Motor evoked potentials in patients with spinal disorders: upper and lower motor neurone affection

Motor evoked potentials (MEP) following magnetoelectric stimulation allow for detection and quantification of a lesion of the central and peripheral motor pathways. MEP latency is a very sensitive parameter, irrespective whether the lesion affects the upper motor neurone, e.g. in cervical myelopathy, or the lower motor neurone e.g. in cervical or lumbar radiculopathy. However, an increase of MEP latency alone does not allow for a distinction of either upper motor neurone or lower motor neurone affection. We have therefore scrutinized MEP latency and MEP waveform (amplitude, duration, and number of phases) in normal subjects, as well as in patients with cervical myelopathy (upper motor neurone affection: UMNA) and in patients with radiculopathies (lower motor neurone affection, LMNA). The increase of central motor latency compared to normal values was significantly higher in UMNA than in LMNA. MEP following transcranial magnetoelectric stimulation were split up and their duration in relation to M-wave duration was significantly increased in patients with UMNA, yet normal in patients with LMNA. MEP amplitude in relation to M-wave amplitude was significantly decreased in patients with UMNA, yet normal in patients with LMNA. We conclude that cervical myelopathy and cervical or lumbar radiculopathies lead to changes of MEP waveform, which are specific for either upper or LMNA. The analysis of amplitude, duration, and number of phases of MEP following transcranial stimulation increases the diagnostic value of MEP in disorders of the spine affecting the central or proximal peripheral motor pathways.