Adhesive and non-adhesive internal hernias: How to distinguish between the two in closed loop small bowel obstruction

A 78-year-old female presented with a one-day history of lower abdominal pain and bilious vomiting. Past surgical history included hernia repair (1989) and colposuspension (1994). On examination, there was tenderness in the right iliac fossa, abdominal distension and elevated inflammatory markers on blood biochemistry.

The surgical team requested CT imaging on suspicion of small bowel obstruction (SBO), in line with guidelines, where clinical signs of ischaemia or strangulation were not immediately present [1,2]. 

The orthogonal reconstructions of CT abdomen and pelvis demonstrated two transition points with twisting of the mesentery forming a ‘beak’ sign within the right iliac fossa (Figure 1).  There was lateral compression of the bowel by possible extra-luminal band demonstrating the “fat-notch sign” (Figure 2). There was associated mesenteric fat stranding, mesenteric hyperaemia and reactionary free fluid along the cluster of involved oedematous, reducing enhancing small bowel loops (Figure 3). There was faecalisation within afferent bowel loops.

The aforementioned findings were suggestive of a closed-loop obstruction with impending small bowel ischaemia and thought to be induced by an internal hernia or adhesional bands requiring emergency laparotomy.

Internal hernias (IH) describe abnormal protrusion of abdominal viscera through a peritoneal or mesenteric aperture. These either arise from idiopathic defects (primary IH) in the mesentery or omentum or acquired (secondary IH) following adhesional band formation, GI reconstructive surgery or trauma. The most common presentation of IH is a strangulating SBO following closed-loop obstruction [3], and internal hernias cause up to 5.8% of all SBO [4,5].

Retrospective analyses of patients operated for SBO found mortality rates were 15% when strangulated and rose where treatment was delayed over 24 hours [6]. SBO secondary to IH has a varied clinical presentation depending on duration, reducibility of the hernia and complications such as strangulation or incarceration [7]. Thus, imaging plays a vital role in the timely diagnosis and surgical management of these patients.

Contrast-enhanced CTAP is the imaging modality of choice to diagnose SBO; meta-analyses of pooled sensitivity and specificity are 91% and 89% respectively [8].  The diagnostic challenge for radiologists is identifying IH and its aetiology, namely in distinguishing between adhesive IH (the most common acquired IH) and non-adhesive IH [9]. Imaging features of SBO secondary to IH include 1) the clustering of dilated small bowel loops in an abnormal anatomical location, 2) mesenteric fat and vessel changes (engorgement and stretching) and 3) altered positioning of surrounding viscera [10].

Imaging studies investigating adhesive IH found the ‘fat-notch sign’ which describes extra-luminal compression of bowel as a distinguishing feature for adhesive IH [9] [11].  Importantly, whilst adhesive IHs are not confined to a specific anatomical location within the abdominal cavity, non-adhesive IHs can be typified. Most common congenital paraduodenal hernias are located within the left upper quadrant [12] and transmesenteric hernias are located at the periphery of the abdominal cavity.

Emergency laparotomy performed revealed an adhesional band causing internal herniation and strangulation of a small bowel loop 45cm from the ileocaecal junction. A 90-cm segment of small bowel was resected, and anastomosis formed. Histopathology confirmed ischaemic changes within the resected small bowel.

Teaching Points

• The presence of the ‘fat-notch sign’ and peritoneal adhesive bands on CT imaging are distinguishing features for adhesive IH.
• SBO secondary to IH can be identified by clustering of small bowel loops, mesenteric changes and altered positioning of abdominal viscera
• Closed-loop SBO secondary to IH are at high risk of strangulation and ischaemia and thus warrant urgent surgical intervention if identified on imaging.