What is the prominence of an interlobular fissure on a chest X-ray called?

Prominence of Interlobular Fissure on Chest X-ray: Radiographic Terminology

The prominence of an interlobular fissure on a chest X-ray is commonly referred to as a "bulging fissure" sign. This radiographic finding represents fluid accumulation or inflammatory exudate within the pleural space of the fissure, causing it to appear more prominent or bulging on imaging 1.

Anatomical and Radiographic Basis

Interlobular fissures are pleural invaginations that separate the lobes of the lungs:

• The right lung has two fissures (major/oblique and minor/horizontal)
• The left lung has one fissure (major/oblique)

When these fissures become prominent on imaging, several key characteristics may be observed:

• Appearance on chest X-ray:

  • Sharp, curvilinear demarcation between lobes
  • Increased opacity along the fissure
  • Concave line with perihilar lucency 2
  • Lateral opacity extending into the fissure

• Appearance on CT:

  • More clearly visualized than on plain radiographs
  • May show thickening of the interlobular septa
  • Can demonstrate fluid or inflammatory changes within the fissure 3

Clinical Significance

The bulging fissure sign is clinically significant as it may indicate:

1. Lobar pneumonia - particularly when caused by organisms like:

   • Streptococcus pneumoniae
   • Klebsiella pneumoniae
   • Legionella pneumophila 1

2. Pleural effusion extending into the fissure

3. Pulmonary edema with fluid accumulation in the fissure

4. Inflammatory processes causing exudative changes in the fissure

Differential Diagnosis of Interlobular Septal Thickening

It's important to distinguish a bulging fissure from interlobular septal thickening, which can appear as:

• Smooth thickening: Seen in pulmonary edema and lymphangitic spread of tumors 4
• Nodular thickening: Associated with lymphangitic carcinomatosis and sarcoidosis 4
• Irregular thickening: Typically seen in interstitial fibrosis 4

Imaging Considerations

When evaluating a prominent interlobular fissure:

• Plain chest X-ray:

  • First-line imaging modality
  • Limited in sensitivity and specificity for mild cases
  • May show the bulging fissure sign, especially on lateral views

• CT/HRCT:

  • More sensitive for detecting fissural abnormalities
  • Can better characterize the nature of the prominence
  • Helps distinguish between fluid, inflammation, or other pathologies 3
  • Can detect incomplete interlobular fissures, which occur in approximately 83% of right lungs and 50% of left lungs 5

Clinical Implications

Recognition of a bulging fissure is important because:

1. It may be the earliest or most prominent radiographic finding in certain infections
2. It can help localize pathology to a specific lobe
3. It may indicate the severity of the underlying condition
4. It can guide diagnostic and therapeutic interventions

Understanding this radiographic sign helps clinicians accurately interpret chest imaging and make appropriate clinical decisions regarding patient management, particularly in cases of suspected pneumonia or pleural effusion.