What are the clinical implications of an abnormal mediastinal to chest width ratio?

From the Guidelines

An abnormal mediastinal to chest width ratio (MCR) greater than 0.25-0.28 on chest radiography is a significant finding that requires urgent evaluation for potentially life-threatening conditions, with the most concerning implication being aortic pathology, particularly aortic dissection, aneurysm, or rupture, which demands immediate CT angiography and surgical consultation 1. The clinical implications of an abnormal MCR are numerous and can be categorized into several key areas, including:

• Aortic pathology: aortic dissection, aneurysm, or rupture, which are potentially life-threatening conditions that require immediate attention 1
• Mediastinal hemorrhage, lymphadenopathy from malignancy or infection, mediastinitis, pneumomediastinum, and anterior mediastinal masses like thymoma or teratoma, which can also be serious and require prompt evaluation and treatment 1 When encountering an abnormal MCR, clinicians should immediately assess for symptoms of chest pain, dyspnea, or hemodynamic instability while obtaining a contrast-enhanced CT scan of the chest 1. The MCR is calculated by dividing the widest mediastinal width by the widest thoracic width at the level of the aortic arch on an upright PA chest radiograph, with normal values typically below 0.25, though this varies slightly by institution and patient factors 1. It is essential to note that a completely normal chest x-ray does lower the likelihood of aortic dissection and may provide meaningful clinical information in very low-risk patients 1. However, if aortic dissection is suspected, blood pressure control with IV beta-blockers (such as esmolol 50-200 μg/kg/min or labetalol 2-8 mg/min) should be initiated to maintain systolic BP between 100-120 mmHg while arranging urgent cardiothoracic surgical consultation 1.

From the Research

Clinical Implications of an Abnormal Mediastinal to Chest Width Ratio

The clinical implications of an abnormal mediastinal to chest width ratio are significant, particularly in the context of blunt chest trauma.

• An abnormal ratio may indicate a high risk of traumatic rupture of the aorta or its major branches 2.
• The study found that 44.4% of patients with blunt chest trauma and a widened mediastinum on chest radiograph had traumatic rupture of the aorta or its major branches 2.
• The sensitivity of aortography for diagnosing traumatic rupture of the aorta or its major branches was 100%, and the specificity was 90% 2.
• The findings suggest that all blunt chest trauma patients with a widened mediastinum on chest radiograph should undergo further investigations to exclude traumatic rupture of the aorta or its major branches 2.

Diagnostic Accuracy of Mediastinal Width Measurement

The diagnostic accuracy of mediastinal width measurement on chest radiographs is an important consideration.

• A study found that the maximal left mediastinal width (LMW) and maximal mediastinal width (MW) were the most powerful parameters for determining acute nontraumatic thoracic aortic dissection on posteroanterior (PA) and anteroposterior (AP) chest radiographs 3.
• The optimal cutoff levels for LMW and MW were 4.95 cm and 7.45 cm, respectively, for PA projection, and 5.45 cm and 8.65 cm, respectively, for AP projection 3.
• The use of LMW alone in PA film allowed for more accurate prediction of aortic dissection 3.

Limitations of Mediastinal Widening as a Diagnostic Criterion

While an abnormal mediastinal to chest width ratio may indicate a high risk of traumatic rupture of the aorta or its major branches, it is essential to consider the limitations of mediastinal widening as a diagnostic criterion.

• A study found that mediastinal width has unacceptable sensitivity for predicting aortic injury, with substantial inter-reader variability 4.
• Another study found that the positive predictive value of a widened mediastinum for aortic injury was less than 1% 5.
• These findings highlight the importance of using a combination of diagnostic criteria and imaging modalities to accurately diagnose traumatic injuries, particularly aortic injuries 5, 4.