Autoimmune > Microlithiasis

Alveolar Microlithiasis:

View cases of alveolar microlithiasis

Clinical:

Alveolar microlithiasis is autosomal recessive disorder characterized by the development of sand-like calcifications composed of calcium and phosphorus within the alveoli [5], however, sporadic cases may occur [6]. The disease is caused by autosomal recessive inheritance of a mutation in the gene that produces the protein that is the only known sodium-dependent phosphate transporter protein in the lungs [5]. As a result, patients are unable to transport phosphorus ions from the alveolar space into type II pneumocytes which leads to the development of calcospherites in the alveolar space [5]. Identification of the SLC34A2 gene mutation is diagnostic of PAM [6].

 

Patients generally present between the ages of 30 to 50 years, although the condition has been identified in utero. There is no sex predilection (although sporadic cases are more common in males) [6]. There is a positive family history in 50% of cases, especially siblings. In the early phase of the disease, the lung tissue remains normal, however, as the microliths expand, the alveolar walls become compressed and damaged and eventually replaced by fibrous tissue [6]. Over time (20-30 years), as the microliths progress to involve all lobes bilaterally, the lungs increase in weight and lose their collapsibility [6]. Most patients are asymptomatic (70%), but DOE or clubbing may be seen (symptoms typically do not develop until the 3rd or 4th decade of life [6]). With advanced disease, patients may expectorate microliths [6].


The disorder is slowly progressive and usually fatal due to respiratory or cardiac failure. Patients have normal serum calcium and phosphorus levels, and parathyroid hormone levels are also normal [6]. On pulmonary function testing a decreased residual volume or restrictive pattern are identified [6]. There is currently no effective treatment for the disorder other than lung transplantation [5,6]. A differential consideration includes talc granulomatosis secondary to IV drug use.

X-ray:

CXR: On plain film there is fine, sand-like micronodulation (<1mm) involving both lungs diffusely (but more prominent in the mid and lower lung zones [6]) and may appear confluent in areas (producing ground glass opacity). There is a predominantly symmetric middle and lower lobe involvement. The heart borders and the diaphragm are usually obliterated. The pattern has been described as ?sandstorm-like? [5]. A "black pleural line" or zone of increased translucence between the lung parenchyma and the ribs has been described and is due to the presence of thin-walled subpleural cysts [6]. Subpleural cyst rupture can lead to pneumothorax [6]. With time, interstitial fibrosis, pulmonary hypertension, and cor pulmonale develop. The major finding on chest radiographs in children are ground-glass opacifications.

Computed Tomography: On CT, there is a gradient in the distribution of the calcifications which tend to cluster in the posterior and inferior subpleural spaces, the anterior segments of the upper lobes, and along the bronchovascular bundles. Additionally, the medial portions of the lung appear to be more heavily involved than the peripheral portions [5]. Calcific polygonal lines can also be seen due to thickening of the interlobular septa producing a crazy paving pattern [6]. Calcified nodules larger than 1 mm (up to 5 mm) are visible on HRCT scans. Microliths with a diameter of less than 1 mm produce areas of ground-glass opacification, but can be discerned as discrete calcifications with appropriate windowing [5]. Numerous, thin walled sub-pleural cysts produce a dark pleural line and are indicative of early fibrosis.

Scintigraphy: There is intense pulmonary uptake of radiotracer on Tc-MDP bone scan [6].

REFERENCES:

(1) AJR 1997; Helbich TH, et al. Pulmonary alveolar microlithiasis in children: radiographic and high-resolution CT findings. Jan 168 (1): 63-65 (No abstract available)

(2) J Comput Assist Tomogr 1991; Pulmonary alveolar microlithiasis: CT findings. 15 (6): 938-42

(3) Pediatr Radiol 1987; Volle E, et al. Pulmonary alveolar microlithiasis in pediatric patients- review of the world literature and two new observations. 17: 439-442

(4) AJR Am J Roentgenol 1992; Korn MA, et al. Pulmonary alveolar microlithiasis: findings on high-resolution CT. 158(5): 981-982 (No abstract available)

(5) Radiographics 2011; Siddiqui NA, Fuhrman CR. Pulmonary alveolar microlithiasis. 31: 585-590

(6) Radiographics 2016; Delic JA, et al. Pulmonary alveolar microlithiasis. 36: 1334-1338

(7) Radiology 2021; Ufuk F. Pulmonary alveolar microlithiasis. 298: 567

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