What Causes the Odd Gait of ‘Sidewinding’?

A stable owner wonders what could be causing this Quarter Horse mare to move crookedly in a persistent ‘shoulder-in’ gait.

Q I recently encountered a Quarter Horse mare with a condition known as sidewinding. Many vets have seen her, but none know what caused the condition or how to cure it. The horse can walk, but not straight. On a longe line, she seems to be in a constant shoulder-in to the right. This has been going on for quite a few months. What can be causing this?


A What you’ve described sounds like a serious problem for this mare. She’s likely suffering from a neurologic disorder that’s resulted in her being unable to ambulate (walk) in a straight line.

When considering the possibility of a problem within the central nervous system (CNS), it’s helpful to understand the composition of this extremely sophisticated organ system. The CNS is broken into three main regions: the cerebral cortex (brain), responsible for emotion and general demeanor; the brainstem, which controls the function of the cranial (skull) nerves and the balance center based in the inner ear; and the spinal cord, the primary highway of communication between the brain and the rest of the body.

The spinal cord is equivalent to a major telegraph station, capable of sending information from the brain and brainstem out to the peripheral regions—such as organs and limbs—then carrying signals back to the brain and brainstem.

An important aspect of evaluating a patient with suspected neurologic disease is determining what part(s) of the CNS are affected. Based on this horse’s description, I believe she has normal mentation (thinking/state of mind), meaning her attitude is normal and she hasn’t had seizure activity. This indicates her cerebral cortex is working as it should. In addition, you don’t mention her having difficulty eating, chewing, or moving any part of her head or eyes. So it’s likely her brainstem and associated cranial nerves are also working properly. This means that the lesion causing the odd gait is probably affecting the spinal cord.

Based on the description of how she walks to the side, it’s likely she has a focal (partial, limited) neurologic lesion that’s causing an asymmetric ataxia—asymmetric because she’s worse on one side. A focal lesion in the spinal cord can cause weakness in the affected region; if a front limb is affected, it results in a drifting in the direction of this weak limb, giving the appearance of the “shoulder-in” gait you describe. More commonly we see this effect in the hind end, which results in the appearance of the hind end drifting to one side, sometimes in a dramatic way. The drifting is a result of the severe weakness of the affected side.

Based on the description and suspected severity of the lesion, I suggest this mare be examined by someone trained in the field of equine internal medicine and board certified through the American College of Veterinary Internal Medicine (ACVIM)—ideally a specialist familiar with equine neurologic disorders.

Causes of a focal lesion resulting in severe gait asymmetry include traumatic events such as a fracture that’s compressed a portion of the spinal cord. A traumatic lesion may be evaluated with ultrasound and/or radiographs. Another possibility is an infectious disease such as EPM (equine protozoal myelitis), which would require specific testing on a sample of cerebrospinal fluid obtained by a spinal tap. Less likely would be something like a mass that could cause compression in a localized region of the CNS.

An evaluation performed by an ACVIM veterinary specialist will provide you with the most accurate information possible. Then you can proceed with appropriate diagnostic testing and treatments aimed at improving the mare’s condition.

Professor and Head, Equine Internal Medicine and Surgery
Kansas State University

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