Chest radiography
Classic radiography demonstrates a number of specific optical phenomena associated with the deposition of calculi. Due to the extremely high density of the affected areas of the parenchyma, the «contour blurring» sign occurs — the boundaries of the cardiac shadow and the diaphragmatic domes are no longer clearly visualized and merge with the pulmonary background.
The calculi (microliths) themselves are so small and have such a high density that their detailed visualization is possible exclusively on overexposed, so-called «hard» radiographs.
An important diagnostic marker is the subpleural line. It appears as a black band of subpleural lung tissue. Its appearance is physically caused by the formation of small subpleural cysts (5 to 10 mm in diameter) or the development of paraseptal emphysema (paraseptal emphysema).
Changes also affect the pleura: interlobar fissures may diffusely thicken or acquire a specific «beaded» contour (beading). Rare radiological findings in this pathology include the formation of apical blebs (apical blebs) and the appearance of thickened septal lines — Kerley B lines (Kerley B lines).
Computed tomography (CT)
Computed tomography allows the most accurate assessment of the microlith distribution pattern and interstitial involvement. The leading CT pattern is widespread ground-glass opacification (ground-glass opacification), which is often combined with micronodular infiltration (micronodular infiltration).
The distribution of changes has certain patterns: the pathological process is most pronounced in the basal (lower lobes) and dorsal (posterior) segments of the lungs. At the same time, a characteristic subpleural low-density zone (subpleural low density) forms along the periphery of the lungs.
In approximately 50% of cases, uniform tissue calcification is observed, which creates a picture of massive dense parenchymal opacification on tomograms (dense parenchymal opacification). The interstitial component of the disease manifests as marked thickening of the interlobar fissures and interlobular septa (interlobular septa). In a small percentage of patients, interstitial changes and ground-glass opacities may combine into the rare crazy-paving pattern (crazy-paving pattern).
Differential diagnosis spectrum
When diffuse and micronodular lesions of the lung tissue are detected, a thorough differential diagnosis must be performed. The spectrum of excluded pathologies includes:
- Idiopathic pulmonary haemosiderosis (Idiopathic pulmonary haemosiderosis).
- Cryptogenic organizing pneumonia (Cryptogenic organizing pneumonia, COP).
- Eosinophilic lung diseases (Eosinophilic lung diseases).
- Drug- and radiation-induced lung disease (Drug- and radiation-induced lung disease).
- Pulmonary alveolar proteinosis (Pulmonary alveolar proteinosis).
- Granulomatosis with polyangiitis (Granulomatosis with polyangiitis).
- Cardiogenic pulmonary edema and acute respiratory distress syndrome (ARDS).
- Infectious processes complicated by diffuse alveolar hemorrhage (Diffuse alveolar hemorrhage).
Briefly: answers to topical questions
What methods are used for the comprehensive diagnosis of pulmonary alveolar microlithiasis?
Diagnostic imaging methods are used to diagnose pulmonary alveolar microlithiasis.
- Chest X-ray — may reveal the sign of blurring of the heart and diaphragm contours, the subpleural line, as well as thickening or «beading» of the interlobar fissures. Small calculi (microliths) may be detected only on overexposed images.
- Computed tomography (CT) — reveals widespread ground-glass opacification (ground-glass opacification) and/or a micronodular pattern (micronodular infiltration), a subpleural low-density zone (subpleural low density), and in approximately 50% of cases, uniform calcification creating a picture of dense parenchymal opacification.
To memorize the CT picture, use the association «Rocky shore in the fog»: stones lie below and in the back (microliths in the lower and posterior regions), fog rolls over them (ground-glass opacity), and a clear strip runs along the shoreline itself (subpleural low-density zone).
Class and exam questions
Why are the heart contours not visible on the X-ray in pulmonary alveolar microlithiasis?
Due to the totally high density of the lung tissue filled with microliths, a «blurring» effect occurs: the pulmonary parenchyma merges in X-ray density with the contours of the mediastinal and diaphragmatic structures.
How can microliths be visualized on a standard X-ray?
The small stones themselves can be reliably distinguished only by using overexposed (hard) radiographs.
What is the typical localization of changes on CT?
Pathological changes are most pronounced in the lower lobes and posterior regions of the lungs, while the presence of a subpleural low-density zone is characteristic.
What else to study on this topic
- Radiological differences between pulmonary alveolar microlithiasis and pulmonary alveolar proteinosis.
- Morphological substrate of the crazy-paving pattern in rare interstitial diseases.
- Assessment and clinical significance of Kerley B lines (Kerley B lines).
- Mechanisms of formation of paraseptal emphysema and apical blebs.
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