Pulmonary Alveolar Microlithiasis

Pulmonary alveolar microlithiasis is a rare autosomal recessive disease that is caused by impaired transfer of phosphorus ions from the alveolar space into type II pneumocytes. This leads to the development of microliths composed of calcium and phosphorus (calcospherites) in the alveoli.

Men and women are affected at an equal rate, and patients tend to be younger than 50 years of age at diagnosis. Most patients have at least one sibling with the disease, and there is a high rate of consanguineous marriages in affected families.

Patients typically present with progressive dyspnea, which, after a protracted course, progresses to pulmonary fibrosis, end-stage lung disease, and chronic pulmonary heart disease.

Initial laboratory evaluation usually reveals no underlying disorder calcium metabolism, and pulmonary function testing reveals a restrictive pattern of lung disease and decreased diffusion capacity (transfer factor).

Radiographs reveal fine calcific micronodules in a diffuse (sandstorm) pattern, but with a more dense involvement of the lower zones (thought to be due to the larger surface area and greater thickness of this part of the lungs). There is usually silhouetting of the mediastinal and diaphragmatic borders and a linear lucency between the ribcage and parenchyma (reflecting subpleural cystic changes). Small apical bullae with or without an associated pneumothorax may also be present.

The findings are more subtle in children. Nodular calcific densities are less prominent on children’s chest radiographs and the major finding may be ground-glass opacities.

HRCT reveal widespread involvement of both lungs with microliths, with a predisposition for the posterior segments of the lower lobes, anterior segments of the upper lobes, and the medial aspects of the lungs.

Confluent calcifications may also be seen, and are often found in the upper lobes. There is also micronodulation and thickening of the interlobular septa and bronchovascular and subpleural interstitium.

Multiple small, thin-walled subpleural cysts can also be seen and are responsible for the lucent subpleural line seen on radiographs. Pleural calcifications have also been described.

When extensive, the calcifications can result in interlobular septal thickening. Microliths smaller than 1 mm can produce a ground-glass appearance. Together, these can give an appearance similar to crazy paving.

In children, a ground-glass pattern may predominate, and microliths are seen to a lesser degree.

Differential considerations include:

• Metastatic calcification: For example, in chronic renal failure and orthotopic liver transplantation.
• Dystrophic calcification: Caused by granulomatous disorders (silicosis, sarcoidosis), DNA viruses, parasitic infections, and pulmonary amyloidosis.
• Pulmonary ossification: Multiple causes. Most pimped: chronic chronic mitral stenosis.

References

• Chan ED, Morales DV, Welsh CH, McDermott MT, Schwarz MI. Calcium deposition with or without bone formation in the lung. Am J Respir Crit Care Med. 2002 Jun 15;165(12):1654-69.
• Gasparetto EL, Tazoniero P, Escuissato DL, Marchiori E, Frare E Silva RL, Sakamoto D. Pulmonary alveolar microlithiasis presenting with crazy-paving pattern on high resolution CT. Br J Radiol. 2004 Nov;77(923):974-6.
• Helbich TH, Wojnarovsky C, Wunderbaldinger P, Heinz-Peer G, Eichler I, Herold CJ. Pulmonary alveolar microlithiasis in children: radiographic and high-resolution CT findings. AJR Am J Roentgenol. 1997 Jan;168(1):63-5.
• Siddiqui NA, Fuhrman CR. Best cases from the AFIP: Pulmonary alveolar microlithiasis. Radiographics. 2011 Mar-Apr;31(2):585-90.