3.1 Clinical & Cross-Sectional Anatomy
Key Takeaways
- The sternal angle (angle of Louis at T4/T5) demarcates the boundary between superior and inferior mediastinum, the tracheal bifurcation, the aortic arch origin/termination, and the thoracic duct crossing from right to left.
- Left recurrent laryngeal nerve loops under the aortic arch posterior to the ligamentum arteriosum; compression by left atrial enlargement (Ortner's syndrome) or thoracic lymphadenopathy causes hoarseness.
- The splenic flexure (Griffiths' point) and rectosigmoid junction (Sudeck's point) are critical mesenteric watershed areas uniquely prone to non-occlusive ischaemic colitis during systemic hypoperfusion.
- Posterior communicating artery (PCoA) berry aneurysms classically cause painful oculomotor (CN III) palsy with pupillary dilatation, whereas anterior communicating artery (ACoA) aneurysms compress the optic chiasm causing bitemporal hemianopia.
- Radial nerve injury at the humeral spiral groove causes wrist drop with sensory loss over the first dorsal web space, but preserves triceps brachii motor extension.
MRCP(UK) Part 1 tests applied clinical anatomy, emphasizing structural relations, vascular territories, and peripheral nerve injury patterns.
1. Thoracic Anatomy
The Mediastinum & Sternal Angle
The horizontal plane from the sternal angle (angle of Louis) to the T4/T5 disc separates superior from inferior mediastinum, marking the tracheal carina, aortic arch concavity, azygos entry into SVC, and thoracic duct crossing right to left.
| Compartment | Key Boundaries | Major Contents | Clinical Relevance |
|---|---|---|---|
| Superior | Thoracic inlet to T4/T5 | Brachiocephalic veins, SVC, aortic arch branches, trachea, oesophagus, thoracic duct, thymus | Retrosternal goitre, thymoma, lymphoma compression |
| Anterior | Sternum to pericardium | Thymic remnants, internal thoracic vessels, lymph nodes | Masses: "4 Ts" (Thymoma, Teratoma, Thyroid, Terrible lymphoma) |
| Middle | Pericardial reflections | Heart, ascending aorta, pulmonary trunk, phrenic nerves (C3–C5) | Cardiac tamponade, LAD occlusion, phrenic palsy |
| Posterior | Pericardium to T5–T12 | Descending aorta, azygos veins, oesophagus, thoracic duct, vagus | Oesophageal rupture, neurogenic tumours, dissection |
Coronary Circulation & Dominance
Coronary dominance is defined by the vessel giving off the posterior descending artery (PDA):
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Right-dominant (~85–90%): PDA arises from the right coronary artery (RCA).
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Left-dominant (~10%): PDA arises from the left circumflex artery (LCx).
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Codominant (~5%): Arises from both RCA and LCx.
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Left Anterior Descending (LAD): Supplies anterior LV wall, anterior two-thirds of septum, and bundle branches (leads V1–V4).
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Right Coronary Artery (RCA): Supplies right ventricle, inferior LV wall, posterior third of septum, SA node (60%), and AV node (90%). Occlusion causes inferior STEMI (leads II, III, aVF) and AV block; ST-elevation in V4R indicates RV infarction (preload-dependent; avoid nitrates).
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Left Circumflex (LCx): Supplies lateral and posterior LV walls (leads I, aVL, V5–V6).
Thoracic Duct & Bronchopulmonary Anatomy
The thoracic duct originates at the cisterna chyli (L1–L2), enters the thorax through the aortic hiatus (T12), ascends between aorta and azygos vein, crosses from right to left at T4–T5, and terminates at the left internal jugular/subclavian vein junction. Disruption below T5 causes a right-sided chylothorax; disruption above T5 causes a left-sided chylothorax (triglycerides > 1.24 mmol/L).
The right main bronchus is wider, shorter, and more vertical (25° vs 45°). In supine posture, aspirated material enters the apical segment of the right lower lobe or posterior segment of the right upper lobe; in upright posture, it settles in the posterior basal segment of the right lower lobe.
Recurrent Laryngeal Nerve (RLN)
Branches of CN X: right RLN loops under the right subclavian artery (T1); left RLN loops under the aortic arch posterior to the ligamentum arteriosum (T4–T5). Both ascend in the tracheo-oesophageal groove to supply all intrinsic laryngeal muscles except the cricothyroid (external branch of superior laryngeal nerve). Left RLN injury causes hoarseness in Ortner's syndrome (left atrial dilation in mitral stenosis) and apical lung tumours.
2. Abdominal & Pelvic Anatomy
Mesenteric Circulation & Watershed Areas
Aortic branches: Coeliac trunk (T12) supplies foregut (stomach to 2nd duodenal part); SMA (L1) supplies midgut (distal duodenum to proximal two-thirds of transverse colon); IMA (L3) supplies hindgut (distal third of transverse colon to upper rectum).
