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Hernias > Why do they occur?

A hernia is formed by the protrusion of part of the abdominal contents (usually fat or bowel) through a defect or weakness in the abdominal wall.

Basically the abdominal wall has been modified at particular sites to developmental requirements. Hernias almost exclusively occur at these well recognised sites of weakness.

All hernias occur at the site of a structural weakness in the abdominal wall - that is where the 3-layer muscle structure of the abdominal wall has been modified developmentally and anatomically. In the case of the umbilical region and epigastrium where the muscle and fascia have fused into one layer, in the midline of the abdomen wall. As shown in A & B - representing midline hernias such as an epigastric hernia or umbilical hernia.

A. Rollover image for explanation of diagram.

B. Rollover image for explanation of diagram.

These diagram shows the general structure of the abdominal wall. It shows the 3 main muscles in brown. These muscles fuse and form a large aponeurotic tendon sheet (termed the rectus sheath) and this encloses a single strap-like muscle - the rectus abdominis muscle - the rectus sheath fuses in the mid-line to form the linea alba (white). Because this is a single layer here this is a potential weakness and thus a common site where hernias develop - termed 'epigastric' hernias.

The commonest site for hernias is the groin. In the groin there are 2 types of hernias - inguinal and femoral. The femoral is lower and relatively rare. It occurs relatively more frequently in females.

The INGUINAL hernia, which is by far the most common, occurs at any age. The descent of the testes and its cord through the muscle wall of the groin into the scrotum at birth has left a potential weakness, through which the hernia descends. Any activity which then constantly or suddenly increases intra abdominal pressure or weakens the muscle may cause a hernia eg, cough, constipation, bladder obstruction or heavy lifting. Ageing weakens the muscles. Obesity increases both the intra abdominal pressure and reduces the muscle strength.

A. Before birth. Testes has not decended.

B. At birth. Testes has decended.

C. Small bowel in an indirect hernia.


There are 2 types of inguinal hernias
(1) Indirect hernia occurs at any age and in females. It may be a developmental type of problem occurring very frequently in the newborn and infants.
(2) Direct hernia - rarely occurs in children or women. Is uncommon under 30-40 years of age. The hernia develops in a lax muscular wall, where the tissues either atrophy or develop a weakness, which allows the abdominal contents to push out.

Generally the indirect hernia is more dangerous, it is protruding through a narrow neck of muscle, which can trap the contents as opposed to, a direct hernia in which is just an enlarging bulge.

This is the reason why direct hernias can be left longer without surgery.

However, it is not always clinically possible to distinguish between direct and indirect hernias.

The thin inner lining of the abdominal wall (the peritoneum) protrudes through a defect or weakens in the muscle and expands like a balloon. The abdominal contents move in and out of the sac.

Direct Inguinal Hernia with the balooning being sutured back in.


1. INGUINAL - in the groin
2. FEMORAL - below the inguinal - relatively more common in females
3. EPIGASTRIC - in the upper abdomen - midline - above the navel
4. UMBILICAL - at the navel - common also in babies
5. INCISIONAL - in the scar of the previous surgery
6. RECURRENT - at the site of a previous hernia operation
7. UNUSUAL HERNIA - lumbar
8. HIATUS HERNIAS - intra-abdominal - different kettle of fish. Sometimes surgery can help.

Hernias are described as:

a) REDUCIBLE: That is those that are not present all of the time and often disappear temporarily when you lie down or can be manipulated back into the abdomen.

The contents may become stuck and the hernia is permanently protruding (an irreducible hernia).


The bowel can be blocked causing abdominal pain and vomiting.
Strangulation and bowel obstruction require emergency surgery.


The contents may be nipped off suddenly at the narrow neck of the sac.

Next: Where do they occur?

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