HBF-II LEC 66 Gross Anatomy Post Abd Wall Diaphragm Notes 2024 Walker PDF

Gross Anatomy: Posterior Abdominal Wall & Diaphragm Page 1 of 16 Dr. Paul Walker Session Learning Objectives By the end of this session, students should be able to accurately: 1. Describe the anatomy of the posterior abdominal wall and diaphragm. Summa...

Gross Anatomy: Posterior Abdominal Wall & Diaphragm Page 1 of 16 Dr. Paul Walker Session Learning Objectives By the end of this session, students should be able to accurately: 1. Describe the anatomy of the posterior abdominal wall and diaphragm. Summarize the anatomy of the posterior abdominal wall and diaphragm. List the bones that provide the structural framework for the posterior abdominal wall. List the muscles that add structure to the posterior abdominal wall and discuss their attachments and innervation. Describe the anatomy of the diaphragm. List the branches of the abdominal aorta and categorize into unpaired and paired branches. Describe the tributaries to the inferior vena cava. Describe the retroperitoneal lymphatic trunks that drain lymph from the abdominal cavity. Identify the nerves of the lumbar plexus in the posterior abdominal wall. 2. Relate posterior abdominal wall and diaphragm anatomy to their functions or positional relationships with nearby structures. Relate retroperitoneal structures found in the posterior abdominal wall region. List actions produced by posterior abdominal wall muscles. Relate position of erector spinae muscles to the posterior abdominal wall. Discuss the location of structures that pass through or posterior to the diaphragm. Relate the development of the diaphragm to its adult structure. Relate the nerves of the lumbar plexus to structures innervated. Relate the positions of preaortic ganglia and autonomic plexus to the sympathetic trunk and abdominal aorta. Review organ relationships in the posterior abdominal wall region. 3. Apply anatomical knowledge to clinical problems and procedures associated with the posterior abdominal wall and diaphragm: Psoas abscess Congenital diaphragm hernia Hiatal hernia Nutcracker vs. SMA syndromes Abdominal Aortic Aneurysm May-Thurner Syndrome. RetroPeritoneal Lymph Node Dissection (RPLND) Axial CT Interpretation Gross Anatomy: Posterior Abdominal Wall & Diaphragm Page 2 of 16 Dr. Paul Walker Session Outline I. Overview and Musculoskeletal Framework of the Posterior Abdominal Wall A. Definition B. Bones C. Muscles D. Clinical Correlation: Psoas Abscess II. Diaphragm A. Anatomy B. Developmental Considerations C. Clinical Correlation: Diaphragmatic Hernias III. Abdominal Aorta A. Anatomy B. Clinical Correlation: Nutcracker vs. SMA syndrome C. Clinical Correlation: Abdominal Aneurysm IV. Inferior Vena Cava A. Tributaries B. Clinical Correlation: IVC Filters C. Clinical Correlation: May-Thurner Syndrome V. Abdominal Lymphatics A. Node Organization B. Clinical Correlation: RPLND VI. Nerves of the Posterior Abdominal Wall A. Lumbar Plexus B. Autonomic Nerves VII. Organ Relationships in Cross Sectional Imaging A. Organs Related to Posterior Abdominal Wall & Diaphragm B. Cross Sectional Anatomy and Axial Imaging Gross Anatomy: Posterior Abdominal Wall & Diaphragm Page 3 of 16 Dr. Paul Walker I. Posterior Abdominal Wall A. Definition Fig 1(Gray’s Anatomy for Students) The posterior abdominal wall (Fig 1) is a musculoskeletal structure that is posterior to the abdominal GI tract, liver, pancreas, and spleen. It includes muscles that move the trunk and lower limbs. Superiorly, the posterior abdominal wall is limited by the diaphragm. Retroperitoneal viscera include the kidneys, ureters, and suprarenal glands (see Dr. Goebel’s lecture). The posterior abdominal wall is also conduit between body regions for structures such as the abdominal aorta, inferior vena cava, lymphatics, and sympathetic trunk. It is also the site of emergence of the lumbar plexus of nerves that innervate the lower trunk and extremities. Fig 2 (Gray’s Anatomy for Students) B. Bones The skeletal framework of the posterior abdominal wall includes the below bones (Fig 2): Ribs XI & XII Crest & fossa of the ilium Bodies & transverse processes of lumbar vertebrae (LI-V) Upper margin (ala) of sacrum The prominence of the lumbar vertebral bodies in this region is due to the secondary curvature (forward convexity) of the vertebral column.  