- Griffiths' point (splenic flexure): Watershed between middle colic (SMA) and left colic (IMA) along the marginal artery of Drummond.
- Sudeck's point (rectosigmoid junction): Watershed between the lowest sigmoid branch (IMA) and superior rectal artery. Both are highly vulnerable to non-occlusive ischaemic colitis during systemic hypoperfusion.
Retroperitoneum & Portosystemic Anastomoses
Retroperitoneal structures (SAD PUCKER): Suprarenal glands, Aorta/IVC, Duodenum (2nd–4th), Pancreas (head/body; tail intraperitoneal), Ureters, Colon (ascending/descending), Kidneys, Esophagus, Rectum (lower two-thirds).
| Anastomosis Site | Portal System | Systemic System | Clinical Manifestation |
|---|---|---|---|
| Gastro-oesophageal | Left gastric vein | Oesophageal azygos tributaries | Oesophageal varices |
| Anorectal | Superior rectal vein | Middle/inferior rectal veins (internal iliac) | Rectal varices |
| Anterior Wall | Paraumbilical veins | Superficial epigastric veins | Caput medusae |
| Retroperitoneal | Colic veins | Retroperitoneal lumbar veins (Retzius) | Occult retroperitoneal bleeding |
Inguinal & Femoral Canals
- Inguinal Canal: Deep ring is an outpouching of transversalis fascia lateral to inferior epigastric vessels (indirect hernias enter here). Direct hernias push through Hesselbach's triangle medial to inferior epigastric vessels.
- Femoral Canal: Medial compartment of femoral sheath (NAVeL lateral to medial: Nerve, Artery, Vein, Empty canal with Cloquet's node, Lacunar ligament). Rigid boundaries (inguinal, Cooper's, and lacunar ligaments) confer high risk of strangulation.
3. Neurovascular Anatomy & Peripheral Nerves
Circle of Willis & Internal Capsule
- ACoA aneurysms (~35%): Cause SAH and optic chiasm compression (bitemporal hemianopia).
- PCoA aneurysms (~30%): Compress CN III, causing painful third nerve palsy with a fixed dilated pupil and down-and-out eye.
- Cortical territories: ACA (contralateral leg motor/sensory); MCA (contralateral face/arm motor/sensory, aphasia); PCA (homonymous hemianopia with macular sparing).
- Internal Capsule: Posterior limb carries corticospinal fibres (anterior two-thirds) and sensory thalamocortical radiations (posterior third). Lenticulostriate artery stroke causes pure motor hemiplegia.
Peripheral Nerve Deficits
- Radial (C5–T1): Spiral groove fracture causes wrist drop and first dorsal web space numbness; triceps extension is preserved.
- Median (C5–T1): Carpal tunnel entrapment causes thenar wasting (LOAF muscles: Lumbricals 1–2, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis) and radial 3.5 digit numbness.
- Ulnar (C8–T1): Cubital tunnel or Guyon's canal compression causes claw hand (ulnar paradox: worse distally), medial 1.5 digit numbness, and positive Froment's sign (adductor pollicis weakness compensated by FPL).
- Common Peroneal (L4–S2): Fibular neck injury causes foot drop (loss of dorsiflexion/eversion) and dorsal foot numbness; inversion is preserved.
- Tibial (L4–S3): Tarsal tunnel syndrome causes loss of plantarflexion, absent ankle jerk, and sole numbness.
A 54-year-old woman presents to the emergency department with an acute, excruciating headache. On physical examination, she is alert and oriented. Her right eye exhibits complete ptosis, and when the eyelid is manually elevated, the right pupil is 6 mm and non-reactive to light, while the left pupil is 3 mm and reactive. The right eye is deviated downward and laterally. Emergent CT angiography confirms a ruptured saccular berry aneurysm. At which anatomical junction is this aneurysm most likely situated?
A 71-year-old man undergoes an open repair of an infrarenal abdominal aortic aneurysm. Postoperatively, he develops profound hypotension requiring vasopressor support. Twelve hours later, he complains of severe lower abdominal cramping pain followed by the passage of moderate-volume bloody diarrhoea. On examination, there is tenderness over the left iliac fossa with guarding. Colonoscopy demonstrates patchy mucosal ulceration and violaceous ischaemic mucosa. Which anatomical region of the colon is most vulnerable to this ischaemic insult?
A 32-year-old cyclist falls onto his outstretched right hand, impacting the hypothenar eminence. Two weeks later, he notes clumsiness when writing and difficulty holding small objects. Physical examination reveals intact sensation over the thenar eminence and lateral three digits, but decreased sensation over the palmar aspect of the fifth digit. When asked to grip a flat piece of paper between his thumb and index finger, the examiner is easily able to pull the paper away unless the patient strongly flexes the interphalangeal joint of his thumb. Which nerve has been injured, and what anatomical landmark corresponds to the entrapment or trauma site?