Understanding Movement Disorders: Shivers and Stringhalt

This article originally appeared in the Summer 2025 issue of EquiManagement. Sign up here for a FREE subscription to EquiManagement’s quarterly digital or print magazine and any special issues.

At the 2024 American College of Veterinary Internal Medicine Conference, Stephanie Valberg, DVM, PhD, DACVIM, DACVSMR, formerly of Michigan State University’s Equine Neuromuscular Research Laboratory, discussed salient features of movement disorders in horses. Specifically, she clarified the differences between two often-confused ­conditions: shivers and stringhalt.

Shivers

For a horse to have a rhythmic gait, both forelimbs and hind limbs must coordinate with opposite limbs and flexor and extensor muscles simultaneously, within nanoseconds, Valberg explained. The cerebrum initiates movement patterns along with inhibitory input. Then, the cerebrum modulates the plan to coordinate a motor pattern; this plan is transmitted down the spinal cord to the flexor and extensor muscles via motor pathways to motor nerves. While forward walking occurs at the spinal level, she explained that backward walking requires anticipatory postural adjustments in the absence of visual cues. 

A horse with shivers has difficulty backing up due to hind-limb hyperflexion or extension. Most of the horse’s forward strides are normal. Valberg described two subcategories of this condition: 

Shivers hyperextension: 

• The horse backs with a stiff gait. 
• Extensors are more active than flexors in both limbs. 
• The back and torso are stiff, but the horse can walk forward and look normal in gait at a trot. 
• The horse has difficulty lifting his hind limbs and holding them in a normal position when raised.

Shivers hyperflexion: 

• When the horse backs up, one or both hind limbs have a degree of hyperflexion with a pause before the horse continues to back up. 
• Walk and trot forward are normal. 
• When a hind limb is lifted and hyperflexes, the leg often trembles. 
• When a handler asks for one hind limb, the horse often offers the other because it is difficult to pick up the requested limb. 
• The horse might slam his leg down to the ground.
• The forelimbs are easy to pick up, but the horse becomes anxious when someone moves toward the hind limbs. 
• The tail is held in ­hyperextension. 

With progression, a horse with shivers might develop intermittent abduction and hyperflexion of the hind limb as he moves forward. In contrast to horses with stringhalt, the forward trot appears relatively normal. A shivers gait abnormality is most pronounced when a horse is turning at a forward gait, going from one surface to another, or stepping over an object while his visual cues are interrupted. A shivers horse holds the limb away from the body; in contrast, a stringhalt horse adducts the limb forward under the body.

Shivers is most common in horses 16.3 hands or taller, such as warmbloods, Thoroughbreds, and draft horses. The incidence ratio is 3:1 males to females. Onset can be as early as two years but tends to be around 4-6 years of age; most horses do not develop the condition after age 10. At least half progress to more pronounced and/or bilateral deficits or have difficulty holding a foot up. Temporary triggers include stress, pain, stall rest, farrier work, movement initiation, varying footing, and feeding off the ground.

Valberg recommends performing a thorough lameness exam to rule out pain or mechanical gait disruption and a neurologic exam for sensory ataxia. Shivers horses do not have sensory ataxia but motor ataxia without proprioceptive ­abnormalities.

Researchers have studied the causes of shivers: Input from the cerebellum comes through deep cerebellar nuclei where Purkinje cell afferents synapse on nuclei to carry impulses to the cerebral cortex or down the spinal cord to motor nerves. Affected horses have significant differences in deep cerebellar nuclei, which have 100 times more spheroids (axonal swelling due to degeneration in nervous system) in shivers horses than in controls. 

Normally, the Purkinje cells’ actions inhibit motor pathways with higher input. Distal Purkinje cell axonopathy in deep cerebellar nuclei initially disrupts cerebellar input onto movement patterns that are not ingrained, said Valberg. She explained that this results in discordant muscle contractions during movement. A shivers horse has anxiety from anticipating impending muscle cramping during backing or when asked to pick up a limb. From recent studies, gene expression within Purkinje cell bodies is not affected. However, expression within axons that travel to deep cerebellar nuclei seem to be disrupted with a degree of degeneration and aberrant regeneration. Focal loss of Purkinje cells affecting backward movement does not result in a full-blown disruption—­proprioception remains, without ataxia. Purkinje cell axon degeneration within motor modules is involved in higher input for backing up. As deterioration progresses, it might also impact modules that control forward movement.

Stringhalt

Stringhalt might occur because of a peripheral neuropathy associated with disruption of the nerve fibers that deliver information to the Golgi tendon apparatus that senses stretch in a muscle to cause relaxation. The neuropathy also affects muscle spindles that sense necessary tension to maintain normal posture and assist in recruiting muscle fibers to maintain that tension. When muscle needs to increase tension, reflexes associated with a sense of stretch amplify contraction and the firing of alpha-motor nerves. Uninhibited firing in the Golgi apparatus or spindles results in an excessive reflex reaction that hyperflexes the hind limb, resulting in stringhalt. 

Valberg described the multiple types of stringhalt:

• Unipedal stringhalt is usually due to trauma, resulting in an adducted forward phase with rapid movement under the belly; the horse’s leg might even strike the belly. Walk and trot reach 78% of a normal stride; backing and standing leg lift can be variable.
• Stringhalt caused by a neurologic disorder like polyneuropathy is accompanied by muscle atrophy and weakness. A distal peripheral neuropathy, especially of the long digital extensor tendon, is often associated with No abnormalities occur in forelimbs, but 20% of affected horses develop laryngeal hemiplegia. 
• Sensory-induced stringhalt manifests when the horse walks on a hard surface. Therefore, it’s important to rule out foot pain.
• Pasture-associated stringhalt in Europe, Australia, and the western and southeastern United States is caused by a toxin, such as in flatweed (aka false dandelion), that induces a peripheral neuropathy. This plant grows during periods of drought stress, producing toxins absorbed by the peripheral nerves. Tall and young horses seem most affected; not all horses on pasture with this plant will develop stringhalt. Signs are variable, but every forward stride is usually affected, with bilateral hind-limb adduction. It might begin as a unilateral gait deficit before progressing to both hind legs. Some horses develop a bunny-hopping gait. Many recover if the offending plant is removed.
• Other forms of bilateral stringhalt might occur at the walk and trot despite no evidence of atrophy on biopsy and no neurologic findings of ataxia or peripheral neuropathy. Unlike shivers, these horses do not have standing hyperflexion but can back up normally. The cause is unknown. 

The information Valberg provided can help practitioners better define which of these syndromes they are observing in horses with movement disorders.

Related Reading

• Researchers Identify a Root Cause of Shivers in Horses
• Differentiating Noninfectious Neurologic Diseases in Horses
• Is That Horse Ataxic or Just Lame?

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