CHAPTER 7 – ELECTRODIAGNOSTIC EVALUATION OF THE SPINAL CORD TRACTS

OBJECTIVES- To provide a brief overview of somatosensory evoked potentials

(SEPs) and motor evoked potentials (MEPs)

- To discuss briefly the different clinical applications of SEPs and MEPs

SOMATOSENSORY EVOKED POTENTIALS

• SEPs are elicited with electrical stimulation delivered transcutaneously to a mixed or sensory nerve, or sometimes to the skin of the territory of an individual nerve or nerve root (dermatome)

• The most commonly stimulated nerves are the median and ulnar nerve at the wrist and the posterior tibial nerve at the ankle

• Recorded with either surface or needle electrodes, as well from the nerve proximal to the site of stimulation, as over the spine and scalp (mostly according to the international 10-20 system)

SOMATOSENSORY EVOKED POTENTIALS

• Clinical Applications

• It provides limited information on the exact location of the lesions proximal to the dorsal root ganglion

• Helpful in diagnosis of numerous spinal disorders, determining prognosis, evaluating treatment, and in following up patients

• SEPs seem to be rather sensitive in predicting outcome in the acute phase of SCI and stroke (when SEPs are absent, prognosis seems to be poor)

• Helpful in evaluation of the severity and level of the lesion in cervical spondylytic myelopathy

• Patients in coma are unlikely to recover from their condition when the cortical responses of SEPs are bilaterally absent

MOTOR EVOKED POTENTIALS

• MEP procedure consists of transcranial stimulation followed by measurement of the compound muscle action potential (CMAP) from different limb and trunk muscles

• Can be elicited with magnetic brain stimulation or transcutaneous electrical stimulation

MOTOR EVOKED POTENTIALS

• Clinical Applications

• Multiple sclerosis – delayed CMCTs, absent MEPs in patient’s with marked clinical disability

• ALS – prolonged or absent MEPs • SCI – strong correlation seen between MEP findings and motor

function• Decreased MEP amplitudes or absent MEP responses are more

frequently seen in neoplastic lesions• MEP with more often increased latencies are seen in

inflammatory lesions

MOTOR EVOKED POTENTIALS

• Clinical Applications

• Spondylotic myelopathy • CMCT has been reported to correlate well with clinical and

radiological signs of cord compression• MEP > SEP abnormality in cervical spondylosis

• Lumbosacral radiculopathies – electrical stimulation is useful versus TMS

• Cervical radiculopathies – both electrical stimulation and TMS are not so useful detectors

• Stroke – prognostic value (if MEPs are present during the acute phase, outcome usually is favorable; however, absent MEPs are not correlated with poor outcome)

SUMMARY

• Somatosensory evoked potentials and its clinical applications ✔

• Motor evoked potentials and its clinical applications ✔

CHAPTER 7 – ELECTRODIAGNOSTIC EVALUATION OF THE SPINAL CORD TRACTS

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