Chapter 8: The abdomen and perineum

Transcription

1 Chapter 8: The abdomen and perineum The abdomen is that part of the trunk between the diaphragm and pelvis. For descriptive purposes the anterior abdominal wall is divided into nine regions by two horizontal and two vertical planes. The horizontal planes are the transpyloric (midway between the jugular notch and the crest of the pubis), and the transtubercular (through the tubercles of the iliac crests). The vertical planes are the right and left lateral which pass through the midinguinal points (between anterior superior iliac spine and symphysis pubis). The regions so formed are - centrally, from above downwards, the epigastric, the umbilical and the pubic. On each side of these there are the hypochondriac, lateral and inguinal regions. The transpyloric plane is coincident with the level of the tip of the 9th costal cartilage anteriorly and the body of the 1st lumbar vertebra posteriorly. In clinical practice, the subcostal plane (through the lowest part of the costal margin) is often used instead of the transpyloric plane. Skeleton: vertebrae, sacrum, ribs and pelvis are described in other chapters. Anterior abdominal wall The skin of the anterior abdominal wall is supplied by the 6th thoracic to 1st lumbar spinal nerves. T6 supplies the epigastric region, T10 the umbilical region, and L1 the groin. The superficial fascia is in two layers. The more superficial is fatty and continuous with the superficial fascia of the thorax and thigh. The deeper is membranous and best developed in the lower abdomen. It is attached inferiorly to the iliac crest, the fascia lata of the thigh and the pubic tubercle. It is continued in front of the symphysis pubis, between and below the two pubic tubercles, and is attached inferiorly to the ischiopubic rami and the posterior border of the perineal membrane. (See in later chapters for relations of this fascia in the perineum.) Fractures of the pelvis may be accompanied by a torn urethra. If such a patient micturated, urine would escape and spread over the lower abdomen deep to this membranous layer. Laterally, the anterior abdominal wall consists of three sheet-like muscles in separate layers, an outer external oblique, a middle internal oblique and inner transversus abdominis. Anteriorly they become aponeurotic, fuse and form the sheath around rectus abdominis. Rectus abdominis Each muscle lies ventrally alongside the midline enclosed in a fibrous sheath. It is formed of vertically running fibres and is wider above than below. Attachments Superior - the anterior surface of the 5th, 6th and 7th costal cartilages and the xiphoid process. Inferior - the pubic crest and the front of the pubic symphysis. There are two or three horizontal tendinous intersections in the muscle which are attached to the anterior layer of the sheath. 1

2 The rectus sheath is formed by the aponeuroses of the three flat abdominal muscles which join in the midline to form a strong fibrous raphé, the linea alba. Above the costal margin the posterior wall of the sheath is absent and the anterior wall is formed by the external oblique aponeurosis. Between the costal margin and a point midway between the umbilicus and the pubic symphysis, the internal oblique aponeurosis splits to enclose the muscle and is reinforced anteriorly by the external oblique and posteriorly by the transversus abdominis. The lower free edge of the posterior wall forms the arcuate line. Inferior to it the rectus muscle lies on the transversalis fascia and peritoneum, and the anterior wall is formed by all three aponeuroses. The sheath contains (i) rectus abdominis, (ii) superior and inferior epigastric vessels (which anastomose in the sheath), (iii) the 7th-11th intercostal and subcostal nerves, (iv) lymph vessels. External oblique Attachments Superior - from the outer surfaces at the lower eight ribs, interdigitating with serratus anterior and latissimus dorsi. Inferior - the fleshy posterior fibres pass downwards to the anterior half of the iliac crest. The muscle has a free posterior border. Anteriorly, the fibres become aponeurotic, pass medially and downwards, in front of the rectus abdominis to be attached to the xiphoid process, the linea alba and the front of the body of the pubis. The pubic crescent forms the base of a triangular deficiency in the aponeurosis, the superficial inguinal ring. The apex of the deficiency points laterally and its thickened borders form the lateral and medial crura. Between the pubic tubercle and the anterior superior iliac spine the thickened lower border of the aponeurosis is known as the inguinal ligament. This free lower border is curved backwards on itself and gives attachment laterally to internal oblique and transversus abdominis. Its medial 2 cm is expanded and attached to the pectineal line on the pubis. It forms the pectineal part (lacunar ligament) which, with the inguinal ligament proper, forms the gutter-like floor of the inguinal canal. Below the inguinal ligament, the aponeurosis is continuous with the fascia lata of the thigh. Internal oblique Attachments Lateral - in a continuous line from the thoracolumbar fascia, the anterior two-thirds of the iliac crest and the lateral two-thirds of the inguinal ligament. Medial - the vertical posterior fibres ascend to the costal margin. Most of the fibres pass upwards and medially and form an aponeurosis which splits around the upper part of rectus abdominis and reaches the linea alba. In the lower abdomen, all the fibres pass in front of rectus abdominis leaving a free edge posteriorly known as the arcuate line. The fibres from the inguinal ligament arch medially over the spermatic cord, unite with the transversus abdominis aponeurosis and form the conjoint tendon which passes anterior to the lower part of the rectus abdominis and reaches the crest and medial part of the pectineal line of the pubis. 2

