How is the mediastinum measured on a chest X-ray (CXR) and what constitutes a widened mediastinum?

Mediastinal Width Measurement on Chest X-Ray and Widened Mediastinum Definition

The most widely used criterion for a widened mediastinum on chest X-ray is a mediastinal width greater than 8 cm on a supine anteroposterior projection, which has high sensitivity but low specificity for detecting thoracic aortic pathology. 1

How to Measure the Mediastinum

Mediastinal width can be measured using several techniques:

1. Standard Mediastinal Width (MW):

   • Measure the transverse width of the mediastinum at its widest point
   • Normal: <8 cm on supine anteroposterior projection
   • A measurement ≥8 cm is considered widened 1, 2

2. Left Mediastinal Width (LMW):

   • Measure from the left lateral border of the aortic knob/arch to the left edge of the tracheal air column
   • Normal: <6 cm
   • A measurement ≥6 cm suggests possible aortic pathology 3

3. Mediastinal Width Ratio (MWR):

   • Calculate the ratio of mediastinal width to chest width at the same level
   • Normal: <0.38 4 or <0.60 3 (different studies use different cutoffs)
   • Higher ratios correlate with increased likelihood of aortic injury

4. Mediastinal-to-Cardiac (MC) Ratio:

   • Calculate the ratio of mediastinal width to cardiac width
   • Normal: <0.555 2

What Constitutes a Widened Mediastinum

A widened mediastinum is defined by:

• Quantitative criteria:

  • Mediastinal width >8 cm on supine anteroposterior chest X-ray 1, 5, 2
  • Mediastinal-to-thoracic width ratio >0.25-0.38 2, 4
  • Left mediastinal width ≥6 cm 3

• Qualitative findings that suggest mediastinal widening:

  • Abnormal aortic contour
  • Loss of the aortic knob definition
  • Left apical cap
  • Depression of the left main bronchus
  • Deviation of an indwelling esophageal tube 1
  • Mediastinal infiltration (irregular, amorphous shape) 1

Clinical Significance and Limitations

1. For Aortic Dissection/Injury:

   • Sensitivity of widened mediastinum for thoracic aortic disease: 64%
   • Sensitivity of abnormal aortic contour: 71%
   • When all abnormal radiographic findings are included: 90% sensitivity 1
   • Specificity for aortic pathology: 86% 1

2. For Trauma Patients:

   • The positive predictive value of a widened mediastinum for aortic injury is <1% 5
   • Combined criteria (LMW ≥6 cm and MWR ≥0.60) improve specificity to 66.7% 3

3. Important limitations:

   • Chest X-ray is inadequately sensitive to definitively exclude aortic dissection except in lowest-risk patients 1
   • In trauma, subjective impression of mediastinal widening by experienced clinicians may be superior to objective measurements 2
   • For trauma patients, chest X-ray is a poor screening test for aortic injury 1

Best Practice Recommendations

1. Use chest X-ray as an initial screening tool, but recognize its limitations
2. A completely normal chest X-ray lowers the likelihood of aortic pathology but does not exclude it 1
3. When mediastinal widening is detected, proceed to more definitive imaging:
   • CT angiography is the preferred next step for suspected thoracic aortic pathology 1
   • For trauma patients with positive findings based on combined LMW and MWR criteria, proceed to CT angiography 3

Remember that while mediastinal measurements are helpful, they cannot replace comprehensive evaluation with advanced imaging when thoracic aortic pathology is suspected.