Posterior abdominal wall and retroperitoneum

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CHAPTER 62 Posterior abdominal wall and retroperitoneum

The posterior abdominal wall consists of fasciae, muscles and their vessels and spinal nerves; the overlying skin is continuous with that of the back. It is not easily defined, and is best described as that part of the abdominal wall lying between the two mid-dorsal lines, below the posterior attachments of the diaphragm and above the pelvis. It is continuous laterally with the anterolateral abdominal wall, superiorly with the posterior wall of the thorax behind the attachments of the diaphragm and inferiorly with the structures of the pelvis. The spinal column forms part of its structure and the muscles and fasciae of the back are closely related to it, especially posterolaterally.

The major vessels and lymphatic channels, in addition to the peripheral autonomic nervous systems of the abdomen, pelvis and lower limbs lie on the posterior abdominal wall. These structures, together with several viscera (including the kidneys [Ch. 74], suprarenal glands [Ch. 72], pancreas [Ch. 70], ureters [Ch. 74] and parts of the gut tube [Chs 66 and 67]), lie beneath the posterior parietal peritoneum. These tissues and their surrounding connective and fascial planes are collectively referred to as the retroperitoneum.

It has been suggested that the retroperitoneum can be divided into several spaces according to their relationships to the fascial layers that surround the kidneys and ureters. In this description, the layers of the perirenal fascia enclose a perirenal space containing the kidney, suprarenal gland, upper ureter and their neurovascular supply. The anterior layer of the perirenal fascia is continuous across the midline anterior to the main neurovascular structures of the retroperitoneum, and the right and left perirenal spaces communicate, although this channel is limited and contains many of the midline neurovascular structures of the retroperitoneum. Behind the posterior layer of the perirenal fascia lies the posterior pararenal space. Anterior to the anterior layer of the perirenal fascia lies the anterior pararenal space, in which lie several retroperitoneal parts of the gut tube, including the duodenum and pancreas. The anterior pararenal spaces are also continuous across the midline and are limited posteriorly by the anterior communicating layer of the perirenal fascia and anteriorly by the parietal peritoneum. This description helps to explain why moderate amounts of fluid, blood or pus collecting in the retroperitoneum tend to remain constrained within the space in which they are formed although, for pathological processes such as tumour invasion, the fascial planes provide a weak barrier to local spread (Figs 62.1, 62.2).

Fig. 62.1 Fascial layers of the upper posterior abdominal wall. A, Transverse section just below the level of the hilum of the kidney. For clarity, the deep muscles of the back have not been identified separately. B, The separate layers of the thoracolumbar fascia.

Fig. 62.2 Fascial layers of the lower posterior abdominal wall. Transverse section at the level of the fifth lumbar vertebra.

Several structures, such as the pancreas, are referred to as being retroperitoneal. Although they are derived embryologically from the gut tube, they are not readily separated from the other retroperitoneal structures. Several other structures, such as the descending colon, are also referred to as being retroperitoneal, but they remain separated from the other retroperitoneal structures by a clearly defined fascial plane, which corresponds with the plane of fusion of their mesentery during development. This is of relevance during surgical exposure of the retroperitoneal organs and in some pathological processes: those defined by clear fascial planes may be mobilized with little or no risk of bleeding, whereas mobilization of the pancreas, for example, is difficult and often very vascular.

SKIN AND SOFT TISSUES

The skin of the back in the region of the posterior abdominal wall is similar to that of the rest of the trunk. It is supplied by vessels from the musculocutaneous branches of the lumbar arteries and veins, and receives its innervation from the dorsal rami of the lumbar spinal and lower thoracic nerves.

The soft tissues of the posterior abdominal wall and retroperitoneum are composed of several distinct layers of fascia, which divide them into anatomically distinct compartments.

THORACOLUMBAR FASCIA

The thoracolumbar fascia in the lumbar region is in three layers (Figs 62.1, 62.2, 62.3). The posterior layer is attached to the spines of the lumbar and sacral vertebrae and to the supraspinous ligaments. The middle layer is attached medially to the tips of the transverse processes of the lumbar vertebrae and the intertransverse ligaments, inferiorly to the iliac crest, and superiorly to the lower border of the 12th rib and the lumbocostal ligament. The anterior layer covers quadratus lumborum and is attached medially to the anterior surfaces of the transverse processes of the lumbar vertebrae behind the lateral part of psoas major. Inferiorly, it is attached to the iliolumbar ligament and the adjoining part of the iliac crest. Superiorly, it is attached to the apex and inferior border of the 12th rib and then extends to the transverse process of the first lumbar vertebra, to form the lateral arcuate ligament of the diaphragm. The posterior and middle layers of the thoracolumbar fascia unite at the lateral margin of erector spinae. At the lateral border of quadratus lumborum they are joined by the anterior layer, to form the aponeurotic origin of transversus abdominis.