3 Transversus abdominis Attachments Lateral - in a continuous line from the inner surfaces of the lower six costal cartilages (interdigitating with the diaphragm), the thoracolumbar fascia, the anterior two-thirds of the iliac crest and the lateral one-third of the inguinal ligament. Medial - the horizontal upper fibres become aponeurotic lateral to the rectus abdominis muscle and reinforce the posterior wall of its sheath from the xiphoid to the arcuate line. Below this the aponeurosis blends with the internal oblique aponeurosis and forms the conjoint tendon. Deep to transversus abdominis lie the transversalis fascia and peritoneum. Actions The muscles form an elastic wall that both supports the abdominal viscera and yet allows changes in their size and position during diaphragmatic movements. They assist in expiration and in expulsive efforts such as defaecation, micturition, parturition (childbirth) and vomiting. The muscles of both sides act together in flexing the trunk; their unilateral action produces lateral flexion of the trunk and the oblique muscles can produce trunk rotation. Nerve supply The lower five intercostal nerves, the subcostal nerve and the 1st lumbar nerve. Blood supply The anterior abdominal wall receives its blood supply from the superior and inferior epigastric, the intercostal and subcostal arteries, and branches of the femoral artery. The veins mainly correspond to the arteries. Surgical access to the abdomen is gained through a variety of incisions. Surgeons operating on the gall bladder, stomach, duodenum and spleen frequently use either midline (through the linea alba) or paramedian incisions. The latter are made vertically about 2 cm from the midline through the anterior rectus sheath and, after the rectus abdominis muscle has been retracted laterally, cutting the attachments of the tendinous intersections when necessary, through the posterior rectus sheath and peritoneum. Oblique muscle splitting incisions in the right iliac region through external and internal obliques and transversus abdominis muscles provide access to the appendix. Longer oblique muscle cutting incisions allow access to the right colon or, on the left side, to the sigmoid colon and rectum. Lower paramedian or midline incisions provide similar access to the latter structures. Pelvic organs can be approached through a low transverse incision through the anterior rectus sheath and peritoneum. The rectus muscles may be divided transversely but it is usually sufficient to retract them laterally. Some pelvis structures, such as the bladder, may be reached extraperitoneally, the peritoneum being swept superiorly, rather than divided. 3

4 The inguinal canal This is an oblique path through the anterior abdominal wall. It extends from the deep inguinal ring, a deficiency in the transversalis fascia just above the midpoint of the inguinal ligament, to the superficial inguinal ring, a deficiency in the external oblique aponeurosis, lying just above and medial to the pubic tubercle. The canal is about 4 cm long and possesses anterior and posterior walls, a floor and a roof. Anterior wall Formed throughout by the external oblique aponeurosis and reinforced laterally by the internal oblique. Posterior wall Formed by the transversalis fascia throughout and reinforced by the conjoint tendon medially. The inferior epigastric artery ascends in this wall, medial to the deep ring. Floor part. The recurved lateral edge of the inguinal ligament and its medial pectineal (lacunar) The roof is formed by the arch of the internal oblique fibres, becoming the conjoint tendon medially. The spermatic cord in the male and the round ligament of the uterus in the female traverse the canal. The spermatic cord The spermatic cord is formed when the testis descends through the inguinal canal into the scrotum, carrying with it its duct, vessels and nerves. A prolongation of peritoneum, the processus vaginalis, precedes the testis down the fetal inguinal canal and is later largely obliterated. The spermatic cord gains three fascial coverings from the layers through which it passes: (i) internal spermatic fascia - from the transversalis fascia, (ii) cremasteric fascia and muscle - from the internal oblique, (iii) external spermatic fascia - from the external oblique. It contains three nerves, three arteries, lymph vessels and three other structures, namely: (i) the genital branch of the genitofemoral nerve, the ilio-inguinal nerve and autonomic nerves, 4

