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Development of the Diaphragm and Its Anomalies

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INTRODUCTION

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EMBRYONIC COMPONENTS (Four Sources)

1. Septum Transversum

• Formed during the 4th week from mesodermal tissue ventrolateral to the body wall
• Grows dorsally, forming a semicircular shelf separating the heart from the liver
• After head folding, it forms a thick, incomplete connective tissue partition between the pericardial and abdominal cavities
• Contributes to the central tendon of the diaphragm
• Large pericardioperitoneal canals persist along the sides of the esophagus at this stage
• The septum subsequently fuses with the dorsal mesentery of the esophagus and pleuroperitoneal membranes

2. Pleuroperitoneal Membranes

• These folds appear at the beginning of the 5th week, extending from the lateral body wall
• They grow medially and ventrally, fusing with the septum transversum and dorsal mesentery of the esophagus by the 6th-7th week
• This fusion completes the partition between thoracic and abdominal cavities, forming the primordial diaphragm
• Although forming large portions of the early fetal diaphragm, they represent relatively small portions of the neonate's diaphragm
• Complete closure of the canal occurs by the 8th week, with the right side closing before the left (clinically important)

3. Dorsal Mesentery of the Esophagus

• The septum transversum and pleuroperitoneal membranes fuse with the dorsal mesentery of the esophagus
• This constitutes the median (mesial) portion of the diaphragm
• The crura of the diaphragm - leg-like diverging muscle bundles crossing in the median plane anterior to the aorta - develop from myoblasts that grow into the dorsal mesentery of the esophagus

4. Muscular Ingrowth from the Lateral Body Walls

• Occurs during the 9th to 12th weeks as the lungs and pleural cavities enlarge, burrowing into the lateral body walls
• Body-wall tissue splits into two layers:
  • External layer - becomes part of the definitive abdominal wall
  • Internal layer - contributes to the peripheral parts of the diaphragm (external to the pleuroperitoneal membrane-derived portions)
• Further extension forms the costodiaphragmatic recesses, establishing the characteristic dome-shaped configuration

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POSITIONAL CHANGES AND INNERVATION

• During the 5th week, myoblasts from cervical somites migrate into the developing diaphragm, carrying their nerve fibers with them - this is why the phrenic nerve arises from C3-5
• The rapid growth of the dorsal embryo causes apparent caudal descent of the diaphragm, explaining why the phrenic nerves are ~30 cm long in adults
• The phrenic nerves pass through the pleuropericardial membranes - this is why they lie on the fibrous pericardium in adults

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ANOMALIES OF THE DIAPHRAGM

A. Posterolateral Defect (Bochdalek Hernia) - MOST COMMON

• Results from defective formation and/or failure of fusion of the pleuroperitoneal membranes with the other three diaphragmatic components
• Creates a large opening in the posterolateral region (lumbocostal triangle/foramen of Bochdalek)
• If closure of the pleuroperitoneal canal has not occurred by the time the midgut returns to the abdomen (9th-10th weeks), abdominal viscera herniate into the thorax
• No hernia sac is present if herniation occurs before complete closure; a non-muscularized membrane forms a sac in ~10-15% of cases

• Left side in 85-90% of cases (right side closes earlier due to earlier right pleuroperitoneal membrane fusion)
• Right in 13%; bilateral in ~2%

• Severe respiratory distress at birth
• Scaphoid abdomen
• Heart and mediastinum displaced to the right (left-sided hernia)
• Absent breath sounds on affected side
• Bowel sounds in chest
• Polyhydramnios in utero

• Pulmonary hypoplasia (most common cause) - both ipsilateral and contralateral, ipsilateral more severe; fewer bronchial branches, reduced alveolar surface area
• Pulmonary hypertension - increased arteriolar smooth muscle thickness, decreased cross-sectional vascular area
• Persistent fetal circulation (right-to-left shunting through foramen ovale and ductus arteriosus)
• Abnormal intestinal rotation and fixation

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B. Retrosternal (Parasternal) Hernia - Morgagni Hernia

• Herniation through the sternocostal hiatus (foramen of Morgagni) - the opening between the sternal and costal parts of the diaphragm, through which the superior epigastric vessels pass
• Results from incomplete fusion of sternal and costal components at the anterior diaphragm

• Often asymptomatic, discovered incidentally on chest X-ray
• May present with epigastric pain, respiratory symptoms, or bowel obstruction
• Anterior mediastinal mass on lateral chest film

