IN-DEPTH INTERACTIVE:
NEUROLOGY
Neurologic Examinations of Horses
Monica Aleman, MVZ Cert., PhD, DACVIM
Neurologic examinations of horses can be done by the practicing clinician and must include the
evaluation of behavior, state of consciousness, cranial nerves, posture and postural reactions,
segmental reflexes, palpation, and gait evaluation. Examiners need to rely on sight (observation
essential), touch (palpation), and hearing. Key points include safety first, observation, knowing
what is normal, tailoring the exam to the individual (safety, domestication, cooperation), and
performing more than one examination. Author’s address: School of Veterinary Medicine,
Department of Medicine and Epidemiology, University of California, Davis, CA 95616;
e-mail: [email protected]. © 2015 AAEP.
1.
Introduction
This section is intended to provide some guidelines or recommendations on how to perform a
neurologic examination and how to localize the
observed deficits. The author strongly believes in
learning functional neuroanatomy because it
helps in understanding and interpreting the findings from the neurologic examination. This is
essential for localizing the affected area within the
nervous system. All clinicians follow their own
method and order when performing a neurologic
examination. The examination will be successful
if it is thorough, follows a consistent order, and
avoids overlooking abnormalities. Several authors have described how to perform neurologic
examinations in horses; each description varies
slightly, and a few references are provided in subsequent sections.1–7 A complete neurologic examination is warranted in horses with a suspected
neurologic condition, unusual gait, nonlocalizable
lameness, unexplainable weakness and muscle atrophy, and altered behavior or sleep, among others. The author also considers it important to
include a neurologic examination in pre-purchase
examinations to determine whether a horse is
neurologically normal.
It is essential before performing a neurologic examination to obtain relevant information about the
horse such as signalment (breed, gender, age); physical activity or intended use (athlete, companion,
breeding, shows); a complete medical history that
includes previous illnesses and/or any illnesses the
horse may have been exposed to while on the farm
(and the age and activity of the animals affected);
and preventive medicine (vaccinations, deworming
programs, oral/dental and hoof care). It is important to know how many animals are affected, how
old they are, and what their diet, water source, and
housing (e.g., stall, pasture, dry lot) consist of.
Nutrition/toxic, infectious, and genetic disorders (in
breeding farms) must be considered as possible
causes of disease if several animals are affected in
close proximity. Duration of disease (acute vs.
chronic), progression of disease (nonprogressive vs.
progressive), fever (especially important because of
the potential risk of an infectious contagious disease), and the presence or absence of apparent pain
NOTES
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are important features that will aid in the investigation of possible causes of neurologic disease.
A full physical examination followed by a thorough
neurologic examination is essential because systemic illnesses can influence the neurologic status of
the horse, particularly the status of neonatal foals.
Furthermore, metabolic, nutritional, and/or toxic
disorders might present with neurologic manifestations. Weakness is a sign that could present with
systemic illness and neurologic disease. Therefore,
it is important to determine the overall health status of the horse before beginning the examination.
2.
Key Points of the Neurologic Examination
Key points of the neurologic examination are as
follows:
1.
2.
3.
4.
Safety first.
Observation is essential.
Know what is normal.
Tailor the exam to the individual horse (safety,
domestication, lack of cooperation).
5. Perform more than one exam.
It is necessary to emphasize the importance of
safety. Because of a horse’s size, level of domestication, and neurologic status, not all components of
the neurologic examination will be feasible (e.g., full
gait evaluation in a severely affected horse or performing a close examination in wild horses). However, one essential component in performing a
neurologic evaluation is observation. Examiners
can learn so much about the neurologic status of any
animal just by carefully observing it. Knowing
what is normal is paramount, particularly when it
comes to different gaits of various breeds. Tailor
the exam to the individual horse. For example, if a
horse is at risk of falling as a result of severe deficits,
it is not necessary to evaluate its gait on a hill or
curb; for safety reasons the examination for that
particular horse is as complete as it can be. Finally, performing more than one neurologic examination is important and helpful. The examiner
might observe deficits that have gone unnoticed
(mild deficits), or, in the case of progressive disease, the neurologic condition might change.
This last point is important because neuroanatomical localization depends on a thorough neurologic evaluation.
