Pulmonary alveolar microlithiasis (PAM) is a rare idiopathic condition characterised by widespread intra-alveolar deposition of spherical calcium phosphate microliths (calcospherites).
A slight female predilection may be present in the familial form 2. Most cases are reported in Asia and Europe 9.
Often discovered incidentally on a chest radiograph. The radiographic features are frequently disproportionate to the clinical symptoms 5.
PAM is believed to be due to a mutation in SLC34A2 that causes inactivation of a sodium-dependent phosphate cotransporter, which itself is found mainly in alveolar type II cells. This cotransporter normally clears phosphate from degraded surfactant, and when inactivated there is accumulation of phosphate in the alveolus, and calcium phosphate microliths are then thought to form 9.
An autosomal recessive inheritance pattern has been proposed given familial occurrence in a majority of cases 2-3,9. Usually, there is no abnormal calcium metabolism.
The pattern in children differs from that in adults 2.
Chest x-ray typically demonstrates:
- sand-like calcification distributed throughout the lungs
- bilateral distribution with middle to lower zone predilection 2,3
- black pleural lines may be evident 2
HRCT better shows numerous sand-like calcifications throughout the lungs with subpleural and peribronchial distribution (typically ~1 mm) 8. Additional accompanying HRCT features include
- crazy paving pattern 1,5
- calcified interlobular septa 1
- small subpleural cysts / emphysema 1
- black pleura sign
- ground glass opacities: tends to be more common in children 2
Treatment and prognosis
No known treatment is available. Overall prognosis is good but occasionally slow progression can result in end-stage lung fibrosis requiring lung transplantation.
Some consider the radiographic appearance to be pathognomonic 3.
Differential for fine granular densities includes
- healed varicella pneumonia
- idiopathic pulmonary haemosiderosis: tend to be large nodular opacities; no calcification 1
- pulmonary stannosis: deposition of tin dust
- pulmonary baritosis: deposition of barium particles
- mitral stenosis: may produce small foci of pulmonary ossification
- 1. Marchiori E, Souza AS, Franquet T et-al. Diffuse high-attenuation pulmonary abnormalities: a pattern-oriented diagnostic approach on high-resolution CT. AJR Am J Roentgenol. 2005;184 (1): 273-82. AJR Am J Roentgenol (full text) - Pubmed citation
- 2. Helbich TH, Wojnarovsky C, Wunderbaldinger P et-al. Pulmonary alveolar microlithiasis in children: radiographic and high-resolution CT findings. AJR Am J Roentgenol. 1997;168 (1): 63-5. AJR Am J Roentgenol (citation) - Pubmed citation
- 3. Korn MA, Schurawitzki H, Klepetko W et-al. Pulmonary alveolar microlithiasis: findings on high-resolution CT. AJR Am J Roentgenol. 1992;158 (5): 981-2. AJR Am J Roentgenol (citation) - Pubmed citation
- 4. Johkoh T, Itoh H, Müller NL et-al. Crazy-paving appearance at thin-section CT: spectrum of disease and pathologic findings. Radiology. 1999;211 (1): 155-60. Radiology (full text) - Pubmed citation
- 5. Gasparetto EL, Tazoniero P, Escuissato DL et-al. Pulmonary alveolar microlithiasis presenting with crazy-paving pattern on high resolution CT. Br J Radiol. 2004;77 (923): 974-6. doi:10.1259/bjr/96331922 - Pubmed citation
- 6. Chandra, Subhash; Mohan, Anant; Guleria, Randeep; Das, Prasenjit; Sarkar, Chitra. Bilateral stony lung: pulmonary alveolar microlithiasis BMJ Case Reports. 2009 doi:10.1136/bcr.09.2008.1012 -
- 7. Chai JL, Patz EF. CT of the lung: patterns of calcification and other high-attenuation abnormalities. AJR Am J Roentgenol. 1994;162 (5): 1063-6. AJR Am J Roentgenol (abstract) - Pubmed citation
- 8. Webb WR, Higgins CB. Thoracic Imaging. Lippincott Williams & Wilkins. (2010) ISBN:1605479764. Read it at Google Books - Find it at Amazon
- 9. Castellana G, Castellana G, Gentile M et-al. Pulmonary alveolar microlithiasis: review of the 1022 cases reported worldwide. Eur Respir Rev. 2015;24 (138): 607-20. doi:10.1183/16000617.0036-2015 - Pubmed citation