Neurology

How to Conduct a Neuro Assessment of Neonatal Foal

By Marie Rosenthal, MS

A neurological examination of the neonatal foal with a physical examination provides a more comprehensive assessment of the young horse, according to Monica Aleman, MVZ Cert., PhD, DACVIM (LAIM & Neurology), a neurologist at the University of California-Davis, School of Veterinary Medicine.

The neurologic examination should assess behavior, state of consciousness, cranial nerves, spinal reflexes, posture and gait, she said, and in the stable, can be done using simple tools in every veterinarian’s toolbox: listening, observation, palpation and patience.

Although it does not matter in which order the examination is performed, she said it was a good idea to develop a routine so that nothing is overlooked.

The goals of the neurological exam are to determine whether the foal is meeting its neuroanatomical milestones, and if not, why not.

Because a horse is a prey animal, the normal transition from intrauterine to extra-uterine life must develop quickly in the foal. Important early milestones include the time to sternal recumbency, when the foal becomes alert and responsive to stimuli, the suckle reflex, nickering in response to the mare, the time it takes to stand and to nurse.

A modified APGAR score can be calculated in the stable; monitoring the heart and respiration rates, mucus membrane color, muscle tone, and the response to several stimuli will quickly indicate if the foal has a neurological issue that requires immediate attention.

During the evaluation, the veterinarian should ob- serve behavior and the animal's state of consciousness, cranial nerves and the foal’s responses, reactions and reflexes, segmental reflexes of level the dressage spine, horse posture, in right gait and nociception, if needed. “The canter: only Ears time back; intense we're stare; going and the bit is pulled through to to evaluate for nociception, however, the right. is if the animal is non-ambulatory, and there is no obvious voluntary movement. If the animal is walking, you don't need to check for this,” Dr. Aleman said.

Behavior

A new foal should be curious and responsive to its environment. It should be investigating, licking and smelling, while bonding with its mother, answering her nickers and suckling. That is normal behavior.

If a light is shone in its eye, it should blink. Check the direct pupillary reflex, which is a response to light that enters the ipsilateral eye and consensual pupillary reflex that responds to bright light shown in the contralateral eye.

Foals should respond to external stimulation with exaggerated movements. Again, in the field, she uses a common tool—she claps. The foal should jerk its head up in response to the sudden noise. “That is normal. This is a remarkable evolutionary adaptation of the foal in order to survive in the wild—the foal is fully hearing and visual,” Dr. Aleman said.

To check the spinal reflexes, the animal must be completely relaxed and be in lateral recumbency. A foal should enter a relaxed state when restrained gently. “If it’s standing, it is going to have increased muscle tone,” she said. “Even if he’s recumbent, he must be relaxed. If he's recombinant, but all rigid, you're going to have a false interpretation of the reflexes, or maybe you are not going to even be able to check the reflexes.”

She gently taps 3 areas of the thoracic limbs: biceps, triceps and flexor; 3 aspects of the pelvic limbs: patellar, gastrocnemius and flexor; and the cutaneous trunci.

The foal will have crossed extensor reflexes; if 1 limb is flexed, the contralateral limb will extendand vice versa, according to Dr. Aleman. “These foals lose that cross extension as they mature,” she said.

Check its posture during stance and urination, she said. “Foals usually have a wider stance, but as they mature, they stand a little bit more square,” she said.

An abnormal stance might not be an indication of a neurological problem. It could signal abnormal growth of the hoof, she said.

The gait is important, she said. Evolution dictates the foal should be fairly coordinated shortly after birth to escape a predator. “These foals usually have a bouncy gate as neonates, but that bouncy gate usually is lost as they mature,” she said.

What’s Wrong?

“If the foal is quiet and obtunded, in lateral recumbency and not responding to external stimuli, such as touch or noise, there is something wrong,” Dr. Aleman said.

A foal that is obtunded will respond to tactile and audio stimuli; a foal in stupor will only be aroused by very painful stimuli and only for as long as that stimulus is applied. “That is the big difference between a stupor and a severely obtunded foal. The obtunded foal is going to respond to non-painful stimuli and the foal in stupor, very painful stimuli. Andin coma, the foal is not going to be arousable under any circumstances, not even to very painful stimulate.”

Dr. Aleman gave some pointers to localize common neurological problems in the foal. First, figure out which part of the brain is at fault.

Cerebral Cortex and Thalamus

The thalamus translates neural impulses from various receptors to the cerebral cortex, which is responsible for awareness and mental activities. Barking or some other type of compulsory vocalizing or other compulsory behavior, star gazing and seizures, are among the signs of a problem in this part of the brain, as are sudden behavioral changes. If the animal is usually aggressive and suddenly becomes subdued or is very docile and suddenly becomes aggressive, that could be a signal of a problem in this area. Cortical blindness, seizures, proprioceptive deficits and altered motor function can also be signs.

