What is the role of motor evoked potentials (MEPs) in the diagnosis and treatment of multiple sclerosis?

Updated: Aug 20, 2019
  • Author: Jasvinder Chawla, MD, MBA; Chief Editor: Selim R Benbadis, MD  more...
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The diagnosis of multiple sclerosis (MS) is based on the detection of multiple inflammatory, demyelinating white matter lesions, which are disseminated in time and space. In many patients, clinical assessment is insufficient and paraclinical studies must be performed. These tests, such as MRI, cerebrospinal fluid (CSF) studies, visual evoked potentials (VEPs), and somatosensory evoked potentials (SEPs), may be used in conjunction with motor evoked potential (MEP) studies to establish the diagnosis of MS.

The duration of MEPs is increased in patients with definite (MS), a phenomenon that is compatible with increased temporal dispersion of the impulses that arrive at the motor neuron pool. MEP studies may have 2 applications in MS: (1) as a diagnostic tool and (2) as an index of corticospinal pathway dysfunction. [8]

In MS, a prolongation of the CMCT may be explained by reduced stimulus conduction in the large-diameter corticospinal fibers; this phenomenon is caused by demyelination or incomplete remyelination. The temporal stimulus summation that is necessary for large motor anterior cells to discharge may be reduced. Dispersion of the conduction velocity of individual axons may be increased.

In patients with definite MS, a correlation between CMCT and manual dexterity (but not muscle strength) has been reported. The lack of correlation between CMCT and isometric muscle strength, hyperreflexia, or spasticity is attributed to the role of the fast-conducting pyramidal tract in generating rapid phasic muscle action important for fine motor skills but not for strength of muscle contraction (which can be carried by different descending tracts).

MEP studies can be useful for monitoring the electrophysiologic correlates of clinical response to treatment. In patients who show clinical improvement after steroid treatment, CMCT decreases toward normal values and can remain stable if disease activity is controlled.

MEPs are more sensitive than SEPs in MS, with an overall incidence of abnormality greater than 70%. In patients with definite MS, prolonged CMCT to muscles in the lower extremities is observed in 77-89% of patients, while abnormal VEP findings are observed in 74.4% of these patients.

Recording MEPs in upper and lower extremities increases the sensitivity of the study, because lesions that are caudal to C8 can be detected. Nociti and colleagues have shown that SEPs reflect the upper limb motor performance in MS. [9, 10]

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