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Year : 2019  |  Volume : 13  |  Issue : 1  |  Page : 88-89

Motor-evoked potential monitoring: A valuable tool for the diagnosis of position-related neurologic deficits

1 Department of Neuroanaesthesiology and Critical Care, All India Institute of Medical Sciences, New Delhi, India
2 Department of Physiology, All India Institute of Medical Sciences, New Delhi, India

Correspondence Address:
Dr. Girija P Rath
Department of Neuroanaesthesiology and Critical Care, Neurosciences Centre, A.I.I.M.S., New Delhi - 110 029
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/sja.SJA_710_18

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Date of Web Publication28-Dec-2018

How to cite this article:
Khandelwal A, Lamsal R, Rath GP, Netam RK, Bir M. Motor-evoked potential monitoring: A valuable tool for the diagnosis of position-related neurologic deficits. Saudi J Anaesth 2019;13:88-9

How to cite this URL:
Khandelwal A, Lamsal R, Rath GP, Netam RK, Bir M. Motor-evoked potential monitoring: A valuable tool for the diagnosis of position-related neurologic deficits. Saudi J Anaesth [serial online] 2019 [cited 2022 Jun 27];13:88-9. Available from:

Transcranial motor-evoked potentials (TcMEPs) monitors integrity of corticospinal tract during spinal surgery and allows corrective measures being ensured before permanent neuronal dysfunction sets in.[1],[2] It has the potential to diagnose position-related complications. We report a case of a 24-year-old male (55 kg/165 cm) who presented with complaints of weakness of bilateral upper limbs and neck pain which was radiating to shoulders for 2 years. There was no bowel or bladder involvement. Radiologic imaging of craniovertebral junction revealed atlanto-axial dislocation (AAD) with cord compression at cervicomedullary junction and C3 level. Dynamic X-ray of neck showed mobile AAD. In the operating room, anesthesia was induced with fentanyl and propofol. Tracheal intubation was facilitated using rocuronium. It was done with neck in neutral position, with the use of video laryngoscope and manual-in-line stabilization. Anesthesia was maintained with oxygen and air (1:1) and continuous infusion of fentanyl and propofol titrated to maintain bispectral index between 40 and 50. No muscle relaxant was administered in view of TcMEP monitoring. The patient was positioned prone and reduction of AAD was done by extension and traction; it was confirmed with an image intensifier. During this positioning, there was a sudden decrease in heart rate from 71 to 44 beats/min along with mild hypotension (94/58 from 104/60 mmHg). Meanwhile, the train-of-four parameter in neuromuscular monitor was achieved more than 85%. An attempt was made to record the baseline amplitude and latency of motor-evoked potential (MEP). However, there was minimal response from the monitored muscles even at a stimulation voltage of 1000 V [Table 1]. The transcranial stimulation was provided in the form of train-of-eight square wave pulses of 75 μs duration each, and a frequency of 300 pulses/second. Due to persistent bradycardia and minimal MEP response from the monitored muscles, the neck extension was reduced which resulted heart rate increasing close to baseline values (68–69 beats/min). The amplitude of MEP was also better appreciated at 400 V [Table 1]. Thus, amplitude readings at 400 V were accepted as baseline and the same stimulation voltage was utilized throughout the surgery. The surgery (occipito-C4-C5 fixation and C1-C2-C3 laminectomy) completed uneventfully without any further episodes of bradycardia. Motor power in few muscles of the upper limbs improved during postoperative period [Table 2].
Table 1: Recording of MEP amplitudes in various muscles at 1000 and 400 V

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Table 2: Muscle power in the preoperative and postoperative period

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This case highlights two important aspects. First, baseline MEP readings should ideally be recorded in supine position before the patient placed prone for spine surgery. We took baseline recordings after making the patient prone in order to utilize time as the effect of muscle relaxant had not worn off during supine position. The effect of position with regard to amplitudes can only be appreciated if the baseline recordings are made in supine. The clinching factor for cervical cord compression was bradycardia in addition to requirements of higher voltage to elicit muscle responses. This prompted us to modify the neck position.

Second, it should be borne in mind that muscles with intact power would not require voltage as high as 1000 V to elicit a response unless they are under the effect of muscle relaxant or have preexisting spasticity. In our case, the muscle power of lower limbs was intact during preoperative period, and thus, the poor response to high MEP currents before the commencement of surgery seemed unjustifiable. Moreover, other factors, such as temperature, oxygenation and ventilation, hematocrit, and anesthetic concentrations were kept constant. Thus, MEP monitoring is a valuable tool for diagnosing position-related neurologic deficits in addition to surgical complications.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

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Conflicts of interest

There are no conflicts of interest.

  References Top

Hilibrand AS, Schwartz DM, Sethuraman V, Vaccaro AR, Albert TJ. Comparison of transcranial electric motor and somatosensory evoked potential monitoring during cervical spine surgery. J Bone Joint Surg Am 2004;86:1248-53.  Back to cited text no. 1
Segura MJ, Talarico ME, Noel MA. A multiparametric alarm criterion for motor evoked potential monitoring during spine deformity surgery. J Clin Neurophysiol 2017;34:38-48.  Back to cited text no. 2


  [Table 1], [Table 2]


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