01Core principlesThe concepts and mechanisms needed to understand the subject.
The superior mediastinum contains thymic tissue, brachiocephalic veins, upper superior vena cava, aortic arch and its branches, trachea, oesophagus, thoracic duct, vagal, phrenic and left recurrent laryngeal nerves. Anterior-to-posterior lists are useful only if paired with a particular level: the left brachiocephalic vein crosses anterior to the arch branches, while trachea and oesophagus maintain a central posterior relationship.
The middle mediastinum is the pericardium and its contents, including heart roots and intrapericardial great vessels. The phrenic nerves descend on fibrous pericardium between mediastinal pleura and pericardium with pericardiacophrenic vessels. By contrast, vagal trunks pass behind the lung roots and contribute to the oesophageal plexus. This anterior-versus-posterior root relation is a high-value operative orientation rule.
The posterior mediastinum contains descending thoracic aorta, oesophagus, azygos and hemiazygos veins, thoracic duct, sympathetic trunks and thoracic splanchnic nerves. The oesophagus shifts relative to the aorta and is accompanied by vagal fibres reorganising into anterior and posterior trunks. The duct’s thin wall and variable duplication make lymphatic injury possible even when the broad route is remembered correctly.
Pleura forms cervical, costal, mediastinal and diaphragmatic surfaces. Visceral pleura follows the lung and fissures, whereas parietal pleura lines the cavity and has somatic pain supply except on central diaphragm/mediastinum via phrenic fibres. Surface levels differ between lung and pleural reflection, particularly inferiorly. A procedural model must include intercostal neurovascular structures, the diaphragm and abdominal organs rather than treating the pleural space as empty.
Key points
- The mediastinum is divided by the transverse thoracic plane from the sternal angle to the T4–T5 disc into superior and inferior regions; the inferior region is further anterior, middle and posterior.
- At the lung root the phrenic nerve passes anteriorly and the vagus passes posteriorly, a relation that predicts which nerve is endangered by dissection on either surface.
- The left recurrent laryngeal nerve loops beneath the aortic arch near the ligamentum arteriosum, linking hoarseness to disease or intervention at the aortopulmonary window.
- The thoracic duct usually enters through the aortic hiatus, ascends between aorta and azygos system behind the oesophagus, crosses to the left and ends at the left venous angle.
- Pleural recesses are potential spaces created by reflections; the costodiaphragmatic recess extends below the lung margin during quiet breathing and changes procedural risk with respiration.
- A cross-sectional mediastinal answer should follow continuity over several levels because a single axial slice can make a vessel, nerve or oesophagus appear to end or change side.
02Mechanisms and patternsImportant relationships and how to distinguish them.
Use the sternal-angle plane, then identify crossing veins, arch and branches, airway, oesophagus and vertebral column; nerves and thoracic duct weave between rather than forming a separate clean layer.
Each phrenic nerve runs on fibrous pericardium anterior to the lung root and supplies the diaphragm; injury can elevate and weaken the ipsilateral hemidiaphragm.
The vagus descends behind the lung root, contributes pulmonary branches and forms the oesophageal plexus; its recurrent branches have asymmetric loops.
The left recurrent laryngeal nerve relates to the inferior aortic arch and ligamentum arteriosum, so pathology or intervention here can present with vocal change.
The duct commonly ascends on the right side of the lower thorax, crosses behind the oesophagus around the mid-thorax and terminates at the left internal-jugular/subclavian venous angle.
A recess is a potential space not occupied by lung during quiet breathing; inferior lung expansion enters the costodiaphragmatic recess during inspiration without abolishing all risk below the lung edge.
03Interpreting evidenceInformation, measurements and their limitations.
Consider the information, its meaning and its limitations before deciding what follows.
- 01
Axial-level localisation - Why
- Assign a mass or structure to a mediastinal compartment at a defined vertebral level.
- Interpretation and limitations
- Use sternum, pericardium, trachea, oesophagus and vertebral body as anchors, then trace the lesion on adjacent slices before attaching a compartment label.
- 02
Nerve-course prediction - Why
- Relate postoperative voice or diaphragmatic change to the side and surface of dissection.
- Interpretation and limitations
- Anterior lung-root work threatens phrenic structures; posterior work approaches vagal structures; arch-region work can affect the left recurrent laryngeal nerve.
- 03
Pleural surface model - Why
- Predict where lung, pleura, diaphragm and abdominal viscera lie during a planned intercostal approach.
