An IH is defined as the protrusion of part of the intestinal viscera through openings, recesses, or fossae within the peritoneal cavity, resulting from congenital, post-traumatic, or post-surgical modifications of the normal peritoneal anatomy. IH are divided following the traditional Welch classification  into various subtypes based on the topographic distribution of herniated bowel loops in relation to the anatomic location of the orifice:

• Paraduodenal hernias: classically considered the most common type of IH with left-sided hernias being the most common subtype. In this type of hernia, intestinal loops herniate through the foramen of Landzert into the fossa of Landzert, an abnormal opening between the small bowel mesentery and the trasverse mesocolon located in the left upper quadrant. Right paraduodenal hernias are far less common and are caused by entrapment of intestinal loops in an abnormal peritoneal recess, the fossa of Waldeyer, located behind the superior mesenteric artery, in the proximal part of the transverse mesocolon, near the medial margin of the ascending colon. In both sub-types, the herniated bowel loops are located typically in the upper quadrants and appear clustered and encapsulated in the abnormal mesenteric recess.

Fig 1: Schematic anatomic illustration depicting both types of paraduodenal hernia: - a: left paraduodenal hernia with herniated bowel loops in the fossa of Landzert. - b: right paraduodenal hernia with herniated bowel loops in the fossa of Waldeyer. References: Kelahan L, Menias CO, Chow L. A review of internal hernias related to congenital peritoneal fossae and apertures. Abdom Radiol (NY). 2021 May;46(5):1825-1836. doi: 10.1007/s00261-020-02829-4. Epub 2020 Oct 30. PMID: 33128101.

• Foramen of Winslow: in this subtype of IH, small bowel loops herniate through an existing anatomical foramen in the lesser sac of the peritoneum, located between the liver hilum and the inferior vena cava, causing a similar appearance to that of the left paraduodenal hernia, but without the bowel encapsulation typically seen in the latter.

Fig 2: Schematic anatomic illustration depicting small bowel herniation through foramen of Winslow. References: Kelahan L, Menias CO, Chow L. A review of internal hernias related to congenital peritoneal fossae and apertures. Abdom Radiol (NY). 2021 May;46(5):1825-1836. doi: 10.1007/s00261-020-02829-4. Epub 2020 Oct 30. PMID: 33128101.

• Pericecal hernia: this type of hernia encompasses various subtypes, all of which involve the abnormal herniation of bowel loops posterior to the right paracolic gutter in the right low quadrant

Fig 3: Schematic anatomic illustration depicting paracolic internal hernia. References: Kelahan L, Menias CO, Chow L. A review of internal hernias related to congenital peritoneal fossae and apertures. Abdom Radiol (NY). 2021 May;46(5):1825-1836. doi: 10.1007/s00261-020-02829-4. Epub 2020 Oct 30. PMID: 33128101.

• Sigmoid-mesocolon related hernia: small bowel loops herniate through defects present in this structure located in the left low quadrant.

Fig 4: Schematic anatomic illustration depicting sigmoid-mesocolon related hernia. References: Kelahan L, Menias CO, Chow L. A review of internal hernias related to congenital peritoneal fossae and apertures. Abdom Radiol (NY). 2021 May;46(5):1825-1836. doi: 10.1007/s00261-020-02829-4. Epub 2020 Oct 30. PMID: 33128101.

• Transmesenteric: bowel loops herniate through defects present in the small bowel mesentery covering the jejunum and ileum; the location of the mesenteric defect can vary, making a key feature of this subtype of IH the displacement of the main mesenteric trunk passing through the small bowel mesentery. Their incidence is gradually increasing as the number of abdominal surgical procedures in the clinical practice continue to rise.

Fig 5: Schematic anatomic illustration depicting transmesenteric internal hernia. References: Kelahan L, Menias CO, Chow L. A review of internal hernias related to congenital peritoneal fossae and apertures. Abdom Radiol (NY). 2021 May;46(5):1825-1836. doi: 10.1007/s00261-020-02829-4. Epub 2020 Oct 30. PMID: 33128101.

• Transomental: as with the transmesenteric IH, even in this case there is not a fixed location for the bowel herniation as the greater omentum extends between the greater curvature of the stomach and the transverse colon, then draping into the anterior inframesocolic abdomen. This subtype of hernia should be strongly suspected if the dilated bowel loops are located in the anterior aspect of the abdomen.

Fig 6: Schematic anatomic illustration depicting trans-omental herniation of small bowel loops. References: Kelahan L, Menias CO, Chow L. A review of internal hernias related to congenital peritoneal fossae and apertures. Abdom Radiol (NY). 2021 May;46(5):1825-1836. doi: 10.1007/s00261-020-02829-4. Epub 2020 Oct 30. PMID: 33128101.

• Supravesical and pelvic hernia: clustered bowel loops are located in the inferior abdomen resulting from herniation trough defects in the broad ligament of the uterus. They are characterized by the presence of bowel loops between the uterus and the pelvic wall usually accompanied by some degree of uterus deviation to the opposite side.

Fig 7: Schematic anatomic illustration depicting broad-ligament defect which is a possible site of small bowel herniation causing supravesical and pelvic internal hernia. References: Kelahan L, Menias CO, Chow L. A review of internal hernias related to congenital peritoneal fossae and apertures. Abdom Radiol (NY). 2021 May;46(5):1825-1836. doi: 10.1007/s00261-020-02829-4. Epub 2020 Oct 30. PMID: 33128101.

The herniation of intestinal loops through these defects, recesses and foramens can distort normal intestinal anatomy and, in some cases, lead to intestinal obstruction and strangulation.