Study design and setting
The study protocol was approved by the appropriate ethics committee (Yokohama City University Certified Institutional Review Board) (Approval No: CRB3180007) and was exempted from obtaining consent directly from patients by using an opt-out method. We conducted a case–control study of patients previously diagnosed with IC at our hospital during an 11-year interval between January 1, 2009, and December 31, 2019. Patients diagnosed with IC were selected from a total of 439,312 patients registered in the electronic medical record database during this period. The follow-up period was from the onset of IC to the date of the last visit (up to December 31, 2019). Patients who had a relapse after resolution of the initial IC episode during the follow-up period were included in the recurrent IC group, whereas those who experienced a single episode of IC during the study period were included in the non-recurrent IC group.
We identified patients with IC in the database using the following International Classification of Diseases-10 codes: K550 (NDNL, GK7L, PR7K), K551 (F1A5, BPJQ, G440), and K559. We counted cases with multiple IC disease classifications as a single case, and a total of 316 patients with IC were identified. The diagnosis of IC was further confirmed based on clinical symptoms and laboratory findings, as there are no internationally standardized diagnostic criteria for this disease. We used the diagnostic criteria proposed by Marston et al., (1) which are commonly followed in Japan: 1) sudden-onset left lower abdominal pain accompanied by diarrhea and bloody stools, 2) typical findings on contrast-enhanced computed tomography (CT) (edematous thickening of the intestinal wall and increased peri-intestinal fatty tissue concentration) or on colonoscopy (CS) (ulceration/erosion, redness, edema, biopsy image), predominantly on the left side of the colon, 3) no autoimmune disease or infection, 4) no trauma or mechanical factors (e.g., hernia), 5) no abdominal surgery within the past 6 months, 6) no evidence of severe acute ischemic changes in other organs, and 7) absence of a secondary onset of IC. We excluded patients who were younger than 20 years, pregnant, or opted out of the study; we also excluded suspected cases that could not be definitively diagnosed, and patients diagnosed with IC at other hospitals, as it was not possible to confirm what diagnostic criteria were used.
We extracted the following data from the patients’ medical records: patient characteristics (sex, age, body mass index [BMI], drinking history, smoking history, medical history, comorbidities, medications, and vital signs), clinical signs and symptoms (abdominal pain, diarrhea, bloody stools, and peritoneal irritation symptoms), laboratory findings (contrast-enhanced CT findings, endoscopic features, and histological features), method of treatment, length of hospital stay, disease course (time of resolution of abdominal pain, time of resolution of bloody stools, and initiation of meals), and the frequency of IC morbidities. Contrast-enhanced CT and/or CS was performed to assess each segment of the colon. Contrast-enhanced CT was performed to detect poor contrast, bowel wall thickening, mesenteric fat stranding, pericolonic free fluid, mesenteric gas, and portal vein gas. CS was performed to assess the presence of mucosal erosion, ulceration, edema, redness, necrosis, and stricture. The frequency of IC morbidities was defined as the number of times the patient was diagnosed with IC at our hospital.
All patient data were entered into a Microsoft Excel 2016 (Microsoft Corporation, Redmond, WA, USA) spreadsheet. The computer containing the data was password-protected and stored in a locked cabinet in the medical office. Continuous variables are expressed as mean ± standard deviation, and categorical variables are expressed as numbers and percentages. The chi-square test was performed to compare proportions between the recurrent IC and non-recurrent IC groups, and the t-test was used to compare means. All statistical analyses were performed using JMP Pro 15 (SAS, Cary, NC, USA), and the level of statistical significance (p) was set at ≤ 0.05. Two separate multivariate analyses were performed including 1) three items that were previously reported to be significantly different (current smoking (24) and two items that were significantly different in the univariate analyses) and 2) all items. These three items were also analyzed for the time to IC recurrence using the Kaplan–Meier method.