- Interpretation and limitations
- Account for respiratory phase and patient position; real procedures require imaging or direct guidance rather than a textbook rib level alone.
- 04
Venous collateral tracing - Why
- Explain how azygos and hemiazygos pathways connect thoracic-wall veins to the superior vena cava.
- Interpretation and limitations
- A collateral route must be traced across side, vertebral level and terminal arch; enlargement may mark upstream obstruction but does not identify its cause alone.
- 05
Lymphatic consequence check - Why
- Recognise when an operative field could disrupt thoracic-duct flow.
- Interpretation and limitations
- Left lower neck and posterior mediastinal surgery are key contexts; laterality and fluid character guide evaluation but anatomical possibility is not a diagnosis.
04Applied reasoningWorked examples connecting principles to decisions.
01Worked caseLocalise hoarseness from an arch-region massA supplied contrast CT description places a mass beneath the left aortic arch beside the ligamentum arteriosum; the patient has new hoarseness but preserved diaphragmatic movement.+
- 1Fix the level at the aortopulmonary window and identify the aortic arch, left pulmonary artery and ligamentum region as the spatial inputs.
- 2Trace the left vagus down the thorax and its recurrent laryngeal branch looping under the arch before ascending in the tracheoesophageal groove.
- 3Use preserved diaphragm movement to reduce support for phrenic dysfunction while recognising that this does not define mass histology.
- 4Conclude that left recurrent laryngeal involvement is anatomically plausible and predicts impaired laryngeal motor function requiring direct assessment.
- 5Verify by checking vocal-fold movement and the full imaging extent; do not infer malignant invasion merely from adjacency and hoarseness.
02Applied anatomyDifferentiate nerves at a lung rootAn operative view labels one nerve on pericardium anterior to the hilum and another passing posterior to the hilum.+
- 1Identify the anterior nerve as phrenic from its pericardial course.
- 2Identify the posterior nerve as vagus from its route toward the oesophageal plexus.
- 3Predict diaphragm weakness after anterior-nerve injury and broader vagal effects after posterior injury.
- 4Confirm laterality and branches before any clinical conclusion.
03Spatial reasoningTrace a postoperative chyle pathwayA supplied postoperative scenario follows left lower-neck dissection and describes milky drain fluid after enteral feeding.+
- 1Reconstruct thoracic-duct ascent and left venous-angle termination.
- 2Relate the field to the final cervical arch of the duct.
- 3Explain why a lymphatic leak is anatomically plausible without claiming the appearance is diagnostic.
- 4Verify fluid identity and clinical consequence through the appropriate team pathway.
05Checking understandingVerify the reasoning, revisit uncertainties and apply feedback.
- Practise the sternal-angle plane on sagittal and axial images, then list only the structures visible at the chosen level.
- Draw phrenic and vagal courses on both sides and check each relation to lung root, pericardium, arch and oesophagus.
- Rehearse thoracic-duct continuity from cisterna chyli to left venous angle, including its variable crossing and terminal branches.
- For each pleural procedure question, state respiratory phase, pleural reflection, intercostal bundle and nearby abdominal viscus.
- When a deficit follows surgery, separate anatomical plausibility from proof and name the examination or imaging that could verify it.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
Left brachiocephalic vein is anterior
The long left brachiocephalic vein crosses obliquely behind the manubrium and anterior to the arch branches, a relationship relevant to superior mediastinal access.
Phrenic pain can refer
Central diaphragmatic pleura and pericardium share phrenic afferents from C3–C5, allowing pain to be perceived in the shoulder region.
Azygos arch is right-sided
The azygos vein arches over the right lung root to enter the superior vena cava, whereas hemiazygos channels cross the vertebral column to join it.
Arch relations explain laterality
The left recurrent laryngeal nerve has an intrathoracic loop under the arch; the right loops under the subclavian artery at the root of the neck.
The thoracic duct varies
Duplication and alternative terminal channels occur. A single schematic predicts the common route but cannot guarantee the exact duct seen in an operation.
07Common pitfallsFrequent interpretation and management errors.
- 01
Listing mediastinal contents without specifying the level at which their anterior-posterior order is being described.
- 02
Reversing the lung-root relation by placing the vagus anterior and phrenic posterior.
- 03
Assuming all vocal change after thoracic surgery proves direct recurrent-nerve transection.
- 04
Using a fixed inferior pleural level for a real needle path without accounting for respiration, position and image guidance.
- 05
Forgetting that thoracic-duct injury can occur at the left venous angle as well as in the posterior mediastinum.