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

The study investigates the neurological mechanisms behind an equine movement disorder known as “Shivers”, finding that it is associated with neuroaxonal degeneration in the deep cerebellar nuclei of horses.

Introduction to “Shivers” and the Study

• “Shivers” is a movement disorder seen in horses, characterized by difficulty in walking backwards, hyperextended limb postures, and hind limb tremors resembling shivering. However, forward movements are generally unaffected, at least initially.
• The exact cause of this disorder and the underlying neuropathic and neuroanatomic mechanisms were previously unknown, prompting this detailed investigation.
• The study carried out an extensive neuroanatomic and neuropathological analysis of the complete sensorimotor system in horses affected by “Shivers”, in comparison with clinically normal control horses.

Investigation and Findings

• No abnormalities were identified in either the hind limb or forelimb skeletal muscles or the associated peripheral nerves in horses with “Shivers”.
• The researchers found eosinophilic segmented axonal spheroids, an abnormal structure found in the axons of nerve cells, to be a common occurrence.
• Calretinin-positive axonal spheroids were present throughout the central nervous system, particularly in the nucleus cuneatus lateralis. However, their numbers were not significantly different from the control horses, suggesting these are not directly related to the Shivers condition.
• On the other hand, calretinin-negative, calbindin-positive, and glutamic acid decarboxylase-positive spheroids were found to be increased by about 80 times in the Purkinje cell axons located 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 horses with Shivers but not in control horses.

Conclusions and Implications

• The immunohistochemical and ultrastructural features of the lesions, along with their functional neuroanatomic distribution, led the researchers to propose that “Shivers” is characterized by end-terminal neuroaxonal degeneration in the deep cerebellar nuclei. This degeneration results in context-specific hypermetria and myoclonus, which are excessive muscle movements and involuntary twitching respectively.
• This study, for the first time, pinpoints a possible pathological process for the “Shivers” equine ailment. The findings could lead to advances in understanding, diagnosing and potentially treating this and related neurological movement disorders in horses.