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C. Eventration of the Diaphragm

• Not a true diaphragmatic hernia - it is a superior displacement of viscera into a sac-like outpouching
• One half of the diaphragm has defective musculature (failure of muscular tissue from the body wall to extend into the pleuroperitoneal membrane on that side)
• The affected half balloons into the thoracic cavity as a thin aponeurotic/membranous sheet
• Abdominal viscera are displaced superiorly into the pocket-like outpouching
• Some cases are acquired (phrenic nerve injury)

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D. Congenital Hiatal Hernia

• Part of the fetal stomach herniates through an excessively large esophageal hiatus
• Usually acquired in adult life, but a congenitally enlarged hiatus may predispose
• The esophageal hiatus transmits the esophagus and vagus nerves

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E. Accessory Diaphragm

• Rare anomaly (>30 cases reported)
• Most often on the right side
• Associated with lung hypoplasia and other respiratory complications
• Diagnosed by MRI or CT; treated by surgical excision

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SUMMARY TABLE OF DIAPHRAGMATIC ANOMALIES

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EMBRYOLOGICAL BASIS OF CLINICAL IMPORTANCE

1. Why left side is affected more - The right pleuroperitoneal membrane closes first; the left side is therefore still open when the midgut returns to the abdomen at 9-10 weeks
2. Why referred pain from diaphragm is felt at shoulder tip - Central diaphragm is supplied by C3-5 phrenic nerve, same segment supplying the shoulder
3. Why phrenic nerve is so long (~30 cm) - The diaphragm descends from its cervical origin (C3-5) to the thoracolumbar junction as the embryo grows
4. Pulmonary hypoplasia in CDH - Visceral herniation at critical phase of lung branching (5-16 weeks) compresses the lung and disrupts both ipsilateral and contralateral development; sonic hedgehog signaling is also delayed

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Applied Anatomy of the Diaphragm

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I. INTRODUCTION

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II. GROSS STRUCTURE

• A central tendinous part (central tendon) - a trefoil (three-leaflet) sheet of strong fibrous tissue
• Peripheral muscular part - radially arranged muscle fibers that arise from the margins of the inferior thoracic aperture and converge into the central tendon

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III. ATTACHMENTS (ORIGIN)

Sternal Part

• Two muscular slips from the posterior surface of the xiphoid process

Costal Part

• Inner surfaces of the lower six costal cartilages and their adjacent ribs (ribs 7-12), interdigitating with transversus abdominis

Vertebral / Lumbar Part - via Two Crura and Three Arcuate Ligaments

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IV. OPENINGS IN THE DIAPHRAGM

• Greater, lesser, and least splanchnic nerves - through the crura
• Hemi-azygos vein - through the left crus
• Sympathetic trunks - posterior to the medial arcuate ligament
• Superior epigastric vessels - anterior to the diaphragm, deep to ribs (between sternal and costal origins = sternocostal triangle / foramen of Morgagni)
• Left phrenic nerve - through the muscular part just anterior to the central tendon on the left side

• IVC = T8, Esophagus = T10, Aorta = T12

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V. RELATIONS

Superior (Thoracic) Surface

• Right dome: Right lung and pleura, right lobe of liver
• Left dome: Left lung and pleura, fundus of stomach, spleen
• Central: Pericardium and heart (fibrous pericardium is adherent to the central tendon)

Inferior (Abdominal) Surface

• Right: Right lobe of liver, right kidney, right suprarenal gland
• Left: Left lobe of liver, stomach, spleen, left kidney, left suprarenal gland
• Centrally: Covered by peritoneum except where bare area of liver is in contact

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VI. BLOOD SUPPLY

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VII. NERVE SUPPLY

• Arises from cervical plexus in the neck
• Descends vertically in the neck on the anterior surface of scalenus anterior
• Enters thorax through the superior thoracic aperture
• Passes through the mediastinum anterior to the lung roots, between mediastinal pleura and pericardium
• Right phrenic enters diaphragm through the caval opening with the IVC
• Left phrenic pierces the muscular part of the left dome separately

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VIII. ACTION AND PHYSIOLOGY

• On contraction: domes descend, vertical diameter of thorax increases, intrathoracic pressure falls - air enters lungs; simultaneously, intra-abdominal pressure rises
• At rest (end-expiration): right dome at rib V, left dome at 5th intercostal space
• After contraction (end-inspiration): both domes descend ~1.5 cm
• The right dome is always higher than the left because of the underlying liver

