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

Background: Cerebellar Purkinje cell axonal degeneration has been identified in horses with shivering but its relationship with abnormal hindlimb movement has not been elucidated. Objectives: To characterise surface electromyographic (sEMG) hindlimb muscle activity in horses with shivering, correlate with clinical scores and examine horses for Purkinje axonal degeneration. Study design: Descriptive controlled clinical study. Methods: The hindlimb of seven shivering and six control draught horses were clinically scored. Biceps femoris (BF), vastus lateralis (VL), tensor fasciae latae and extensor digitorum longus were recorded via sEMG during forward/backward walking and trotting. Integrated (iEMG) and peak EMG activity were compared between groups and correlated with clinical locomotor exam scores. Sections of the deep cerebellar nuclei (DCN) of six of the seven shivering horses were examined with calbindin immunohistochemistry. Results: In control horses, backward walking resembled forward walking (right hindlimb peak EMG: backward: 47.5 ± 21.9%, forward: 36.9 ± 15.7%) but displayed significantly higher amplitudes during trotting (76.1 ± 3.4%). However, in shivering horses, backward walking was significantly different from forward (backward: 88.5 ± 21.5%, forward: 49.2 ± 8.9%), and resembled activity during trotting (81.4 ± 4.8%). Specific to backward walking, mean sEMG amplitude fell outside two standard deviations of mean control sEMG for ≥25% of the stride in the BF for all seven and the VL for six of the seven shivering horses. Locomotor exam scores were correlated with peak EMG (r = 0.87) and iEMG (r = 0.87). Calbindin-positive spheroids were present in Purkinje axons in DCN of all shivering horses examined. Main limitations: The neuropathological examination focused specifically on the DCN and, therefore, we cannot fully exclude additional lesions that may have influenced abnormal sEMG findings in shivering horses. Conclusion: Shivering is characterised by abnormally elevated muscle recruitment particularly in BF and VL muscles during backward walking and associated with selective Purkinje cell distal axonal degeneration.