000 Miliary Pattern

      • 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.