Data about DRA are too scarce and heterogeneous for a systematic review
The need for deeper clarity and uniformity in DRA has been well understood by the German Hernia Society (DHG) and the International Endohernia Society (IEHS). They, therefore, proposed a classification for DRA [
1]. Nevertheless, strong indications about the definition and evaluation of DRA are lacking.
The prevalence rate of DRA varies with measurement method, measurement site and judgment criteria. The finger-width method is widely used, as it is relatively convenient and economical compared with other methods. However, due to individual differences in finger width, the accuracy of outcome is not reliable. In addition, certain conditions, such as thick subcutaneous fat and significant abdominal slack, can make diagnosis difficult with this technique. Using ultrasound or CT to measure IRD is more accurate than using finger width.
Moreover, the lack of a uniform definition and classification is a barrier to comparing outcomes.
Most researchers focused on pregnant women and postpartum women but paid little attention to women more than 1 year postpartum or middle-aged and elderly women.
No significant differences were found for age as a risk factor, by Mota and Sperstad, but they only included primiparous women in their studies, so this was a series with a small age span. Conversely, age is considered a risk factor by Spitznagle, but a protective factor by Wu et al., who explained the high incidence of DRA in young women because pregnancies are mostly concentrated during this period, but women with a pregnancy or postpartum within 1 year were excluded in the study by Wu, and the IRD tends to become significantly smaller during the first 12 months postpartum, and the greatest recovery tends to occur between day 1 and 8 weeks after delivery when the IRD reached a plateau [
17].
Pregnancy is confirmed to be a risk factor for DRA and, moreover, the more pregnancies a woman has had, the more likely she is to develop DRA. Cesarian section only seems to be a risk factor for women who have given birth twice. The viscoelastic properties inherent to the collagen make the linea alba prone to increasing length when the mechanical stress is prolonged in time, as in the case of lasting increased intra-abdominal pressure [
18]. Long-lasting increased intra-abdominal pressure from a growing fetus and expanding uterus combined with hormonal changes (increasing in secretion of relaxin, progesterone and estrogen) on connective tissue create a physiological (normal) widening of the IRD, creating a DRA during pregnancy. The anterolateral abdominal wall undergoes dramatic changes as the pregnancy progresses. The two muscle bellies of the rectus abdominis elongate and curve round as the abdominal wall expands, similar to suspenders on obese men. At 38 weeks gestation, the length of the abdominal muscles increases to a mean of 115% compared to the beginning of pregnancy. The infra-umbilical (from umbilicus to symphysis pubis) region of the linea alba has a greater number of transverse fibers, which provides a greater ability to resist the tensile stresses imposed thereon [
19,
20]. Liaw et al. [
21] noted that, during pregnancy, the infraumbilical region could sustain a longer duration of stretch during pregnancy (as the growing uterus rises out of the pelvis at 12 weeks and makes contact with the abdominal wall). Their data indicated that IRD values were larger for locations above the umbilicus compared to those below the umbilicus, and suggested that the infraumbilical region of the linea alba has a greater ability to resist stresses imposed over a longer period of time [
21].
According to Wu et al., BMI is an influential factor for the occurrence of DRA. The possible reason is that obese people usually have more adipose tissue in the abdominal cavity, such as greater omentum and mesentery, resulting in an increase in abdominal contents and pressure on the abdominal wall, which—in turn—causes the separation of rectus abdominis to both sides. In addition, obesity may occur at the same time as muscle loss [
22]. Moreover, similar results from the study proposed by Grossi et al. showed that the amount of collagen in the linea alba above the umbilical region in morbidly obese patients is smaller than in non-obese cadavers of the same age group [
23]. Therefore, all these factors probably result in the occurrence of DRA in obese patients.
Moreover, Wu indicated that DRA was related to diabetes. Diabetes can cause loss of muscle mass and function, and cause sarcopenia [
24,
25]. The changes in the rectus abdominis caused by diabetes may have the following two mechanisms: diabetes could contribute to impaired mitochondrial oxidative phosphorylation and hypercomplex assembly in rectus abdominis muscle fibers [
26]. It could also induce muscle structure change by reducing fast fibers and increasing slow fibers [
27].
The high incidence of diastasis recti in subjects with AAA suggests an underlying weakness of connective tissue. Smoking causes an acquired weakness of connective tissue, and it has also been associated with an increased risk of incisional and recurrent groin hernia [
28,
29]. Hydroxyproline, a compound early in the collagen synthesis pathway, has been described as constituting 80% of the dry weight of the rectus sheath, with lower levels in those with inguinal hernias [
30]. Smokers have been shown to produce less hydroxyproline than nonsmokers [
31]. These findings could explain weaknesses in both the linea alba and the aortic wall.
With the aim of exploring the anatomical interactions between the rectus muscle and lateral muscles (external oblique, internal oblique and transverse abdominal muscles) and of understanding the real etiopathogenesis of DRA, we carried out some cadaveric dissections. During these anatomical evaluations, we noted that the internal oblique aponeurosis can join the rectus sheath in two ways, namely (a) splitting its fibers in an anterior and posterior layer, as classically described, or (b) joining only the posterior rectus sheath without an anterior layer.
As usually described, at the lateral margin of the rectus sheath, the lateral muscles aponeurosis join themselves in the semilunar or spigelius line. The external oblique aponeurosis constantly passes in front of the rectus muscles, composing the anterior lamina of the sheath. The internal oblique aponeurosis splits its fibers in an anterior and a posterior layer. The anterior layer joins the fibers of the external oblique in front of the rectus muscle to constitute the anterior lamina. However, some centimeters below the umbilicus, no split in the fibers is evident, and all the aponeurosis of the internal oblique join the external oblique and transverse aponeurosis in constituting the anterior sheath. The transverse muscle aponeurosis also behaves differently at a cranial level compared to the caudal level. Cranially, the fibers constantly remain posterior to the rectus and constitute the deep layer of the sheath, but—at a variable level—some centimeters below the umbilicus, they present anteriorly with all other flat muscle aponeurosis.
To confirm our new variation in the semilunar line, we conducted a retrospective review of abdominal CT images. A blinded trained radiologist evaluated the CT abdominal images of 100 patients (men and women), randomly selected from patients who had been referred to the Radiology Service for any pathological indication (abdominal wall pathology, vascular pathology, urological pathology, gastrointestinal pathology). In compliance with privacy laws, no information about personal details was available.
In 89/100 (89%) patients, the classic insertion (anterior and posterior) was present, but in 11/100 (11%) patients, only a posterior insertion was present. In patients presenting the classical insertion, only 23/89 (26%) CT images showed a rectus diastasis (according to the Rath definition [
15]), whilst in patients with only the posterior insertion, diastasis was present in all patients (100%).
It seems that only the posterior insertion could be considered as a risk factor for diastasis.
A descriptive observational cross-sectional prospective study with a large sample size will soon begin, with the aim of confirming our hypothesis.