Intraoperative neurophysiological monitoring of the bulbocavernosus reflex during surgery for conus spinal lipoma: what are the warning criteria?

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OBJECTIVE

Despite the surge in the intraoperative use of the bulbocavernosus reflex (BCR) during lumbosacral surgeries, there are as yet no widely accepted BCR warning criteria for use with intraoperative neurophysiological monitoring (IONM). The author’s aim was to find clinically acceptable warning criteria for use in IONM of the BCR.

METHODS

Records of IONM of the BCR in 164 operations in 163 patients (median age 5 months) with a conus spinal lipoma who underwent surgery between August 2002 and May 2016 were retrospectively analyzed. The outcomes of IONM of the BCR were grouped by the residual amplitude at the end of surgery: group 1, ≥ 50%; group 2, 25%–50% (including the lower bound, but not the upper); and group 3, < 25%. Cases in which the BCR was lost were separately assessed as a subgroup of group 3. The postoperative urinary complication rate was used to verify the warning criteria zones.

RESULTS

The BCR could be monitored in 149 surgeries (90.9%). There were 118 surgeries (79.2%) in group 1, 18 (12.1%) in group 2, and 13 (8.7%) in group 3. Two surgeries (11.1%) in group 2 and 6 (46.2%) in group 3 resulted in urinary complications. In the group 3 subgroup (lost BCR), all 5 surgeries resulted in urinary complications. The cutoff value of the BCR amplitude reduction was placed between groups 1 and 2 (zone 1: cutoff 50%), groups 2 and 3 (zone 2: cutoff 25%), and group 3 and its subgroup (zone 3: cutoff zero, present or lost). In zone 1, the positive predictive value (PPV) was 25.8% and the negative predictive value (NPV) was 100%. In zone 2, the PPV was 53.8% and the NPV 98.5%. In zone 3, the PPV was 100% and the NPV 97.9%. The PPV was highest in zone 3. The NPV was highest in zone 1, but its PPV was low (25.8%).

CONCLUSIONS

The “lost or remained” criterion of BCR amplitude (zone 3: cutoff zero) can be used as a predictor of postoperative urinary function. As a warning criterion, the cutoff value of the BCR amplitude reduction at 75% (zone 2) may be used. This preliminary clinical report on the warning criteria for the BCR may contribute to improving the safety of surgery for conus spinal lipoma.

ABBREVIATIONS BCR = bulbocavernosus reflex; CEPA = continuous evoked potential array; CIC = clean intermittent catheterization; IONM = intraoperative neurophysiological monitoring; NCCHD = National Center for Child Health and Development; NGB = neurogenic bladder; NPV = negative predictive value; PPV = positive predictive value; TMCMC = Tokyo Metropolitan Children’s Medical Center; UDS = urodynamic study.

Article Information

Correspondence Nobuhito Morota: Kitasato University School of Medicine, Sagamihara, Kanagawa, Japan. nobu.m01@gmail.com.

INCLUDE WHEN CITING Published online February 22, 2019; DOI: 10.3171/2018.12.PEDS18535.

Disclosures The author reports no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.

© AANS, except where prohibited by US copyright law.

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Figures

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    Classification based on BCR amplitude changes. Outcomes of IONM of the BCR were grouped according to the rate of closing BCR amplitude compared to the baseline: group 1, closing BCR amplitude ≥ 50% of baseline; group 2, 25%–50% of baseline (including the lower bound but not the upper bound); group 3, < 25% of baseline. Cases in which the BCR was lost (closing BCR = 0) were separately assessed as a subgroup of group 3. The cutoff value for IONM of the BCR was assumed to lie between groups 1 and 2 (zone 1: cutoff 50%), groups 2 and 3 (zone 2: cutoff 25%), and group 3 and its subgroup (zone 3: cutoff zero, present or lost).

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    Illustrative case 1: 10-year-old boy with type 2 spinal lipoma. A: Baseline and closing BCR readings. The R1 was used for IONM and measurement of the BCR amplitude change. In this case, the left BCR was used to measure the BCR amplitude change (group 1). B: Continuous evoked potential array (CEPA) of the BCR was stable during surgery (arrow indicates the time trend from bottom to top). C: Preoperative and postoperative axial T1-weighted MR images. The postoperative image demonstrates sufficient removal of the spinal lipoma and untethering of the spinal cord.

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    Illustrative case 2: type 1 spinal lipoma in a 2-month-old boy. A: Baseline BCR reading with the stimulation from 5 mA to 40 mA. The baseline BCR had a small amplitude but was monitorable. B: CEPA showed stable BCR readings during radical removal of the spinal lipoma (arrow indicates time trend from bottom to top). C: Closing BCR reading with the stimulation from 5 mA to 40 mA showing improved BCR amplitude (group 1). D: Preoperative and postoperative sagittal T1-weighted MR images. The postoperative image demonstrates near-total removal of the spinal lipoma.

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    Illustrative case 3: type 1 spinal lipoma in a 2-month-old girl. A: Three sequences of CEPA were composed (arrow shows time trend from the bottom to top). The BCR fluctuated during surgery. It was lost once and partially recovered. Then, following an increase in amplitude, the BCR waveform disappeared and recovered again with reduced amplitude. B: The baseline and closing BCR records with the stimulation from 5 mA to 30 mA showed the amplitude decreased more than 75% but remained (group 3). C: Preoperative and postoperative sagittal T1-weighted MR images. The postoperative image demonstrates partial removal of the spinal lipoma.

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    Illustrative case 4: type 3 spinal lipoma in a 3-year-old boy. A: Two sequences of CEPA are shown (arrow on left shows time trend from bottom to top). The BCR was stable at the beginning but was abruptly lost near the end of lipoma removal. The BCR never recovered (group 3). B: The baseline and closing BCR recordings with the stimulation from 5 mA to 40 mA demonstrated complete loss of BCR in the closing recording. C: Preoperative and postoperative sagittal T1-weighted MR images. The postoperative image demonstrates partial removal of the spinal lipoma.

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