5 (ii) the testicular artery (from the aorta), the cremasteric artery (from the inferior epigastric artery), and the artery to the ductus deferens (from the inferior vesical artery), (iii) lymph vessels draining to the para-aortic lymph nodes round the renal vessels, (iv) the pampiniform plexus of veins, the ductus (vas) deferens, and the remains of the processus vaginalis. Hernias are a protrusion of the lining membrane and contents of any body cavity, through its wall. They are common in the inguinal region and particularly so in males because the descent of the testis into the scrotum during development carries with it a tongue of peritoneum, the processus vaginalis. Normally part of this forms the tunica vaginalis of the testis and the remainder disappears but on those occasions where the process remains patent there remains a hernial sac of peritoneum extending from the deep inguinal ring into the inguinal canal. This is a congenital indirect (because of its oblique course) inguinal hernia and can appear at any age from birth onwards. In middle-aged adults another variety of inguinal hernia, the direct hernia is common. This results from weakness of the posterior wall of the inguinal canal, the fascia transversalis, and protrusion of a peritoneal sac directly forwards. Femoral hernias are rather less common. They occur through the narrow unyielding femoral ring and may extend superficially through the saphenous opening to lie inferior to the pubic tubercle, distinguishing them from inguinal hernias which are always felt superior to that structure. Femoral hernias are more common in women because of the greater width of the female pelvis and the consequent larger size of the femoral canal. Congenital umbilical hernias occur in infants through the umbilical opening or a weakened umbilical scar. In adults, hernias in this region are usually through a weakened linea alba, close to but above the umbilicus and are called para-umbilical hernias. Less frequently, adults may present with small hernias, through the linea alba, midway between the umbilicus and the xiphoid. These are termed epigastric hernias. It is advisable to repair surgically all hernias in children and young people for they tend to enlarge as time passes. Any adult hernia producing pain also requires repair. The principles of hernia repair are to remove the sac (herniotomy) and to repair the defect (herniorrhaphy). The testis and epididymis The two testes are oval glandular organs. Each is suspended in the scrotum by the spermatic cord and is about 4 cm long and 2.5 cm in diameter. They have upper and lower poles. The epididymis, with the ductus deferens on the medial side, is applied to its posterolateral aspect. The other aspects of the gland and the sides of the epididymis are covered by the visceral layer of a closed serous sac, the tunica vaginalis, the lower end of the processus vaginalis. The tunica vaginalis, testis, epididymis and ductus deferens are surrounded by extension of the spermatic cord coverings, namely: (i) internal spermatic fascia; (ii) cremaster muscle and cremasteric fascia; (iii) external spermatic fascia; (iv) superficial fascia which contains some smooth muscle (the dartos); and (v) scrotal skin. Each testis is 5

6 separated from its fellow by the median scrotal septum. Vessels and nerves enter the gland at its lower pole posteriorly. The epididymis This is a tightly coiled tube about 6 cm long applied to the posterolateral surface of the testis. It consists of an enlarged upper extremity (the head), a body and a tapering lower extremity (the tail). The head is attached to the upper pole of the testis by the efferent ducts of the testis. The lumen of the tube is continuous with these efferent ducts above and with the ductus deferens below. Blood supply This is through the testicular artery, a branch of the aorta, and to a lesser extent, the artery to the ductus, a branch from a vesicular artery. Venous blood drains via the pampiniform plexus to the right and left testicular veins. The right drains into the inferior vena cava and the left into the left renal vein. Nerve supply Sympathetic fibres, which originate in the 10th thoracic segment pass to the gland along the testicular artery. Afferent fibres enter the spinal cord in spinal nerves, especially the genitofemoral and ilio-inguinal nerves. Lymph drainage Lymph vessels pass with the arteries to para-aortic nodes in the region of the renal arteries. Histology The testis is covered by thin mesothelium (the tunica vaginalis), continuous with the peritoneum in the embryo. Under this is a dense fibrous coat, the tunica albuginea which is thickened posteriorly to form the mediastinum testis. From the mediastinum incomplete septa radiate into the gland and divide it into compartments, each of which contains two to four convoluted seminiferous tubules. These tubules open into the plexiform rete testis in the mediastinum and about 20 efferent ducts pass from the rete to the head of the epididymis where they become convoluted prior to forming the duct of the epididymis. The seminiferous tubules consist of a thin basement membrane and, after puberty, contain developing spermatozoa. Scattered groups of interstitial cells are present between the tubules. The efferent ducts and the duct of the epididymis have muscular walls and are lined by ciliated columnar and pseudostratified epithelium respectively. The ductus deferens has muscular walls and the lining is much folded and mostly nonciliated. Embryology The testis develops from the coelomic mesothelium of the posterior abdominal wall between the mesonephros on each side of the dorsal mesentery. The tubules of the 6

