Pulmonary alveolar microlithiasis

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 familial form 2. Most cases are reported in Asia and Europe 9

Often discovered incidentally on a chest radiograph. The radiographic features are frequently out of proportion to 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.

Plain radiograph

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

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
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Article information

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

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Cases and figures

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    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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    Alveolar microlit...
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    Case 7: pulmonary alveolar microlithiasis
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