By Sarah Evers Conrad
Is it a limp or a stagger, a guarded step or a sway? We watch our horses carefully, maybe obsessively, and puzzle over gait nuances when a mount comes up lame. Standing in the barn aisle, heads tilted, we watch a horse walk and trot, again with our friends also watching, and we wonder aloud if it’s a stone bruise, a soft tissue injury, or something else -altogether.
In the something-else-altogether category lives a sizable list of neurologic conditions that can be tricky to spot early and difficult for a veterinarian to diagnose. Further complicating things, neurologic issues might first be apparent as signs other than an altered gait.
Therefore, it’s important that horse owners and caretakers understand the signs of neurologic disease and keep an eye out for them just as they do indications of gastrointestinal or infectious disease. And as soon as clinical signs or unusual behavior appear, it’s crucial they call a veterinarian.
Two equine practitioners with vast neurology experience shared with us what neurologic signs owners might see, what practitioners look for during lameness and neurologic exams, and how they diagnose.
Lameness, or an abnormal stance or gait caused by either a structural or a functional disorder of the locomotor system, is a clinical sign, not a disease. Neurologic lameness, generally seen as ataxia, or incoordination, can be caused by bacterial, viral, protozoal, or rickettsial infections; trauma or congenital or developmental diseases; degenerative diseases or cancers affecting the brain or spinal cord; and toxicities.
Musculoskeletal vs. Neurologic
It can be difficult to detect whether a horse’s lameness is musculoskeletal or ataxic, says Stephen Reed, DVM, Dipl. ACVIM, of Rood & Riddle Equine Hospital, in Lexington, Kentucky. Reed is a recognized authority on equine neurology who has spoken extensively on wobbler syndrome, EPM, and head and spinal cord trauma, as well as peripheral neuropathy, all of which can cause neurologic gait deficits.
He says differentiating between the two is challenging because deficits can be similar and distinguishing features subtle. “The often simple sentence used to describe the difference between lameness and ataxia is that ‘A lame horse is regularly irregular, and an ataxic horse is irregularly irregular,’ ” says Reed. “The deficits one looks for in an ataxic horse are weakness, ataxia, dysmetria, and spasticity.
“Weakness is characterized by knuckling, stumbling, dropping, or dragging of a toe and can be seen with lame and neurologic conditions,” he continues. “Ataxia is abnormal foot placement or lack of coordination in foot placement, circumduction (circular movement of the limb) when turning with the outside limb, and sometimes pacing (the gait, in a horse that does not usually pace). Dysmetria is characterized by variation in the height and length of an individual limb while walking or trotting. And finally, spasticity is characterized by lack of joint flexion or a ‘tin-soldier’ way of moving.”
Amy Johnson, DVM, Dipl. ACVIM, assistant professor of large animal medicine and neurology at the University of Pennsylvania’s New Bolton Center, in Kennett Square, also looks for the irregularly irregular or regularly irregular distinction.
“Musculoskeletal lamenesses tend to be predictable (happening with every step), and neurologic lamenesses are more likely to be unpredictable,” she says. “It is not unusual to see a horse that has both problems, neurologic disease and musculoskeletal disease. Those horses can be even trickier to figure out.”
Johnson adds that one clinical sign that indicates a lameness is neurologic in nature is neurogenic atrophy, which is severe muscle wasting that has developed quickly due to a loss in neural stimulation that helps maintain muscle fiber mass.
Additional clinical signs owners might notice include an abnormal stance, proprioceptive (awareness of one’s limbs) problems, paralysis, muscle twitching or spasms, falling, and problems lying down and/or getting back up.
Despite these red flags, owners still sometimes think they’re dealing with a simple musculoskeletal lameness and put off calling the vet. “A veterinarian should be called whenever the owner becomes concerned, if it worsens or doesn’t improve quickly, or if there is no obvious explanation for the lameness,” says Johnson.
Because certain neurologic conditions can progress quickly, it is important to get a veterinarian involved as soon as signs appear so the horse has the best chance at recovery. Early intervention can also cost less in the long run.
The Neurologic Exam
Reed says the physical exam is key to distinguishing whether a lameness is neurologic or musculoskeletal. Anytime a veterinarian needs to examine a horse, he or she first gets a history, asking the owner what the horse is like when healthy and what changes he or she has noticed, such as if the horse is urinating and defecating normally. The veterinarian also records the horse’s vital signs and evaluates overall health.
Then it is on to the neurologic exam, which Reed begins at the head and proceeds toward the rear.
The exam includes the following steps, as described by Reed and Johnson, although the order might vary among veterinarians:
- The veterinarian examines the horse’s attitude, behavior, and mental status/state of consciousness, looking for signs of dullness, lethargy, stupor, or unresponsiveness. He or she might present different stimuli to see how the horse reacts. For instance, normal horses should be bright, alert, and responsive and blink if someone waves a hand in front of their eyes.
- The vet then performs a detailed cranial nerve exam to evaluate the 12 nerves that supply the head structures. With these tests he or she assesses all motor (muscle) and sensory functions, including of the eyes, ears, muzzle, jaw, and tongue. The veterinarian checks the size of the pupils and whether they respond to light. He or she looks at skin and muscle sensation on the face, cheek, and up the nose. In addition, he or she watches how the horse swallows.
- The practitioner evaluates the horse’s posture, stance, and musculature. He or she palpates muscles and joints looking for any pain reaction, asymmetry, lack of muscle tone, muscle wasting, swelling, heat, and unusual lumps. The vet might also test the horse’s range of motion through flexion and extension of the joints and by moving the horse’s neck.
- Then the veterinarian tests reflexes by stimulating an area of skin with a ballpoint pen or a similar object and looking for an appropriate muscle reaction. For example, during the cervicofacial reflex, the veterinarian stimulates the neck and observes the face for appropriate ear twitching and lip grimacing.;
- The practitioner tests the horse’s proprioception by placing his feet in unusual positions and observing how the horse places his limbs during ­movement.
- Finally, he or she examines the horse’s gait looking for irregularities, head-bobbing, head-tilting or -shaking, toe-dragging, or incorrect placement of limbs during movement. The veterinarian has the horse walk and trot in a straight line, including over different surfaces. He or she also has the horse walk in circles, a zig-zag, and in a serpentine; with his head and neck elevated; while the tail is pulled; over a curb; and up and down hills with the head in a normal as well as an elevated position. If a horse is ataxic, the veterinarian might try to determine if the horse is hypermetric (has a long-strided, spastic gait), hypometric (stiff or spastic movement with limited joint flexion), or dysmetric.
Based on this information, the veterinarian scores the horse’s ataxia on a scale of 0 to 5, with a Grade 0 being normal and 5 being recumbent and unable to rise
Grading System for Ataxia
|Grade 0||Normal strength and coordination|
|Grade 1||Subtle mild neurologic deficits only noted under special circumstances (e.g. when walking in circles)|
|Grade 2||Mild neurologic deficits apparant at all times/gaits|
|Grade 3||Moderate deficits at all times/gaits that are obvious to all observers regardless of expertise|
|Grade 4||Severe deficits with tendency to buckle (at the knees), spontaneous stumbling, tripping, and falling|
|Grade 5||Recumbent, unable to stand|
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