Duncan F. Peters, Nicole Rombach
Neck Pain and Stiffness
In the sport horse, neck pain is one of the common complaints affecting athletic performance. Finding the origin of the pain can be difficult, however, even when the external signs are easy to recognize. A number of clinical signs, including sensitivity to grooming, resistance to rider commands, stiffness, pain on manual palpation, bit or bridle problems, and poor performance, can suggest to the rider or trainer that a horse has neck pain; however, although the neck is the pain source in some horses, many of these clinical signs arise secondary to other problems, such as a subtle lameness, that can be difficult to find.
Within the neck, pain or stiffness may have acute or chronic causes, such as a forced or tense head and neck position in training, vertebral fractures and soft tissue injuries from an accident or fall, primary muscle soreness, or inflammation of cervical spinal joints and supportive soft tissue inflammation. Osteoarthritis, other bony proliferative problems, and soft tissue inflammation can also cause nerve root compression at the intervertebral foramen and result in forelimb lameness that eludes the investigator using the usual flexion tests and regional nerve blocks. The cervical region can also be the source of apparent “mouth problems.” Such horses do not engage the bit and are constantly resisting work by being “behind the bit.” One should therefore rule out dental issues and temporomandibular joint (TMJ) disease as a source of pain. Despite all these possible causes, a thorough physical examination and targeted diagnostic modalities, coupled with a complete therapeutic approach, can most often identify and effectively relieve the signs of neck or cervical pain.
Functional Anatomy, Biomechanics, and Neuromotor Control in the Cervical Portion of the Spine
The head and neck together comprise approximately 10% of the total body mass. Together with the trunk, and aided by the special mobility in the cervical joints, the head and neck provide stability for the cephalic organs of proprioception so that the horse can maintain spatial orientation during locomotion. The length, mass, and orientation of the cervical spine, size of the vertebral bodies, and orientation of the articular process articulations pose some unique problems in terms of dynamic stabilization and susceptibility to injury. The neck is a cantilevered beam that supports the head, and exercise imposes substantial biomechanical demands for stabilization needed to counteract the increased forces imposed by athletic activity.
The horse has seven cervical vertebrae. The first two (the atlas and axis) are atypical in shape, compared with the remaining vertebrae. The third through seventh cervical vertebrae (C3 to C7) are uniform in appearance, having large oval articular processes and roughened surfaces that allow muscular and ligamentous attachment. The morphology of the vertebral bodies does not differ from C3 to C7, but the shape and size of the transverse processes is different in C5, C6, and C7. The vertebrae are articulated to form an S-shaped curvature, with a kyphotic and lordotic shape in the cranial and caudal portions of the neck, respectively. The cervicothoracic junction forms a hinge that facilitates movements of the entire neck. Adjacent cervical vertebrae articulate in three places: at the intercentral joint and at a single pair of articular facet joints. The latter connect the caudal articular process of the more cranial vertebra with the cranial articular process of the more caudal vertebra. Each of the articular process articulations, which consist of two opposing and incongruent articular processes, is capable of a certain range of motion in flexion, extension, lateral bending, and axial rotation and in transverse shearing, longitudinal compression, and vertical shearing. Changes in the shape of the neck are created through summation of the independent motions of individual cervical intervertebral joints.
In the standing horse, flexion, extension, and lateral bending are greatest in the more mobile joints of the cranial and caudal parts of the cervical spine, with only moderate motion observed in the midcervical region. The atlantooccipital (AO) joint accounts for 32% of the dorsoventral flexion and extension ranges and also allows a substantial degree of lateral bending and gliding. The atlantoaxial (AA) joint is responsible for 77% of axial rotation. Lateral bending is uniform along the length of the cervical spine caudal to the AA joint (C1 to C2). During locomotion, the effects of gravity and inertia on the cervical spine are opposed by a combination of passive (ligamentum nuchae) and active (muscular) support. These forces result in distinct oscillations of the neck and dorsoventral displacements of the sternum that can be observed in the sound horse at all gaits. When the sound horse walks, there are two longitudinal oscillations per full stride; in the trot, there are two vertical oscillations per full stride in addition to the two longitudinal oscillations; and in the canter, there is one downward–upward rotation of the cranial part of the neck per full stride. These oscillations can be altered in the presence of disease.
