MATERIALS AND METHODS
This work has been carried out in accordance with the Declaration of Helsinki (2000) of the World Medical Association. This study was approved ethically by Chang Gung Memorial Hospital (98-0044B).
Between November 2002 and August 2009, ten patients with surgically and pathologically confirmed NSC were recruited from the Clinico-Pathologic-Radiologic conference at Chang Gung Memorial Hospital, Taoyuan, Taiwan. We reviewed their abdominal radiographs, CT images, and medical records retrospectively.
All of these patients underwent CT examinations of the abdomen and pelvis before surgical exploration, while they stayed in the ER. CT examinations were performed by four-detector CT (LightSpeed QX/i Scanner, General Electric Medical Systems, Milwaukee, WI, USA). Helical CT images were acquired using either 7- or 5-mm slice collimation, reconstruction interval of 5 mm, pitch of 1.5-2, 120 kV, and 200-240 mA. One hundred milliliters of intravenous (IV) contrast agent was used routinely.
Several CT findings of fecal impaction in the colon, thickening of the colon wall, and pericolonic stranding indicated SC, whereas the presence of extraluminal gas bubbles or an abscess suggested that perforation had occurred.
The CT examinations were retrospectively reviewed by two independent board-certified abdominal radiologists who were blinded to the CT official reports and the surgical and pathologic findings. They viewed the CT images on a picture archiving and communication system (PACS) independently and discussed the findings until consensus was reached. If consensus could not be reached, a third abdominal radiologist was consulted. All abdominal radiographs were reviewed for abnormal gas. They were also requested to determine the presence or absence of the CT features of NSC, including location of fecal impaction, proximal colon dilatation, colon wall thickening, dense mucosa, mucosal sloughing, perfusion defect, pericolonic stranding, pericolonic abscess, and abnormal gas with or without pneumo-mesocolon. Vascular ischemic colitis was excluded based on patency of the inferior mesenteric artery and vein.
Definition of CT signs
The individual CT signs were defined as follows - Fecal impaction: distended colon with much feces or packing of dehydrated fecaloma in the colon; Proximal colon dilatation: a distended left-sided colon with a cylindrical shape and cross-sectional diameter > 6 cm; Colon wall thickening: regional wall thickness > 3 mm in the obstruction site; Dense mucosa: increased mucosal lining density on pre-contrast CT; Mucosal sloughing: mucosa dislodged into the lumen; Perfusion defect: discontinuity of the enhancement of colon mucosa or apparently decreased enhancement as compared with adjacent small bowel loops; Pericolonic stranding: increased streaks of pericolonic fat; Pericolonic abscess: pericolonic loculated fluid or mottled substance; Abnormal gas: gas migrating into or beyond the colon wall as pneumoperitoneum or pneumoretroperitoneum, i.e. pneumo-intestinalis coli: gas entrapped in the mural wall; pneumo-mesocolon: gas confined inside the mesocolon; and portal vein gas: air leakage into the porto-mesenteric vessels.
Stercoral ulcer with perforation was first described by Berry in 1894, and to date, fewer than 150 cases have been reported. The incidence of perforated stercoral ulcer at autopsy ranges from 0.04% to 2.3%. Pre-mortem diagnosis is even less frequent, which suggests that the incidence of this condition is often underestimated. One study reported that stercoral perforation of the colon was found in 0.5% of all surgical colorectal procedures, 1.2% of all emergency colorectal procedures, and 3.2% of all colonic perforations.
Fecal impaction and perforation occur most often in the sigmoid colon. The sigmoid colon is the narrowest region of the entire colon, and passage of stools with a more solid consistency can be difficult. In such cases, fecaloma exerts localized pressure on the walls of the sigmoid colon, the area with the most precarious vascular supply, especially the vascular region known as Sudeck’s point. Prolonged localized pressure and ischemia can give rise to pressure ulceration[7,8].
Distention predisposes the colon to insufficient perfusion, leading to slight, moderate, or severe ischemic lesions. Ischemic colitis will occur when intraluminal pressure exceeds 35 cm H2O for hours. Maurer et al have postulated that colonic dilatation and the presence of multiple fecalomas indicate additional stercoral ulceration and carry the risk of secondary perforation. This view was supported by Huang et al by visualization of stercoral ulceration during intraoperative colonoscopy.
Chronic constipation (n = 8) and systemic disease (90%, n = 9) were the common clinical problems of the patients in this study, some of them (50%, n = 5) presented with multiple necrotic foci involving long segmental bowel that spanned the territory of the superior and inferior mesenteric arteries. It is probable that long-term systemic disease weakens the colon, while stool impaction causes bowel dilatation and increases wall tension, which worsens perfusion insufficiency and leads eventually to necrosis and potentially to fatal perforation. Unfortunately, the early clinical signs such as fever (20%, n = 2), peritoneal signs (20%, n = 2), and leukocytosis (70%, n = 7) are insufficient to diagnose this severe condition in order to prompt appropriate intervention in these patients.
