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

Abstract

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 TMMEPs 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. Extensor carpi radialis 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 that those with mild clinical signs. Significantly longer CT latencies were found in dogs with moderate and severe SC compression compared to dogs with mild compression. CT TMMEPs could not be recorded in 8/9 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.

Transcranial magnetic motor evoked potentials and magnetic resonance imaging findings in paraplegic dogs with recovery of motor function

“…In a previously published study, TMMEPs could be recorded only in ambulatory dogs after IVDH, which differs from findings in this study, as 7/14 non‐ambulatory dogs had detectable TMMEPs at least in 1 pelvic limb at 1st follow‐up . The reason that TMMEPs cannot be generated from limbs, although motor function is present, remains subject to assumption . It is suspected that the propagating impulses could be insufficient to depolarize the postsynaptic membrane of the motor neuron and thus, propagation of impulses stops and no muscle contraction is detectable …”

“…This finding is in accordance with studies in human cervical spondylotic myelopathy, where TMMEPs could not be elicited in all individuals although motor function was present . Additionally, similar findings were reported in Doberman Pinschers, Great Danes, and horses with cervical spinal cord lesions . In a previously published study, TMMEPs could be recorded only in ambulatory dogs after IVDH, which differs from findings in this study, as 7/14 non‐ambulatory dogs had detectable TMMEPs at least in 1 pelvic limb at 1st follow‐up .…”

“…Dogs were sedated with acepromazine (Vetranquil, CEVA Tiergesundheit GmbH, Düsseldorf, Germany; 0.02‐0.05 mg/kg) and levomethadone/fenpipramide (L‐Polamivet, Intervet Deutschland GmbH, Unterschleißheim, Germany; 0.2‐0.4 mg/kg) IV and TMS was performed in lateral recumbency or sternal positioning. TMMEPs were recorded as described in previous studies with minor modifications . A transcranial magnetic stimulator (Magstim 200², Magstim, Carmarthenshire, UK), capable of producing a maximum 4.0 Tesla (T) magnetic field (correlates with a 100% intensity) with a 50 mm ring coil was used.…”

“…Peak‐to‐peak amplitudes were measured in microvolts (mV) and calculated from the peak of the negative wave to the nadir of the 1st positive wave. Neuronal path length was measured starting at the vertex via the estimated course of nerve fibers to the muscle needle positioned within the cranial tibial muscle, contralateral to the stimulation site …”

“…In veterinary medicine application of TMS has been described as an ancillary tool for evaluation of clinical signs in horses and dogs with spinal cord diseases . Associations of TMMEP data with severity of clinical signs and magnetic resonance imaging (MRI) findings has been described in cervical spondylomyelopathy in Great Danes and Dobermann Pinschers …”