Hind End Hitch
While there are any number of ailments and injuries that can cause a horse to “go off” behind, some neurological and mechanical problems create specific abnormalities of movement you can learn to distinguish – if you know what to look for. The following guide will help you recognize the abnormalities caused by three ailments often mistaken for one another: upward fixation of the patella (UFP), shivers, and stringhalt. I’ve even included video links so that you can check out the differences for yourself!
UPWARD FIXATION OF THE PATELLA
Incidence: Fairly common
Etiology: Mechanical, possibly linked to conformation in some cases
Breeds affected: Any
What it is: When in a standing position, the horse has the ability to temporarily “lock” or fixate its stifle joints, allowing the horse to stand and even sleep standing up with minimal muscular exertion. The joint locks when the patella (knee cap) slides into the upward position, and the medial patellar ligament (MPL) shifts slightly to the inside and hooks over a notch in the medial femoral trochlea, a knob-like structure located at the bottom end of the femur. The patella is then fixed in its upward position, keeping the limb extended. Fixation only becomes a problem when the locking mechanism doesn’t release properly when the limb needs to flex.
There are a variety of factors that may contribute to UFP, but one of the most common is thought to be a lack of tone or condition in the muscles and ligaments in the stifle area In the most extreme cases of UFP, the horse is completely unable to flex the affected leg without assistance. In the mildest cases of UFP, there is only a brief delay in flexion, without any actual locking of the limb. Horses with UFP may fall anywhere in between mild and severe. Locking or less severe “catching” may be intermittent.
UFP is usually present in both stifles, though symptoms are generally seen only on one side. Examination most often shows that the other side is strongly predisposed to develop the problem, and it is not unusual for the “good” side to become symptomatic after treatment of the initially affected side.
What it looks like: The most severe cases of UFP are easy to distinguish: the “locked” leg will hyperextend (stick out behind the horse), and cannot release forward. In moderate cases where the leg “catches” noticeably at times but releases on its own, the leg will remain extended behind the horse for longer than normal, then “pop” out of the locked position, often with exaggerated flexion of the hock. There may actually be an audible pop or click when this occurs. Very mild cases can easily go undetected, as there may not be any observable abnormality in the gait. What you may notice instead is that the horse is reluctant to perform movements that require it to push off with or increase flexion of the affected limb. Whatever the severity, it is quite common for signs of UFP to come and go.
Because of its variability, some cases of UFP can be difficult to diagnose. “The most common way UFP is identified is by manually pushing the patella up into the fixed position and then having the horse take a step forward,” says equine surgeon and lameness expert, Dr. Dennis Meagher, DVM, PhD, Dipl. ACVS, Professor Emeritus at UC Davis. “If the horse is normal, the patella will unlock once the horse takes a step forward. But if they are having problems with it catching intermittently, you can usually make it catch manually. In some cases, it is also possible to feel sort of a clicking or catching if you keep your hand on the area as the horse is walked forward.”
Prognosis: The prognosis in most cases of UFP is very good. Mild to moderate cases can often be improved with a program of specific exercises, and if that doesn’t work, success is often achieved with counterirritant injections or acupuncture. Severe cases or those that don’t respond to conservative treatment can usually be corrected with a simple, minimally invasive surgery known as ultrasound guided splitting of the medial patellar ligament. This surgery is a great improvement over the older method of desmotomy, which cuts the ligament entirely and has the potential for serious complications down the line.
Most often confused with: Moderate cases of UFP are often confused with stringhalt (see below), as the hyperextension of the limb can release into hyperflexion, which looks somewhat similar to the hyperflexion caused by stringhalt.
Video links: https://www.youtube.com/watch?v=A8uloBlHDZ8 (moderate)
https://www.youtube.com/watch?v=VmEMdcMyGr4 (more severe)
Incidence: Fairly uncommon
Etiology: Unknown, possibly neurological
Breeds affected: Mainly draft horses, but can occur in QHs, warmbloods, TBs
What it is: There are several theories as to what type of ailment shivers is. While it is generally categorized as a neuromuscular problem, the cause has yet to be determined. Some researchers believe neuropathy is involved, while others are exploring possibilities such as myopathy (muscle disease), genetic and metabolic disorders
What it looks like: According to a 2006 review by the AAEP, “Shivers is characterized by periodic, involuntary spasms of the muscles in the pelvic region, pelvic limbs, and tail. Mildly affected horses show a tenseness or trembling of the hind limbs and sudden jerky extensor movements of the tail that cause it to elevate. The degree of tail elevation varies considerably in different cases.” In more severe cases, backing up causes the hind limb to raise suddenly upwards and to the side, and the hoof is held in the air for up to several minutes. The raised limb will tremble, and the tail will usually elevate and tremble as well. The horse may exhibit symptoms in one or both legs. More rarely, shivers can affect the front legs, neck, trunk or facial muscles.
Prognosis: Horses with mild cases of shivers may perform well for years, though the disease is typically progressive, with spasms occurring more often and with greater severity over time. Muscle atrophy and weakness of the hind end is an almost universal consequence as the disease progresses. There is no known treatment or cure, though anecdotal evidence suggests that a low-carb, high fat diet may help some horses.
Most often confused with: The sudden raising of the limb caused by shivers can resemble stringhalt (see below), but horses with stringhalt do not hold the limb in suspension as horses with shivers do. Stringhalt also does not affect that tail, which is a characteristic sign of shivers.
Video link: https://www.youtube.com/watch?v=2l90K9NoZ6s (moderate)
Incidence: Rare in North America, very rare in Canada
Etiology: Neurological, some cases thought to be caused by ingestion of toxic plants
Breeds affected: Any
What it is: Explains Andrea Finnen DVM, DES, MSc, Diplomate ACVIM, a specialist in equine neurology at the Caledon Equine Hospital in Ontario, “'Stringhalt' is the lay term for equine reflex hypertonia. There are two forms of the disease; idiopathic and acquired. In idiopathic disease, signs are usually asymmetric with one hind limb affected only whereas in acquired (also known as Australian stringhalt) disease, both hind limbs are affected and a toxic plant etiology is suspected.”
What it looks like: Stringhalt is characterized by spasmodic, rapid and excessive flexion (lifting) of one or both hind limbs when attempting to move, most often seen at the walk, when turning or when backing. Says Finnen, “The flexion can be so severe that the horse's hoof hits the abdomen.” In mild cases or in the early stages of disease, symptoms may be hard to spot. Severely affected horses, however, will exhibit obviously abnormal movement, with the legs jerking upwards hard and fast, and possibly a plunging or hopping gait. Hind limb muscle atrophy is present in some cases, and laryngeal paralysis (roaring) is sometimes an accompanying symptom in Australian stringhalt. Symptoms may appear and disappear intermittently.
Prognosis: Some cases of stringhalt will resolve spontaneously, but others will progress over time. “It is difficult to predict if a horse will respond to treatment and what the prognosis is,” says Finnen. “Some cases will take a long time to recover and may never regain 100% function. There are reports of clinical improvement with conservative treatment such as anti-inflammatory and anti-oxidant medications. In severe cases, myotenectomy can be performed to relieve the tension on the flexor tendon to prevent the mechanical movement of the limb.”
Most often confused with: The hyperflexion that is the characteristic symptom of stringhalt is most often confused with shivers, but can also be taken for UFP, especially in less severe instances.
Video link: https://www.youtube.com/watch?v=Cii3DVIxp7w