Muscles of the neck have two functions. The more superficial m semispinalis capitis and m splenius provide antigravitational support and aid in raising the head and the neck during locomotion. The deeper m multifidus and m longus colli are perivertebral muscles. Atrophy of the latter two muscles is associated with neck pain in humans. The fascicles of these perivertebral muscles span up to three vertebrae and have direct attachments onto the articular process articulations (m multifidus) and onto the transverse processes in the cervical and intervertebral disks and medial costovertebral joints in the cervicothoracic junction (m longus colli). This structure is consistent with provision of dynamic segmental stability and support in the equine cervical and cranial thoracic spine segments. Dysfunction of m multifidus and m longus colli may be associated with instability, dysfunction, and pain as well as proprioceptive deficits and compromised neuromotor control in the cervical spine. In humans, it has been proposed that suboptimal activation or atrophy of m multifidus cervicis and m longus colli is associated with osseous degenerative lesions in the articular processes at the corresponding spinal levels.
In humans and experimental animals, neuromotor control, proprioception, and articular stability are affected by dysfunction of the stabilizing paravertebral musculature. In the horse, m multifidus cervicis and m longus colli both have the largest cross-sectional area in the midcervical region between C3 and C5, which is also the area with the highest prevalence of the most severe grades of osseous degenerative lesions in the articular process articulations.
Osseous lesions of the cervical part of the spine are equally distributed between the left and right sides and are more prevalent in older and larger horses. On the basis of joint morphology, muscular architecture, and biomechanical properties of motion in the horse’s cervical spine, it has been proposed that osseous degenerative lesions in the cervical vertebrae are not the start of a problem but instead represent an end stage of slow-onset joint failure secondary to inadequate neuromuscular control of the perivertebral muscles. In humans with some types of neck pain, specifically designed therapeutic exercises are suggested to strengthen the perivertebral musculature to provide increased intervertebral stability and slow the process of osseous degeneration. The relationship between neuromotor control of the deep perivertebral musculature and osseous degenerative lesions in the equine cervical spine is worthy of further investigation.
Clinical Evaluation and Diagnosis
Clinical diagnosis of cervical pain necessitates observation and examination of the whole horse. Mild neck pain or stiffness often arises secondary to other conditions affecting various systems of the horse. The veterinarian should definitely rule out lower limb pain, which may affect the carriage of the horse and can result in soreness of the neck and shoulder musculature. Chronic forelimb soreness or lameness may contribute to neck pain in the performance horse because, in attempting to protect the forelimbs, the horse creates rigidity of the neck muscles. Poorly fitting tack and equipment, mouth pain originating from dental disease or oral cavity ulcers, and poorly adjusted bits and bridles can be a source of discomfort, which may lead to positional preferences and protective postures that end in cervical pain. Equine gastric ulceration syndrome and other sources of abdominal pain also may lead to body postural changes and associated neck stiffness or pain.
Observation
Examination of the horse’s neck should include a combination of observation and palpation techniques, coupled with movement tests to assess range of motion and discomfort. Initially, the horse should be observed in hand, or in the stall, so that posture and position of the head and neck can be observed. The horse should appear at ease and flexible in its movements of the head and neck from side to side and in a dorsoventral plane. The latter movement should include a rotational component that complements and smooths lateral bending. The surface of the neck over the muscles should appear relaxed and soft. In response to nearby activity or noise, most horses will freely turn their head and neck to fully observe that activity. Any indication of neck rigidity, stiffness, or staged or jerky movement should be noted. Musculature that seems taut or bunched may be an indication of a protective posture. Unusual patches of perspiration may be indicative of some underlying nerve function damage and may need further exploration. By observing the horse in its attempts to remove annoying stimuli (e.g., flying insects), one can determine whether there are any limitations in movement of the head and neck or difficulty in maintaining balance of the body during the movement.
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