain, so affected horses are not necessarily uncomfortable.

There are two major categories of stringhalt: acquired (Australian, plant-associated, pasture-associated, or sporadic) and idiopathic (true, classic or atypical). The acquired form results from plant (often flatweed/false dandelion, Hypochaeris radicata) toxicity, often occurs as an outbreak in horses on pasture in late summer or fall, and is usually temporary. Although it is known as Australian stringhalt, it can occur worldwide and cases have been reported in California. The idiopathic form can be caused by injury or trauma, specifically to the back, neck, or leg, and can improve once the injury is healed. There are no other identified causes for the idiopathic form, and cases typically do not resolve on their own.

What are the clinical signs of stringhalt?

Clinical signs of stringhalt can arise suddenly and include hyperflexion of one or both of the hindlimbs, especially at the walk. Signs may be mild or more severe, with the hoof lifted sharply to the belly and forcefully stomped on the ground. In these cases, the concussive forces may cause secondary injury. One hind leg may be more severely affected than the other. Cold weather, hard exercise, anxiety, or excitement can intensify clinical signs in some cases.

Acquired stringhalt affects the peripheral muscles and nerves. It usually affects both hindlimbs nearly equally and may also affect the forelimbs and neck. The affected nerve fibers of this form of stringhalt are also found in the larynx, so affected horses may exhibit abnormal vocalization due to laryngeal paralysis. With idiopathic stringhalt, a stronger effect is frequently seen in one leg and clinical signs can progress over time.

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How is stringhalt diagnosed?

Stringhalt is typically diagnosed based on clinical signs. A diagnosis of acquired stringhalt may be more apparent than the idiopathic form if evidence of the associated plants is found in pastures or hay. X-rays and/or ultrasound may be required to rule out injuries to the muscles, tendons, or hocks of the hindlimbs. Additional diagnostic tests may be utilized to rule out muscle diseases such as polysaccharide storage myopathy (PSSM) and infectious diseases such as equine protozoal myeloencephalitis (EPM). Gait analysis may be performed to distinguish stringhalt from shivers (Draper et al. 2014. Equine Vet J 47(2): 175-181).

How is stringhalt treated?

There is no definitive treatment for stringhalt. Some horses may recover spontaneously. In cases of plant poisoning or intoxication, horses should be removed from areas containing the toxic plant. Surgical resection of part of the muscle and tendon (myotenectomy) of the lateral digital extensor at the level of the hock may alleviate signs of chronic stringhalt, but the success rate is variable. Administration of muscle relaxants, anticonvulsants (phenytoin), tranquilizers (acepromazine), and other medications can be helpful in some cases, but the effects are often temporary. One study reported fewer stringhalt-like steps after repeated injections of Botox (Wijnberg et al. 2009. Equine Vet J 41:313-318).

What is the prognosis for stringhalt?

The prognosis for horses with stringhalt varies depending on a number of factors. Horses may not return to performance, but they can still have a good quality of life. Some horses respond to treatment, whereas others do not. Most horses with acquired stringhalt improve within days, but recovery can take months or years in some cases. Relapses may occur, but even severely affected horses may return to normal. Idiopathic stringhalt is usually irreversible without surgery.

Proper management of pastures, paddocks, and turnouts to prevent ingestion of toxic plants can prevent acquired stringhalt. Since other cause(s) of stringhalt are unknown, there is no way to prevent the idiopathic form.

*This article may not be reproduced without the written consent of the UC Davis Center for Equine Health. Please email requests to cehadmin@ucdavis.edu.