A Cautionary Note About Rabies
Even if the local prevalence of rabies is low or you
have not seen it in your area, if the horse develops
acute progressive neurologic signs you should consider it as a possibility. The presence of wildlife,
lack of vaccination in endemic areas, and increased
traffic of animals across geographical areas (from
endemic areas) can pose a risk. Protect yourself
and others, minimize the number of personnel handling the horse, and wear protective clothing (do not
touch without gloves). In endemic areas with a
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horse for which routine vaccination practices are
minimal or nonexistent, wear gloves, protective
clothing, and face shields when dealing with horses
with acute neurologic disease. Rabies often presents initially with signs that are not perceived as
being neurologic, such as lameness, fever, and colic;
however, if undiagnosed, more obvious neurologic
signs will quickly begin to show.
Goals
The first goal of the neurologic examination is to determine whether a neurologic abnormality is present.
The second goal is to determine the neuroanatomical
localization, which refers to the affected area within
the components of the nervous system. Remember
that the nervous system consists of central and peripheral parts. The central components include the brain
and spinal cord. The peripheral parts include the
nerve roots and ganglia, nerves, and neuromuscular
junction. The neuromuscular system consists of central and peripheral components. All nerves are peripheral (cranial nerves, spinal nerves). The cauda
equina is formed by sacral and caudal nerve roots and
nerves. Do not forget the autonomic nervous system,
which includes the sympathetic, parasympathetic, and
intrinsic/enteric plexuses. Functional neuroanatomies
for neuroanatomical localizations are discussed later.
3.
How to Perform a Neurologic Examination
This section is merely a guideline and does not represent the only way to perform a neurologic examination. The first thing to note is that minimal
equipment is needed. Remember that the most important equipment is your senses (i.e., sight [observation], touch [palpation], and hearing). Useful
tools include a strong light source (transilluminator,
ophthalmoscope, or pen light) and hemostats or a
pen to assess pain sensation and to induce segmental (spinal) reflexes. Additional tools could include
a sound source such as car keys or a ringing object
(bell) to evaluate for hearing deficits. Examination
under saddle is not recommended because of safety
concerns. However, complicated cases or gait deficits only observed under saddle might require a
rider, but the horse must be evaluated in hand before being ridden to determine whether it is safe.
An example of a neurologic examination form the
author has used is provided in the Appendix.
The examination consists of evaluating the neurologic status while the horse is at rest (static) and during movement (dynamic). Some authors divide the
examination in 4 broad categories: (1) evaluation of
mental status and behavior; (2) cranial nerve
examination; (3) standing exam of posture, segmental
reflexes, postural reactions, and muscle; (4) examination of gait and posture during movement.5 The author divides the examination in the following manner:
1. Behavior and mentation
2. Cranial nerves
3. Posture (head, neck, trunk, limbs, tail)
IN-DEPTH INTERACTIVE:
4. Postural
reactions
(i.e.,
positioning)
5. Segmental reflexes
6. Palpation
7. Gait evaluation
8. Nociception (pain perception)
proprioceptive
Behavior and Mentation
Try to observe the horse in its own environment and
note its behavior and how it interacts with others
(humans and animals). In referral institutions, allow
the horse to get some habituation (sometimes not possible) to the surroundings and observe it in confined
and open areas. Keep an open mind but listen to
what the owner has to say about the behavior of the
horse. Usually, subtle changes in behavior are first
noted by the owner. Note that behavior alterations
might not always be caused by a neurologic disease
(i.e., pain, learned behavior). Behavior is important
even if the horse is apparently bright and alert. For
example, sudden aggression, fear, or docile behavior in
horses that usually have the opposite behavior should
raise a concern. Horses with encephalopathies (brain
disease) can manifest compulsive behavior such as
compulsive walking, yawning, biting, circling, head
pressing, and appearing sleepy or blind.
The 4 states of consciousness (mental status) are
as follows:
1. Normal: Bright, alert, responsive.
2. Obtunded: Quiet because of a neurologic disease—not a systemic one (this would be lethargic). Do not use the word “depressed” because
this is a neurologic disorder in humans. In
this state, the horse remains responsive to
stimuli (visual, tactile, aural) and reacts to the
environment. There could be degrees of obtundation such as mild, moderate, or severe on
which the stimuli might have to be stronger for
a response. These degrees of obtundation are
subjective. In mild cases, the horse might not
respond to someone walking in the stall but
might react to a loud sound.
