Sidewinder is a term describing a neurologic older horse with a sudden alteration in gait that causes the hindquarters to list to one side when the horse walks.
Dr. Amy Johnson (New Bolton Center) has seen horses with this problem, but it is not common, and can be challenging to diagnose, partly because there has been little research on this condition.
"Syndrome" is a just description because the condition is a symptom, not a disease in itself. Many things could result in a horse having this type of gait. Dr. Johnson notes that, “In my experience this syndrome is much more common in older horses (late teens or older) than young horses, and usually has a sudden onset. The horse seems normal one day and the next day is walking abnormally,” she says.
Some people call these horses "crab walkers" or "side walkers". Hind legs move on a different track than the front legs—offset to one side or the other. “When you watch from behind, the front legs are moving straight forward and the horse is walking on three tracks. If the hind end is drifting to the left, the left hind foot will be moving out to the side and the right hind will be in line with the left front. The right front will be on a third track,” says Johnson.
When the horse is standing, it tends to weight one hind leg more than the other. If the hind end is offset to the left, the left hind leg would be held away from the body and resting, with most of the weight on the right hind—which is positioned more centrally.
Sometimes these horses spin in a circle when they try to walk, and may try to lean against a wall or fence to support that weak side. Hind leg locomotion is affected, which is different from some of the other situations that might cause ataxia in a horse (like a neck problem that affects all four legs).
The veterinarian will check for EPM because it is one of the causes of this side-walking syndrome. EPM can cause different signs because it can affect the brain, the spinal cord, or different areas of the spinal cord. “If it affects the spinal cord in the thoracolumbar region (between the front legs and back legs) it would not affect the front legs but could cause this abnormal gait in the hind legs,” Johnson explains.
It is important to do tests to rule in or rule out some possibilities. “The cause of the side-walking may be neurologic or musculoskeletal. I’ve seen more cases due to neurologic, spinal cord problems but have also seen a fair number due to musculoskeletal problems--usually involving the pelvis or the hind legs. Things like pelvic fractures or hip arthritis can cause this odd gait,” she says.
Sometimes we wonder whether a fracture in an older horse is cause or effect. Perhaps they fell because they have this abnormal gait, and fracture the pelvis. Or did they break the pelvis and then it causes them to walk like this?” Johnson says.
"Arthritis in the vertebral column in the back region, or intervertebral disk degeneration may be a cause—similar to a degenerative back arthritis or bulging disks in a person. “I sometimes make that comparison to explain to clients; the person with a bad back wakes up one morning with severe muscle spasms when they get out of bed, and maybe they just slept in a position that causes a problem. Then they walk around all day kind of crooked to one side,” she says.
She has seen horses that developed this syndrome acutely, suddenly walking with the hind end drifting to one side consistently and the gait stays like that for a while. Then they seem to improve and recover almost back to normal, and then the abnormal gait may return.
When it comes back, sometimes the hind end is drifting to the same direction as it did before, and sometimes it swaps. “I wonder, in some of these cases, if it is not a spinal cord lesion that is permanent, but rather some degenerative old-age changes that create intermittent muscle spasms on one side,” says Johnson.
“Unfortunately, many of these cases are old horses that have other problems as well, and end up being euthanized. Sometimes even when we do post mortem exams, we can’t figure out what the actual cause was. Other times, we see multiple old-age changes in the vertebral column but not necessarily anything in the spinal cord that explains the dramatically abnormal gait,” she says.
In her geographic region there are more cases in winter and she wonders if it’s because cold weather aggravates arthritis pain in older horses or whether the horses are slipping on ice and bad footing.
Prognosis for complete recovery is poor. Very few horses recover enough to go back to work. Some can be comfortable in pasture retirement, but it depends on the case.
“To figure out what the problem is, usually we look for EPM (with a spinal tap, if that’s feasible) and/or treat for EPM if they have a positive blood test. With some horses, it’s challenging to try to do a spinal tap because they are so unstable. We might do ultrasound or a bone scan (nuclear scintigraphy) to look for pelvic fractures. Radiographs can detect some pathology in the pelvic region but a lot of times we cannot get good views unless we put the horse under general anesthesia and that’s often the last thing you want to do with old horses with problems,” she says.
Muscle biopsies or electro diagnostics like EMGs can possibly pinpoint the source of the problem by looking for alterations in electrical signal from the muscles. Electromyography (EMG) is sometimes used to assess health of muscles and the nerve cells that control the muscles. EMG results can reveal nerve dysfunction, muscle dysfunction or problems with nerve-to-muscle signal transmissions.
“This can help us figure out the area of the spinal cord affected, if it is in fact a spinal cord disease. There are many diagnostic modalities we can utilize to look for either a primary spinal cord problem or a primary musculoskeletal problem like the pelvis. Then potentially we can address whatever it is, but sometimes we can’t, even if we know what the problems are. Sometimes the only treatment is rest—for a pelvic fracture, for instance,” explains Johnson.
Anti-inflammatory medications may be useful. “With many neurologic problems horses tend to better if you keep them moving. Some degree of physical therapy and mild exercise like turnout or hand walking is helpful for horses recovering from EPM, but sometimes the sidewinding horses do better with exercise restriction."
"If you put them on stall rest with anti-inflammatories and pain control, they get better faster than if you leave them out at pasture wandering around and reinjuring themselves or continually exacerbating the condition. It’s variable, however, from horse to horse, whether they get worse or improve with exercise,” she says.
Steroids can sometimes be helpful, but with older horses you may want to avoid the risk for endocrine problems and laminitis. “If the horse has arthritis in a certain location and you can target that joint with a steroid (rather than using it systemically), this might be beneficial,” Johnson says.
The only disease that can be specifically treated, that is commonly associated with this syndrome, is EPM. Thus it is important early on to test for EPM—because if it is EPM and you put them on treatment, there is a good chance of improving that horse. If the horse does not have EPM, you can decide how much time and money you want to invest in dealing with less-treatable causes or supporting the horse to give it more time—especially if it’s an older horse with multiple problems. You have to weigh the option of euthanasia or trying to keep him comfortable a bit longer.
Unfortunately it’s hard to predict which ones will improve. “Sometimes you just have to give them time and see. Some horses with severe signs recover temporarily. An old horse at my barn had a few good years after his first episode; he was happy and comfortable in his pasture before the problem occurred again.”
Some people report that these horses seem dull and depressed, but this may depend on what the actual cause might be. “It’s not really a brain disease but there may be a pain component that might make some horses dull,” Johnson says.