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IX. APPLIED ANATOMY (CLINICALLY IMPORTANT POINTS)

1. Referred Pain to the Shoulder Tip

• Central diaphragm is supplied by the phrenic nerve (C3, C4, C5)
• Any irritation of the central diaphragm (subphrenic abscess, blood, free air under the diaphragm, ruptured spleen, ectopic pregnancy, perforated peptic ulcer) refers pain to the tip of the shoulder (C4 dermatome) and the root of the neck
• The peripheral diaphragm is supplied by intercostal nerves T7-T12; irritation here refers pain to the lower chest and abdominal wall
• Clinically: Post-operative free gas under the right diaphragm causes right shoulder tip pain. Splenic injury causes left shoulder tip pain (Kehr's sign)

2. Hiccup (Singultus)

• A sudden, involuntary contraction of the diaphragm, causing rapid inhalation stopped by glottis closure
• Triggered by stimulation of the phrenic nerve, vagus nerve, or direct irritation of the diaphragm
• Causes include gastric distension, uremia, CNS lesions, pericarditis
• Persistent hiccup may indicate a serious underlying pathology (e.g., esophageal carcinoma, mediastinal tumor, brainstem lesion)

3. Diaphragmatic Paralysis (Phrenic Nerve Palsy)

• Causes: Lung cancer (most important - never miss), malignant mediastinal lymph nodes, aortic aneurysm, postviral neuropathy (varicella zoster), trauma, iatrogenic (cardiac surgery, neck dissection), cervical spondylosis (C3-C5 root compression)
• Signs: Elevated hemidiaphragm on the affected side on chest X-ray; paradoxical movement (rises on inspiration = "paradoxical" - because intra-thoracic negative pressure sucks the flaccid diaphragm up)
• Unilateral: Usually asymptomatic; mild dyspnea on exertion
• Bilateral: Severe respiratory distress; orthopnea prominent (diaphragm falls away from chest wall when supine)
• Treatment: Surgical plication if symptomatic (creates folds in the paralyzed diaphragm)

4. Subphrenic Abscess

• Collection of pus between the diaphragm and the underlying viscera
• Right subphrenic space (between right lobe of liver and diaphragm) is most common
• The diaphragm is irritated from below, causing referred right shoulder pain, hiccup, and pleural effusion
• Chest X-ray shows elevated hemidiaphragm, pleural effusion, and air-fluid level below the diaphragm

5. Diaphragmatic Hernias (Applied)

6. Aortic Aneurysm and the Median Arcuate Ligament

• The aorta passes behind the diaphragm through the aortic hiatus (T12) - it is NOT compressed during diaphragmatic contraction (important surgical and physiological point)
• Median arcuate ligament syndrome: Compression of the celiac axis by a low-placed median arcuate ligament causes postprandial angina and mesenteric ischemia

7. Thoracoabdominal Wounds

• Stab wounds below the level of the nipple (anteriorly, 4th intercostal space) or the tip of the scapula (posteriorly) can penetrate both the thoracic and abdominal cavities, injuring the diaphragm
• This must always be suspected in lower thoracic trauma; laparoscopy is used to assess the diaphragm

8. Costodiaphragmatic Recess

• The potential space between the lowest extent of the parietal pleura and the uppermost extent of the diaphragm
• Clinically: Site of pleural effusion accumulation; requires ~300 mL of fluid before visible on erect chest X-ray (blunting of costophrenic angle); site for diagnostic thoracocentesis

9. Pneumoperitoneum

• Free gas under the diaphragm (best seen under the right dome on erect CXR) is a classical sign of hollow viscus perforation (perforated peptic ulcer)
• The gas irritates the diaphragm, causing referred right shoulder tip pain

10. Liver and Right Dome

• The right dome of the diaphragm sits directly on the right lobe of the liver - elevation of the right dome on CXR may indicate hepatomegaly, right lobe abscess, or right subphrenic collection rather than a primary diaphragmatic problem

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X. EMBRYOLOGICAL BASIS OF CLINICAL ANOMALIES (Brief)

• Diaphragm develops from four components: septum transversum (central tendon), pleuroperitoneal membranes, dorsal mesentery of esophagus, and muscular ingrowth from lateral body walls
• Failure of fusion of any component causes the corresponding hernia at that site
• The diaphragm's cervical origin (C3-5) explains the long phrenic nerve and shoulder-tip referred pain

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SUMMARY TABLE

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