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Anatomy clinical correlates: Peritoneum and diaphragm

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The peritoneum is a transparent serous membrane that lines the abdominal cavity and covers the abdominal organs, whereas the diaphragm is a skeletal muscle that separates the thoracic cavity from the abdominal cavity and plays an important role in respiration.

Understanding the anatomy of these structures allows us to better understand the unique clinical conditions that affect them.

Let’s start off by reviewing the innervation of visceral and somatic pain in the abdomen. The peritoneum is made up of two continuous layers, the visceral peritoneum and parietal peritoneum.

The inner visceral layer mainly covers the abdominal organs and has a visceral nerve supply, where the outer parietal layer mainly covers the internal surface of the abdominopelvic walls and has a somatic nerve supply.

Furthermore, both the visceral and somatic nerve supply of the peritoneum sense pain during different clinical conditions; however, the way in which they sense pain is different.

The visceral peritoneum is innervated by the autonomic nervous system. When the visceral peritoneum is irritated, pain sensation is detected by visceral afferent fibers which travel with the autonomic nerves, mainly sympathetic nerve fibers, back to the spinal cord.

Visceral pain is typically sensed as a diffuse, dull pain which is not very well localized, and the pain is often referred to the midline region of the dermatomal areas of the same spinal level as the autonomic nerve supply.

Typically, pain sensed by the foregut visceral peritoneum is referred to the epigastric region, midgut visceral pain is referred to the umbilical region, and hindgut visceral pain is referred to the hypogastric region.

Patients who experience visceral pain may also present with nausea, vomiting, or sweating due to activation of the autonomic nervous system.

On the other hand, the parietal peritoneum has a somatic nerve supply, which is the same as the adjacent abdominopelvic wall in which it lines.

When the parietal peritoneum is irritated, pain sensation travels back with these somatic nerves to the spinal cord.

Somatic pain is typically sensed as a sharp, localized pain specific to the spinal level innervating that irritated area.

To illustrate these types of pain, let’s use appendicitis as an example, where appendicitis refers to inflammation of the appendix, and can cause both visceral and somatic pain.

Visceral pain occurs due to irritation of the visceral peritoneum of the appendix and surrounding areas which travel with sympathetic fibers back to the level of T10.

As a result, there may be vague pain in the right lower quadrant where the appendix is, as well as anywhere along the T10 dermatome level, frequently being felt around the umbilicus.

As the appendix becomes more and more inflamed, it can also irritate the parietal peritoneum, causing somatic pain; specifically, a sharp, localized pain to the right lower quadrant over the appendix to an area classically known as McBurney point.

The McBurney point is situated at two-thirds of the distance between from the umbilicus to the anterior superior iliac spine, and tenderness due to palpation over this area is a classic sign of appendicitis.

In addition to appendicitis, there are many situations that can cause irritation and inflammation of the peritoneum, and this is called peritonitis.

Often, peritonitis is the result of bacterial contamination, such as during open surgery; when the gut is penetrated during trauma; or as the result of a ruptured internal organ, such as the appendix.

This exposes the peritoneum to gas and fecal matter which is usually contained within the gastrointestinal tract, and allows bacteria to enter the otherwise sterile peritoneal cavity, leading to infection and subsequent inflammation of the peritoneum.

Peritonitis causes intense pain in the abdomen and results in increased tone of the anterolateral abdominal muscles, which is a mechanism that aims to protect the contents of the abdominal cavity.

Therefore, on examination the abdomen is extremely tender to palpation, resulting in patient discomfort and voluntary guarding.

Other signs include abdominal wall rigidity which is due to involuntary increased muscle tone as we just mentioned, as well as another clinical sign called rebound tenderness.

Rebound tenderness is when you slowly perform deep palpation on the abdomen, and then quickly remove your hand which elicits a stabbing pain over the area of palpation, resulting in a ‘rebound’ tenderness indicating peritonitis.

Symptoms of peritonitis include severe abdominal pain, nausea, vomiting, fever and constipation.

Now, if the peritoneum is damaged or infected because of trauma, surgery or peritonitis, fibrin starts to deposit on the inflamed peritoneal surface which results in fibrous tissue formation during healing.

This fibrous tissue, often referred to as scar tissue, can lead to the formation adhesions, which are abnormal fibrous attachments between the visceral peritoneum of nearby organs or between the visceral peritoneum of an organ and the parietal peritoneum of the nearby abdominal wall.

Abdominal adhesions are often the result of an abdominal operation such as an appendectomy, and adhesion formation can limit the normal movements of the abdominal viscera.

Adhesions can cause chronic pain, or complications such as intestinal obstruction as the intestine can become tethered or twisted around to an adhesion, and can also result in a potential volvulus.

This is one reason why less invasive surgical techniques, such as laparoscopy, are preferred to open surgery, when possible.

Before we move on, we can’t discuss the peritoneum without mentioning how one of its really cool features is that it can be used for peritoneal dialysis!

The peritoneum is a semipermeable membrane with a large surface area rich in blood and lymphatic capillary beds.

The fact that the peritoneum is semipermeable means that it allows certain substances or molecules to be rapidly absorbed, just like how the information in this Osmosis video is absorbed into your brain! Ok, maybe it’s not exactly the same but you get the point.

Now, in individuals with end-stage renal disease, waste products like urea accumulate in the individuals blood and tissues because they cannot be excreted, causing signs and symptoms like nausea, vomiting, pruritus, mental status changes or even pulmonary edema.

If the individual is unable to overcome their renal dysfunction, dialysis may be needed. There are two main types of dialysis: peritoneal dialysis which takes advantage of the semipermeable peritoneal membrane, and hemodialysis where waste products are removed from the blood.

Peritoneal dialysis involves inserting a large indwelling catheter in the peritoneal space, where a dilute sterile solution is introduced in the peritoneum through the catheter which creates a concentration gradient between the blood and the fluid in the peritoneal cavity.

This concentration gradient pulls waste and excess fluid from the blood and into the peritoneal cavity, where the fluid is then removed through the indwelling catheter.

This procedure relies on the peritoneum’s large surface area and semipermeable membrane to eliminate waste.

Quick quiz before we move on to the next section: what is the difference between visceral and somatic abdominal pain?

Now let’s talk about one of the unknown heroes of the peritoneal cavity: the greater omentum. The greater omentum is a large, four layered peritoneal fold containing a large amount of fat that hangs from the greater curvature of the stomach.

It is a very mobile structure which covers a large portion of the abdominal organs, and in doing so prevents the visceral peritoneum from sticking to the parietal peritoneum.

Furthermore, it has a protective mechanism where it can form adhesions and surround inflamed organs, such as the appendix, in order to wall them off and keep inflammation from spreading to other abdominal organs and viscera.

The greater omentum is also a cushion for the abdominal organs against injury and also forms insulation against loss of body heat.

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