“They can have 1 sign, 2 signs, 3, or all of these,” she said. “But just having 1 of these signs is enough to localize to this area.”

She elaborated a bit on seizure activity. “We could have extracranial causes of seizure or intracranial causes,” she explained. She discussed a case of an orphan with an extracranial cause. The owner prepared the milk replacement for the foal, but made an error in the preparation and the milk contained too much salt causing hypernatremia.

There are many causes for motor disorders, including neuroglycopenia, which is a shortage of glucose in the brain, typically from hypoglycemia. This will often accompany hypernatremia, which is an electrolyte imbalance. Even though the foal is recumbent, the joints are not the problem, and they are not contracted, yet, the foal will remain recumbent and will not initiate movement and doesn’t stand, she said.

“So that is a big clue—you have all the apparently normal functions there, but it is simply not happening. So that is another big red flag for disease,” she said.

Brainstem Disorders

The brainstem controls many basic functions of the nervous system in the horse. There are multiple issues that can be affected by the brainstem, including altered states of consciousness: obtundation, stupor and coma.

There are also multiple cranial nerve deficits, such as sleep disorders, central vestibular disease, cardiovascular, respiratory and proprioceptive deficits. The prognosis for cardiovascular and respiratory disorders associated with the brainstem are poor, she said.

Foals can have 1 or more signs if they suffer from cranial nerve deficits. Often cranial nerve deficits will be accompanied by an altered state of consciousness. .

Abnormal sleep can also be a signal, she said. A slow wave sleep in a foal occurs when the foal is lying down. “The foal should never sleep standing. The neonatal foal does the entire sleep in recumbency,” she said. “So this is very abnormal to see a foal standing and sleeping,” she said. “As they mature, they will start sleeping more like the adult horse by 2 or 3 months of age.”

Cerebellum Disorders

The cerebellum controls movement and motor control. Signs of cerebellum disorders are hypermetria, ataxia, intention tremors, lack of menace, anisocoria and proprioceptive deficits. A foal with cerebellar hypoplasia is born with a small cerebellum compared with a normal foal. Although different from cerebellar atrophy, both diseases will have the same clinical signs: depressed or absent menace reflex and ataxia without weakness.

Intention tremors occur during intended movement. The animal will experience tremors, usually of the face or head, when it tries to move, walk or eat. Cerebellar tremors are not intention tremors.

A common problem is vestibular disease. Signs include a head tilt, nystagmus, ventral strabismus, leaning, circling and ataxia. “These are the common clinical signs that you could see with either peripheral or central vestibular disease,” she said.

Animals with vestibular disease rely on visual input, she said. If you remove the visual input, say by putting a towel over the animal's face, it’s defects will be obvious. “You will definitely be able to tell if the animal has vestibular disease because the change will be quite dramatic.” She said.

“The majority of the cases you're going to see out on the field, I would say probably 99% of the cases that you are going to see with vestibular disease are going to be ipsilateral to the site of the injury,” Dr. Aleman said.

Ipsilateral vestibular disease usually affects 1 side of the brainstem; paradoxical, the caudal cerebellum.

Vestibular disease that is affecting the peripheral nerves is ipsilateral. She offered some tips to tell central versus peripheral disease: Looking at nystagmus will be helpful, she said. If the head is moved to different positions, it changes in central disease, but not in peripheral disease.

The state of consciousness is another indicator, she said. “Even if the animal is bright and alert, it doesn’t rule out a central problem, but if it's obtunded, has stupor or coma, there is central vestibular disease.”

Although facial nerves could be affected by either because the facial nerve and vestibular nerve pathways are close, if there are multiple cranial nerve abnormalities, it’s going to be associated with central vestibular disease.

Signs of spinal cord dysfunction could be sensory or motor, depending on what parts of the spinal cord are affected. Sensory deficits include proprioceptive ataxia or proprioceptive deficits; motor issues include dysmetria, paresis and paralysis and weakness. Motor deficits can be caused by an upper motor neuron injury or a lower motor injury. Another spinal problem are compressive myopathies, which can occur at various areas along the spinal cord.

Unfortunately, proprioceptive deficits can be affected by every brain region, she said. “So that means that proprioceptive deficits alone cannot help you to localize the disease. You have to see what other signs you have,” she said.

The neuromuscular junction can be involved in many functional and physical deficits, including paresis, paralysis, dysphagia, dysphonia, dyspnea, proprioceptive deficits, gait deficits, reflex deficits and muscle atrophy, she said.

Once a neurological problem has been determined, the animal requires a complete diagnostic work up, which should be tailored toward the specific signs the foal is having. Full blood work, urinalysis, electrolyte analysis and other tests will be required. Cerebrospinal fluid might be needed for certain problems. Imaging will also be helpful, radiography and ultrasonography can be easily performed in the field, she said, but advanced tests such as electroencephalography will need to be done at a large academic institution.