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Correlating MEPs and Preop Motor Function

Posted By Lanjun Guo, MD, MSc, DABNM, FASNM, Tuesday, June 5, 2018

This blog post will summarize a paper recently published by a member of the ASNM's Board of Directors, Lanjun Guo, MD, MSc, DABNM, FASNM. Dr. Guo trained as a neurosurgeon in China and is now a prominent clinical neurophysiologist practicing in California. She is active in multiple societies, including the ASNM and ISIN. The post below was written by Dr. Guo. Thanks for reading! RV

The Correlation Between Recordable MEPs and Motor Function During Spinal Surgery for Resection of Thoracic Spinal Cord Tumor

This paper examined the association between preoperative motor function of patients’ lower extremities and intraoperative motor evoked potential (MEP) recording.

 Patients undergoing thoracic spinal cord tumor resection were studied. Patients’ motor function was checked immediately before the surgical procedure. MEP responses were recorded from the tibialis anterior and foot muscles, and the hand muscles were used as control. Electrical current with train of eight pulses, 200 to 500 volts was delivered through two corkscrews placed at C3’ and C4’ sites. Anesthesia was maintained by total intravenous anesthesia (TIVA) using a combination of propofol and remifentanil after induction with intravenous propofol, remifentanil, and rocuronium. Rocuronium was not repeated. Bispectral Index was maintained between 40 to 50.

From 178 lower limbs of the 89 patients,  myogenic MEPs (m-MEPs) could be recorded from 100% (105/105) of the patients with 5 out of 5 motor strength in lower extremity; 90% (36/40) from the patients with 4/5 motor strength; only 25 % (5/20) with 3/5; and 12.5% (1/8) with 2/5 motor strength; None (0/5) were able to be recorded if the motor strength was 1/5. Therefore, it was concluded that the ability to record m-MEPs is closely associated with the patient’s motor function. They are difficult to obtain if motor function is 3/5 motor strength in the lower extremity. They are almost impossible to record if motor function is worse than 3/5.

Excerpt: Manual Muscle Test Grading Scale

Number Clinical Exam 
 0  No muscle movement. Flaccid paralysis.
 1  Visible muscle twitch, but no movement at the joint.
 2  Able to move in horizontal plane, but not against gravity.
 3  Able to move against gravity, but not against resistance.
 4  Able to move against resistance, but less than normal.
 5  Full strength against resistance

 Generation of m-MEPs depends on the excitability of the alpha-motor neurons in the anterior horns and excitability of the neuromuscular junction. Muscle MEPs can be generated only if the resting potential of alpha-motor neurons reaches the firing threshold, and thus, transmits this activity via motor axons of the peripheral nerves and neuromuscular junctions to the muscle.

The m-MEPs are affected by anesthetic drugs. Anesthetics impair the motor cortex’s ability to generate multiple descending volleys, the I waves. They also depress the excitability of the entire spinal cord, including the alpha-motor neuron pool. Because the D wave is resistant to anesthetic depression, the anesthetic effect at the alpha-motor neurons can be overcome at low anesthetic concentrations by high-frequency multipulse stimulation through transcranial stimulation.   The multiple D waves followed by stimuli to the motor cortex summate at the anterior horn cell to generate a subsequent myogenic response. The temporal accumulation of several cortico-motoneuronal excitatory postsynaptic potentials (EPSPs) is necessary to bring motor neurons from the resting state to the firing threshold during general anesthesia.

 However, transcranial stimuli only activate a small and variable subpopulation of the lower motor neuron pool to generate MEPs. Therefore, the m-MEPs are substantially more difficult to record in patients with underlying neurological abnormalities, such as spinal cord tumor. In practice, although a patient may maintain some motor function and can move their legs, MEPs may still not be recordable from muscles of the lower extremity.  There are previous studies correlating intraoperative recordings of m-MEPs during different types of spine surgery with the preoperative motor function, although the detailed information about the relation between the recordable MEPs and the grade of motor function were not reported.

 There are a number of methodological considerations in this study. The number of lower limbs with poor grade function was relatively small, only 33 lower limbs with 3/5 grade or less compared to 145 lower limbs with 5/5 or 4/5 grade. Different stimulation methods, such as different stimulating sites on the skull, different stimulation inter-stimulus interval, and /or different stimulating pulses, were not compared. Therefore, the recordable m-MEPs rate in clinical practice may be higher if different stimulating montages were tested.

 The current study provided evidence and confirmed the clinic experience that it can be difficult to obtain m-MEPs during a surgery when the patient has motor weakness, even the patient could still move legs. It also indirectly provide the information that if MEPs lost during surgery due to surgical manipulation, the patients may still have some motor function postoperatively, but most likely that would be worse than 3/5 motor strength.

References:

Guo L, Li Y, Han R, Gelb AW. The Correlation Between Recordable MEPs and Motor Function During Spinal Surgery for Resection of Thoracic Spinal Cord Tumor. J Neurosurg Anesthesiol. 2018 Jan;30(1):39-43.

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