Gross Anatomy: Posterior Abdominal Wall & Diaphragm Page 4 of 16 Dr. Paul Walker C. Muscles Fig 3 (Gray’s Anatomy for Students) There are 4 muscles of the posterior abdominal wall. You have already learned two of them. Muscles Learned in HBF-I Psoas major Iliacus New Muscles Psoas Minor (sometimes absent) Quadratus lumborum Fig 4 Muscle Proximal Attachment Distal Attachment Actions Innervation Vertebral Column Femur Thigh Flexion Ventral rami Psoas Major (TV12-LV4) (Lesser Trochanter) Trunk Flexion L1-L3 Pelvis Femur Thigh Flexion Femoral nerve Iliacus (Iliac Fossa) (Lesser Trochanter) Trunk Flexion (L2-L4) Vertebral Column Pelvis Trunk Flexion Ventral rami Psoas Minor (TV12-LV1) (Iliopubic eminence) (weak) L1 Rib 12 & Vertebral Pelvis Trunk Lateral Flexion Ventral rami Quadratus Lumborum Column (TV12-LV5) (Iliac crest) Stabilizes 12th Rib T12-L4 Fig 5 (WSU cross section & Gray’s Anatomy for Students) Recall that the erector spinae muscle group is located posterior to the vertebral column and is also posterior to the musculoskeletal framework of the posterior abdominal wall. Can you find the psoas major, quadratus lumborum, and erector spinae muscles in cross section 14 provided as part of Fig 5? Gross Anatomy: Posterior Abdominal Wall & Diaphragm Page 5 of 16 Dr. Paul Walker D. Clinical Correlation- Psoas Abscess Pus accumulates between the psoas muscle and its fascia (Fig 6). Causes severe pain when moving the thigh at the hip joint. Fever, weight loss, malaise also accompany the pain symptoms. Causes: Certain infections target the intervertebral discs such as tuberculosis of the vertebral column (rare in the US) or Salmonella discitis. In the US, it may be more common to find cases caused by Staphylococcus aureus or Group A Streptococcus. In the case of the CT to right, it was a Strep infection following a right nephrectomy for angiomyolipoma (benign mesenchymal kidney tumor). The arrow indicates the abscess spreading laterally against the surface of the psoas major muscle. Compare this CT to the previous cross section shown in the Fig 5. You should also be able to identify the quadratus lumborum and erector spinae muscles. The gut is bright because a contrast agent was administered to the patient prior to the CT. Fig 6 II. Diaphragm A. Anatomy Fig 7 (Gray’s Anatomy for Students) The diaphragm forms the superior boundary of the abdominal cavity, and thus the superior boundary of the posterior abdominal wall. It consists of a central tendinous region into which the circumferential muscles attach. The diaphragm is anchored posteriorly to the lumbar vertebrae by musculotendinous crura, which blend with the anterior longitudinal ligament of the vertebral column (discussed by Dr. Goebel in HBF-I). Right crus: attached to LI-III Left crus: attached to LI-II The crura are connected across the midline by a tendinous arch Median arcuate ligament. Two additional ligamentous arches on each side: Medial arcuate ligament Lateral arcuate ligament Gross Anatomy: Posterior Abdominal Wall & Diaphragm Page 6 of 16 Dr. Paul Walker Fig 8 (Gray’s Anatomy for Students) Under Median Arcuate Ligament Aorta Thoracic duct Under Medial Arcuate Ligament Psoas Major Sympathetic trunk Under Lateral Arcuate Ligament Quadratus lumborum The aorta enters the abdominal cavity at TXII. Just below the median arcuate ligament, the inferior phrenic arteries branch from the aorta to supply the inferior surface of the diaphragm. The esophageal hiatus is at TX. Passing with the esophagus is the anterior and posterior vagal trunks and the esophageal branches from the left gastric artery and vein. The caval opening is at the right side of the vertebral bodies at TVIII. Passing with the IVC through this opening is the right phrenic nerve. The left phrenic nerve passes through its own opening in the diaphragm just anterior to the central tendon on the left side. Structures that also pass directly through diaphragm muscle posteriorly include the azygos and hemiazygos veins and the thoracic splanchnic nerves. B. Developmental Considerations The adult structure of the diaphragm develops from 4 different parts (see Dr. Goebel’s Embryology LEC 26: Respiratory & Body Cavity Development). 