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• • • What is it:
      The miliary pattern refers to a radiological finding of numerous, small, uniformly sized nodules (typically <2 mm) distributed randomly throughout the lungs. It is classically associated with hematogenous spread of infection or disease.
    • Etymology:
      The term “miliary” is derived from the Latin word milium, meaning millet seed, as the nodules resemble these small grains.
    • AKA:
      Miliary nodules, Miliary dissemination.
    • How does it appear on each relevant imaging modality:
      • Chest X-ray:
        • Diffuse, finely granular, or reticulonodular opacities uniformly distributed across both lungs.
        • Difficult to detect in early stages due to limited resolution.
      • Chest CT:
        • Micronodules (<2 mm) appear as well-circumscribed, randomly distributed opacities.
        • The nodules are uniform in size and involve all lung zones, including subpleural and fissural regions.
        • A random distribution without predilection for lymphatic or bronchovascular structures is key.
      • MRI:
        • Rarely used but may show tiny nodules as hyperintensities on T2-weighted imaging in specific conditions.
    • Differential diagnosis:
      • Infection:
        • Miliary tuberculosis (TB): The prototypical cause due to hematogenous spread of Mycobacterium tuberculosis.
        • Disseminated fungal infections (e.g., histoplasmosis, coccidioidomycosis).
        • Viral pneumonitis (rare).
      • Neoplasm:
        • Hematogenous metastases (e.g., thyroid cancer, renal cell carcinoma).
        • Melanoma (rarely presents with this pattern).
      • Infiltrative disorder:
        • Pulmonary amyloidosis (diffuse micronodular type).
      • Immune-related or inflammatory:
        • Sarcoidosis (rare miliary form).
        • Hypersensitivity pneumonitis (chronic phase).
      • Metabolic:
        • Pulmonary amyloidosis (classified here as well).
      • Circulatory:
        • Pulmonary hemosiderosis (uncommon).
      • Other rare causes:
        • Pneumoconioses (e.g., silicosis in an early nodular phase).
      • Idiopathic:
        • Idiopathic miliary interstitial lung disease.
      • Iatrogenic:
        • Drug-related pneumonitis (e.g., amiodarone, methotrexate).
    • Recommendations:
      • When a miliary pattern is detected:
        • Clinical correlation: Look for systemic symptoms (e.g., fever, weight loss, night sweats).
        • Laboratory workup: Tuberculin skin test, interferon-gamma release assay (IGRA), fungal serologies, or tumor markers.
        • Advanced imaging: Consider PET-CT for malignancy evaluation if metastatic disease is suspected.
        • Histological confirmation: Transbronchial or surgical lung biopsy may be required if non-invasive tests are inconclusive.
    • Key points and pearls:
      • The miliary pattern is classically associated with miliary TB, making it the first diagnosis to consider, especially in patients from endemic regions or with immunosuppression.
      • Hematogenous spread is the unifying mechanism for diseases causing this pattern, explaining the random and diffuse distribution.
      • Early detection on imaging may be challenging, particularly on chest X-ray; chest CT is more sensitive and diagnostic.