Mediastinum - Contents and Syndrome (Short Answer)

Contents by subdivision

• Thymus
• Right and left brachiocephalic veins, left superior intercostal vein, superior vena cava
• Arch of aorta and its three branches (brachiocephalic, left common carotid, left subclavian)
• Trachea and esophagus
• Phrenic and vagus nerves, left recurrent laryngeal nerve
• Thoracic duct, other small nerves/vessels/lymphatics (Gray's Anatomy for Students, p. 223)

• Inferior extension of the thymus gland
• Fat, connective tissue, lymph nodes
• Mediastinal branches of internal thoracic vessels
• Sternopericardial ligaments (Gray's Anatomy for Students, p. 220)

• Pericardium (fibrous + serous layers) and pericardial cavity
• Heart
• Origins/roots of the great vessels
• Phrenic nerves and associated smaller vessels (Gray's Anatomy for Students, p. 220-222)

• Esophagus and its nerve plexus
• Thoracic (descending) aorta and branches
• Azygos venous system
• Thoracic duct and associated lymph nodes
• Sympathetic trunks and thoracic splanchnic nerves (Gray's Anatomy for Students, p. 223)

Mediastinal Syndrome

• Common causes: mediastinal tumors (non-Hodgkin lymphoma, ALL, neuroblastoma, germ cell tumors in children; lung cancer/lymphoma in adults), neurofibroma ("mediastinal syndrome" is a described complication of neurofibromatosis with a mediastinal dumb-bell tumor) - S. Das Manual on Clinical Surgery, p. 54.
• Signs/symptoms (from compression of SVC + trachea + esophagus + nerves):
  • Dyspnea, cough, stridor, wheeze (tracheal compression)
  • Facial/upper body edema and plethora, distended neck and chest wall veins, collateral vein formation (SVC obstruction)
  • Chest discomfort, syncope (reduced cardiac output)
  • Dysphagia (esophageal compression)
  • Hoarseness (recurrent laryngeal nerve involvement)
  • Neurologic symptoms from cerebral venous congestion: headache, confusion, altered mental status
• Danger: Considered a true medical emergency - young children are especially at risk because of small, compliant airways; sedation or intubation attempts can be lethal if the endotracheal tube cannot bypass the site of compression.
• Diagnosis: Chest X-ray or CT; prone positioning preferred over supine for imaging in cooperative children to avoid airway compromise.
• Management: Elevate head of bed, high-flow oxygen, BiPAP, heliox if available; definitive treatment targets the underlying malignancy (chemo/radiotherapy) to shrink the mass; fibrinolytics if thrombotic; emergency stenting in select centers. (Tintinalli's Emergency Medicine, p. 1005)

Pericardial Sinuses and Intercostal Space (Short Answer)

Pericardial Sinuses

• A passage/tunnel lying posterior to the ascending aorta and pulmonary trunk, and anterior to the superior vena cava.
• Formed embryologically by degeneration of the dorsal mesocardium, which connects the right and left sides of the pericardial cavity.
• Clinical importance: During cardiac surgery, a finger or clamp can be passed through this sinus to encircle the aorta and pulmonary trunk together (e.g., to apply a single cross-clamp).

• A blind-ending, J-shaped cul-de-sac located posterior to the left atrium, bounded by the reflections of serous pericardium around the pulmonary veins and venae cavae.
• Formed similarly during development as the pericardium wraps around the venous end of the heart tube.
• Clinical importance: On imaging (CT/echo), fluid or a mass here can mimic a bronchogenic cyst, esophageal lesion, or be mistaken for other posterior mediastinal pathology.

Intercostal Space

1. External intercostal - fibers run obliquely anteroinferiorly; most active in inspiration (elevates ribs); becomes the external intercostal membrane anteriorly.
2. Internal intercostal - fibers run obliquely in the opposite direction; most active in expiration (depresses ribs).
3. Innermost intercostal (+ subcostales and transversus thoracis as related deeper layers) - acts with the internal intercostal muscles.

• Vein (intercostal vein) - most superior
• Artery (intercostal artery)
• Nerve (intercostal nerve) - most inferior

• Chest tube/needle insertion: Always insert just above the rib margin (not at the inferior border) to avoid the neurovascular bundle, classically at the 4th-5th intercostal space, midaxillary line for thoracostomy, or 2nd intercostal space midclavicular line for needle decompression.
• Intercostal nerve block: Performed near the inferior rib margin, respecting the VAN order.

Urogenital Diaphragm - Short Answer

• Deep transverse perineal muscle
• Sphincter urethrae (external urethral sphincter)

• Pierced by the urethra (and vagina in females).
• The urogenital diaphragm marks an important landmark in urethral trauma classification (e.g., Goldman classification of pelvic fracture urethral injury) - injuries are staged as above, at, or extending through the urogenital diaphragm, which determines whether the injury is to the posterior or anterior urethra.
• Exists in a funnel-shaped configuration (rather than a flat "sandwich") according to more recent anatomic descriptions, and is unique to humans due to the upright bipedal perineal anatomy.

Pelvic Diaphragm - Short Answer

1. Levator ani - made up of three parts:
   • Pubococcygeus (including pubovaginalis and pubourethralis)
   • Puborectalis
   • Iliococcygeus
2. Coccygeus muscle

• Plus the superior and inferior fasciae covering these muscles.

• Origin: tendinous arch (arcus tendineus levator ani) - a thickened band of obturator fascia running from the pubic body to the ischial spine.
• Insertion: perineal body, wall of the anal canal, anococcygeal ligament/raphe, coccyx, and (in females) the vaginal wall.
• Its circular line of attachment to the pelvic wall passes between the greater and lesser sciatic foramina on each side.

• Supports pelvic viscera (bladder, uterus/vagina, rectum) against intra-abdominal pressure.
• Contributes to urinary and fecal continence (puborectalis sling around the anorectal junction).
• Forms the roof/ceiling of the ischioanal (ischiorectal) fossa.