The Equine Movement Disorder “Shivers” Is Associated With Selective Cerebellar Purkinje Cell Axonal Degeneration

Abstract

''Shivers'' is a progressive equine movement disorder of unknown etiology. Clinically, horses with shivers show difficulty walking backward, assume hyperflexed limb postures, and have hind limb tremors during backward movement that resembles shivering. At least initially, forward movements are normal. Given that neither the neurophysiologic nor the pathologic mechanisms of the disease is known, nor has a neuroanatomic locus been identified, we undertook a detailed neuroanatomic and neuropathologic analysis of the complete sensorimotor system in horses with shivers and clinically normal control horses. No abnormalities were identified in the examined hind limb and forelimb skeletal muscles nor the associated peripheral nerves. Eosinophilic segmented axonal spheroids were a common lesion. Calretinin-positive axonal spheroids were present in many regions of the central nervous system, particularly the nucleus cuneatus lateralis; however, their numbers did not differ significantly from those of control horses. When compared to controls, calretinin-negative, calbindin-positive, and glutamic acid decarboxylase-positive spheroids were increased 80-fold in Purkinje cell axons within the deep cerebellar nuclei of horses with shivers. Unusual lamellar or membranous structures resembling marked myelin decompaction were present between myelin sheaths of presumed Purkinje cell axons in the deep cerebellar nuclei of shivers but not control horses. The immunohistochemical and ultrastructural characteristics of the lesions combined with their functional neuroanatomic distribution indicate, for the first time, that shivers is characterized by end-terminal neuroaxonal degeneration in the deep cerebellar nuclei, which results in context-specific hypermetria and myoclonus.

Abnormal locomotor muscle recruitment activity is present in horses with shivering and Purkinje cell distal axonopathy

“…However, because our neuropathological examination focused specifically on the DCN, we cannot fully exclude additional lesions that may have influenced the abnormal sEMG findings in shivering horses. Conversely, the presumed distal axonal degeneration, when severe, may be expected to result in Purkinje cell chromatolysis, or other evidence of degeneration, none of which could be detected .…”

“…Furthermore, the numbers of end terminal synapses within the DCN appeared to be reduced with degeneration being most evident in the lateral nuclei (dentate and interpositus). Given the findings of the study by Valberg et al (2015), we evaluated only the region of the DCN with HE and calbindin stains [8], which is a potential limitation of this study. Another limitation of the current study is that control horses were not subjected to euthanasia to evaluate the DCN and thus comparisons are made with a limited number of control horses from a previous study.…”

“…Although rare, axonal spheroids have been found in control horses. However, shivering horses exhibited greater than an 80‐fold increase in the number of axonal spheroids compared with controls . Apart from lesions in the DCN, no other pathological lesions within the muscular, central and peripheral nervous systems specific to horses with shivering were identified.…”

Assessing pathological changes within the nucleus ambiguus of horses with recurrent laryngeal neuropathy: An extreme, length‐dependent axonopathy

“…Specific stains were chosen to identify common pathological features (recorded on a continuous scale of how many cell body profiles were affected or as a dichotomous result) associated with central nervous system degeneration; cell body profile numbers, chromatolysis, nuclear margination or fragmentation, a shrunken or swollen cell body profile, presence of axonal spheroids, vacuolation (present or absent), inclusion bodies, demyelination, or a glial response (present or absent) were all assessed. Sequential brainstem sections were stained with hematoxylin and eosin (H&E), LFB, and cresyl violet (CV); astrocytes were identified by immunohistochemical labeling of glial fibrillary acidic protein (GFAP) . Slides were first dewaxed in xylene for at least 20 minutes at room temperature, before being rehydrated through a graded series of alcohols (100%, 100%, 90%, and 70%), and finally placed in Tris‐buffered saline (TBS) for 5 minutes, and then treated with 3% hydrogen peroxide (in methanol) for 15 minutes, at room temperature, before being rinsed in TBS.…”

“…Sequential brainstem sections were stained with hematoxylin and eosin (H&E), LFB, and cresyl violet (CV); astrocytes were identified by immunohistochemical labeling of glial fibrillary acidic protein (GFAP). 34 Slides were first dewaxed in xylene for at least 20 minutes at room temperature, before being rehydrated through a graded series of alcohols (100%, 100%, 90%, and 70%), and finally placed in Tris-buffered saline (TBS) for 5 minutes, and then treated with 3% hydrogen peroxide (in methanol) for 15 minutes, at room temperature, before being rinsed in TBS. Blocking with 10% goat serum in TBS was then performed for 60 minutes.…”