Page 1212 - Adams and Stashak's Lameness in Horses, 7th Edition

1178 Chapter 12

symmetrical ataxia and hypermetria in all four limbs, with a locked stifle. In a QAR or BAR horse, the poll of
but without loss of strength. It is routinely accompanied the head should be above the level of the withers. The
VetBooks.ir and absence of a menace response. Forebrain ataxia is sternal position or in a flat lateral position. Minimal dis­
normal recumbent/resting horse is usually found in a
by an intention tremor of the head, a base‐wide stance,
turbance is typically sufficient for arousal and getting
accompanied by abnormal mentation and, sometimes,
cranial nerve deficits. Spinal ataxia is the most frequently up. A horse extends its forelimbs first, followed by an
encountered form of ataxia in the horse. Its cause is a abrupt extension and lift‐off with both hindlimbs. An
disruption of ascending proprioceptive flow of informa­ abnormal horse is either stuporous or (semi)comatose;
tion toward the cerebellum. As a result, descending wanders in circles within its confined space; may appear
information cannot appropriately be fine‐tuned, which blind, agitated, or violent; or has a seizure. All clinical
influences initiation/maintenance/termination of motion signs relate to a problem rostral to the foramen mag­
through muscle tone and motor function in neck, trunk, num. An abnormal stance can be wide based (sawhorse
or limbs. Alternatively, as the descending motor path­ stance) or narrow based (horse‐on‐a‐ball or goat‐on‐a‐
ways are most likely damaged by the same processes as rock stance). The sawhorse stance is typical for a horse
the ascending pathways, ataxia and dysmetria occur with a cerebellar lesion, whereas the horse‐on‐a‐ball
simultaneously. If fore‐ and hindlimbs are affected, this stance is seen with diseases collectively affecting the
is either caused by a focal cervical or by multifocal LMN. Whenever a horse places its limbs randomly, once
lesions throughout the spinal cord. 1,2,5,10 it comes to a standstill (hence, an asymmetrical limb
Dysmetria—Dysmetria collectively describes a state placement), this may be a sign for a lack of ascending
of rigidity, spasticity, and decreased or exuberant limb proprioceptive information or interpretation in the fore­
flexion. It is associated with UMN conduction disrup­ brain. A conscious horse that cannot rise from recum­
tion in the spinal cord, interneuron circuits, or cerebellar bency is probably affected by profound weakness, which
disease. UMN tracts have two major functions: some can be (focal/multifocal) UMN or (diffuse) LMN weak­
are involved in the initiation of movement, whereas oth­ ness. Whether the horse can or cannot lift its neck from
ers have an inhibitory effect on a peripheral reflex loop the ground, maintain a sternal position, or sits like a dog
(e.g. a patellar reflex). Dysmetria is caused by a reduced is a critical observation in determining where the lesion
inhibition of the peripheral reflex loop, and the uninhib­ of the problem is located—the cranial or caudal neck,
ited reflex is observed. A hypermetric gait is character­ thoracolumbar spinal cord, multifocal, or diffusely
ized by an increased range of motion and excessive joint involving gray and white matter of the spinal cord.
flexion. A hypometric gait is typified by limb stiffness Horses with a head tilt and vestibular ataxia often stand
and reduced joint flexion, particularly of the tarsal and slightly wide based, and the body may be concavely bent
carpal joints. toward the side of the lesion. If recumbent, a horse with
Weakness or Paresis/Paralysis—Weakness or paresis a head tilt is more comfortable while lying on the
is the decreased ability to initiate gait, maintain posture, affected side. These clinical signs worsen when the horse
or resist gravity. Paralysis is the inability to do all three. is blindfolded.
Paresis can be further divided into UMN and lower
motor neuron (LMN) paresis. UMN paresis, or flexor
weakness, presents with a delay in initiating movement, Head and Body Symmetry
followed by longer and, typically, lower stride. LMN The normal horse shows symmetry in the head posi­
paresis is usually observed as an antigravity or flexor tion and the posture of the neck, limbs, and trunk.
weakness, presenting as short‐strided, poor swing‐phase Symmetry also extends to facial expression and the car­
gait; trembling due to muscle fatigue (muscle fascicula­ riage of the tail. A head tilt (a sign of central or periph­
tions); and lowered neck carriage while standing. Muscle eral vestibular damage), a dropped ear, and paralysis of
atrophy is more pronounced and often more localized in facial muscles (facial nerve paralysis) are examples of
LMN than in UMN paresis; for both forms it may take head asymmetry. Regional muscle atrophy or complete
2–3 weeks before the muscle atrophy becomes noticea­ unilateral atrophy is more likely a sign of UMN paresis.
ble. Toe dragging and abnormal hoof wear can be seen Defined muscle atrophy of a single muscle belly is best
with either form of paresis. Weakness in all four limbs, explained by peripheral nerve damage. However, gener­
but no ataxia or dysmetria, indicates diffusely affected alized symmetrical muscle atrophy without signs of
neuromuscular units and is by definition an LMN ataxia or dysmetria is the result of diffuse LMN disease
weakness. 1,2,5,10 if neurological and needs to be distinguished from a
horse with chronic malnutrition. 1,2,5,10

FINDINGS FROM A NEUROLOGICAL EXAM THAT Manipulations in the Standing Horse
CAN HELP IN LOCALIZING A LESION The normal horse is able to extend and flex its head

Level of Consciousness and Body Position and neck dorsally and ventrally. During lateral flexion
of the head and neck, the horse’s muzzle should touch its
The normal horse is quiet or bright, alert, and respon­ ribs at either side. Some horses with cervical vertebral
sive (QAR or BAR); it pays attention to its surround­ stenotic myelopathy (CVSM) may be reluctant or resent
ings, recognizes visitors, and finds water and food. lateral flexion due to pain in the intervertebral (facet)
A horse puts weight on all four limbs, which are verti­ joints. Both sides of the horse’s neck and back muscula­
cally placed underneath the body (aka the columns of a ture, from front to back, should be probed with a blunt
Greek temple). The exception to this is a standing horse instrument such as the handle of a neurology hammer or