Locating the source of lameness can be challenging, especially when signs are subtle. Sue Dyson FRCVS outlines the best routes to an accurate diagnosis

Lameness is the most common reason for days lost from work, for horses from all sports.

Generally, the quicker an accurate diagnosis is achieved, the more likely treatment is to be successful.

Without an accurate diagnosis, treatment cannot be appropriately targeted. So is there a best way to reach an early, correct diagnosis — and when should we turn to sophisticated and expensive imaging techniques such as scintigraphy (bone scanning), magnetic resonance imaging (MRI) or computed tomography (CT)?

Success depends on an owner first recognising that their horse has a problem. Any investigation must then be carried out by a vet with the necessary expertise and experience — not all vets are specialists in either lameness recognition or lameness diagnosis.

Just because a vet examines the horse and cannot detect lameness when it moves in straight lines or on the lunge, it does not mean that the horse is sound. If a rider can feel a problem then there probably is an underlying reason.

Investigating further with a screening bone scan, however, is highly unlikely to pinpoint the problem correctly and may well confuse matters.

There are many occasions when lameness is only apparent when the horse is ridden — even then, signs can be subtle but may cause a dramatic alteration in performance. This type of problem ideally needs to be investigated by a vet with knowledge of riding and how pain can influence a horse’s performance.

Detective work

The principles of any lameness investigation are to determine the lame limb(s) and to identify the source — or sources — of pain.

A detailed clinical examination is the fundamental start point. Besides looking at and feeling the limbs, neck and back, this involves assessing the horse while he moves under a variety of circumstances in conjunction with flexion tests.

Low-grade lameness is often accentuated under saddle. Unless this might be harmful for the horse, ridden assessment is an important part of lameness examination.

A recent development in lameness evaluation is the use of technical devices (inertial measurement units) attached to the poll and pelvis for accurate detection of asymmetry of either the fore or hindlimbs. Sometimes called a “lameness locator”, this system has the potential to detect subtle asymmetries.

While these systems can be helpful in some circumstances, potential limitations must be recognised. A horse may be symmetrically short-striding, which the technology cannot detect. Neither can the system differentiate between a head nod induced by hindlimb lameness and concurrent fore and hindlimb lameness.

Once the lame limb has been identified, nerve blocks can be used to locate the source of pain. Some vets are reluctant to perform nerve blocks on mildly lame horses, because interpretation can be challenging unless you have a very good eye for lameness. In skilled hands, however, this is a valuable diagnostic tool.

Nerve blocks can be used together with a ridden assessment — a marked change in the horse’s performance or attitude is sometimes apparent when the pain causing the lameness is abolished. Nerve blocking can sometimes be dangerous because of the horse’s temperament.

If clinical signs suggest a fracture, a bone scan may be the most appropriate way to investigate further. This may also be helpful to detect bone injury if X-rays of a region known to be the source of pain are negative.

Hi-tech imaging options

With the source of pain identified by nerve blocks, the next goal is to establish the cause of the pain. This requires diagnostic imaging.

The standard of X-rays has risen enormously with digital technology and this remains the number one technique for investigation of bone injuries and identification of osteoarthritis. Yet interpretation of X-rays is not always straightforward.

Whereas very early X-rays may be negative, a bone scan may at that stage identify a “hot spot” indicative of bone injury. Some bone injuries never develop changes visible on X-ray. And while X-rays may show signs of arthritis, the joint is not necessarily the source of pain.

Ultrasonography remains invaluable for assessment of soft tissues including tendons, tendon sheaths, ligaments, muscles and joint capsules.

An X-ray can give information about the presence of soft tissue swelling but not the nature of that swelling, so a combination of both methods may yield the most accurate answer.

If X-rays and ultrasonography are negative, the use of MRI or CT may be indicated in some cases.

MRI provides images of both soft tissues and bones, allowing us to identify some types of damage in bone that cannot be detected by either X-ray or CT. A particular area must be targeted, however, as image acquisition by MRI is time-consuming and expensive.

Both MRI and CT are particularly valuable for foot-related lameness, because certain soft tissue structures within the hoof capsule cannot be assessed using ultrasonography.

There is not necessarily a correlation between MRI findings and the degree of lameness, however, and not all abnormalities cause pain. As with all imaging techniques, anomalous results may reflect the way in which the image was acquired or the presence of adjacent blood vessels, resulting in alteration in signal intensity that may mimic a genuine injury.

Again, correct interpretation of the results — by an experienced human eye — remains key.

Ref: H&H 8 January, 2015