Obstructive colitis differs from colonic cancer with marginal ulceration at aspects of normal mucosa distal to cancer and, frequently, centimeters immediately proximal to the carcinoma are free of ulceration and inflammation. As an example of this, NSC was diagnosed in our case number 4.
Fecal impaction was present in all our patients and was located mostly at the sigmoid colon (90%, n = 9), which was highly correlated with surgical findings and a result which agrees with other studies. Proximal dilatation was observed in two patients (20%), and was less frequent than we expected. It is possible that the colon could have ruptured prior to the CT scan, thus relieving the luminal pressure. This could also be due to the possible fulminant course which did not allow time for the colon to dilate. None of these two patients with proximal colon dilatation showed abnormal gas that would have indicated whether the colon had ruptured. Probably owing to absence of proximal colon dilatation in NSC, clinicians underestimate the stool impaction.
Colon wall thickening (60%, n = 6) is an indicator of stercoral colitis caused by edema or acute inflammation. Dense mucosa as a result of mucosal hemorrhage has been reported to be a sign of ischemic bowel[13,14]. This was one of the most frequently observed signs of NSC and occurred in 62.5% (5 of 8) of our cases. Mucosal sloughing (10%, n = 1) and perfusion defect (70%, n = 7) indicated status of ischemia progressing to infarct of the colon. The radiologists’ disagreement over wall thickening and perfusion defect may have been the result of subtle and localized changes. These findings indicate that the CT signs of NSC are not obvious, and that radiologists must be aware of the signs to make an early diagnosis. Pericolonic fat stranding was the most frequent CT sign of NSC observed in our patients (8 of 10, 80%). Intraoperative findings indicated that pericolonic fat stranding was the result of pericolonic inflammation and edema. The pericolonic reaction was most likely the cause of the intolerable abdominal pain experienced by these patients.
NSC with abnormal gas (50%, n = 5) often appears on CT scans as small gas bubbles in the proximity of the colon wall: pneumo-intestinalis coli or pneumo-mesocolon. This is usually undetected by radiography and differs from gastroduodenal perforation that usually presents massive pneumoperitoneum. Intraoperative findings indicate that the perforation can be temporarily plugged by a fecaloma. In our sample, a standing radiograph was not often obtained, partly because pneumoperitoneum was not suspected clinically, and the elderly patients were usually in a weakened state that impeded their assuming a standing posture. Pneumo-mesocolon was not always evident on the radiograph because it was obscured by the presence of a lot of fecal material in the abdomen. This explains why only one (20%) of five cases with abnormal gas was detected by radiography. Thus, abdominal CT, with meticulous searching for signs of abnormal gas, is required. Pericolonic abscess formation was seen in two (20%) of our patients. When the NSC is perforated, the viscous nature of the fecal material causes it to further impede the peritoneum with soiling.
NSC differs from other colitis by absence of diarrhea clinically. It can be confirmed by intraoperative and histological findings. At surgery, stercoral ulcers and perforations are usually found on the anti-mesenteric side; ulcerations usually have sharp margins and measure 1-10 cm, and are occasionally multiple. Histological findings include sharp demarcation without undermining at ulcer margins, and transmural necrosis at the perforated site. Treatment is usually resection of the affected bowel, colostomy, and Hartmann’s procedure[1,5].
Typically, only the more severe cases in this sample would have been discussed at the conference, and this resulted in a high mortality rate among our patients (70%; 7/10, which is higher than previously reported).
In the elderly and in nursing home patients, ascites associated with liver cirrhosis or malnutrition is often encountered. This could obscure the significance or specificity of pericolonic fluid accumulation for colonic pathology. Thus, we did not investigate this factor for NSC.
This retrospective study consisted of a small population of patients with NSC; thus, the statistical significance and likelihood ratios of each CT sign for NSC could not be determined appropriately. Owing to the nature of the retrospective study, some important clinical data and imaging were unavailable. This study aimed to alert clinicians to the CT findings of NSC, a potentially fatal condition. A further study with a larger number of patients is needed to validate the accuracy of our CT findings.
In summary, elderly patients with a history of chronic constipation and systemic disease, presenting with fecal impaction and acute abdomen with indeterminate leukocytosis, are at risk of NSC. CT is justified to be suggested to investigate the possibility of NSC. Pericolonic stranding, perfusion defect and dense mucosa were the most sensitive CT measures for NSC, detecting about 80%, 70%, and 62.5% of the cases, respectively. Awareness of NSC and familiarity with these CT signs enables us to make a differential diagnosis between this fatal condition and benign stool impaction.