Rectus diastasis (RD) describes the separation of the rectus abdominis muscles and is characterized by thinning and widening of the linea alba.

This causes the midline to bulge when intra-abdominal pressure is increased. RD is not a hernia because it does not have a true fascial defect.

Normal anatomy of the anterior abdominal wall

The anterior abdominal wall has three layers. The most superficial layer consists of the skin and adipose tissues.

The middle layer is the myofascial layer, which consists of muscles and their fascial envelopes.

The deep layer is formed by the transversalis fascia, preperitoneal fat and the parietal peritoneum.

The most superficial layer provides coverage for the underlying tissues.

The middle myofascial layer has two vertical paramedian muscles, the rectus abdominis and its accessory pyramidal muscle and three lateral muscles, the external oblique, the internal oblique and the transversus abdominis.

The rectus abdominis muscles extend from the last costal cartilage to the upper edge of the pubis. They are composed of several muscular bodies separated by three or four tendinous intersections. 

Laterally, the external oblique, the internal oblique and the transversus abdominis muscles extend from the lateral edge of the rectus to the flanks with three overlapping layers.

The aponeuroses of these muscles form the rectus sheaths, enveloping the right and left rectus muscles and forming the linea alba. The linea alba opposes the diastasis of the rectus abdominis muscles.

The deep layer of the anterior abdominal wall, formed by the transversalis fascia, preperitoneal fat and the parietal peritoneum, is quite elastic and covers and protects the underlying viscera.

The physical examination

Patients are often referred to surgeons for an upper abdominal bulge that is concerning for a hernia. Distinguishing whether the fascia is intact or disrupted is critical in the diagnosis of DR or hernia, respectively.

Diastasis recti is an aponeurotic laxity of the linea alba, with the muscles themselves that can be normal in thickness or, more frequently, atrophic (myoaponeurotic laxity).

The best manner to diagnose patients with DR is to place them supine and then have the patient engage their core. This can be done with a half sit-up or with a leg raise.

In patients with DR, this will demonstrate a smooth bulgin in the upper midline that tends to have a pyramidal shape. DR can extend from the subxiphoid to below the umbilicus in men and women.

During a physical exam, it is important to recognize that patients may have both DR and associated abdominal wall hernias. The finding of a hernia within a DR may affect the surgical plan of care for the hernia repair.

Obesity is a well-established risk factor for DR. It also makes the diagnosis of DR more difficult on examination.

Use of imaging

Imaging is the most accurate manner to diagnose and qualify a diagnosis of DR. These include the use of ultrasound, computed tomography (CT) and magnetic resonance imaging (MRI).

Classification schemes for diastasis recti have been created based on the inter-rectus distance and location of the defect. Though imaging is not mandatory, imaging can help classify DR and thus guide surgical planning for a patient.

There is no consensus of opinion regarding the normal distance between the rectus muscles.

According to the Beer classification, the surgical definition of an abnormal inter-rectal distance is a width larger than 22 mm when measured 3 cm above the navel and larger than 16 mm when measured 2 cm below the navel.

Below the navel the collagen fibres of the linea alba are arranged in a more transverse way, compared to their arrangement above the navel.

The Rath classification submits that pathological DR changes with age, and thus the classification is age-dependent.

The Nahas classification categorizes DR based on the underlying cause of the myofascial deformity in order to help in surgical planning for its correction.

According to a recent meta-analysis, the available information supports US as an adequate method to assess diastasis of the recti abdominis muscles.

US measurement of inter-rectal distance has proven to have good to excellent intraobserver and interobserver reproducibility, particularly in the above-navel level of the abdominal wall.

US measurement has proven reliable, particularly in the above-navel area, while at the below-navel level there is a slight tendency to underestimate the separation between the two muscles.

The same applies to MRI measurements and to CT measurements, both of which underestimate somehow the extent of muscles separation.

Predisposing factors in rectus diastasis

Rectus diastasis in women

Pregnancy is the predominant cause of RD in women. The growing foetus causes mechanical strain on the abdominal wall and increasing weight gain and displacement of abdominal organs may also play a role.

The structure and function of the abdominal muscles also undergo significant change during pregnancy.

Stretching of the linea alba is further facilitated by hormonal changes during pregnancy, causing elastic connective tissue changes.

After the delivery there is a physiological reduction of the recti muscles’ thickness and an increase in the normal distance between the two muscles.

This change usually persists for some months but, normally, within one year from the delivery the distance between the two muscles returns to the normal, pre-pregnancy value.

Rectus diastasis in men

Suggested risk factors are increasing age, obesity, raised abdominal wall circumference, weight training and abdominal aortic aneurysm.

What symptoms are associated with RD?

A range of symptoms is reported to be associated with RD. Impaired body image and core instability seem to be the most common symptoms.

Patients with RD also experience more back pain due to the instability of the abdominal wall. 

Rectus diastasis as a risk factor in the incidence of abdominal wall hernias

DR is characterized by a protruding midline as a result of an increase in intra-abdominal pressure. DR involves a gradual thinning and widening of the linea alba, combined with a general laxity of the ventral abdominal wall muscle.

The musculofascial continuity of the midline and subsequent absence of a true hernia sac distinguishes DR from a ventral hernia.

However, thinning and stretching of the linea alba is an important risk factor for the actual development of midline hernias (umbilical, epigastric, trocar and incisional hernias) due to deterioration of the connective tissue and pulling of the abdominal muscles.