An internal hernia happens when part of the intestine, usually the small bowel, pushes through an opening inside the abdomen and becomes trapped in a space where it doesn’t belong. Unlike the hernias most people picture, which create a visible bulge under the skin, internal hernias are entirely hidden within the abdominal cavity. They account for less than 1% of all abdominal hernias but carry serious risks: if the trapped bowel loses its blood supply, mortality rates can exceed 50%.
How an Internal Hernia Differs From Other Hernias
Most hernias push outward. An inguinal hernia, for example, creates a lump in the groin where tissue pokes through a weak spot in the abdominal wall. An internal hernia does the opposite. Loops of intestine slip through a gap in the membranes that hold your organs in place (the mesentery, peritoneum, or omentum) and migrate into a pocket or compartment that already exists inside your abdomen or pelvis. Because nothing reaches the surface, there’s no bulge to see or feel.
This hidden nature makes internal hernias notoriously difficult to diagnose. They often mimic other, more common causes of bowel obstruction, such as scar tissue from prior surgeries. Many cases are only identified during emergency surgery or on a CT scan ordered for unexplained abdominal pain.
What Causes an Internal Hernia
The openings that allow an internal hernia can be either congenital or acquired. Some people are born with natural pockets or gaps in the membranes lining the abdomen. Defects in intestinal rotation during fetal development can leave these spaces wider or more accessible than normal. In other cases, a hernia develops through a gap created by previous surgery, trauma, or inflammation.
The single biggest acquired risk factor today is laparoscopic gastric bypass surgery. When surgeons rearrange the digestive tract during a Roux-en-Y procedure, new openings are created in the mesentery. Because laparoscopic surgery produces fewer internal adhesions (scar tissue) than open surgery, there’s less natural “sealing” of those gaps afterward. In a study of over 1,600 patients who underwent laparoscopic gastric bypass, roughly 6% needed reoperation for a confirmed internal hernia. A longer-term analysis estimated that about 11% of patients will develop a clinically significant internal hernia within five years. The reported incidence across studies ranges from 0.2% to 9%, depending on the specific surgical technique used.
Among people who have never had abdominal surgery, internal hernias are rare and usually involve congenital anatomical pockets. Some contributing factors include elevated pressure inside the abdomen, unusually mobile intestines, a long small bowel mesentery, or a thin layer of the fatty tissue that normally drapes over the intestines.
Types of Internal Hernias
Internal hernias are classified by where the bowel herniates. The most common congenital types are paraduodenal hernias, where small bowel loops slip into a pocket near the duodenum (the first section of the small intestine). Left-sided paraduodenal hernias involve bowel entering a space behind the descending colon, while right-sided ones push bowel behind the ascending colon, sometimes displacing major blood vessels forward.
Other recognized types include:
- Transmesenteric hernias: Bowel pushes through a gap in the small bowel mesentery, typically near where the small intestine begins or ends. These are especially common after bariatric surgery and carry a high risk of the bowel twisting on itself (volvulus) and losing blood flow. In one imaging study, volvulus complicated five of 14 transmesenteric hernias, and bowel ischemia occurred in six of 14.
- Pericecal hernias: A segment of ileum (the lower small intestine) slips into the right side of the abdomen through a defect in the tissue around the cecum, often caused by adhesions.
- Intersigmoid hernias: Small bowel enters a pocket formed between loops of the sigmoid colon and their supporting tissue. This pocket is found in 50% to 75% of people at autopsy, meaning many people have the anatomical setup without ever developing a hernia.
- Foramen of Winslow hernias: Bowel slips through a small natural opening that connects two compartments of the abdominal cavity. These make up about 8% of internal hernias, with roughly 200 cases described in the medical literature.
- Broad ligament hernias: Seen only in women, where bowel passes through a defect in the ligament supporting the uterus.
- Supravesical hernias: Intestine drops into a space around the bladder.
Symptoms to Recognize
Internal hernias can be completely silent until they cause a bowel obstruction. When symptoms do appear, they typically resemble a blockage in the small intestine: crampy or colicky abdominal pain, bloating, nausea, vomiting (often bile-stained), and an inability to pass gas or have a bowel movement. The pain can come on suddenly and severely, or it may be more intermittent, with episodes that flare up and then resolve on their own.
Some people experience recurrent bouts of abdominal pain over months or even years before a hernia is finally diagnosed. Between episodes, they may feel completely normal, which distinguishes this pattern from chronic partial bowel obstruction, where symptoms tend to linger. A history of similar pain that resolved spontaneously in the past is a common finding when these hernias are eventually caught.
In the acute setting, worsening pain, a rigid or increasingly distended abdomen, fever, or signs of shock suggest the trapped bowel has lost its blood supply. This is a surgical emergency.
How Internal Hernias Are Diagnosed
CT scans are the primary diagnostic tool. Radiologists look for a cluster of characteristic signs: small bowel loops bunched together in an unusual location, stretched and engorged blood vessels in the mesentery, and displacement of surrounding structures like the colon. In transmesenteric hernias, the clustered bowel loops may sit directly against the abdominal wall without the normal layer of fatty tissue in front of them. In left paraduodenal hernias, a sac-like mass of small bowel appears wedged between the stomach and the tail of the pancreas, pushing the stomach forward.
Even with CT imaging, diagnosis is challenging. In one review of 17 confirmed cases, CT findings were considered definitive or strongly suggestive in 15 patients, meaning two were still unclear on imaging. The overall incidence of internal hernias is low enough (0.5% to 0.9% of all causes of bowel obstruction) that clinicians may not think of them initially, especially in patients without a surgical history.
Why Timely Treatment Matters
The risk of a trapped bowel segment becoming incarcerated (stuck) and then strangulated (losing blood flow) is approximately 50% in untreated cases. Once blood supply is cut off, the bowel tissue can die within hours, leading to perforation, widespread infection, and potentially death. Reported mortality rates for complicated internal hernias reach 45% to over 50%.
This is why any confirmed or strongly suspected internal hernia is treated as urgent. Surgery is the only definitive treatment. In straightforward cases, surgeons can operate laparoscopically, using small incisions and a camera to guide the bowel back into its proper position and close the defect that allowed the herniation. If the bowel has already developed ischemia or gangrene, open surgery (exploratory laparotomy) is typically necessary so the surgeon can assess the full extent of damage and remove any non-viable tissue.
The key to a good outcome is speed. Reducing the trapped bowel loops as early as possible preserves blood flow and prevents tissue death. When surgery happens before ischemia develops, outcomes are generally favorable. Delays, on the other hand, dramatically increase the risk of needing bowel resection and the chance of life-threatening complications.
After Bariatric Surgery: What to Watch For
If you’ve had a laparoscopic Roux-en-Y gastric bypass, internal hernia is a known long-term risk. These hernias can develop anywhere from a few months to several years after surgery, with a mean onset around 13 months in one large study (range of 4 to 43 months). Significant weight loss after bariatric surgery may increase the risk further, because as fat within the mesentery shrinks, previously closed gaps can open up.
Recurring episodes of crampy abdominal pain, especially pain that comes and goes and is accompanied by nausea or vomiting, warrant investigation. Many bariatric surgery programs now routinely close mesenteric defects during the initial operation to reduce this risk, but no technique eliminates it entirely. The retrocolic surgical route, where the new intestinal limb is passed behind the colon, is associated with higher hernia rates than the antecolic route, which passes it in front.