A hernia is a protrusion of a tissue, structure, or part of an organ through the muscular tissue or the membrane by which it is normally contained. The hernia has three parts: the orifice through which it herniates, the hernial sac, and its contents.

Pathophysiology

By far most hernias develop in the abdomen, when a weakness in the abdominal wall evolves into a localized hole, or "defect", through which adipose tissue, or abdominal organs covered with peritoneum, may protrude. Another common hernia involves the intervertebral disc, and causes back pain or sciatica.

Hernias may present either with pain at the site, a visible or palpable lump, or in some cases by more vague symptoms resulting from pressure on an organ which has become "stuck" in the hernia, sometimes leading to organ dysfunction. Fatty tissue usually enters a hernia first, but it may be followed by or accompanied by an organ.

Most of the time, hernias develop when pressure in the compartment of the residing organ is increased, and the boundary is weak or weakened.

• Weakening of containing membranes or muscles is usually congenital (which explains part of the tendency of hernias to run in families), and increases with age (for example, degeneration of the annulus fibrosus of the intervertebral disc), but it may be on the basis of other illnesses, such as Ehlers-Danlos syndrome or Marfan syndrome, stretching of muscles during pregnancy, losing weight in obese people, etc., or because of scars from previous surgery.
• Many conditions chronically increase intra-abdominal pressure, (pregnancy, ascites, COPD, dyschezia, benign prostatic hypertrophy) and hence abdominal hernias are very frequent. Increased intracranial pressure can cause parts of the brain to herniate through narrowed portions of the cranial cavity or through the foramen magnum. Increased pressure on the intervertebral discs, as produced by heavy lifting or lifting with improper technique, increases the risk of herniation.

Characteristics

Hernias can be classified according to their anatomical location:

Examples include:

• abdominal hernias
• diaphragmatic hernias and hiatal hernias (for example, paraesophageal hernia of the stomach)
• pelvic hernias, for example, obturator hernia
• hernias of the nucleus pulposus of the intervertebral discs
• intracranial hernias

Each of the above hernias may be characterised by several aspects:

• congenital or acquired: congenital hernias occur prenatally or in the first year(s) of life, and are caused by a congenital defect, whereas acquired hernias develop later on in life. However, this may be on the basis of a locus minoris resistentiae (Lat. place of least resistance) that is congenital, but only becomes symptomatic later in life, when degeneration and increased stress (for example, increased abdominal pressure from coughing in COPD) provoke the hernia.
• complete or incomplete: for example, the stomach may partially herniate into the chest, or completely.
• internal or external: external ones herniate to the outside world, whereas internal hernias protrude from their normal compartment to another (for example, mesenteric hernias).
• intraparietal hernia: hernia that does not reach all the way to the subcutis, but only to the musculoaponeurotic layer. An example is a Spigelian hernia. Intraparietal hernias may produce less obvious bulging, and may be less easily detected on clinical examination.
• bilateral: in this case, simultaneous repair may be considered, sometimes even with a giant prosthetic reinforcement.
• irreducible (also known as incarcerated): the hernial contents cannot be returned to their normal site with simple manipulation.