The resection of lesions within the substance of the cord is more complex. It carries a risk of selective damage to the motor tract, which may not be reflected by SEP changes,² and SEPs can even be recordable in paralysed individuals prior to surgery.³ Moter-evoked potential (MEP) monitoring is based on the cumulative understanding of the motor system acquired since the 1950s,^4,5 when a small but essential fibre population in the corticospinal tract was identified and found to give rise to a recordable travelling wave, then termed the D-wave. After the development of transcranial electrical motor cortex stimulation in humans,⁶ this knowledge was applied in the operating room.^7,8 Muscle recording techniques⁹ were hampered by the effects of general anaesthesia on the α-motor neurons. This was resolved by the multipulse stimulation technique.¹⁰ Thus, two techniques to monitor the functional integrity of the motor system are now available: the D-wave and muscle MEPs. The practical application of these in various types of spine and spinal cord surgery were refined during the 1990s.3,^11–14 More recently, very strong evidence for the benefit of MEP monitoring for spinal cord surgery was reported.¹⁵

Neurophysiology
Motor potentials are evoked with transcranial (through the skin and skull) electrical stimulation of the motor cortex of the brain. Electrical stimulation is then performed with rectangular constant current impulses of 500μs duration and intensities between 15 and 200mA. Individual stimuli elicit D-waves,⁴ which can be recorded directly from the spinal cord caudal to the site of surgery. Depending on the level on the spinal cord where the recording electrode is placed, the latencies are quite short, never exceeding 20ms. Baseline recordings are obtained before the opening of the dura. The stimulations are repeated at a rate of 0.5–2Hz during the critical part of the procedure. This provides fast, ‘online’ feedback. The important D-wave parameter is its amplitude. A decrease of more than 50% of the baseline value is associated with a long-term motor deficit.¹⁶ Latency changes of the D-wave are rare and are the result of non-surgical influences such as temperature.¹⁷ Higher stimulation intensities lead to shorter latencies, implying that the corticospinal fibre activation occurs deeper in the white matter of the brain.⁷

Muscle MEPs are elicited in the same way, although not with single stimuli but with a short train of five to seven stimuli with 4ms interstimulus intervals.^18,19 Therefore, this is called the multipulse technique.¹⁴ Compound muscle action potentials are recorded with needle electrodes from target muscles in all four extremities (thenar, anterior tibialis and abductor hallucis). Other muscles, such as the quadriceps, hamstrings, biceps or the diaphragm, and even the anal sphincter, can be used if required. Realtime feedback is possible here as well, and in most cases is even easier than the D-wave. Muscle MEPs are recorded in an alternating fashion with D-waves. An individual electrical stimulus on the motor cortex, either with exposed cortex or transcranial stimulation,²⁰ elicits a D-wave in the corticospinal tract. A fast train of five stimuli at 250Hz elicits five D-waves, which then travel down the corticospinal tract 4ms apart. The spinal α-motor neurons are hit by five consecutive D-waves elevating their membrane potential above firing threshold.⁵ The parameter monitored is the presence or absence of muscle MEPs in the target muscles within a stimulus intensity range of approximately 15–200mA. This all-or-nothing concept has been adopted because of the tremendous variability of muscle MEP amplitudes^11,21,22 and because a motor deficit occurred only when the muscle response was lost.^3,9,11,22 To define a threshold amplitude below which one expects an intraoperative injury1⁴ is difficult, even though it appears logical that a stimulus threshold increase vis-à-vis stable anaesthetic depth may indicate some degree of subclinical injury.