Fig. 62.3 Axial T2 weighted MRI scan showing the fascial layers of the posterior abdominal wall at the level of the renal hilum.

(Courtesy of Dr Louise Moore, Chelsea & Westminster Hospital, London.)

OTHER FASCIAL LAYERS

Psoas fascia

Psoas major is enclosed within a layer of fascia over its anterior surface. The medial border is continuous with the attachments of the muscle to the transverse processes of the lumbar vertebrae, the bodies of the lumbar vertebrae and the tendinous arches. Superiorly, the fascia forms part of the medial arcuate ligament. Laterally, the fascia blends with the fascia over quadratus lumborum in the upper part of the muscle and is continuous with the iliac fascia lower down. It separates the anterior mass of psoas major from the retroperitoneal structures lying on it. The fascia extends down into the thigh. Inflammatory collections arising from the paraspinal tissues or the retroperitoneal tissues that penetrate through the fascia tend to be confined by it, and they may track down the length of the muscle, to appear in the groin where the fascia is thinnest.

Iliac fascia

The iliac fascia is continuous with and indistinguishable from the psoas fascia. The fascia blends with the anterior layer of the thoracolumbar fascia over quadratus lumborum in the upper retroperitoneum. Lower down, it is attached firmly to the inner aspect of the iliac crest and medially to the periosteum of the ilium at the pelvic brim. It is also attached in the abdomen to the iliopectineal eminence.

Perirenal fascia

The perirenal fascia is a multilaminated fascial layer that surrounds the kidney, suprarenal glands, upper ureter and associated fat, which all lie in the perirenal space (see Ch. 74). Although described as having anterior and posterior layers, these are continuous with each other laterally, and give rise to the lateroconal fascia at this point (Figs 62.1–62.3). The posterior layer of the renal fascia is adherent to the fascia over psoas major, the iliac fascia and the anterior layers of the thoracolumbar fascia. In the obese, there may be some loose adipose tissue between these layers, but it is rarely thick. The anterior part of the renal fascia separates the kidney and the perirenal space from the overlying anterior pararenal space and its associated viscera (on the right the duodenum, ascending colon and right colonic mesentery and on the left the duodenum, descending colon and left colonic mesentery). Inferiorly, the perirenal fascia continues down and encloses the ureter. It becomes progressively thinner towards the brim of the pelvis, where it is no longer distinguishable from the loose general connective tissue of the retroperitoneum.

Lateroconal fascia

The lateroconal fascia is formed from the lateral aspect of the perirenal fascia and extends anterolaterally to fuse with the fascia over transversus abdominis. It divides the anterior and posterior pararenal spaces from each other, but is thinnest in the inferior part of the retroperitoneum.

POSTERIOR EXTRAPERITONEAL CONNECTIVE TISSUE

The retroperitoneum usually contains loose connective tissue between the fascial layers. This is particularly true around the renal fascia and anterior to the psoas and iliac fascia. In all but the thinnest individuals, there is some adipose tissue present in these areas, and in the obese it may be markedly thickened. The retroperitoneal arteries and veins lie within this tissue, but the branches of the lumbar plexus of nerves lie deep to it, beneath the iliac and psoas fascia.

BONES

The posterior abdominal wall is supported by the bony structures of the vertebral column and bony pelvis. These include the lower two ribs, the 12th thoracic and five lumbar vertebrae, and the sacrum and ilium, in addition to their interconnecting ligaments.

MUSCLES

The majority of the muscles of the posterior abdominal wall are functionally part of the lower limb or vertebral column. They provide the surface against which the neurovascular structures of the retroperitoneum lie, and they are supported and separated from the majority of the retroperitoneal structures by fascial layers (Figs 62.4, 62.5; see Fig. 62.14).

Fig. 62.4 Bones and deep muscles of the posterior abdominal wall. Left psoas major and the diaphragm have been removed.

(Adapted from Drake, Vogl and Mitchell 2005.)

Fig. 62.5 Muscles of the posterior abdominal wall. A, Coronal True FISP MRI. B, Axial T2 weighted MRI. (Provided by Dr Louise Moore Chelsea & Westminster Hospital.)

Fig. 62.14 Muscles and nerves of the posterior abdominal wall. The left psoas major has been removed to expose the origins of the lumbar plexus and quadratus lumborum.

Erector spinae

The muscles of the erector spinae group (see Ch. 42) do not form part of the posterior abdominal wall itself, but are closely associated with the fascial layers of the posterior wall.