3. Stuporous: Severely altered mental status; unresponsive to minimal-to-moderate stimuli. Profound painful stimuli (pinching the skin
or foot with hemostats) generates a response of
the animal (“waking up”), but the response dissipates (goes back to stuporous) as soon as the
stimuli stop. Horse appears as if dead.
4. Comatose: The horse is unresponsive to any
kind of stimuli, including profound painful
stimuli.
Do not confuse response and reaction with reflexive
movement when applying a painful stimulus. Alterations in behavior and/or state of consciousness
indicate intracranial disease (forebrain, brainstem).
Cranial Nerves
Responses, reactions, and reflexes can be evaluated
in the standing horse at rest or in the recumbent
NEUROLOGY
horse. Cranial nerves can be evaluated in order
from CN I to XII to avoid forgetting one or more or
could be evaluated to include all functional regions.
The author prefers functional regions, starting with
sense of smell (subjective); all eye functions (menace, palpebral fissure, palpebral reflex, corneal reflex, dazzle reflex, pupillary light reflex, adaptation
to light and darkness, eye globe position and retraction, physiologic nystagmus, tear production); jaw/
facial motor, sensation, and symmetry; and eating/
drinking (prehension, suction, tongue tone and
movement, gag reflex). Do not change your method
of evaluation, and always be consistent to avoid
overlooking abnormalities. The examination specifically covers the following:
●
●
●
●
●
●
●
●
●
●
●
●
●
Olfaction (smell): CN I, subjective evaluation
and interpretation. Horses with interest in
food might have normal olfaction.
Menace response: CN II and VII, cerebral
cortex, and cerebellum.
Palpebral fissure: CN III and VII, sympathetic innervation.
Palpebral reflex: CN V and VII (check medial
and lateral palpebral).
Trigeminal facial reaction/reflex: CN V and VII
(touch face, nasal mucosa, and inner pinnae).
Facial and nasal sensation: CN V (VII for
inner pinnae).
Pupillary light reflexes (direct and indirect):
CN II and III.
Corneal reflex: CN V, VI, and VII.
Eye globe position: CN III, IV, and VI (VIII
also contributes; rule out extraocular muscle
disease
or
retrobulbar
or
periocular
mass). Upon head elevation, horses have
mild ventral strabismus that is considered normal, but if all you see is the sclera that is
considered abnormal.
Eye globe retraction: Gently press the eye
globe for retraction; do not perform this task if
ocular disease (i.e., corneal ulceration) or eye
contamination is a concern.
Tear production: CN VII (Schirmer’s test on
both eyes— compare).
Physiological nystagmus: Turn head side to
side and observe ocular movements; movements should always be horizontal if head is
moved from side to side and synchronous with
the fast phase toward the direction of the head
turn. Pathologic nystagmus, occurring at
rest or when the head is held in a certain
position, is an indication of vestibular disease
(central or peripheral).
Gag reflex: Offer food and observe swallowing or palpate the larynx externally, not directly (orally) like in small animals, to induce a
gag—this is very subjective and might not reflect an obvious abnormality where there is
one and definitively does not determine degree
or stage of dysfunction.
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Fig. 1.
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Horses displaying proprioceptive deficits.
Note the abnormal limb placement.
Offering food also helps to test olfaction (subjectively— could cover eyes to see if the horse would
smell), vision (visual perception of food), prehension
(CN VII—the horse has particularly mobile lips, especially the upper lip), mastication (CN V), pushing
bolus back to the throat (CN XII with contributions
of V), and swallowing (CN IX–XII). Food packing
can also be seen with CN VII dysfunction. The
author provides small amounts of food of various
kinds (pellets, cookies, grain, mash, hay) and water
to look for abnormalities or difficulty from suction,
prehension, masticating, and swallowing that might
be apparent with one type of food but not with others. The author has found this very useful for identifying subtle deficits.
