000 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.
Etymology:
- Derived from the Greek word mikros (small) and Latin word nodulus (small knot), describing their minute size and nodular appearance.
AKA:
- Pulmonary micronodules.
Abbreviation:
- MN (Micronodules).
How does it appear on each relevant imaging modality:
- Chest CT (preferred):
  - Parts: Discrete or clustered micronodules distributed within the lung parenchyma.
  - Size: Less than 3 mm in diameter.
  - Shape: Round or slightly irregular, depending on the etiology.
  - Position:
    - Distribution can be centrilobular, perilymphatic, or random, providing clues to the underlying cause.
    - Centrilobular micronodules are located in the central part of the secondary pulmonary lobule, sparing the pleura.
    - Perilymphatic micronodules involve interlobular septa, subpleural regions, and fissures.
    - Random distribution is associated with hematogenous spread.
  - Character: Ground-glass or solid attenuation, with or without calcifications.
- Chest X-ray:
  - Rarely visible due to their small size; subtle, reticulonodular patterns may indicate micronodular involvement.
Differential diagnosis (starting with the most likely causes):
- Infection:
  - Tuberculosis: Centrilobular micronodules in active or miliary TB (random distribution).
  - Fungal infections: Histoplasmosis, coccidioidomycosis, or pneumocystis pneumonia.
  - Viral infections: Measles or varicella pneumonia.
- Inflammation:
  - Hypersensitivity pneumonitis: Centrilobular micronodules associated with ground-glass opacities.
  - Sarcoidosis: Perilymphatic micronodules along interlobular septa, subpleural regions, and bronchovascular bundles.
- Small Airway Disease and Bronchiolitis:
  - Infectious bronchiolitis: Caused by viral or bacterial infections (e.g., respiratory syncytial virus, influenza).
  - Inflammatory bronchiolitis: Seen in conditions like hypersensitivity pneumonitis or connective tissue diseases (e.g., rheumatoid arthritis, Sjögren’s syndrome).
  - Toxic inhalation: Exposure to environmental irritants (e.g., smoke, fumes, or dust).
  - Post-infectious bronchiolitis: Residual small airway damage after acute infections.
  - Bronchiolitis obliterans: Fibrotic obliteration of small airways, often associated with transplant rejection, drug toxicity, or inhalational injuries.
- Neoplasm:
  - Lymphangitic carcinomatosis: Perilymphatic micronodules from metastatic spread.
  - Miliary metastases: Randomly distributed micronodules (e.g., thyroid or renal cell carcinoma).
- Idiopathic:
  - Idiopathic interstitial pneumonia (e.g., NSIP, RB-ILD).
Recommendations:
- Further evaluation:
  - High-resolution CT (HRCT) to assess nodule distribution (centrilobular, perilymphatic, or random).
  - PET-CT for metabolic activity if malignancy is suspected.
  - Biopsy (transbronchial, surgical) for definitive diagnosis in unclear cases.
- Laboratory workup:
  - Tuberculin skin test (TST) or interferon-gamma release assay (IGRA) for TB.
  - Fungal serologies for endemic mycoses.
  - Autoimmune testing (e.g., ANA, ANCA) for inflammatory causes.
Key considerations and pearls:
- The distribution pattern of micronodules (centrilobular, perilymphatic, or random) is critical in narrowing the differential diagnosis.
- Centrilobular nodules sparing the pleura strongly suggest small airway diseases or hypersensitivity pneumonitis.
- Perilymphatic nodules are characteristic of sarcoidosis and lymphangitic spread of cancer.
- Randomly distributed micronodules often indicate hematogenous spread, seen in miliary TB or metastatic disease.
- Clinical history, including exposure to infectious agents, occupational triggers, or systemic symptoms, is essential for accurate diagnosis.

