Saber-Sheath Trachea

Aqeel A. Chowdhry, MD

Tan-Lucien H. Mohammed, MD, FCCP

Key Facts

Terminology

Trachea in which coronal dimension ≤ 2/3 of sagittal dimension
- Extrathoracic trachea normal
Probably acquired deformity of trachea, usually secondary to chronic obstructive lung disease

Imaging Findings

Normal trachea
- Sagittal diameter is 13-27 mm in men and 10-23 mm in women
- Coronal diameter is 13-25 mm in men and 10-21 mm in women
Tracheal index (coronal diameter)/(sagittal diameter) usually measured 1 cm above aortic arch
In saber-sheath trachea, tracheal index ≤ 2/3
- Specificity for chronic obstructive pulmonary disease (COPD) (95%)
- Sensitivity for COPD < 10%

Top Differential Diagnoses

Pathology

Saber-sheath deformity is sign of hyperinflation
Tracheal index correlates with functional residual capacity
May be related to abnormal pattern and magnitude of intrathoracic pressure changes in COPD

Frontal radiograph shows the typical features of tracheal narrowing

from a saber-sheath trachea. The deformity is often overlooked on chest radiographs.

Axial CECT shows a normally shaped extrathoracic trachea

and saber-sheath deformity

of the intrathoracic trachea.

TERMINOLOGY

Abbreviations and Synonyms

Definitions

Trachea in which coronal dimension is ≤ 2/3 of sagittal dimension
- Extrathoracic trachea normal

IMAGING FINDINGS

General Features

Best diagnostic clue
- Marked decrease in coronal diameter with increase in sagittal diameter
- Inward bowing of lateral tracheal wall usually worsened with forced expiration
Patient position/location
- Intrathoracic airway
  - Main bronchi and extrathoracic trachea are usually normal
Size
- Normal trachea
  - Sagittal diameter is 13-27 mm in men and 10-23 mm in women
  - Coronal diameter is 13-25 mm in men and 10-21 mm in women
  - Tracheal index (coronal diameter)/(sagittal diameter) usually measured 1 cm above aortic arch
  - Round or horseshoe-shaped
- Saber-sheath tracheal measurements
  - Tracheal index ≤ 2/3
Morphology
- Saber-sheath deformity
  - Narrowed trachea on frontal view, widened on lateral view

CT Findings

Morphology
- Side-to-side narrowing of trachea at and below thoracic inlet
- Tracheal index ≤ 2/3
  - Specificity for chronic obstructive pulmonary disease (COPD) (95%)
  - Sensitivity for COPD < 10%
Wall
- No tracheal wall thickening
- Inner wall of trachea usually smooth
- Tracheal cartilage usually calcified
Lung
- Hyperinflated, usually from centrilobular emphysema and bullous lung disease
Extrathoracic trachea remains normal in configuration
Tracheal index may increase following lung reduction surgery
- Does not return to normal, however

Imaging Recommendations

Best imaging tool: HRCT, as tracheal deformity often overlooked on chest radiographs
Protocol advice: CT during forced expiration or Valsalva maneuver shows inward bowing of tracheal walls

Radiographic Findings