Movement Disorders in Horses: Shivers, Stringhalt and other conundrums

摘要

In human medicine, movement disorders comprise any type of disorder that causes abnormal movement or control of a part of the body. In veterinary medicine, movement disorders are better characterized in small animal compared to equine medicine, however, a number of equine movement disorders have been described. Although diseases can be characterized into distinct categories, it is important to appreciate that overlap may occur with dysfunction of both the sensory and motor systems. From an anatomic point of view, equine movement disorders may occur as a result of;1. Abnormal muscle cell depolarization. Disorders in the sarcolemma often produce muscle fasciculations or sustained contraction, however, this rarely results in abnormal limb movement. Equine examples include;Myotonia congenita = sarcolemmal chloride channelsHyperkalemic periodic paralysis = sarcolemmal sodium channels2. Abnormal firing within peripheral nerves. Sustained firing of alpha motor neurons can produce painful sustained contractions. Equine examples include;Muscle cramps due to electrolyte derangementsNeuromyotonia caused by ear tick otobius megnini3. Dysfunction of sensory systems. Includes abnormalities of the spinocerebellar and spinothalamic tracts. Abnormalities of the spinocerebellar tracts often result in ataxia and potentially tremor if the cerebellum is involved. Equine examples include;Neuroaxonal dystrophy/equine degenerative myeloencephalopathy, cervical vertebral compressive myelopathy, equine protozoal myeloencephalitis, cerebellar abiotrophy4. Dysfunction of pyramidal system. Abnormalities of the corticospinal tracts (information to motor neurons of the spinal cord) and corticobulbar tracts (information to motor neurons of the cranial nerves) Equine examples include;Cervical vertebral compressive myelopathy, equine degenerative myeloencephalopathy, equine protozoal myeloencephalitis5. Dysfunction of extrapyramidal system. Dystonia characterized by sustained muscle contractions, usually producing twisting and repetitive movements or abnormal body positions with a sustained directional quality. Equine examples includeFluphenazine reactions6. Dysfunction of interneurons within the spinal cord. Abnormalities disrupt smooth coordination of agonist and antagonist muscle contractions often causing myoclonus. Myoclonus is characterized by variable short irregular bursts of muscle contraction. Equine examples include;Tetanus, myoclonus and stiff horse syndrome which affect glycine or GABAergic receptorsHorses have other unique forms of movement disorders that are triggered by a particular form of locomotion and are stylized and repeatable. Stringhalt for example is triggered by walking forward and Shivers is triggered by walking backward or by manuallypicking up the limb. These abnormal movement patterns disappear when the horse trots or canters. Such movement disorders fall into a grey area between the lameness focus of surgeons and the neurologic focus of internal medicine and remain poorly understood. The purpose of our research is to develop a clinical characterization of locomotor-induced movement disorders in horses and to identify pathophysiologic bases for these conditions. In this proceeding, we will provide a clinical characterization of locomotor induced movement disorders, an overview of the neural control of locomotion and some potentially novel hypotheses for the cause of locomotor-induced movement disorders.