1. Septum Transversum (forms central tendon) 2. Pleuroperitoneal Folds (close the right and left pericardioperitoneal canals) 3. Posterior Esophageal Mesentery (forms right & left diaphragmatic crura) 4. Peripheral Rim (infiltrating myocytes from cervical somites C3-C5) Fig 9 (Sadler) Gross Anatomy: Posterior Abdominal Wall & Diaphragm Page 7 of 16 Dr. Paul Walker C. Clinical Correlation: Diaphragmatic Hernias Fig 10 (Gray’s Anatomy for Students) Congenital Diaphragm Hernia- happens when the pleuroperitoneal folds fail to close the pericardio- peritoneal cavity (usually on the left). This results in a herniation of gut into the left thoracic cavity (MRI in Fig 10) which interferes with the normal development of the lung (lung hypoplasia) and can be fatal if not surgically corrected. Hiatal Hernia- occurs because of a lax esophageal hiatus and results in a portion of the stomach (cardia and sometimes fundus) herniating into the thoracic cavity (Fig 11). Can be associated with GERD. Fig 11 (Gray’s Anatomy for Students) III. Abdominal Aorta A. Anatomy Retroperitoneal structure that enters abdominal cavity posterior to the median arcuate ligament (Fig 12) Enters at TXII and bifurcates into right and left common iliac arteries at LIV. 3 unpaired branches supply GI structures. Paired branches travel laterally or posteriorly and supply non-GI structures. Unpaired Branches Celiac (TXII) SMA (LI) IMA (LIII) Paired Branches Inferior phrenic Middle suprarenal Renal Gonadal Lumbar (4 pairs) Fig 12 (Gray’s Anatomy for Students) Gross Anatomy: Posterior Abdominal Wall & Diaphragm Page 8 of 16 Dr. Paul Walker B. Clinical Correlation: Nutcracker vs. SMA Syndrome Fig 13 (Moore) Small angle between SMA and aorta can compress structures of passage: Nutcracker Syndrome: Compression of left renal vein between SMA and aorta. Causes flank pain and perhaps blood in urine (hematuria) and left-sided varicocele. SMA Syndrome: Compression of 3rd part of duodenum between SMA and aorta causing intermittent intestinal pain (primarily after eating). Occurs more often in patients with low body weight. C. Clinical Correlation: Abdominal Aortic Aneurysm (AAA) Ruptured AAA is the 13th-leading cause of death in the United States, causing an estimated 15,000 deaths per year. In a large US Veterans Affairs screening study, the prevalence of an abdominal aortic aneurysm was 1.4%. Several of our cadavers this year show clear evidence of AAA. Below Fig 14 shows a volume-rendered multidetector CT reconstruction of an unruptured AAA before (left) and after (right) endovascular graft installation. The appearance of the arteries is due to the intraluminar contrast (walls of the arteries are not visible). White patches on the aneurysm (left) are calcium deposits within the vessel walls. The preloaded compressed graft with metal support struts is passed through the femoral artery into the abdominal aorta on a large catheter. X-ray guidance is used during graft opening and the result is a metal support lining inside of the aorta. Limb attachments are made to the graft that extend into the common iliac vessels. Fig 14 (Gray’s Anatomy for Students) Gross Anatomy: Posterior Abdominal Wall & Diaphragm Page 9 of 16 Dr. Paul Walker IV. Inferior Vena Cava A. Tributaries The IVC lies to the right of the abdominal aorta and is formed by the convergence of the 2 common iliac veins at LV and travels superiorly to pierce the central tendon of the diaphragm at TVIII (Fig 15 left). Tributaries include: Common iliac veins Lumbar veins Right gonadal (testicular or ovarian) vein Rena veins Right suprarenal vein Inferior phrenic veins Hepatic veins The ascending lumbar veins are long venous channels that provide collateral venous return when the IVC is blocked. They connect the common iliac, iliolumbar, and lumbar veins with the azygos and hemiazygos veins (Fig 15 right). Fig 15 (Gray’s Anatomy for Students) Gross Anatomy: Posterior Abdominal Wall & Diaphragm Page 10 of 16 Dr. Paul Walker B. Clinical Correlation: IVC Filters Deep vein thrombosis (DVT) is potentially fatal and results from a clot formed in the deep venous system of the legs or pelvis. Positive D-dimer blood test showing fibrin degradation product has high association with DVT. Consequences can be pulmonary embolism (fatal) or destruction of the venous valves of the legs leading to ulcerations. Heparin and compression socks are common prophylactic measures. The installation of an IVC filter introduced through the femoral vein to catch the thrombus prior to entering the heart has also become more common. Fig 16 (left) shows a drawing of what an IVC filter looks like. Incidentally, the patient with the psoas abscess described in Fig 6 previously had an IVC filter installed when her kidney was removed. Can you see it