GÉNEROS DISCURSIVOS

Many back problems seem to be associated with chronic and persistent injuries and multiple lesions are common in horses with back problems (Jeffcott and Haussler 2004). However, in general, there appears to be a correlation between the conformation of the horse, sex, breed or type of use and the type of injury which is sustained (Jeffcott 1979, 1980). A whole range of underlying causes can affect the vertebral column and adjacent structures. They can be separated into three major categories: primary back problems, which can be subdivided in soft tissue injuries, osseus injuries, neurologic disorders and tack related problems; secondary back problems and presumed back problems. The specific lesions and problems associated with the different categories are displayed in Table 3 (Jeffcott and Haussler 2004).

Table 3 Differential diagnoses for horses with back problems (by Jeffcott and Haussler, 2004)

Type of back problem General category Specific lesion or problem Primary back problems Soft tissue injury Longissimus muscle strain

Supraspinous ligament sprain or desmitis Dorsal sacroiliac ligament sprain or desmitis

Exertional rhabdomyolysis

Non-specific soft tissue injury

Osseous injury

Conformational or developmental abnormality

Over-riding or impinged dorsal spinous processes

Osteoarthritis (e.g. articular processes)

Vertebral fracture

Spondylosis

Diskospondylitis

Spinal neoplasia

Neurogenic disorders

Equine protozoal myeloenecephalitis (EPM)

Equine degenerative myeloencephalopathy (EDM)

Equine herpesvirus myeloencephalitis (EHV-1)

Equine motor neuron disease (EMND) Tack associated Poor saddle fit or excessive pressure Secondary back

problems Hindlimb lameness

Forelimb lameness

Neck problem

Acute sacroiliac injury

Chronic sacroiliac disease

Pelvic fracture

Presumed back

problems Bad temperament

Lack of ability (rider or horse) Lack of fitness

Improper tack fit or use

The clinical symptoms for these lesions and problems vary widely in their nature and severity. Depending on the underlying reason horses are presented to the veterinarian because of poor performance, intolerance of exercise, stiffness of the back and hindlimbs and back pain. Soft tissue injuries usually occur as a result of slips, falls or poorly performed jumps which may be caused by fatigue or inadequate fitness of the horse. These incidences can lead to uni- or bilateral strains of the epaxial muscles and ligaments. Clinical signs of muscle strain may include an acute onset of poor performance accompanied by changes of the horses temperament, localised heat and swelling. Gait abnormalities such as stiffness of the hindlimb and the back, disunited canter and frequent breaking of strides are commonly seen in horses with back pain. There can be a marked reduction of thoracolumbar flexion and obvious pain palpating this area. In some acute cases an elevated level of muscle-derived enzymes can be measured after mild exercise. Metabolic disorders of the back muscles show similar clinical signs but are usually characterized by high levels of muscle-derived enzymes (CK and AST) after exercise or characteristic levels of metabolic substances seen in longissimus dorsi muscle biopsy samples (Quiroz-Rothe et al. 2002). Back muscle tension or strain may also be diagnosed secondary to lameness. Other soft tissue injuries such as strain of spinal ligaments are usually caused by the same traumatic reasons as muscle strain. A common site of injury is the supraspinous ligament which is adherent to the summits of the thoracic and lumbar dorsal spinous processes. Clinical signs are similar to those caused by back muscle strain. However, additional signs that may be visible include swelling, pain on palpation, and reduced lateral flexion. Severe injuries to spinal ligaments are characterised by complete ligamentous disruption and joint laxity (Jeffcott and Haussler 2004).

Osseous pathology of the vertebral column and the sacroiliac articulation can also be a major cause of chronic poor performance in horses (Gills 1999; Jeffcott 1980; Jeffcott et al. 1985; Jeffcott and Haussler 2004). The most common locations for degenerative changes of the spine are the facet joints and the sacroiliac articulation (Denoix, 1999a; Denoix, 1999b; Haussler et al, 1999). These synovial joints undergo progressive stages of degeneration: dysfunction, instability and degeneration. They are characterised by restricted mobility, localised pain and inflammation which can cause hypertonicity of the paraspinal muscles. With continuing joint immobilisation bone demineralisation, capsular adhesions and loss of ligamentous strength take place. Initial musculotendinous contractures are followed by capsular, periarticular and intra-articular adhesions. The dysfunctional joint is unable to sustain normal biomechanical stress. Consequently changes of subchondral bone, cartilagenous, capsular and ligamentous deformation and degeneration result in joint

instability which again affects proprioception and the central neuromotor control of movement and posture. Radiographic findings often include osteophyte formation, spinal ligament ossification, spinal ankylosis and other signs of advanced osteoarthritis (Jeffcott and Haussler 2004; Ross and Dyson 2002).

Impinged or over-riding dorsal spinous processes (kissing spines) were reported to be another frequent cause of back pain in horses and occur commonly between T10 and T18 (Denoix 1999a). However, a high percentage of horses with no clinical signs of back pain also show radiographic osseous changes of the dorsal spinous processes which complicate precise diagnostic (Ross and Dyson 2002). A higher prevalence of spinous process impingement was observed in Thoroughbreds due to narrower interspinous spaces and misshapen dorsal apices (Jeffcot L 1979, Townsend et al, 1986). Competitive jumping horses also seemed to have a higher incidence of pathological changes of the spine induced by increased dorsoventral flexion and demanding spinal manoeuvres (Jeffcott 1979).

Conformational deformities of the spine such as scoliosis, lordosis and kyphosis can be congenital. Lordosis can also be found in aged horses or brood mares after several foals. The alterations of the spinal curvature can lead to an extra stress on the vertebral column and adjacent soft tissue structures which can result in soft tissue strain and alteration of the vertebral bones (Ross and Dyson 2002).

In humans, the pain element of a back problem is considered to be as important as the original injury or disease. Primary back pain is caused by the irritation of spinal nerve roots, branches of the spinal nerves and their communicating receptor system (nociceptive receptors). The nociceptive receptors are represented as freely ending nerve fibres distributed throughout all kinds of tissue (skin, muscle, joint and viscera). These nerve endings are activated by chemical, mechanical and traumatic factors. Unlike in humans the unspecific nature of clinical signs associated with back problems contribute to the challenge of diagnosing back pain in horses.

To evaluate the factor pain in horses with back problems, back pain was induced by injections of concentrated lactic acid into the equine back. This resulted in local pain, stiffness and noticeable reduction in performance but did not induce lameness (Jeffcott et al. 1982). A more recent study, however, found an increase in back motion after pain induction on one side (Gomez Alvarez 2007).