Pulmonary alveolar microlithiasis

Pulmonary alveolar microlithiasis (PAM) is a rare idiopathic condition characterized 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

On some occasions may show high FDG uptake but mainly in the lung regions showing sparing calcification 10.

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.

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Article information

rID: 8842
System: Chest
Synonyms or Alternate Spellings:
  • Alveolar microlithiasis
  • Pulmonary alveolar microlithiasis (PAM)

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Cases and figures

  • Case 1: on chest radiograph
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  • Case 1: on HRCT mediastinal window
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  • Case 2: on chest radiograph
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  • Case 2: on CT - lung window
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  • Case 3
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  • Case 4
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  • Case 5
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  • Case 6
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  • Case 7: pulmonary alveolar microlithiasis
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