in her axial CT provided again in Fig 16 (right)? Fig 16 C. Clinical Correlation: May-Thurner Syndrome Venous return from the left lower extremity may be affected by compression of the left common iliac vein by the right common iliac artery against the vertebral body of L5. This results in pain and swelling of the left lower limb (Fig 17) and is called May-Thurner (MT) syndrome. MT syndrome has a higher incidence in women in the 2nd-3rd decade of life. Fig 17 Gross Anatomy: Posterior Abdominal Wall & Diaphragm Page 11 of 16 Dr. Paul Walker V. Abdominal Lymphatics A. Node Organization There are 2 channels of abdominal lymphatics: Pre-aortic. Nodes along the abdominal aorta. These receive lymph from the GI tract and accessory organs (liver, gallbladder, pancreas, spleen). Para-aortic (lateral aortic, also called right and left lumbar nodes). Nodes lateral to the abdominal aorta and IVC. These receive lymph from the body wall, kidneys, suprarenal, and testes/ovaries. The pre-aortic nodes form a central intestinal trunk and the para-aortic nodes form right and left trunks. The trunks converge at the right posterior surface of the aorta near LI-II and form a saccular dilation (cisterna chyli) that marks the beginning of the thoracic duct. Fig 18 (Gray’s Anatomy for Students) Gross Anatomy: Posterior Abdominal Wall & Diaphragm Page 12 of 16 Dr. Paul Walker B. Clinical Correlation: RPLND Fig 19 Testicular cancer is well-known (1:250) and is largely a disease of young and middle-aged men. This type of cancer can usually be treated successfully, and the mortality rate is 1:5000. Depending on the stage of disease progression when detected, treatment involves both chemotherapy and surgical removal of retroperitoneal lymph nodes associated with testicular lymph drainage. The route of lymph drainage along the para-aortic channel is well-characterized (see above), so the para-aortic nodes are targeted for removal from the common iliac artery bifurcation inferiorly to the renal veins superiorly using a surgical approach called RetroPeritoneal Lymph Node Dissection (RPLND). There are several approaches to RPLND. The open surgical approach involves a long incision from sternum to pubic bone. The anterior abdominal wall muscles are divided and the surgeon takes an extraperitoneal route along the posterior abdominal wall toward the vertebral column by pushing the peritoneum and underlying abdominal contents toward midline. Because the peritoneal cavity is not entered, this lowers risk of intraperitoneal infections and other complications. However, the large incision site and injury to the anterior abdominal wall musculature are negative consequences for many patients. Laparoscopic surgical approaches have reduced the cosmetic and musculoskeletal problems associated with open RPLND surgery. This approach has been modified to include robotic assistance (Fig 19 middle and bottom), with both extraperitoneal or transperitoneal approaches. There is much debate about the success of robot assisted RPLND, which can be highly dependent on the training and experience of surgical teams. However, you’ll likely see the adoption of robotics (including AI) into many types of surgery in your careers. Gross Anatomy: Posterior Abdominal Wall & Diaphragm Page 13 of 16 Dr. Paul Walker VI. Nerves of the Posterior Abdominal Wall Fig 20 (Gray’s Anatomy for Students) A. Lumbar Plexus Previously, you were introduced to the femoral & obturator nerves during the HBF-I course as related to the lumbar plexus but were unable to see their posterior abdominal wall origins. One of the main goals of the posterior abdominal wall dissection is to identify these nerves and learn their relationships to other structures as they course toward their targets. The lumbar plexus forms within the psoas major and the nerve branches emerge from the muscle: Anteriorly- genitofemoral Medially- obturator Laterally- iliohypogastric, ilioinguinal, femoral, lateral femoral cutaneous. Fig 21 (Gray’s Anatomy for Students) The subcostal nerve (T12) is associated with the 12th rib and quadratus lumborum muscle. The T12 dermatome is suprapubic (between umbilicus and pubic region). Both iliohypogastric and ilioinguinal nerves originate from L1 and also cross the quadratus lumborum muscle as they course laterally. They both send motor branches to the anterior abdominal wall muscles. Sensory branches from the iliohypogastric nerve distribute to the L1 dermatome