Anatomy clinical correlates: Anterior and posterior abdominal wall

When we think about the clinical conditions affecting the abdomen, we immediately think about injury to organs such as the liver, intestines or stomach. However, there are also many conditions that affect the anterior and posterior abdominal wall, as well as the structures adjacent to them. So let's get started!

When examining the gastrointestinal system, first, it’s important to do a general inspection of the abdomen which includes looking for scars and incisions which can indicate what surgeries the patient has had in the past. Typically, the location of the abdominal incision indicates what type of surgery the patient has had.

First, there’s the Kocher incision, also called the subcostal incision, which is made parallel and at least 2.5 centimeters below the costal margin in order to avoid the 7th and 8th thoracic spinal nerves. Injury to these nerves results in decreased sensation of the anterior abdominal wall supplied by those respective branches. This type of incision is usually found on the right side and provides access to the gallbladder and biliary ducts, and can be extended towards the left side of the abdomen to access the spleen.

Next up, there are longitudinal incisions, which include midline incisions and paramedian incisions. They can be used in almost all abdominal surgeries, especially exploratory surgeries which are sometimes needed to diagnose uncertain abdominal pathologies. Now, longitudinal incisions provide great exposure of the abdominal viscera and can also be extended as needed.

The midline or median incision is made along any part or length of the linea alba, so from the xiphoid process all the way towards the pubic symphysis. This type of incision is preferred because the linea alba only has small vessels and nerves, so there’s a lower risk of bleeding and nerve damage.

A paramedian incision, which is lateral to the median plane, is made in a sagittal plane and can extend anywhere from the costal margin to the pubic hairline. The paramedian incision would therefore go through the anterior rectus sheath, the rectus abdominis muscle, and the posterior rectus sheath.

Other surgical procedures may require oblique incisions, depending on muscle fiber orientation and surrounding nerves. Oblique incisions include the McBurney, or Gridiron incision, and the Rutherford Morison incision.

So first, the McBurney incision is typically needed for appendectomies and it’s done inferomedially in an oblique orientation to follow the muscle fiber orientation of the anterior abdominal wall, specifically the external oblique. This is typically done over McBurney point, which is one third of the way from the anterior superior iliac spine to the umbilicus.

This incision can be extended, or mirrored on the left side of the abdomen as a Rutherford Morison incision, which is an oblique incision used for right and left sided colonic resection. Instead of an oblique incision, surgeons can also use transverse incisions, which are often made through the anterior layer of the rectus sheath and rectus abdominis muscle. They provide good access and are less likely to damage the nerve supply of the rectus abdominis as the incision is parallel to the nerves, and are used in pediatric or vascular repair cases.

Another type of transverse incision is a Lanz incision, which may also be used for appendectomies, and is located in the right iliac fossa and has a more transverse orientation. These types of incisions leave the anterior branch of the iliohypogastric nerve at risk of injury, which would cause decreased sensation over the suprapubic region.

Next up are suprapubic or Pfannenstiel incisions, also called bikini incisions. These are made at the pubic hairline. These types of incisions are horizontal with a slight convexity and are used for most gynecological and obstetrical operations, like a cesarean section. Once this incision is made, the linea alba and anterior rectus sheath is resected, and the rectus muscles are retracted laterally.

Care must be taken to not damage the iliohypogastric or ilioinguinal nerves, as damage to these could result in sensory deficits of the suprapubic and genital region. Sometimes, during a cesarean section, there isn’t enough space to properly finish the procedure and take the baby out safely. In this case, a horizontal transection of the rectus abdominis muscle might be done in addition to the vertical incision along the linea alba.

In order to perform a horizontal incision, first you need to locate the arcuate line, which lies below the umbilicus posterior to the rectus abdominus and marks the inferior limit of the posterior rectus sheath. This is done in order to identify the inferior epigastric arteries, which supply the inferior portion of the rectus abdominis as they ascend on its posterior surface to enter the lateral aspect of the muscle at the arcuate line.

Since there’s no supporting posterior rectus sheath below the arcuate line, this is where the inferior epigastric arteries are most susceptible to injury, so they need to be located and ligated when performing a horizontal incision in order to prevent significant hemorrhage.

Now, bear in mind that any abdominal surgery comes with a risk of nerve injury. The inferior thoracic spinal nerves, those from T7 to T12, and the iliohypogastric and ilioinguinal nerves provide innervation of the abdominal muscles. They are distributed across the anterolateral abdominal wall where they run oblique, but mostly horizontal. They can be injured during any surgical incision or from trauma at any level, leading to muscle weakness of that area of innervation. Muscle wall weakness can lead to herniation of underlying abdominal viscera through the abdominal wall.

In addition to a general inspection of the abdominal wall, another key step in clinical examination is abdominal palpation, which is when you palpate the abdominal wall and the abdominal viscera while the individual lies in a supine position with exposure from their nipples to their pubic symphysis. One clinically relevant sign to look out for when palpating the abdomen is guarding, which is when the muscles of the abdomen get tense and produce spasms.

Guarding can be either voluntary or involuntary. Voluntary guarding is a voluntary contraction of the abdomen performed by the patient in order to avoid pain during examination and usually represents a diffuse abdominal pain, meaning the entire abdomen tenses up.