Do not forget to observe cervical musculature (CN
XI). Do a fundic exam since the retina and optic
nerve are a few of the structures from the nervous
system that can be visualized. Nerves located
within the guttural pouch can be visualized through
endoscopy. The slap test in the horse to assess
possible laryngeal function is subjective and can be
evaluated in conjunction with endoscopy. Sympathetic innervation to the eye is also important (third
eyelid, pupil size, palpebral fissure, eye lashes).
Sympathetic denervation of the head in horses presents as miosis, ptosis, protrusion of the third eyelid
with eyelashes pointing down, and sweating of the
head ipsilaterally.
Posture and Postural Reactions
The posture of the head, neck, trunk, tail, and limbs
is important. Head tilt, neck turn, whole body
leaning, trunk turn or scoliosis, lordosis, or kyphosis, wide- or narrow-based stance, or tail down or up
or pulled to the side could be observed with neurologic disease. Postural reactions, including proprioceptive placing and hopping tests, can be
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performed in the horse, but examiners must practice
caution because of safety concerns. Proprioception
is the sense of knowing the relative position of a
particular point or region in space (e.g., limbs; Fig.
1). Foot placement is evaluated for the thoracic
and pelvic limbs. Examiners can place one limb at
a time in an unusual or uncomfortable position to
assess recovery and how quickly the horse returns
the limb to a normal position. This can be achieved
by either placing one limb far apart from the other or
by crossing one limb in front of the other and observing when and how the horse replaces the limb.
The author does this when proprioceptive deficits
are not obvious and prefers simply to observe the
horse throughout the exam for foot placement.
A repeated series of maneuvers followed by rest for
several seconds to minutes to note foot placement
provides reliable information of proprioception without exposing the handler and/or examiner to the
possibility of being crushed if a horse were to collapse. This also applies for horses for whose lack of
cooperation makes assessment difficult. Observing
how these horses stand at multiple time points during the examination gives an idea whether proprioception might be abnormal. Note that horses with
painful limbs might stand in an unusual or abnormal position to protect the affected limb. Furthermore, a compliant or trained horse might hold an
abnormal stance and thereby complicate how the
limb placement is interpreted. Horses with a mechanical lameness can also present with abnormal
limb placement that does not result from neurologic
dysfunction. In some cases, although not always,
these problems may be obvious. Proprioceptive
deficits alone are not a localizable neurologic deficit
to a specific area because these areas can be seen in
brain, spinal cord, and peripheral diseases.
IN-DEPTH INTERACTIVE:
Segmental Reflexes
The evaluation of segmental reflexes is limited in
horses but should be tested whenever possible. Segmental reflexes that can be readily evaluated include
cervicofacial/auricular, cutaneous trunci (panniculus),
perianal, and perineal reflexes. Tendon and flexor
(withdrawal) reflexes are not usually performed in ambulatory horses. If the horse is young or recumbent,
reflex testing, which includes triceps, biceps, patellar,
and gastrocnemius reflexes, can be performed. Tendon reflexes might be difficult to interpret in recumbent adult horses, but withdrawal reflexes should be
assessed. In general, the larger the horse, the harder
tendon reflexes are to test and interpret. Remember
that the horse must be relaxed and recumbent for
adequate assessment.
Palpation
Palpation of the horse’s body, including the head,
can reveal abnormalities that are not obvious upon
visual inspection. Check for symmetry, shape,
pain, swelling, temperature, sweating, and masses.
Palpate muscles, bones, and joints; flex and extend
the joints to assess for pain and mobility. Examine
the horse carefully for any muscle atrophy. Check
tail and anal tones. Look for any loss of skin sensation (hypalgesia or analgesia), increased sensitivity (hyperesthesia), or abnormal sweating
(sympathetic denervation).
Gait Evaluation
The causes of lameness or irregular gaits could
stem from an orthopedic, musculoskeletal, or neurologic disease. More than one system could be
involved, which complicates gait evaluation and
interpretation. If safe, a full lameness exam
should be performed, including observations at
the walk, trot, and canter, to investigate how
other systems involved contribute to the abnormal
gait. Horses with pain might alter their gait to
protect a painful limb. Caution must be taken
when assessing gait because certain breeds of
horses have been bred for a particular desired gait
(e.g., “floaty” gait in Warmbloods, hyperflexion
gaits in Paso Fino and Peruvian Paso, pacing in
Standardbreds). There could be a fine line between what is considered a desired (i.e., normal)
and undesired gait when breeding horses for specific gaits. Examiners must be familiar with the
gaits of different breeds and be aware of what is
considered an acceptable gait for a specific breed.
