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Table of Contents    
Year : 2021  |  Volume : 69  |  Issue : 5  |  Page : 1354-1355

Unilateral External Anal Sphincter MEP Monitoring in a Case of Conus Medullaris Tumor

1 Associate Staff Physician, Anaesthesia Institute, Cleveland Clinic, Abudhabi, UAE
2 Neuroanesthesia and Neurocritical Care, Yashoda Hospitals, Secunderabad, Telangana, India

Date of Submission26-Dec-2019
Date of Decision12-Mar-2020
Date of Acceptance14-Jun-2020
Date of Web Publication30-Oct-2021

Correspondence Address:
Nitin Manohar
Department of Neuroanesthesia and Neurocritical Care, Yashoda Hospitals, Secunderabad - 500 003, Telangana
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0028-3886.329530

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 » Abstract 

The use of intraoperative neurophysiological monitoring is a helpful tool during surgeries of conus medullaris tumors and helps in reducing the risk of post-operative functional compromise like paraparesis, sensory deficits, or urological dysfunctions. External anal sphincter (EAS) motor evoked potential (MEP) monitoring is usually done for monitoring sacral nerve roots and to prevent post-operative neurological deficits. Here we describe a case where unilateral motor cortical stimulation aided us to differentiate the laterality of TcMEP responses in EAS.

Keywords: Conus medullaris tumor, external anal sphincter monitoring, motor evoked potentials
Key Message: Unilateral EAS stimulated MEP responses can differentiate false transmitted opposite side reponses.

How to cite this article:
Manohar N, Palan A, Manjunath S T, Manchala RK. Unilateral External Anal Sphincter MEP Monitoring in a Case of Conus Medullaris Tumor. Neurol India 2021;69:1354-5

How to cite this URL:
Manohar N, Palan A, Manjunath S T, Manchala RK. Unilateral External Anal Sphincter MEP Monitoring in a Case of Conus Medullaris Tumor. Neurol India [serial online] 2021 [cited 2021 Nov 30];69:1354-5. Available from:

Surgeries for Conus Medullaris tumors can cause anal sphincter dysfunction and External anal sphincter (EAS) monitoring through Transcranial motor evoked potentials (TcMEP) is a frequently performed modality for Intraoperative neurophysiological monitoring (IONM)[1],[2] in these cases. We describe a case where unilateral motor cortical stimulation aided us to differentiate the laterality of TcMEP responses in EAS.

A 25-year-old female with low back ache radiating to left lower limb since 2 years was posted for laminectomy and excision of tumor under IONM. Magnetic resonance imaging (MRI) showed 1.66 × 1.07 cm Conus Medullaris tumor with focal cystic areas at L2-L3 level. She had no preoperative neurological deficits. General anesthesia was induced with IV fentanyl 2 mcg/kg, propofol 2 mg/kg, atracurium 0.5 mg/kg and intubated. Anesthesia was maintained with Bispectral index (BIS) guided total intravenous anesthesia (TIVA) with 1% propofol (Target controlled infusion with Schinder model and target plasma concentration kept between 3 and 3.5 mcg/ml, titrated to maintain BIS between 30 and 40) with mixture of 50:50 oxygen and air. We placed C3, C4 Corkscrew electrodes for transcranial motor evoked potential (TcMEP) stimulation according to 10–20 international system. We placed paired needle electrodes in four muscle groups of bilateral lower limbs (Vastus lateralis, Tibialis anterior, Abductor hallucis longus, Extensor hallucis longus) and one muscle in right upper limb (adductor policis brevis) as control for recording of TcMEP. We also placed paired needle electrodes at 2 O' clock 4 O' clock on right side and 8 O' clock 10 O' clock on left side for recording MEP responses from bilateral EAS.

For TcMEP stimulation, we used lowest voltage (starting from 100 V to 150 V) to obtain satisfactory baseline responses of anal sphincter MEP to avoid to bilateral response due to ipsilateral anodal stimulation of cortex and contralateral cathodal stimulation of the sub-cortical descending fibers at higher voltage stimulation. We got good baseline TcMEP recordings at monophasic stimulation of 150 Voltage, pulse duration of 75 microseconds and train count of 8. Intraoperatively during resection of the lesion we observed disappearance of left lower limb muscle groups TcMEP responses except the left EAS [Figure 1]a. After performing a detailed checklist to rule out hemodynamic, metabolic, anesthetic, and technical factors we performed a unilateral C4 stimulation to check whether the EAS response on left side was a true response or due to concomitant stimulation of right EAS caused by transmitted response due to proximity of electrodes placed around the EAS. When we stimulated unilateral motor cortex with the same stimulation parameters on right side (C4), we could record response in the left side EAS only and not in any other muscle group [Figure 1]b. Then on stimulating unilateral C3 we could record TcMEP in all muscle groups of right side and also right side EAS and no responses were recorded from left lower limb muscle groups including left side EAS [Figure 1]c. With selective unilateral transcranial motor cortex stimulation and obtaining responses from contralateral side, we could confirm that the initial response on left side EAS was a true response. We warned the surgeon about the disappearance of MEPs in left-sided muscle groups except EAS. The surgeon also confirmed that retraction was being applied on left lumbar nerve roots to approach the tumor. Immediate corrective measures (Release of retraction, Warm saline irrigation) were taken and TcMEP responses reappeared bilaterally including the left lower limb muscle groups [Figure 1]d. The surgery was completed thereafter with limited retraction. Satisfactory TcMEPs were recorded thereafter in all the muscle groups bilaterally and patient woke up without any neurodeficits.
Figure 1: (a) Disappearance of left lower limb TcMEP responses except EAS on bilateral motor cortex (C3 and C4) stimulation. (b) Unilateral C4 motor cortex stimulation with only left side EAS TcMEP response. (c) Unilateral C3 motor cortex stimulation with TcMEP responses in all right-sided muscle groups. (d) Gradual reappearance of left lower limb TcMEP responses after corrective measures

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The external anal sphincter in humans is a circular muscle and both the right and left part of this muscle function together.[3] The external anal sphincter has been shown to have bilateral representation and issues related to laterality of innervation are not yet resolved.[3],[4],[5] Alteast in primates, overlapping innervation is seen. Previous human data shows asymmetric dominance in some subjects.[4],[5] Further larger studies are required to elicit technique of unilateral EAS TcMEP.

 » Conclusion Top

In our case unilateral transcranial stimulation successfully differentiated a true drop from a transmitted false response in anal sphincter monitoring and thus helped us in taking corrective measures.

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

Kothbauer KF, Deletis V. Intraoperative neurophysiology of the conus medullaris and cauda equina. Childs Nerv Syst 2010;26:247-53.  Back to cited text no. 1
Haghighi SS, Zhang R. Activation of the external anal and urethral sphincter muscles by repetitive transcranial cortical stimulation during spine surgery. J Clin Monit Comput 2004;18:1-5.  Back to cited text no. 2
Wunderlich M, Swash M. The overlapping innervation of the two sides of the external anal sphincter by the pudendal nerves. J Neurol Sci 1983;59:97-109.  Back to cited text no. 3
Hamdy S, Enck P, Aziz Q, Uengoergil S, Hobson A, Thompson DG. Laterality effects of human pudendal nerve stimulation on corticoanal pathways: Evidence for functional asymmetry. Gut 1999;45:58-63.  Back to cited text no. 4
Turnbull GK, Hamdy S, Aziz Q, Singh KD, Thompson DG. The cortical topography of human anorectal musculature. Gastroenterology 1999;117:32-9.  Back to cited text no. 5


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