7 mesonephros become the efferent ducts and the head of the epididymis. The mesonephric duct becomes the ductus deferens. The testis is attached by its lower pole to a mesodermal mass, the gubernaculum. During development, the testis, preceded by the gubernaculum, descends and passes through the inguinal canal. At birth it lies in the scrotum. A tubular process of peritoneum, the processus vaginalis, passes into the scrotum with the descending testis. Its neck is usually obliterated at the time of birth but the distal part remains as the tunica vaginalis of the testis. Undescended testis: usually the testis has descended through the inguinal canal into the scrotum by the time of birth, but it may be retarded or impeded along the line of descend and so remain undescended. It may pass through the external inguinal ring but fail to enter the scrotum and remain in an ectopic position. It often lies in a subcutaneous fascial pouch just lateral to the external ring. Hydrocele of the testis: is an accumulation of liquid in the tunica vaginalis. Hydrocele of the cord also occurs when fluid accumulates in an unobliterated portion of the processus vaginalis which lies within the spermatic cord. Torsion (twisting) of the cord: results in impairment of the blood supply to the testis and rarely occurs outside the 10 to 20-year-old age group. Malignant tumours of the testis usually occur in the young adult. They are important to detect as they have a good prognosis when treated in the early stages. Peritoneal cavity Peritoneum Peritoneum is a thin serous membrane (mesothelium) enclosing the peritoneal cavity. The part lining the abdominal walls is known as parietal and that covering the viscera as visceral peritoneum. The visceral and parietal layers are separated by a small amount of serous fluid. Viscera invaginate the sac to a varying extent. Some are almost completely invested in peritoneum and carry with them double layers known as mesenteries, ligaments, folds or omenta. Others are incompletely invested and have bare areas in contact with the abdominal walls. This invagination of the viscera decreases the size and increases the complexity of the peritoneal cavity. Arrangements Near the midline and between the umbilicus and the oesophagus, a double layer of peritoneum passes back from the anterior abdominal wall and the diaphragm, and separates to enclose the liver. Anteriorly these two layers are in apposition and form the falciform ligament. Superiorly the right layer is pulled from the diaphragm to form the coronary and right triangular ligaments and it encloses much of the bare area of the liver. The left layer forms the left triangular ligament. On the visceral surface of the liver, the two layers reunite along the fissure for the ligamentum venosum and the porta hepatis to form the lesser omentum. The two layers of the lesser omentum pass back to the oesophagus, the stomach and the beginning of the duodenum, and separate to enclose these three structures. The layers 7

8 come together again along the greater curvature of the stomach. The peritoneum is attached to the posterior abdominal wall from the descending (2nd) part of the duodenum, along the anterior border of the pancreas, the anterior surface of the left kidney and the under surface of the diaphragm as far as the oesophageal opening. Between these attachments and the greater curvature of the stomach, the greater omentum hangs down in a lax double fold in front of the transverse colon and mesocolon, and small intestine. Between the stomach and the anterior surface of the left kidney, the peritoneum encloses the spleen and so is divided into gastrosplenic (gastrolienal) and lienorenal ligaments. The four peritoneal layers of the greater omental fold fuse with each other and become adherent to the upper surface of the transverse mesocolon, so that the transverse colon appears to be enclosed within the layers of the greater omentum. The transverse mesocolon is attached to the posterior abdominal wall horizontally across the descending (2nd) part of the duodenum, the anterior border of the pancreas and the anterior surface of the left kidney. The parietal peritoneum covering the posterior abdominal wall below the transverse mesocolon also covers the horizontal (3rd) part of the duodenum and the anterior surfaces of the ascending and descending colons, as these structures lie on the posterior abdominal wall. Peritoneum forms a mesentery for the sigmoid colon. This is attached along a ^-shaped line whose apex lies over the left sacroiliac joint. This same parietal peritoneum forms a mesentery for the jejunum and ileum which is attached to the posterior abdominal wall along the line joining the duodenojejunal flexure and the ileocolic junction. The omental bursa (lesser sac) of peritoneum is a diverticulum of the peritoneal cavity lying behind the stomach and in front of the posterior abdominal wall. Its opening, the epiploic foramen, is directed to the right, and is bounded by the liver superiorly, the inferior vena cava posteriorly, the duodenum inferiorly and the free edge of the lesser omentum anteriorly. The omental bursa extends downwards into the greater omentum, to the left as far as the hilus of the spleen and superiorly into a recess between the liver (caudate lobe) and the diaphragm. Its anterior and posterior relations are shown in the figure. The abdominal peritoneum extends down into the pelvis to cover the pelvic walls and the upper surfaces of the viscera. It covers the superior surface of the bladder. In the male it covers the upper end of the seminal vesicles and dips down into the rectovesical fossa, whence it ascends to the front and sides of the rectum. In the female, peritoneum also covers the anterior and posterior surfaces of the uterus and uterine tubes. The peritoneal fold over each tube extends to the pelvic walls and is known as the broad ligament of the uterus. It separates a shallow uterovesical pouch in front (which is present between the uterus and bladder) from a deeper rectouterine posteriorly (pouch of Douglas). This latter pouch has the rectum behind and the uterus and upper part of vagina in front. Embryology The development of the abdominal alimentary tract can be described in five stages. Stage 1, elongation: most parts increase in length especially between four relatively fixed regions, (a) the oesophagus where it passes through the developing diaphragm, (b) the proximal part of the duodenum lying on the posterior abdominal wall, (c) the region where 8