For example, a floaty gait in an American Quarter
Horse will be considered abnormal. An acceptable (or normal) gait is a topic of discussion and
debate among breeders, owners, trainers, and veterinarians. Gait evaluation could be challenging
for the reasons previously outlined but also because horses could present with a concurrent orthopedic or muscle disease that can make gait
evaluation even more challenging. There will be
situations in which full gait evaluation might not
NEUROLOGY
be possible because of safety concerns, horses at
risk of falling as a result of the severity of deficits,
or lack of cooperation.
Movement abnormalities include dysmetrias (abnormal range of movement: hypermetria, hypometria), ataxia (incoordination), paresis (decreased
voluntary movement), weakness (lack of strength),
hyperextension/hyperflexion, and specific gait deficits associated with nerves deficits (e.g., radial, femoral, sciatic, peroneal, tibial). Horses can be
evaluated at the walk, trot, and canter when safe.
Helpful maneuvers include the following: walk
and trot in straight line, walk in serpentine (zigzag),
walk with head elevated, walk while pulling tail in
each direction, spin in tight circles, walk on uneven
ground (back and forth over curb or cavaletti, up and
down hill), and walk backward. Using different
surfaces (soft, hard, uneven, colored) can be very
helpful to challenge locomotion skills, especially in
horses with subtle deficits.
If ataxia is noted, try to determine whether it is
caused by cerebellar, vestibular, or general proprioceptive (GP; commonly known as spinal) disease.
To determine the type of ataxia, look at the rest of
the neurologic status of the horse. Does the horse
have cerebellar (hypermetria, intention tremors,
lack of menace response) or vestibular (pathologic
nystagmus, head tilt, body lean, circles in the direction of the head tilt) signs? If the answer is yes to
one of these questions, then you can answer the first
question. Horses with general proprioceptive
ataxia can present with toe scuffing, dragging of the
feet, delayed protraction, knuckling over, crossing
over, stepping on itself, pivoting (leaving the foot
stationary), circumduction (“swinging” of the limb),
or uneven/irregular stride length. Examples of GP
ataxia include horses with spinal cord disease
caused by equine protozoal myelopathy (EPM), cervical vertebral compressive myelopathy, neuroaxonal dystrophy (NAD), and intervertebral disc
disease with compression of the spinal cord, among
others.
In addition to ataxia with spinal cord disease,
paresis and upper motor neuron (UMN) or lower
motor neuron (LMN) deficits depending on the location within the specific spinal cord segments can be
seen. With UMN involvement, the gait/stride
could be exaggerated (“upper” ⫽ hyper, exaggerated,
elongated), whereas with LMN, the gait/stride is
short and choppy with weakness that can result in a
base-narrow stance and muscle fasciculations if severe. In addition, pronounced muscle atrophy can
be observed. Horses with UMN deficits can also
have muscle atrophy, but this will be more gradual
over time compared with atrophy from LMN injury.
In chronic cases, the presence of muscle atrophy
alone cannot help in distinguishing UMN from LMN
injury. See section on the spinal cord for more
information.
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Nociception (Pain Perception)
Nociception or pain perception (conscious perception
of pain ⫽ cerebral awareness of pain) must be tested
in horses with no voluntary movement or when voluntary movement is questionable or not observed.
When testing for nociception, two things are normally expected: (1) a conscious response to painful
stimuli (e.g., head turn or any other response that
demonstrates that the horse feels what we are doing), and (2) a reflex movement away from the painful stimulus. To clarify, voluntary movement does
not equal reflexive movement. Both are different,
and each tests different things and final pathways.
This means that if you see movement of a limb being
tested in a recumbent horse, this might not necessarily be voluntary—it may be a reflex.
already distinct histologically at birth, whereas cerebellar layers are not completely differentiated at
birth in predators. Cerebellar development and
myelination in various parts of the nervous system
might explain the “bouncy” gait in neonatal foals.