of the pubic region. Sensory branches from the ilioinguinal nerve accompany the spermatic cord in the inguinal canal and innervate the skin of the anterior scrotum (or mons pubis and labium majus) and the upper medial thigh. The genitofemoral nerve (L1-L2) exits anteriorly out of the main belly of the psoas major muscle. Its genital branch provides motor innervation to the cremasteric muscle of the spermatic cord and sensory to the skin of the anterior scrotum (or mons pubis and labium majus). The femoral branch provides cutaneous innervation of the upper anterior thigh. Gross Anatomy: Posterior Abdominal Wall & Diaphragm Page 14 of 16 Dr. Paul Walker The lateral femoral cutaneous nerve (lateral nerve of thigh, L2-L3) follows the contour of the iliac fossa and iliacus muscle and travels into the lateral thigh deep to the lateral attachment of the inguinal ligament on the ASIS. This nerve innervates the skin of the proximal lateral thigh. The muscles and skin regions innervated by the femoral nerve (L2-L4) and obturator nerve (L2-L4) were learned in the HBF-I course and will not be repeated here. However, you should be ready to identify both nerves in lab 20 as related to the psoas major muscle. The obturator nerve is medial, and the femoral nerve is lateral to the inferior part of the psoas major muscle and can be easily identified (and tagged). The lumbosacral trunk (L4-L5) can also be visualized medial to the psoas major muscle as it courses into the pelvic cavity to contribute to the sacral plexus. B. Autonomic Nerves The other nerves encountered during dissection of the posterior abdominal wall are the abdominal autonomic nerves described earlier during this course. The lumbar sympathetic trunks lie anterolateral to the lumbar vertebral bodies. The lumbar sympathetic ganglia are flat and difficult to distinguish. The pre-vertebral plexus includes pre-vertebral ganglia around the aorta and its large branches (celiac, SMA, IMA, renal). The plexus continues inferiorly as the superior hypogastric plexus and divides into left and right hypogastric nerves that become continuous with the inferior hypogastric plexus of the pelvis. More information about the pelvic autonomics and the innervation of the distal GI tract and reproductive organs will be given as part of the HBF-III course in January. Fig 22 (Gray’s Anatomy for Students) Gross Anatomy: Posterior Abdominal Wall & Diaphragm Page 15 of 16 Dr. Paul Walker VII. Organ Relationships in Cross Sectional Imaging A. Organs Related to the Posterior Abdominal Wall & Diaphragm As we close out the abdomen, it’s a good idea to review relationships between posterior abdominal wall structures and the other abdominal viscera you’ve learned during the HBF-II course. Fig 23 below shows some associated structures, but as you review images from other lectures in preparation for the exam, try to incorporate your understanding of the posterior abdominal wall into the picture. Fig 23 (Gray’s Anatomy for Students) Intraperitoneal structures such as the liver, stomach, and small intestine are not included in this diagram, but you can visualize their positions and relate to other structures at various superior to inferior levels. Gross Anatomy: Posterior Abdominal Wall & Diaphragm Page 16 of 16 Dr. Paul Walker B. Cross Sectional Anatomy & Axial Imaging Make sure to review the PDF study guides on Canvas that compare cross-sectional anatomy to axial images. Go to the Anatomy Resources module on Canvas and click on Study Guides for Medical Images on the Practical Exam. Below Fig 24 shows one such image example. Locate the posterior abdominal wall and find the psoas major and quadratus lumborum muscles. You should also see the erector spinae muscle group and the muscles of the anterolateral abdominal wall. Can you see the inferior part of the right kidney? What parts of the GI tract can you identify? One important relationship is the position of the ureters near the psoas major muscle. The below matching medical image is an axial CT with contrast agent given to the patient so the ureters will look like bright dots. Can you identify them? Also locate the abdominal aorta and IVC. Even though the pre-aortic and para-aortic lymph nodes are not visible, you can appreciate the complex route of RPLND surgery in these images. Fig 24 Challenge: Learn as much 3D anatomy of the abdomen as you can in preparation for the Radiology of the Abdomen session on Wed December 8.