Our study was approved by the hospital’s Ethics Committee on August 22, 2019 (protocol no. 2105). It followed the recommended Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guidelines Informed consent was obtained Opt-out on the web-site of Department of Anesthesia, University of Yamanashi. Those who rejected were excluded. This was approved by the ethic board. All methods in the study were conducted in accordance with all requirements outlined by the Ethic Committee and the ethical standards laid down in the amended 1964 Declaration of Helsinki.

Study design

This study used a retrospective, case-control design to assess the patient- and anesthesiologist-related risk factors for ADP in surgical patients.

Patient population

We recruited patients with ADP from April 2012 to March 2019 using the incident reports system of our institute. All patients who experienced ADP during epidural anesthesia who were registered in the incident reporting system were enrolled. Patients who opted out were excluded.

It is mandatory to submit these reports whenever an anesthesia-related ADP occurs in our hospital. Two authors extracted and browsed all anesthesia-related reports and identified those that included the phrase “accidental dural puncture.”

The electronic incident reporting system used for this study is operated by the University Hospital. Any healthcare provider can access the platform without restrictions and submit a report. These reports provide a description of the incident cases.

Following the selection of ADP incidents, we obtained data on the patients’ background. We also collected relevant information from the anesthesia and medical records.

In addition, we conducted a retrospective search for all the surgical patients who underwent epidural anesthesia or combined spinal-epidural anesthesia during the same period. We matched each ADP patient with their non-ADP counterpart in a 1:3 ratio to assess the risk factors. We have described the matched factors in Statistical analysis section.

Data collection

We collected data on age, sex, height, body weight, body mass index, comorbidities, medications, diagnosis, surgical procedure, and the American Society of Anesthesiologists physical status (ASA-PS). In addition, we obtained information on the experience of the anesthesiologist, the number of epidural puncture attempts, number of epidural anesthesia providers, approached intervertebral level, method of approach (median/paramedian), and depth to the epidural space, from the anesthesia records. We also collected additional information that described the event from the incident reports, if available. Missing data were excluded from the analysis.

Epidural anesthesia and accidental dural puncture

All procedures related to epidural anesthesia were performed in a lateral decubitus position using a 17-gauge Tuohy needle (B Braun Medical Inc., Tokyo, Japan) before anesthetic induction. Each anesthesia provider chose the method of approach—median or paramedian—and approached the intervertebral level. They detected the epidural space using the “loss-of-resistance” technique. After placement of the epidural catheter, we injected 3 mL of 2% mepivacaine as a test dose. The anesthetic dermatomal levels achieved were determined using a cold test. Each attending anesthesiologist defined the occurrence of ADP based on cerebrospinal fluid discharge, unexpected blood pressure drops, and unexpected motor block.


The primary hypothesis was that the experience of the anesthesiologist reduces the incidence of ADP in surgical patients. The secondary hypothesis was that there are potential risk factors for ADP.

Sample size calculation

We calculated the sample size using G*Power (version 3.1) (University of Düsseldorf, 2020) [10]. According to the “G*power” calculation, a sample of 32 patients with ADP and 64 patients without ADP in each group would be required to have a large effect (d = 0.50), with α = 0.05, β = 0.2, and patients ratio = 1:2, using the Wilcoxson–Mann–Whitney test.

We estimated that 32 patients should be identified from the incident report for the past 8 years based on the incidence of ADP (0.5%) [6, 7] and the number of patients who underwent epidural anesthesia each year in our hospital (n800): 6400 cases of epidural anesthesia in the past 8 years.

Statistical analysis

Data are presented as mean ± standard deviation or median (interquartile range) for continuous variables and frequencies (percentages) for categorical variables.

We matched the cases and controls in a 1:3 ratio based on the type of surgery (gynecologic, obstetric, upper/lower abdominal intestinal, urological, or thoracic), sex, age (within 10 years), height (within 10 cm), weight (within 10 kg), year of surgery (within 1 year), ASA-PS (within 1 point), and emergency status, considering 30% of unmatched pairs. Patients who experienced ADP were matched with those who did not experience ADP, based on characteristics such as age, sex, weight, and depth to the epidural space, to exclude patient-related factors discussed previously [5,6,7]. In instances where patients who experienced ADP were matched with fewer than three patients who did not experience ADP, we did not recruit additional patients who did not experience ADP.

Continuous variables were compared between the two groups using Student’s t-test or Mann–Whitney U test. Frequencies were compared between the two groups using the chi-squared test or Fisher’s exact test.

Logistic regression analysis was used to estimate unadjusted and adjusted odds ratios with 95 confidence intervals for ADP in surgical patients. Experience of anesthesiologist and variables with P < 0.05 were included in the regression model, which was adjusted for potential anesthetic confounders (anesthesiologists’ experience, number of epidural puncture attempts, and number of epidural anesthesia providers). Statistical analyses were performed using SPSS (version 23) (IBM Japan, Tokyo, Japan). Significance was set at P < 0.05.

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