Understanding Stringhalt (Equine Reflex Hypertonia)

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. 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.

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.

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