What is Stringhalt?

The rhythmic movement of a horse at trot is mesmerizing in its symmetry. When this harmonious gait goes awry, you’ll want to know why it’s happening and what it means for the horse’s athletic ability.
Stringhalt – What is it?
One such locomotor abnormality is recognized as an abrupt lifting of one or both hind legs in an exaggerated manner, particularly as a horse begins to move from a standstill, turns or backs up. This can be due to a syndrome known as stringhalt. Symptoms may be relatively mild with only a slight elevation of the hock; but they can be much more pronounced as the hind leg jerks upward as high as the abdomen, even as extreme as hitting the belly. What occurs when a horse makes this involuntary motion is that the digital extensor muscles act in excess or are unopposed by the flexor tendon muscles – the result is a lack of checks and balances on hind leg muscular movement such that the limb makes an abnormally high upward excursion. On the surface, this may appear as a muscular problem when in fact, such uncontrollable and exaggerated motions are attributable to nerve degeneration that alters regulation of changes in muscle length.

A horse displays the sudden snapping up of the hind legs characteristic of stringhalt. The movement is especially noticeable when backing up or when making tight turns.

Nerve Degeneration – Why?
What causes this nerve degeneration? There are a variety of possibilities. Ingestion of a toxic plant such as sweet peas or vetch can cause injury to the peripheral nerves (outside the central nervous system) that regulate hind limb movement in an individual horse.
Another form of stringhalt, Australian stringhalt, may cause an "outbreak” of stringhalt in multiple horses in a herd. This occurs when horses are pastured in areas with access to flatweed (Hypochoeris radica),dandelion, or little mallow (cheeseweed), particularly in conditions that cause the pasture grasses to be sparse in the midst of an abundance of weeds. Although the name implies a specific geographic location, this can occur in horses anywhere in the world. In addition to affecting the large nerves of both hind legs, flatweed also affects the nerves to a horse’s larynx so instead of a whinny, he may make a hoarse, roaring sound. Rapid recognition as to the problem may help minimize persistent damage. The faster the horses are removed from the pasture containing the noxious weeds, the more favorable the outcome. If left too long, there may be irrevocable damage to the larynx that could affect airway efficiency and thereby impact exercise tolerance and performance.
Besides consumption of toxic plants, another cause of stringhalt is created by traumatic injury – a wound or an impact – to nerves around the hock or upper part of the cannon bone. In this case, the horse usually experiences the problem only in one leg.
Classic stringhalt that affects both hind limbs may also be caused by injury to not just the peripheral nerves but due to injury within the central nervous system within the neck or thoracic spine of the back. While still a neurologic problem, the approach to managing the abnormality requires a careful diagnostic exam and addressing the particular lesions that affect hind leg motor control.
Diagnosis
An accurate diagnosis of classic stringhalt relies on a thorough orthopedic exam as well as diagnostic imaging with ultrasound and radiographs, and electromyogram (EMG) studies that measure electrical signals and nerve conduction in the muscles at rest and during contraction. Australian stringhalt is more readily apparent in its cause because of its effect on multiple horses at once within a herd.
Other nerve or muscular problems may resemble stringhalt and should be differentiated. One such syndrome that can be confused with stringhalt is fibrotic myopathy, which causes the limb to slap hard to the ground due to restricted motion from scar tissue in the hamstring muscles from a previous injury. Shivers can often occur in conjunction with stringhalt – shivers describes tremors in the upper hind leg muscles and involuntary flexion of the hind limb, usually seen in Warmblood or Draft breeds. Even polysaccharide storage myopathy (PSSM) may create hind limb motor coordination abnormalities and this can be evaluated with muscle biopsy.
Solutions
Stringhalt can be difficult to control and when it can’t be controlled, it certainly can affect a horse’s athletic career. In some cases, it may spontaneously resolve. In others, it becomes a long-standing problem. Surgical resection (cutting) of the lateral extensor muscle and tendon has had varying results. Modifying the function of the lateral extensor muscle through surgery has the effect of reducing the force with which the hind limb moves forward, thereby reducing the spastic stringhalt gait.
If the lesion is in the spine due to arthritis or disk disease, this may be manageable with vertebral joint injections and other anti-arthritic medications. Drugs that act as muscle relaxants on the central nervous system have an unwanted effect of sedation so are not particularly in the best interests of a performance athlete.
For injury to the peripheral nerves of the hind leg, there has been some recent research with the use of Botox injected under EMG guidance into the three lateral extensor muscles. The objective is to reduce the spasticity of these muscles so a horse can have more control over movement of the affected hind leg(s). Thus far, this technique is still in research phases.
The important take home message is to call for a thorough veterinary exam for a horse that is displaying signs of stringhalt. The more rapid the recognition, the greater the possibility of an accurate diagnosis so that some therapeutic approach may be possible to keep the horse engaged in active athletic pursuits.
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Nancy S. Loving, DVM, is a performance horse veterinarian based in Boulder, Colo., and is the author of All Horse Systems Go.