Transcranial magnetic motor evoked potentials in Great Danes with and without clinical signs of cervical spondylomyelopathy: association with neurological findings and magnetic resonance imaging

Transcranial magnetic motor evoked potentials (TMMEPs) assess the functional integrity of the descending motor pathways, which are typically compromised in canine cervical spondylomyelopathy (CSM). The objective of this prospective study was to establish the reference ranges of TMMEP latency and amplitude in clinically normal (control) Great Danes (GDs), compare TMMEPs obtained in GDs with and without CSM, and determine whether there is any association between TMMEP data and severity of neurological signs or magnetic resonance imaging (MRI) findings. Twenty-nine client-owned GDs were enrolled (15 controls, 14 CSM-affected). All dogs underwent TMMEPs under sedation, and latencies and amplitudes were recorded from the extensor carpi radialis (ECR) and cranial tibial (CT) muscles. MRI of the cervical vertebral column was performed to evaluate the presence and severity of spinal cord (SC) compression, and the presence of SC signal changes. ECR and CT latencies were significantly longer in CSM-affected than control GDs. No significant differences between groups were found for amplitudes or neuronal path lengths. For the CT TMMEPs, CSM-affected GDs with moderate and severe clinical signs had significantly longer latencies than those with mild clinical signs. Significantly longer CT latencies were found in dogs with moderate and severe SC compression compared with dogs with mild compression. CT TMMEPs could not be recorded in 7/9 CSM-affected GDs with SC signal changes. These results provide a reference range for TMMEPs of clinically normal GDs. The use of TMMEPs is a valid ancillary test to assess the integrity of motor pathways in GDs with CSM.