The equine neck consists of seven vertebrae, which articulate in different directions via intercentral joints and articular process or facet joints.
The greatest range of motion — extension and flexion, lateral bending and rotation — occurs between the fifth cervical and the first thoracic vertebrae. Even clinically normal horses will show X-ray abnormalities involving the articular process joints between these vertebrae, reflecting osteoarthritis.
Various muscles and ligaments attach to the neck vertebrae and influence their movement. In most horses, there is left-right symmetry between the vertebrae and the muscular attachments. Some, however, are born with asymmetry of both the bones and the soft tissues. Suffice to say, some world-class horses have had such abnormalities with no associated clinical signs.
The spinal cord lies within a central channel in each vertebra called the vertebral foramen. The cervical nerve roots, which supply the horse’s neck and forelimbs with nerves, pass through additional passages at the level of the articular process joints, known as intervertebral foramina.
Narrowing of these channels by new bone formation or soft tissue proliferation can result in abnormal pressure on nerve roots and either neurological dysfunction, such as stumbling, or pain that causes forelimb lameness. Pressure on the spinal cord can cause ataxia (wobbliness).
Taking shape
The horse’s neck is mobile and its position has a major influence over his movement. If his neck is too high, the back will be forced into extension and will hollow, so the range of motion of the back will be reduced. If his neck is too flexed and his chin is too close to his chest or sternum, forelimb step length will be shortened.
The shape of the neck and the development of its muscles reflect a horse’s nutritional history. A horse prone to obesity will have excessive fat deposits that give the neck a “cresty” appearance.
High head and neck carriage during work may lead to excessive development of the muscles on the underside of the neck and a ewe-neck appearance. A horse with a high but curved, peacock-like neck will have a concave dip in the upper contour of the neck just in front of the withers.
Forward movement of the forelimbs is, in part, mediated by the long brachiocephalicus muscles on the underside of the neck. Soreness of these muscles, which sit in front of the shoulders, is seen in association with other causes of forelimb lameness, such as front foot pain, so in this case the neck pain is secondary.
Primary unilateral (one-sided) muscle injury can occur, however, resulting in pain as the forelimb on the side of the injury is brought forwards, causing the horse to elevate his head and neck, mainly in walk.
Although many riders may incriminate neck pain as a cause of uneven rein tension or difficulties in turning the horse in a particular direction, such clinical signs are more associated with hindlimb lameness.
A horse may also adapt his posture secondary to primary forelimb lameness — turning his head and neck to the outside of a circle when on the lunge, for example, to protect the painful inside forelimb.
There is an increasing body of evidence that some forelimb lamenesses are associated with nerve root pain. An affected horse may have lameness that is only present when he is ridden, and this may be influenced by his head and neck position. Lameness may be more apparent when the horse is ridden to a contact, or evident only when worked on a loose rein.
Such horses may have a head tilt, with the nose pointing away from the direction of the lame limb. There may be a propensity to a stumble in front, or a rotational fall in the first few steps after landing over a fence.
Rarely, the horse may exhibit a posture called root signature, where he stops and holds a forelimb in a semi-flexed position, with weight only on the toe of the limb, and the fetlock and carpus (knee) partially flexed. Sweating may occur repeatedly in the same position on one side of the neck.
Alternatively, the horse may stop with his head and neck fixed in a low position and will be reluctant to move — so-called “neck locking”, which may also be associated with patchy sweating and signs of extreme pain. Although the head and neck are low, the horse may be unable to reach the ground to graze, believed to be due to transient nerve root compression and pain.
When exhibiting these signs, horses with root signature or neck locking may have intense pain when the affected area on the side of the neck is lightly touched.
Elimination process
Diagnosis of primary neck pain is based on clinical features and the elimination of other potential causes of pain. One of the unusual characteristics of forelimb lameness associated with primary neck pain is that the lameness gets substantially worse after nerve blocks of the lame limb.
Diagnosis cannot be based on radiographs (X-rays) alone, as many clinically normal horses have advanced osteoarthritic abnormalities of the articular process joints of the neck. In addition, some with clinical signs of nerve root compression have no definitive X-ray changes.
In a few clinics worldwide, it is possible to obtain three-dimensional images of the base of the neck using CT (computed tomography), which may provide additional information. In some horses, diagnosis may be verified by assessing the response to ultrasound-guided injection of the affected articular process joints on the side of the lame limb.
Nuclear (skeletal) scintigraphy or bone scanning can be misleading. There is generally greater radiopharmaceutical uptake (areas sometimes termed “hot spots”) in the bones that form the articular process joints at the base of the neck, compared with elsewhere, reflecting increased bone modelling because of their mobility. This natural process may result not only in a change in the internal architecture of a bone, but also in its shape and size.
Loss of coordination (wobbler syndrome) may also be associated with a neck problem that causes compression of the spinal cord. This is generally not a pain-related condition but reflects compromise in information being relayed via the spinal cord in the neck region to the brain and back. Ataxia may occasionally be associated with pain if there is a fracture of an articular process, secondary to pre-existing osteoarthritis.
Pinpointing the problem
A nine-year-old gelding came to the clinic after a recent onset of episodic left forelimb lameness and stumbling. It was found the muscles at the base of the horse’s neck on the left side were less well-developed than those on the right. He was abnormally reactive to palpation in this region, indicating discomfort.
The gelding adopted an abnormally base-wide stance with the hindlimbs while grazing. When pushed backwards, he placed his left forelimb in abnormal positions, and his ears and anxious eye expression were consistent with pain.
Radiography of the horse’s neck revealed marked subluxation (misalignment) of the sixth and seventh cervical vertebrae (Ce6 and Ce7). In addition, the arrows highlight extensive enlargement of the articular process joints between Ce6 and Ce7, and between Ce7 and the first thoracic vertebra (T1), indicating advanced osteoarthritis.
Unfortunately, this misalignment cannot be successfully treated, ruling out any ridden work.
Ref Horse & Hound; 7 November 2019
