Widened Mediastinum: Definition and Clinical Approach
A widened mediastinum is defined as a mediastinal width >8 cm on supine anteroposterior (AP) chest radiographs or >4.95 cm left mediastinal width on posteroanterior (PA) films, but this finding has poor specificity and requires immediate CT angiography (CTA) in any patient with suspected acute aortic syndrome.
Definition and Measurement Thresholds
The definition varies by radiographic projection:
- AP supine films: Mediastinal width >8 cm 1
- PA films: Left mediastinal width >4.95 cm (sensitivity 90%, specificity 90%) 2
- For aortic dissection specifically: >87 mm on AP supine films (sensitivity 81%, specificity 89%) 3
PA chest radiographs are significantly more accurate than AP films for detecting aortic pathology 2. The mediastinum is measured at the level of the aortic knob at its maximal width.
Critical Clinical Context
The vast majority of widened mediastina are NOT aortic injuries. In blunt trauma patients with widened mediastinum, only 0.4% actually have aortic injury 1. Of 502 trauma patients with mediastinal width >8 cm, 74% had no traumatic findings at all—most had either normal CT scans or nontraumatic findings like pericardial effusion or lymph nodes 1.
Radiographic Signs Suggesting Aortic Dissection
When widened mediastinum is present, look for these specific findings that increase suspicion for aortic dissection 4:
- Disruption of the aortic knob contour
- "Calcium sign" (intimal calcification separated from aortic wall by >5 mm)
- Double density appearance within the aorta
- Tracheal deviation to the right
- Nasogastric tube deviation to the right
- Bloody pleural effusion
However, 36-38% of patients with confirmed aortic dissection have completely normal chest radiographs 5, and abnormal findings are present in only 52% of type A and 39% of type B dissections 5.
Decision Algorithm
Step 1: Assess Clinical Probability
Use the AORTAs score to stratify risk 4:
- Hypotension/shock (2 points)
- Known aneurysm (1 point)
- Pulse deficit (1 point)
- Neurologic deficit (1 point)
- Severe pain (1 point)
- Sudden-onset pain (1 point)
Score ≥2 = high probability; proceed directly to CTA regardless of chest X-ray findings
Step 2: Imaging Strategy
If widened mediastinum is present on chest X-ray:
- Do NOT rely on chest X-ray alone—sensitivity is only 70.8% and specificity 82.5% for all acute aortic syndromes 5
- Proceed immediately to CT angiography of chest, abdomen, and pelvis with IV contrast 5
- CTA is the definitive test with near 100% sensitivity and specificity
- Do not obtain non-contrast CT—it is inadequate for ruling out aortic dissection 5
If chest X-ray is normal but clinical suspicion remains:
- Still proceed to CTA—normal chest X-ray does NOT exclude aortic dissection 5
- Lower threshold for CTA in elderly patients and those with AP films showing any mediastinal prominence 2
Step 3: Alternative Imaging (Limited Scenarios)
- Transesophageal echocardiography (TEE): Sensitivity/specificity up to 100% 5, but has blind spots in distal ascending aorta and proximal arch. Use only if CTA unavailable or contraindicated.
- Transthoracic echo (TTE): Sensitivity only 59-85% 5—inadequate for ruling out dissection
- MRA: Equivalent accuracy to CTA but slower and less available in emergency settings 5
Critical Pitfalls to Avoid
Do not use mediastinal width alone to rule out aortic injury—the positive predictive value is <1% in trauma 1
Do not skip CTA based on a normal chest X-ray if clinical suspicion exists—over one-third of dissections have normal radiographs 5
PA films are more reliable than AP films—if using AP supine films (common in trauma/ED), maintain even lower threshold for advanced imaging 2
Body habitus matters—obese patients (BMI 28±5 kg/m²) commonly have widened mediastinum without pathology 1
In trauma, a normal non-contrast CT chest effectively rules out aortic injury 6, but contrast-enhanced CTA remains the standard of care
Non-Aortic Causes to Consider
When CTA is negative, common benign causes include:
- Pericardial effusion
- Mediastinal lymphadenopathy
- Mediastinal lipomatosis (obesity)
- Venous anomalies (azygos continuation of IVC) 7
- Sternal fractures
- Vertebral fractures
- Technical factors (patient positioning, body habitus)
The key principle: widened mediastinum is a nonspecific finding that mandates CTA in any patient with clinical concern for acute aortic syndrome, but should not trigger reflexive imaging in asymptomatic patients with clear alternative explanations.