Joanne Hardy
DVM, PhD, Dip ACVS, The Ohio State University

Introduction

When called upon to examine an acutely neurologic horse, the possibility of rabies should be kept in mind until proven otherwise, as well as the risk of injury to the patient, other animals or humans. If rabies is a possibility, the number of people attending the animal should be minimized, and a list of people having been in contact with the horse should be kept. Gloves should be worn by all attendants, and if possible only people that have been vaccinated and have a current titer should attend the horse. If the horse is unsteady, it should not be moved until knowledgeable personnel are available.

Examination

The goals of the examination in acute neurologic disease are to:
1- Confirm that the nervous system is involved
2- Rule out involvement of other body systems
3- Arrive at a list of possible differentials
4- Initiate treatment and supportive care until further diagnostic tests can be performed
5- Facilitate referral of the horse for further diagnostic and/or treatment

Data base

Signalment
-Age
Weanlings are more at risk for cranial trauma during halter breaking or handling. Young horses (<2 y-o) suffering from ataxia without cranial nerve signs should be examined for CVI/CVM, although lower cervical vertebral lesions (C6 to T1) are more common in older horses. Equine protozoal myelitis (EPM) is more common in young adult horses, and is rarely seen in horses <2 years of age.
-Breed
Arabian foals are at risk for cerebellar abiotrophy. Miniature horses can suffer from narcolepsy, which can mimic neurologic disease. EPM is more common in Thoroughbreds, Stanndardbreds and Quarter horses.
-Sex
There is no sex predisposition for neurologic disease, although males are more at risk for CVI/CVM.

History

The geographic location of the animal should be ascertained. For example, encephalitides are more common in the southern states such as Florida. Rabies is more endemic in certain states, as is botulism. A complete vaccination history should be obtained, including particularly rabies, herpes virus and encephalitides if in an endemic area. Stabling practices, access to pasture, quality of pastures, need to be documented. The possibility of wound botulism through recent injections, wounds or castration should be documented. Although many neurologic disesases may appear acute in onset, careful questioning can reveal the presence of subtle deficits that were overlooked.

Physical examination

A complete physical exam should be performed, paying attention to all body systems, as acute laminitis or rhabdomyolysis can mimic neurologic disease. Similarly, horses that are affected with botulism and are unable to stand for long periods can appear to have abdominal pain. Horses that have been recumbent, are blind or have facial nerve damage can have corneal ulcers that will require treatment. Finally, neurologic deficits such as inability to urinate will need to be addressed as part of the treatment.
Neurologic examination

The purpose of the neurologic examination is to arrive at a neuroanatomic localization, and formulate a list of differentials that will dictate further diagnostics and allow implementation of initial treatment. The basis of the neurologic examination has been well described elsewhere. Essentially, the neurologic examination is divided into:

1- Assessment of mental status and behavior
2- Cranial nerve examination
3- Evaluation of gait and posture
4- Neck and forelimbs
5- Trunk and hindlimbs

Laboratory data

Initial laboratory data include PCV/TP and creatinine for assessment of hydration status. If the physical examination indicates it, CK and AST can be evaluated to rule out rhabdomyolysis, SDH, GGT, Alk Phos and bile acids and ammonia levels to evaluate the possibility of hepatoencephalopathy. Ancillary diagnostics include radiography, myelography, CSF analysis, electromyography and EEG. These procedures are performed as indicated by the physical and neurological exam findings. CSF analysis is usually not performed on an emergency basis, as cytologic examination of spinal fluid needs to be performed on a fresh sample.

The following is brief description of conditions that can result in acute ataxia in horses.

NEUROLOGIC DISEASES

When called upon to examine a neurologic horse, the clinician should rule out diseases that can mimic neurologic diseases such as laminitis (specially if recumbent), rhabdomyolysis (recumbency, stiff gait), colic (recumbency), HYPP (acute recumbency) or hypocalcemia.

Cranial trauma

Cranial trauma most commonly follows falling over backwards and hitting the pole of the head. The most common fracture accompanying this injury is a basisphenoid fracture. The following signs can help localize the location and severity of the injury. Cerebral syndrome: Transient unconsciousness followed by aimless wandering and temporary blindness. PLR and cranial nerves are normal, and if no progression, recovery is complete.
Midbrain syndrome: depression, recumbency, mydriasis, absent plr, irregular respiratory pattern, arrhythmia, bradycardia. Poor prognosis.
Medullary-inner ear syndrome: Head-tilt, ear hemorrhage, facial nerve paralysis, intention tremors. Prognosis guarded.