000 Mosaic Attenuation Caused by Small Blood Vessel Disease

  • Etymology
    “Mosaic” derives from the Greek word mousaikos, meaning “pertaining to the muses,” and refers to the patchy or varied pattern seen on imaging. “Small blood vessel disease” relates to abnormalities in the pulmonary microvasculature that lead to perfusion mismatches.
  • AKA and abbreviation
    Mosaic attenuation (MA) due to vascular abnormalities.
  • What is it?
    Mosaic attenuation caused by small blood vessel disease refers to a patchy lung attenuation pattern observed on CT scans, primarily resulting from perfusion abnormalities due to vascular occlusion or constriction.
  • Characterized by
    • Patchy areas of differing attenuation on CT scans due to regional variations in blood flow.
    • Hypodense regions correlate with hypoperfused areas caused by small vessel obstruction or vasculopathy.
    • No significant air trapping on expiratory CT scans, differentiating it from airway-related causes.
  • Caused by
    • Most Common Cause(s): Chronic thromboembolic pulmonary hypertension (CTEPH), congestive heart failure (CHF).
    • Other Causes Include:
      • Inflammation/Immune: Pulmonary vasculitis (e.g., granulomatosis with polyangiitis).
      • Neoplasm: Pulmonary tumor thrombotic microangiopathy.
      • Inherited: Hereditary hemorrhagic telangiectasia (HHT).
      • Congenital: Pulmonary arteriovenous malformations (PAVMs).
      • Hematologic: Sickle cell disease (SCD), due to vaso-occlusive crises and microvascular injury.
  • Resulting in:
    • Regional hypoxia and impaired gas exchange.
    • Progressive pulmonary hypertension.
  • Structural changes:
    • Occlusion, narrowing, or destruction of pulmonary arterioles and capillaries.
    • Regional ischemia of affected lung tissue.
  • Pathophysiology:
    Mosaic attenuation in small blood vessel disease arises from ventilation-perfusion imbalance. Regions with reduced perfusion due to vascular obstruction or constriction appear hypodense on imaging because of diminished blood volume. In areas with underperfused segments, ventilation persists without significant gas exchange, causing the retained air to accentuate lucency. Reflex mechanisms such as hypoxic vasoconstriction may further exacerbate ventilation-perfusion mismatch. This pathophysiology differentiates vascular causes from airway diseases, where air trapping plays a primary role.
  • Pathology:
    • Endothelial damage or thrombosis within small pulmonary vessels.
    • Chronic vascular remodeling with intimal fibrosis and medial hypertrophy.
    • In CHF, redistribution of blood flow and pulmonary venous hypertension contribute to mosaic attenuation.
  • Diagnosis:
    • Clinical presentation: Dyspnea, exercise intolerance, signs of pulmonary hypertension, or CHF (e.g., edema, orthopnea).
    • Imaging: HRCT, VQ scan, CT pulmonary angiography (CTPA), echocardiography for CHF.
    • Laboratory tests: Markers of vasculitis or coagulopathy; BNP or NT-proBNP for CHF.
  • Clinical:
    Symptoms include progressive dyspnea, fatigue, chest pain, and in advanced cases, right heart failure.
  • Radiology Detail:
    • CXR
      • Findings: Subtle signs of pulmonary hypertension, including enlarged pulmonary arteries, or features of CHF such as cardiomegaly, pulmonary edema.
      • Associated Findings: Possible oligemia in affected regions.
    • CT
      • Parts: Diffuse, lobar, segmental, or subsegmental involvement.
      • Size: Variable, depending on the underlying etiology.
      • Shape: Patchy or lobular.
      • Position: Distribution varies with disease but typically involves areas of vascular compromise or venous congestion.
      • Character: Hypodense regions reflecting perfusion defects; absence of air trapping on expiratory scans.
      • Time: Chronic conditions lead to stable findings; acute cases may evolve.
      • Associated Findings: Enlarged central pulmonary arteries, evidence of thromboembolic disease, or signs of pulmonary venous congestion in CHF.
    • Other relevant Imaging Modalities
      • VQ Scan: Highly sensitive for detecting perfusion defects.
      • Angio: Essential for identifying vascular occlusions or malformations.
      • MRI: Rarely used but can assess perfusion in certain cases.
      • Echocardiography: Critical for evaluating left ventricular function and pulmonary venous hypertension in CHF.
  • Pulmonary function tests (PFTs):
    May show mild restrictive or normal patterns, often non-specific.
  • Recommendations
    • Perform a VQ scan for suspected perfusion abnormalities.
    • Use CTPA to confirm vascular occlusion or assess thromboembolic disease.
    • For CHF, echocardiography and BNP/NT-proBNP levels are essential.
    • Manage underlying causes with anticoagulation, vasodilators, or surgical interventions for malformations, and diuretics or afterload reduction in CHF.
  • Key Points and Pearls
    • Mosaic attenuation due to vascular causes must be differentiated from airway-related and interstitial lung diseases.
    • Expiratory CT can help exclude air trapping, confirming a vascular origin.
    • Chronic vascular changes or venous congestion in CHF may progress to pulmonary hypertension if untreated.