Appropriate neurologic function is not the only
important component for successful effector functions such as suckling, swallowing, locomotion, and
autonomic functions, among many others. All the
anatomical and functional components must be intact (e.g., musculoskeletal, visceral). Failure of any
of these components will result in dysfunction.
The next section briefly reviews key points of the
neurologic examination in neonatal foals. As mentioned for adult horses, the autonomic nervous system (parasympathetic, sympathetic, and intrinsic/
enteric) is also part of the nervous system.
Neonatal Foals
It is paramount to become familiar with what constitutes normal neurologic status according to age,
especially during the evolving neonatal period.
A comprehensive clinical history for both mare and
foal and periparturient events must be obtained in
the case of neonatal foals. A full physical examination is essential for determining the overall health
status of the foal. It is important to determine
whether systemic disease is present and whether
there is any cause or effect, association, or contribution to the neurologic status. Common neonatal
disorders manifest with similar clinical signs such
as weakness, reduced muscle tone, recumbency,
reduced or absent suckle reflex, and dysphagia.
Therefore, a meticulous clinical evaluation of the
horse and diagnostic workup to rule out common
neonatal disorders are crucial for directing proper
therapy in addition to supportive care. There are
important functional differences in the neurologic
examination of foals, especially in neonatal foals as
they mature, and adult horses. Therefore, recognizing what is normal according to age is essential.
Alertness, responsiveness to the environment, and
movement are different in utero, during birth, and
in extra-uterine life. Foals respond and move in
utero but not to the extent of extra-uterine life.
In the birth canal, foals appear to be in a drowsy
state and become minimally responsive; movement
is also depressed. During the first few hours of
extra-uterine life, neonates must hit key milestones
that will result in successful functioning and survival. Furthermore, evolutionary differences between prey and predators have resulted in
differences in neurologic function. For example,
the menace response (a learned response) develops
earlier in prey (e.g., 7–10 days in horses and cattle)
compared with predators (several weeks). Prey
such as horses and cattle are born with more developed brains than predators’ brains. Prey have
functional vision and hearing and can stand and
nurse in a relatively short time after birth compared
with predators. Although brain development continues after birth, the cerebellar layers in prey are
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Neurologic Examination in Neonatal Foals
Here again, observation is paramount. The neurologic status of the normal neonatal foal goes through
a transition from in utero to ex utero life. The
APGAR (appearance, pulse, grimace, activity, respiration) score developed for the assessment of neonates in the postfoaling period (one minute
postfoaling) consists of the following variables:
1. Heart rate (normal: regular, 60 beats per
minute; abnormal: undetectable, irregular, or
⬍60 beats per minute)
2. Respiration (normal: regular, 60 breaths per
minute; abnormal: undetectable, irregular, or
⬍60 breaths per minute)
3. Mucous membranes (normal: pink)
4. Muscle tone (normal: strong enough to be
able to be in sternal recumbency)
5. Responsiveness: nasal stimulation (expected
response: strong grimace, sneeze); ear tickle
(expected response: head shake); back scratch
(expected response: attempts to stand)
This evaluation can be repeated at 5 and 15 minutes
postfoaling to determine whether veterinary intervention is needed. Important milestones include
time to sternal recumbency within 1 to 2 minutes,
alert and responsive to external (tactile, visual, auditory) stimuli within 5 minutes, suckle reflex present within the first 20 minutes, vocalizing in
response to the dam’s nickering within 30 minutes,
time to stand within 60 minutes (longer than 2
hours is considered abnormal), and time to nurse
within 2 hours (more than 3 hours is abnormal) after
birth. The author performs a neurologic evaluation
concurrently with the physical examination in neonatal foals. Similarly, multiple assessments of the
neurologic status are done per physical examinations. The neurologic examination must cover all
areas cited for adult horses.