Iliacus

Iliacus is a triangular sheet of muscle that arises from the superior two-thirds of the concavity of the iliac fossa, the inner lip of the iliac crest, the ventral sacroiliac and iliolumbar ligaments and the upper surface of the lateral part of the sacrum. It reaches as far as the anterior superior and anterior inferior iliac spines, and receives a few fibres from the upper part of the capsule of the hip joint. Most of its fibres converge into the lateral side of the strong tendon of psoas major. It lines the posterior wall of the lesser pelvis formed by the ilium.

Posterior abdominal wall hernias

Herniation through the posterior abdominal wall is extremely rare because the fascial layers usually provide an excellent protection against protrusion of the posterior abdominal viscera, which are relatively immobile. However, the posterior free border of external oblique and the inferior free border of latissimus dorsi create an area of potential weakness, referred to as the lumbar triangle. Spontaneous hernias through this tissue are very rare in the absence of previous surgical access such as a nephrectomy.

VASCULAR SUPPLY AND LYMPHATIC DRAINAGE

ABDOMINAL AORTA

The abdominal aorta begins at the median, aortic hiatus of the diaphragm, anterior to the inferior border of the 12th thoracic vertebra and the thoracolumbar intervertebral disc (Figs 62.6, 62.7). It descends anterior to the lumbar vertebrae to end at the lower border of the fourth lumbar vertebra, a little to the left of the midline, by dividing into two common iliac arteries. It diminishes rapidly in calibre from above downward, because its branches are large; however, the diameter of the vessel at any given height tends to increase slightly with age. The cadaveric superior and inferior calibres are between 9–14 mm and 8–12 mm, respectively, with little difference between the sexes. The angle of the aortic bifurcation varies widely, particularly in the elderly. It has been suggested that the relationship between aortic size and shape is a possible causative factor in the development of abdominal aortic aneurysm (Newman et al 1971). The reflection of transmitted pressure waves at junctions between vessels (of which the abdominal aorta has many) may focally weaken the intimal lining, e.g. at the aortic bifurcation, pressure oscillations and possibly turbulence may be set up as a result of differences in the luminal diameters of the common iliac arteries, producing reflected waves that may injure the intima of the distal abdominal aorta. The role of the relative calibres of the iliac arteries remains uncertain (Shah et al 1978).

Fig. 62.6 The abdominal aorta, inferior vena cava and their branches. The fasciae, lymphatics and connective tissue have been removed for clarity.

Fig. 62.7 Magnetic resonance aorto-iliac angiogram. A, Coronal reformat. B, Sagittal reformat.

Relations

The upper abdominal aorta is related anteriorly to the coeliac trunk and its branches. The coeliac plexus and the lesser sac lie between it and the left lobe of the liver and lesser omentum. Below this, the superior mesenteric artery leaves the aorta, crossing anterior to the left renal vein. The body of the pancreas, with the splenic vein on its posterior surface, extends obliquely up and to the left across the abdominal aorta, separated from it by the superior mesenteric artery and left renal vein. Below the pancreas, the proximal parts of the gonadal arteries, and the third part of the duodenum, lie anteriorly. In its lowest part it is covered by the posterior parietal peritoneum and crossed obliquely by the origin of the small intestinal mesentery.

The thoracolumbar intervertebral discs, the upper four lumbar vertebrae, intervening intervertebral discs and the anterior longitudinal ligament are all posterior to the abdominal aorta. Lumbar arteries arise from its dorsal aspect and cross posterior to it. The third and fourth (and sometimes second) left lumbar veins also cross behind it to reach the inferior vena cava. The aorta may overlap the anterior border of the left psoas major.

On the right, the aorta is related above to the cisterna chyli and thoracic duct, the azygos vein and the right crus of the diaphragm, which overlaps and separates it from the inferior vena cava and right coeliac ganglion. Below the second lumbar vertebra, it is closely applied to the left side of the inferior vena cava. This close relationship occasionally allows the formation of an aorto-caval fistula, particularly after surgery for aneurysmal disease or trauma to the aorta.

On the left, the aorta is related above to the left crus of the diaphragm and left coeliac ganglion. Level with the second lumbar vertebra, it is related to the duodenojejunal flexure and the left sympathetic trunk, the fourth part of the duodenum and the inferior mesenteric vein.

Branches

The branches of the aorta are described as anterior, lateral and dorsal (Fig. 62.8). The anterior and lateral branches are distributed to the viscera and the dorsal branches supply the body wall, vertebral column, vertebral canal and its contents. The aorta terminates by dividing into the right and left common iliac arteries.