South African Equine Veterinary Association Congress 2015 Protea Hotel Stellenbosch
Paresis
Weakness in horses may be associated with dysfunction of upper or lower motor neurones,
neuromuscular transmission or the muscles themselves. Weakness is characterised by knuckling,
stumbling, toe dragging and inability to bear weight. In horses, most neuromuscular and primary
muscle disorders that cause weakness are generalised conditions such as botulism and
hyperkalaemic periodic paralysis. Disorders of lower motor neurones may be generalised (e.g.
equine motor neurone disease) or localised (e.g radial nerve paralysis). Chronic, lower motor
neurone dysfunction and disease results in muscle atrophy of the supplied muscle groups. Muscle
fasciculations are a common feature of generalised disease, whereas localised peripheral nerve
involvement tends to be associated with flaccidity (and focal muscle atrophy). Upper motor
neurone dysfunction can be difficult to distinguish from the general proprioceptive deficits that
characterise spinal ataxia, particularly given that both abnormalities frequently occur together.
Ataxia
The commonest cause of ataxia in horses is cervical vertebral malformation / stenosis. Most
moderate-severe cases are readily recognised by varying degrees of erratic (abducted and
adducted) foot placement, truncal sway, hypermetria, hypometria and dysmetria. In mild cervical
compressive disease, the abnormalities may only be evident within the pelvic limbs; these cases
can be confused with pelvic limb lameness, especially when the signs are asymmetric and even
experienced clinicians sometimes disagree as to the underlying cause. Gait abnormalities
associated with ataxia are usually variable and do not respond to analgesic therapy. Often these
cases require assessment for musculoskeletal and neurological disease.
Dysfunction of myotactic reflex pathways
Syndromes such as Stringhalt are thought to be due to dysfunction of the afferent (sensory)
component of normal myotactic reflexes that normally maintain motor tone in posture and gait.
Although Stringhalt is probably the most widely recognised of these disorders, other related
disorders with variation of the gait abnormality exist. These gait abnormalities may be
asymmetric, and when subtle they can be suggestive of a musculoskeletal lameness. Often
however with true neurological disease, signs may be influenced (presumably via upper motor
neurone control) and vary (often becoming exaggerated) when the horse is led over obstacles.
Similarly, Shivers syndrome, associated with a quivering of the (usually) pelvic limb when lifted
off the ground, or when the horse is backed was thought to relate to a defect in the myotactic
reflex, though some researchers now believe the disorder has a central cause.
Conclusion
Differentiating neurological disease from musculoskeletal disease is usually possible on the basis
of comprehensive lameness and neurological examinations. Subtle cases are the biggest
challenge. In these, a methodical approach, combined with ancillary testing can help in
differentiating the underlying cause although in some cases, progression of signs may be
necessary before a diagnosis is reached. The clinician should also be aware of the possibility of
concurrent conditions contributing to the gait abnormality.jmc
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