Editor's note: This article is part of TheHorse.com's ongoing coverage of topics presented at the 2012 American College of Veterinary Internal Medicine Forum, held May 30 - June 2 in New Orleans, La.

While some equine gait irregularities can go unnoticed by the untrained eye, most equine movement disorders, such as shivers and stringhalt, are obvious.

At the 2012 American College of Veterinary Internal Medicine Forum, held May 30-June 2 in New Orleans, La., Stephanie Valberg, DVM, PhD, Dipl. ACVIM, ACVSMR, reviewed what researchers know about equine movement disorders and what they're still trying to understand.

Valberg, director of the University of Minnesota Equine Center, first explained that, anatomically, there are several different causes of equine movement disorders and gave examples of each:

• Abnormal muscle cell depolarization, which causes hyperkalemic periodic paralysis (HYPP) and congenital myotonia (a condition that renders muscles unable to relax immediately after contracting);
• Abnormal firing in peripheral nerves, which causes painful muscle cramps. Causes can include electrolyte imbalances and ear ticks;
• Upper motor neuron dysfunction, which causes weakness and uncoordinated movement. Equine protozoal myeloencephalitis (EPM) and cervical vertebral compression myelopathy (commonly known as wobbler syndrome) commonly lead to this disorder;
• Extrapyramidal system dysfunction, which causes specific abnormal postures and can be caused by reactions to medications, such as fluphenazine; and
• Spinal cord interneuron dysfunction, which causes abnormal regulation of muscle contraction and relaxation and can results from diseases such as tetanus and conditions such as myoclonus (involuntary twitching of a muscle or muscle group). 

But there are quite a few movement disorders that don't necessarily fit into one of the aforementioned groups, Valberg said.

"Horses have other unique forms of movement disorders that are triggered by a particular form of locomotion and are stylized and repeatable," she explained. "Stringhalt, for example, is triggered by walking forward, while shivers is triggered by walking backwards or by manually picking up the limb.

"Such movements fall into a grey area between the lameness focus of surgeons and the neurologic focus of internal medicine and remain poorly understood," she said. That said, researchers have made great strides in understanding these complicated disorders.

Researching Disorders

Seeking to gather information on affected horses from around the world, Valberg and colleagues from the University of Minnesota Neuromuscular Diagnostic Laboratory collected information from more than 200 horse owners via an online survey; many owners also submitted videos of their horses.

"Based on examination of this material, several distinct categories of movement disorders were apparent," she said, including the following categories and characterizations: -

• Head and neck dystonia (abnormal involuntary muscle movements) is characterized by resistance to manual forelimb flexion; forelimb trembling when flexed; head and neck extension and torsion with lip and eyelid movement when forelimbs are flexed; and normal movement forward and backward.
• Limb hyperflexion only occurs when the limb is flexed manually; these horses have normal movement walking forward and backward;
• Hitch is characterized by hind limb hyperflexion and a pause before placing the hoof back on the ground normally; generally occurs intermittently when walking forward, but horses move backward normally;
• Shivers is characterized by hyperflexion and hind limb trembling when backing as well as when the hind limb is lifted manually;
• Advanced shivers is characterized by hyperflexion and hind limb trembling when walking backward, hind limb hyperextension and inability to flex the limb when walking backward, and occasional upper lip tremors; these horses are incapable of flexing and holding the limb and, thus, can’t have their hind feet trimmed; and
• Stringhalt is characterized by unilateral or bilateral hind limb hyperflexion in a forward direction under the belly while walking and a rapid return of the hoof to the ground. This can occur for several or all strides walking forward and a few strides walking backward. 

Locomotion Control’s Relation to Movement Disorders

"One of the perplexing features of the locomotor-induced movement disorders is why they only occur within a specific gait," Valberg said. "For example, why is shivers only apparent in horses when they walk backwards, whereas stringhalt occurs consistently when horses are walking forwards? Shivers horses always trot normally and in both shivers and stringhalt, the locomotor-induced abnormality disappears when the horses canter.

"A potential explanation for this might lie in the neural mechanisms that control locomotion," she relayed.

To understand this theory, it's important to have at least a basic understanding of those neural mechanisms. Valberg explained the function of central pattern generators (CPG) in locomotion. CPGs are neural networks that produce rhythmic patterned outputs—essentially, they're the signals in the nervous system that regulate the timing and pattern of muscular activities such as walking, trotting, or cantering.

Valberg then shared that she and colleagues hypothesized that the CPGs that control walking backward and forward might have separate components in horses.

"Dysfunction in separate aspects of CPGs might provide an explanation for why stringhalt horses are affected when walking forward and shivers horses are affected walking backwards," she said. "Further, separate aspects of CPGs might explain why horses with both these conditions can trot and canter normally but cannot back or life a hind limb while standing."

While research has yet to confirm Valberg's hypothesis, her research group is fully engaged in identifying the basis for movement disorders in horses.

Take-Home Message

"It appears that there are many unexplored neural pathways involved in gaits that might be implicated in locomotor-induced movement disorders in horses," she said. More research is needed to fully understand the role CPGs play in equine locomotor-induced movement disorders.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.