Anatomy Exam 2: Mediastinum

What plane divides the superior from the inferior mediastinum?

The transverse thoracic plane: sternal angle to the T4/5 IV disc

The inferior mediastinum divides into which three compartments (front to back)?

Anterior, Middle, Posterior

List everything that happens at the sternal-angle (T4/5) plane

Trachea bifurcates (carina); aortic arch begins & ends; azygos arches into SVC; thoracic duct crosses R→L; superior/inferior mediastinum boundary; level of 2nd costal cartilage

Normal contents of the ANTERIOR mediastinum

Fat, lymph nodes, sternopericardial ligaments, internal thoracic vessel branches, lower thymus remnant (nearly empty drawer)

Differential for an ANTERIOR mediastinal mass (the "4 T's")

Thymoma, Teratoma, Thyroid (retrosternal goiter), Terrible lymphoma

Which mediastinal compartment contains the heart & pericardium?

Middle mediastinum

Contents of the POSTERIOR mediastinum

Thoracic (descending) aorta, esophagus + vagal plexus, thoracic duct, azygos/hemiazygos veins, sympathetic trunk + splanchnic nerves

Why is there no trachea in the posterior mediastinum?

The trachea bifurcates at T4/5 (the superior/inferior divider), so below that it's already split into main bronchi (which sit in the MIDDLE compartment)

Which structures run through BOTH superior and posterior mediastinum?

Esophagus, thoracic duct, vagus nerves (long vertical structures)

The 3 branches of the aortic arch, in order (L to R)

Brachiocephalic trunk, Left common carotid, Left subclavian

Which vein joins the subclavian vein to form the brachiocephalic vein?

Internal jugular vein

How is the SVC formed?

Right + left brachiocephalic veins merge (behind the right 1st costal cartilage)

Phrenic nerve: relationship to the lung root

ANTERIOR to the lung root ("phrenic = phront")

Vagus nerve: relationship to the lung root

POSTERIOR to the lung root

Phrenic nerve roots and function

C3, C4, C5 — sole MOTOR to the diaphragm; also sensory to diaphragm/pericardium

Why does diaphragm irritation cause shoulder-tip pain?

Phrenic (C3-5) carries diaphragm sensation; shares roots with shoulder-tip skin dermatome → referred pain

Left recurrent laryngeal nerve: where does it loop?

Under the arch of the aorta (beside ligamentum arteriosum) at T4/5

Right recurrent laryngeal nerve: where does it loop?

Under the right subclavian artery (in the neck)

Aortic arch aneurysm → hoarseness. Which nerve?

LEFT recurrent laryngeal (hooked under the arch — the "fishhook")

Which structure lies immediately posterior to the LEFT ATRIUM?

Esophagus (LA enlargement → dysphagia)

Most POSTERIOR chamber of the heart

Left atrium

Most ANTERIOR chamber of the heart

Right ventricle

Swallowing problem vs breathing problem — which tube?

Dysphagia = esophagus; stridor/breathing = trachea/bronchus

Diaphragm apertures and their levels

T8 caval (IVC + R phrenic); T10 esophageal (esophagus + vagal trunks); T12 aortic (aorta + thoracic duct + azygos)

Mnemonic for diaphragm openings

"I 8 (ate) 10 EGGs AT 12" — IVC T8, Esophagus T10, Aorta T12

What passes through the T10 esophageal hiatus?

Esophagus + anterior & posterior vagal trunks ("T10 = X = vagus")

Anterior vs posterior vagal trunk — which vagus?

LARP: Left = Anterior, Right = Posterior

What passes through the T12 aortic hiatus?

Aorta, thoracic duct, azygos vein (aorta lies BEHIND the muscle)

Thoracic duct: origin

Cisterna chyli in the abdomen (~L1-L2)

Thoracic duct: entry into thorax

Through the T12 aortic hiatus

Thoracic duct: course

Ascends on the RIGHT, crosses midline to the LEFT at T4/5

Thoracic duct: drainage site

Junction of left subclavian & left internal jugular veins (left venous angle)

What does the thoracic duct drain?

Lymph from the entire body EXCEPT the right upper quadrant

What drains the right upper quadrant lymph?

The right lymphatic duct

Thoracic duct injury (e.g., esophageal surgery) causes what?

Chylothorax (milky chyle in pleural cavity)

Azygos vein: side and course

Ascends on the RIGHT of the vertebral column; arches over the right lung root at T4/5 into the SVC

Azygos vein: clinical importance

Caval-to-caval collateral (bridges IVC and SVC) — critical in SVC syndrome/obstruction

Greater splanchnic nerve: levels and ganglion

T5-T9 → celiac ganglion

Lesser splanchnic nerve: levels and ganglion

T10-T11 → aorticorenal / superior mesenteric ganglion

Least splanchnic nerve: level and ganglion

T12 → renal plexus

A 63-year-old presents with a 3-week history of hoarseness. Chest CT shows a thoracic aortic aneurysm at the arch. Which nerve is compressed, and by which anatomical relationship?

Left recurrent laryngeal nerve. It hooks under the aortic arch beside the ligamentum arteriosum; an arch aneurysm stretches/compresses it → paralysis of the left vocal fold → hoarseness

A patient has facial swelling, distended neck veins, and edema of both arms. Imaging shows an anterior mediastinal mass. Name the compressed structure.

Superior vena cava (SVC syndrome). The SVC is the common venous drain for head, neck, and both upper limbs — obstruct it and everything upstream engorges.

“I 8 10 Eggs at Table 12.” What does this mean?

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