9 the midgut becomes the hindgut (future splenic flexure), and (d) the rectum where it passes through the pelvic diaphragm. Stage 2, herniation: the midgut becomes too large for the abdominal cavity, and a loop passes into the body stalk. The proximal limb of the loop is supplied by pre-arterial branches of the superior mesenteric artery. Stage 3, rotation: in the foregut, this is through about 90 in an anticlockwise direction, so that the right side of the upper coelomic cavity comes to lie behind the stomach, and the left side extends in front. The ventral mesentery lies obliquely across the upper abdomen and is much distorted by the enlarging liver. The midgut loop rotates anticlockwise through about 270 round the axis of the superior mesenteric artery so that the pre-arterial limb of the loop comes to lie below and to the left. The postarterial limb comes to lie above and to the right. Stage 4, retraction: on its return, the midgut loop fills much of the abdomen; the prearterial loop (future small intestine) lies below and to the left of the postarterial loop (future ascending and transverse colons). The caecum and appendix are at first high in the abdomen, under the developing liver, but later elongation of the ascending colon and the fixation of the hepatic flexure of the colon brings the appendix and caecum into the region of the right iliac fossa. Stage 5, fixation: in some regions the gut loses its dorsal mesentery, is covered in front only by peritoneum and has a relatively large bare area against the posterior abdominal wall. These changes in the size and disposition of the abdominal alimentary tract mainly take place during the second and third months of fetal life. The stomach, jejunum, ileum, transverse and sigmoid colons retain their mesenteries; the duodenum, ascending and descending colons lose their mesenteries and lie on the posterior abdominal wall covered only anteriorly by peritoneum. The primitive stomach rotates, its left surface becoming anterior. The spleen develops in the left side of its dorsal mesentery and in so doing divides this mesentery into gastrosplenic and lienorenal ligaments. Below the greater curvature of the stomach the dorsal mesentery increases in size and forms a double fold - the greater omentum - in front of the transverse colon and mesocolon. The liver develops in the ventral mesentery which provides its peritoneal covering and grows towards the right causing the lesser omentum and stomach to trap behind them a pouch of the peritoneum called the omental bursa (lesser sac). Minor anomalies are common and are due to defects in the timing of the stages and the direction and degree of rotation. Major anomalies, eg situs inversus, also occur and may involve reversal of all processes (clockwise, and no rotation). Occasionally incomplete retraction into the abdomen occurs and a part of the cavity or of the gut may be in the 9

10 umbilical cord at birth. This is usually associated with maldevelopment of the anterior abdominal wall. The peritoneal spaces Peritoneal anatomy ensures that fluid or pus tends to collect in certain regions or potential spaces within it. The right and left subphrenic spaces lie between the diaphragm and the liver, separated from each other by the falciform ligament. Behind the right space is the coronary ligament, behind the left, the left triangular ligament. Below the liver lies the right subhepatic space, the most dependent part of the abdominal peritoneal cavity, when one lies on ones back. To the lateral side of the ascending colon is the right paracolic gutter - a potential route for the transmission of infection from the pelvis or the appendix to the right subhepatic space. The lesser sac or left subhepatic space may initially limit the inflammation arising from the pancreas. The Pouch of Douglas lies low in the pelvis bounded by the rectum and bladder. Peritoneal infections frequently gravitate and localise as abscesses in this space. They can be palpated by rectal or vaginal examination. 10