• Etymology: Derived from the Greek word mikros (small) and Latin word nodulus (small knot), describing their minute size and nodular appearance.

  AKA: Pulmonary micronodules.

  What is it? Micronodules are small, well-defined, round opacities in the lung parenchyma, typically measuring less than 3 mm in diameter. They represent various pathological processes, including infectious, inflammatory, small airway diseases, or neoplastic conditions.

  Caused by:

  • Most commonly caused by:
    • Infection (e.g., tuberculosis, fungal infections, viral pneumonias)
    • Inflammation (e.g., sarcoidosis, hypersensitivity pneumonitis)
  • Other Causes:
    • Small airway diseases (e.g., bronchiolitis, bronchiolitis obliterans)
    • Neoplasm (e.g., lymphangitic carcinomatosis, miliary metastases)

  Resulting in:

  • Increased lung opacity on imaging
  • Variable distribution based on etiology (centrilobular, perilymphatic, random)

  Structural changes:

  • Small, round opacities with smooth or slightly irregular margins
  • Ground-glass or solid attenuation

  Pathophysiology: Micronodules result from:

  • Cellular infiltration of the alveolar spaces (e.g., inflammatory or infectious processes)
  • Tumor cell spread along lymphatics (e.g., lymphangitic carcinomatosis)
  • Small airway involvement due to inflammation or fibrosis

  Pathology:

  • Granulomatous inflammation (e.g., TB, sarcoidosis)
  • Malignant cells (e.g., miliary metastases)
  • Small airway inflammation (e.g., bronchiolitis)

  Diagnosis:

  • Imaging-based identification (chest CT preferred)
  • Confirmatory testing for specific etiologies: TST for TB, serologies for fungal infections, biopsy when needed

  Clinical:

  • Symptoms vary based on etiology: asymptomatic, dyspnea, cough, fever
  • Exposure history and systemic signs guide diagnosis

  Radiology: Chest CT (preferred):

  • Parts: Discrete or clustered micronodules within the lung parenchyma
  • Size: Less than 3 mm
  • Shape: Round or slightly irregular
  • Position: Centrilobular, perilymphatic, or random distribution
  • Character: Solid or ground-glass attenuation

  Chest X-ray:

  • Rarely visible due to small size
  • Subtle reticulonodular patterns if numerous

  Labs:

  • Tuberculin skin test (TST) or IGRA for TB
  • Fungal serologies
  • Autoimmune panels for inflammatory causes

  Management:

  • Treat underlying cause (e.g., antituberculous therapy, corticosteroids for inflammation)
  • Surveillance imaging for unresolved micronodules

  Radiology Detail:

  • CXR: Rarely visible, subtle reticulonodular opacities
  • CT:
    • Parts: Lung parenchyma
    • Size: <3 mm
    • Shape: Round or oval
    • Position: Centrilobular, perilymphatic, random
    • Character: Solid or ground-glass
    • Time: Acute or chronic depending on cause
    • Associated Findings: Septal thickening, lymphadenopathy
  • Other Imaging:
    • PET-CT for metabolic activity in suspected malignancy

  Pulmonary Function Tests (PFTs):

  • May show restrictive or obstructive patterns depending on underlying disease

  Recommendations:

  • High-resolution CT for distribution assessment
  • PET-CT for malignancy workup
  • Biopsy in uncertain cases

  Key Points and Pearls:

  • The distribution of micronodules helps narrow the differential diagnosis.
  • Centrilobular nodules sparing the pleura suggest small airway diseases.
  • Perilymphatic nodules often indicate sarcoidosis or lymphatic malignancy.
  • Random nodules suggest hematogenous spread (e.g., miliary TB, metastases).
  • “Micronodules measuring <3 mm are commonly identified in granulomatous, infectious, or neoplastic processes depending on their distribution and imaging features.” Radiopaedia
  • “Centrilobular micronodules sparing the pleura often indicate small airway disease like hypersensitivity pneumonitis.” Radiographics

   

   

Random distribution

• • random nodules in  the vascular distribution and lymphatic
  • lower lobes (that is where blood flow goes)
  • solid
  • well defined
  • +/- feeding vessel sign
  • +/- cavitation
  • +/- lymphangitic appearance
• Causes
  • Metastases
    • renal
    • melanoma
    • thyroid
    • testicular
  • Miliary
    • TB
    • Fungal
      • • coccidiomycoses
        • histoplasmosis
        • pneumocytis
      • Viral

TB

Transbronchial Spread of TB with Extensive Tree in Bud Changes Masquerading as Miliary TB

Acute Miliary Histoplasmosis

 

Histoplasmosis

MICRONODULES IN ILD

Silicosis

Silicosis vs Sarcoidosis

42-year-old cement worker presents with dyspnea .
A CXR performed 5 years prior was close to normal with possible right hilar prominence.
The CT scan, shows diffuse micronodular lung disease, predominantly in the upper lobes with mediastinal widening consistent with mediastinal lymphadenopathy, dominant in the right paratracheal region and in the subcarinal region.
Lung windows show the presence of extensive diffuse micronodular disease accumulating along lymphatics along fissures and pleural surfaces, and along the bronchovascular bundles. Although there is diffuse disease, the upper lobes are slightly more involved than the lower lobes. The extensive thickening along bronchovascular bundles and prominent adenopathy favors a diagnosis of sarcoidosis but with a work history of being a cement worker, silicosis still remains in the differential diagnosis as a less likely possibility.

Hypersensitivity Pneumonitis

 

 

Bilateral Lymphangitis Carcinomatosis in a Patient with Adenocarcinoma

References and Links

Videos

See around 25minutes

• TCV
  • Cases
    • 012Lu Sarcoidosis vs Silicosis in Cement Worker