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/Posterior abdominal wall and retroperitoneum/SKIN AND SOFT TISSUES

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).

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.

  

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.

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.)

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