The suspensory ligament can be divided into three separate regions, all of which can become injured: the proximal (upper) part, the body and the branches. Proximal suspensory desmitis (PSD) or high suspensory disease, is a common injury in both the forelimbs and the hindlimbs of athletic horses and may occur in one limb or in both the forelimbs or both the hindlimbs at the same time.
Proximal suspensory desmitis in the forelimb results in a sudden onset of lameness which can be remarkably temporary, resolving within 24 hours unless the horse is worked hard. Lameness varies from mild to moderate and is rarely severe unless the lesion within the ligament is extensive. PSD in both front limbsmay result in loss of action rather than observable lameness. This occurs more commonly in racehorses, probably because of the failure to recognize earlier, subtle lameness of just one limb.
Lameness is usually worse on soft ground, especially with the affected limb on the outside of the circle. When subtle, the lameness may be more easily felt by a rider than seen by an observer. Lameness may not be apparent at working trot but may be detectable at medium or extended trot. Flexing the lower limb often temporarily worsens the lameness. It may be undesirable to work the horse hard to reproduce lameness because of the risk of worsening the injury.
Nerve Block (Local Diagnostic Analgesia)
If PSD is suspected, local diagnostic analgesia is indicated. This should result in substantial improvement in, or complete alleviation of, lameness within about 10 minutes, assuming PSD is the only cause of lameness. However, none of the different ways of blocking the proximal suspensory ligament are necessarily specific. That is, pain in other structures such as the knee or foot may also be alleviated. On the other hand, it is possible for the blocking solution to be injected into structures next to the proximal suspensory ligament. Thus, even if the lameness was not improved or alleviated with a PSD block, the lameness may still be due to this condition.
Diagnostic ultrasonography is essential to accurately diagnose PSD. The limb should be evaluated in two different planes and careful comparisons should be made to the opposite limb. High quality images are needed since lesions can be subtle and easily missed. Measurements of the ligament may be extremely valuable since, especially in acute cases, enlargement of the ligament may be the only detectable ultrasonographic abnormality. It may be difficult to evaluate an acute case accurately if the horse has had a previous injury to the suspensory ligament because the tissue may not have healed fully. Also, local diagnostic analgesia may allow air to enter the area, making visualization difficult.
The degree of ultrasonographic abnormality usually reflects the severity of the lameness. In acute cases the ultrasonographic abnormalities may be very subtle. Since they may worsen over the next 10 to 14 days, re-evaluation may be useful to confirm the diagnosis.
Other Imaging Techniques
There are usually no detectable radiographic abnormalities of the cannon bone in acute cases of PSD. In chronic cases however, certain parts of the bone may appear “whiter” or “darker” in certain views. When these secondary bony changes occur in a forelimb, a more guarded prognosis is given. Nuclear scintigraphy is generally unnecessary for diagnosis if good quality ultrasonographic images are obtained, but it may add information about secondary bony changes.
It should be kept in mind that there may be more than one source of pain contributing to lameness. For example, PSD and foot pain occur together quite commonly. There may also be hindlimb lameness occurring at the same time as PSD, especially in the opposite hindlimb, so it is important both to assess and to re-evaluate the horse as a whole.
Most cases of acute forelimb PSD respond well to stall rest and controlled walking exercise for three months. Attention to correct foot balance is important. Although starting the horse back to work too soon usually results in recurrent injury, approximately 90 percent of horses do resume full athletic function without injuring themselves again. More chronic cases may require longer rehabilitation; in a small proportion of cases lameness persists. Extracorporeal shock wave treatment has been successful in some chronic cases which had failed to respond to conservative management.
In some horses the lesions disappear completely upon follow-up ultrasonography. In others, the appearance of the suspensory ligament never returns to normal. Rest should be continued until the appearance of the ligament on ultrasound remains stable.
By Sue Dyson, FRCVS
Courtesy of AAEP