The determination of mentation, behavior, and
posture can be done as the history is being taken or
as the foal is being examined. States of conscious-
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ness include bright alert and responsive, obtunded,
stuporous, and comatose. Behavior includes bright
alert and responsive to the environment, mare attachment, udder seeking, nursing, curious of the
environment, and sleep. Head posture in neonatal
foals has a “flexed” appearance at the atlanto-occipital joint compared with adults, and their stance is
wide-based and becomes narrower within days after
birth. Cranial nerve deficits might be apparent
during the initial observation before approaching
the horse. Palpation is essential for detecting areas of apparent pain, local temperature, muscle tone
and symmetry, joint extension and flexion, and tail
tone, among others. Tactile stimuli result in brisk
exaggerated responses and reactions in normal foals
compared with older animals. Segmental reflexes
that can be evaluated in foals include cervicofacial,
cutaneous trunci, bicep, triceps, patellar, gastrocnemius, flexor (withdrawal), anal, and perianal reflexes. The cross-extensor reflex may or may not be
present in the neonatal period. If present, it is not
considered abnormal and will not be apparent
within a few days after birth. An extensor thrust
reflex can also be seen in normal neonatal foals.
Neonatal foals have a hypermetric gait that becomes
more coordinated within 3 days after birth. Effects
of systemic disease, orthopedic disease, congenital
anomalies, motor deficits (from initiation of movement by the forebrain all the way to the nerves,
neuromuscular junction, and muscle as the executers), and weakness can result in recumbency. Cutaneous sensation can be evaluated to investigate
the presence or absence of sensory function. Nociception (conscious perception of pain) is only evaluated if voluntary motor function is absent or difficult
to interpret. For the purposes of this article, diseases of neonatal foals will not be discussed, but
consider congenital/hereditary disorders and other
common neonatal diseases that affect the overall
neurologic condition in foals.
Neuroanatomical Localization
There are several ways of classifying the nervous
system. This section will divide the nervous system into its functional areas to ease the interpretation of neurologic findings and to localize the lesion.
The functional areas and their subdivisions are
shown in Fig. 2.
When localization to a specific or single area is not
possible, consider diffuse or multifocal localization.
Neuroanatomical localization along with the signalment and medical history (including information of
duration, progression, fever, and apparent pain) of
the horse will aid in the formulation of a list of possible causes of the disease. Based on this list of
possible causes, a targeted diagnostic plan can be
made. Without an appropriate neuroanatomical
diagnosis, it is pointless to start thinking about diagnostic tests or treatments. As previously mentioned, do not forget the autonomic nervous system
(parasympathetic, sympathetic, intrinsic/enteric),
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Fig. 2. Neuroanatomical localization based on major functional
areas: brain, spinal cord, and peripheral.
which is also part of the nervous system. Next, the
most common signs of each of the functional areas
will be mentioned. As a reminder, proprioceptive
deficits alone are not a localizing sign because these
deficits could be seen in brain (all major functional
divisions), spinal cord, and peripheral diseases.
Brain
The brain has three primary divisions: prosencephalon (forebrain), mesencephalon (midbrain), and
rhombencephalon (hindbrain). It consists of three
main functional areas: cerebrothalamic, brainstem,
and cerebellum (Fig. 3).
The cerebrothalamic (prosencephalon or forebrain)
area includes the cerebrum, basal nuclei, limbic system, and thalamus. One or more signs might be observed, such as behavior alterations (compulsive,
bizarre, manic); lack of initiation of movement; ignoring one side of the head and body (contralateral); central blindness; wide circles ipsilateral to the lesion;
seizures; contralaterally decreased nociception; and/or
contralateral proprioceptive deficits. Note that the
thalamus belongs to the brainstem but behaves functionally more like the forebrain. The thalamus is an
important relay center. Examples of diseases include
hypoxic/ischemic encephalopathies, metabolic encephalopathies (ammonia, sodium disorders, bilirubin
[neonatal isoerythrolysis in neonatal foals], viral encephalitis (Eastern, Western, Venezuelan equine encephalitis), trauma, neoplasia (rare), and hereditary
epilepsies (Egyptian Arabian foals), among others.
The brainstem has components in the forebrain
(thalamus), midbrain, and hindbrain (pons and medulla). Signs that might be observed include an
altered state of consciousness or mental status (obtunded, stuporous, comatose), altered sleep, and
multiple cranial nerve deficits. Ataxia can be observed with the involvement of tracts within the
brainstem. Proprioceptive deficits are ipsilateral
in brainstem disease (except for the thalamus—
functionally include thalamus as part of the cerebrothalamic area). Vestibular (central) dysfunction
can also be observed with brainstem disease because
the vestibular nuclei and part of the vestibular tracts
are within the brainstem. Examples of diseases inAAEP PROCEEDINGS Ⲑ Vol. 61 Ⲑ 2015
187
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Fig. 3.