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

CT Active TB Cavitation and Ipsilateral Transbronchial Spread with Extensive Tree in Bud Pattern
39-year-old immigrant Vietnamese male presents night sweats fever and cough
CXR shows a cavitating lesion in the apex of the right lung (magnified lower image, right) associated with an ipsilateral micronodular pattern (magnified lower image, left)
Although the right lung has the appearance of a “miliary” pattern, this term is usually referred to the hematogenous spread of the disease
Ashley Davidoff MD TheCommonvein.net 131708cL

CT Active TB Transbronchial Spread with Extensive Tree in Bud Changes
39-year-old immigrant Vietnamese male presents night sweats fever and cough. CT in the axial plane through the mid chest shows innumerable micronodules resulting from transbronchial spread with resultant tree in bud pattern scattered through the right lung (magnified in the right lower image). There are minimal similar changes in the lingula (magnified left lower image)..
Ashley Davidoff MD TheCommonvein.net 135789c 006Lu

Acute Miliary Histoplasmosis

CAVITATING LUNG NODULE – ACUTE PULMONARY HISTOPLASMOSIS
22-year-old female presents with flu like symptoms. 3 weeks later a chest CT shows a cavitating nodule in the left upper lobe, and extensive diffuse bilateral micronodular miliary disease.
Ashley Davidoff MD Ashley Davidoff MD TheCommonVein.net 131706

MILIARY NODULES – ACUTE PULMONARY HISTOPLASMOSIS
22-year-old female presents with flu like symptoms. CT shows extensive diffuse bilateral micronodular miliary disease.
Ashley Davidoff MD Ashley Davidoff MD TheCommonVein.net 131707

Histoplasmosis

MIP IMAGE SHOWING EXTENSIVE MICRONODULES – ACUTE MILIARY PULMONARY HISTOPLASMOSIS
22-year-old female presents with flu like symptoms and has a normal CXR
3 weeks later a chest CT shows extensive diffuse bilateral micronodular military disease associated with mediastinal lymphadenopathy, and hepatosplenomegaly.
A week later she was admitted to the ICU with confluent pneumonic infiltrates with air bronchograms in the lower lobes.
Later that month her CXR started to improve but still showed military disease.
A CXR 9 months later shows resolution.
Ashley Davidoff MD

ACUTE PULMONARY HISTOPLASMOSIS – PNEUMONIA
22-year-old female presents with flu like symptoms and has a normal CXR
3 weeks later a chest CT shows extensive diffuse bilateral micronodular military disease associated with mediastinal lymphadenopathy, and hepatosplenomegaly.
A week later she was admitted to the ICU with confluent pneumonic infiltrates with air bronchograms in the lower lobes.
Later that month her CXR started to improve but still showed military disease.
A CXR 9 months later shows resolution.
Ashley Davidoff MD

ACUTE PULMONARY HISTOPLASMOSIS – 1 WEEK LATER IMPROVING
22-year-old female presents with flu like symptoms and has a normal CXR
3 weeks later a chest CT shows extensive diffuse bilateral micronodular military disease associated with mediastinal lymphadenopathy, and hepatosplenomegaly.
A week later she was admitted to the ICU with confluent pneumonic infiltrates with air bronchograms in the lower lobes.
Later that month her CXR started to improve but still showed military disease.
A CXR 9 months later shows resolution.
Ashley Davidoff MD

ACUTE PULMONARY HISTOPLASMOSIS RESOLUTION 9 MONTHS LATER
22-year-old female presents with flu like symptoms and has a normal CXR
3 weeks later a chest CT shows extensive diffuse bilateral micronodular military disease associated with mediastinal lymphadenopathy, and hepatosplenomegaly.
A week later she was admitted to the ICU with confluent pneumonic infiltrates with air bronchograms in the lower lobes.
Later that month her CXR started to improve but still showed military disease.
A CXR 9 months later shows resolution.
Ashley Davidoff MD

MICRONODULES IN ILD

NODULAR PATTERN ON CXR IN ILD
Frontal view exemplifies a diffuse nodular pattern of ILD such as is seen in silicosis and sarcoidosis

NODULES IN ILD
Micronodules in ILD is another CT feature of interstitial lung disease and is characterised by nodules of a variety of shapes and sizes and likely centrilobular in origin. Sometimes they are ill defined such as in this case.

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.