Texas Equine Veterinary Association

Neurological diseases or disorders in horses is not uncommon. One report indicated that 180,000 horses are diagnosed with some neurologic disease annually. These numbers were based on a broad distribution of neurologic diseases beginning with inherited and developmental conditions and continuing through infectious, traumatic and degenerative conditions. The neurological examination is undertaken to determine whether any deficit is due to a lesion in the nervous system and, if so, where within the nervous system any possible lesion or lesions are located. The examination of horses has challenges not encountered when doing the equivalent examination in dogs and cats, principally that spinal reflexes and postural reactions are impossible/difficult to assess in most animals. Nevertheless, assessment of the function of the nervous system is possible given sufficient care and observations. A detailed knowledge of neuroanatomy is not essential as long as all the deficits are noted. The anatomy book can be consulted later but at the end of the neurological examination the clinician then should be able to determine broadly which area of the nervous system is affected: cerebrum, brain stem, cerebellum, spinal cord, peripheral nerve, muscles and neuromuscular junction. A sudden onset of neurologic disease in horses can pose challenges to both veterinarians and horse owners. Swift and appropriate therapeutic intervention might have a profound influence on a patient's outcome, but advanced diagnostics are frequently necessary to achieve a definitive or presumptive diagnosis. Moreover, neurologic disease has the potential to affect multiple horses on the same premises and can cause rapid deterioration of an affected horse's status. It is critical for the equine practitioner to be familiar with neurologic assessment and the process of neurolocalization. Possession of these skills can facilitate timely recognition of common equine neurologic diseases and an appropriate list of differential diagnoses. Tetanus and botulism are two common and potentially fatal equine neurologic diseases that result in starkly different clinical signs. However, they are frequently described alongside one another because they are both caused by toxins produced by clostridial organisms. In addition, tetanus and botulism produce characteristic neurologic deficits that usually allow for a provisional diagnosis based on a thorough physical examination. In many, if not most cases of neurologic disorders in horses the focus of the neurologic examination is an evaluation of gait. The basic neurologic circuit required for locomotion consists of afferent nerves ascending proprioceptive information and spinal interneurons that receive afferent information and connect with alpha motor neurons (lower motor neurons) that initiate muscle contraction. An alpha motor neuron supplies a specific subset of muscle fibers, and the nerve together with those muscle fibers comprise a motor unit. The impact of multiple motor units acting in concert during muscle contractions ensures stable force production and smooth initiation of movement. This rhythmic locomotor pattern can be maintained at a spinal level without higher input; this is due to a specific network of spinal interneurons termed pattern generators that rhythmically transmit OVERVIEW OF THE NEUROLOGIC EXAM, MOVEMENT DISORDERS AND COMMON CAUSES OF ATAXIA STEPHEN M. REED, DVM, DACVIM ROOD & RIDDLE EQUINE HOSPITAL LEXINGTON, KENTUCKY 64

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