Functional areas of the brain:
NEUROLOGY
cerebrothalamic, brainstem, and cerebellum.
clude EPM, West Nile virus and other types of viral
encephalitis, trauma, and neoplasia (e.g., melanoma in
grey horses), among others.
The hallmark signs of disease in the cerebellum
include intention tremors (tremors upon intended
movement), hypermetria of all limbs (but particularly
in thoracic limbs), ataxia, and plus/minus menace deficits. Furthermore, horses might rear up when being
walked backward as a result of exaggerated and uncoordinated movement of the thoracic limbs. Examples of diseases include cerebellar abiotrophy in
Arabian horses and cerebellar hypoplasia.
Spinal Cord
Gait deficits such as general proprioceptive ataxia,
paresis, and UMN or LMN deficits depending on the
location within specific spinal cord segments will be
observed. With the involvement of LMN, weakness
will also be observed. Lesions localized in the following spinal cord segments (not vertebral bodies)
will present thusly:
C1-C5/6: UMN deficits (normal to exaggerated) of
thoracic and pelvic limbs (sometimes also tail)
C6-T2: LMN signs of thoracic limbs; UMN of
pelvic limbs
T3-L3: Normal thoracic limbs; UMN of pelvic
limbs
L4-S2: Normal thoracic limbs; LMN of pelvic
limbs (plus or minus urinary/rectal depending
on location—sacral segments)
S-caudal: Normal thoracic limbs; normal or
LMN of pelvic limbs depending on location because the sciatic nerve originates from cranial
sacral segments; cauda equina signs (e.g., urinary/rectal incontinence)
UMN deficits of the tail can be observed with
lesions cranial to the sacral segments. A grading
system for neurologic gait deficits and ataxia was
developed by Dr. Mayhew and is currently the
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2015 Ⲑ Vol. 61 Ⲑ AAEP PROCEEDINGS
method used by most clinicians.2 The author follows his grading system but have modified it to help
teach students and residents and to minimize disagreement among examiners. Even then, the grading is still subjective and prone to individual
interpretation. The deficits in gait/locomotion in
the author’s modified system includes postural reactions, paresis, and ataxia. Simple grading system
is as follows:
Grade 0: Normal.
Grade 1: Subtle deficits visible only under special circumstances and not always consistent.
Grade 2: Mild deficits but visible at all gaits and
tests, including walking in a straight line.
Grade 3: Moderate deficits visible to any untrained eye and from a distance. Anyone can
tell that something is wrong with the way the
horse walks.
Grade 4: Severe deficits with risk of falling easily even if just standing. Do not get close.
Grade 5: Recumbent and unable to stand.
Examples of diseases that affect the spinal cord
include cervical vertebral compressive myelopathy, EPM, NAD/equine degenerative myelopathy
(EDM), herpesvirus myelopathy, other infectious
causes, trauma, disc disease, neoplasia, and vascular anomaly, among others. Polyneuritis equi
is an example of a disease that affects the cauda
equina.
Neuromuscular System
As mentioned earlier, the neuromuscular system
has central (lower motor neurons) and peripheral
(nerve roots, ganglia, nerves, neuromuscular junction) components. Neuromuscular disorders can be
diffuse or can involve only a single nerve. Diffuse
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neuromuscular disease induces generalized weakness, difficulty supporting weight, base-narrow
stance, paresis or paralysis, muscle fasciculations,
and tendency to become recumbent. Segmental reflexes can be decreased or absent in neuromuscular
disease. The two most common diffuse neuromuscular diseases of horses are equine motor neuron
disease and botulism. Focal LMN disease or neuropathy lead to specific signs that pertain to the
affected region, such as specific gait deficits and
focal muscle atrophy.
NEUROLOGY
Acknowledgments
Declaration of Ethics
The Author declares that she has adhered to the Principles of Veterinary Medical Ethics of the AVMA.
Conflict of Interest
The Author declares no conflicts of interest.
Appendix
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