Alveolar microlithiasis: causes, symptoms, diagnosis, treatment

Alveolar microlithiasis is a rare, inherited disorder in which calcium phosphate microliths gradually deposit in the alveoli, leading to diffuse lung tissue damage and progressive restrictive respiratory failure. The typical radiographic finding is described as a "sandstorm" effect, and the diagnosis is often made incidentally in the early stages in largely asymptomatic individuals. [1]

The key to understanding the disease is a defect in phosphate transport in alveolar type II cells due to mutations in the SLC34A2 gene, which encodes the sodium-phosphate cotransporter Npt2b. Excess phosphate from broken down surfactant phospholipids is not properly removed, forming hydroxyapatite stones in the alveoli. [2]

The disease often progresses slowly and remains asymptomatic for a long time, but eventually leads to shortness of breath during exertion, a dry cough, decreased tolerance to physical activity, and, in some patients, the development of pulmonary hypertension and respiratory failure. In a small number of patients, the disease may begin in childhood and progress more rapidly. [3]

Since there is no etiotropic drug therapy yet, the approach is built around early recognition, genetic confirmation, observation in a specialized center for rare lung diseases, timely respiratory support and consideration of lung transplantation in terminal cases. [4]

Epidemiology

Alveolar microlithiasis is a rare disease with an estimated prevalence of less than one case per million population, although the incidence varies greatly by region due to founder effects and diagnostic availability. The largest number of descriptions come from Mediterranean countries, the Middle East, South Asia, and Japan. [5]

The age of onset is wide, from early childhood to late adulthood. Many patients are diagnosed incidentally during radiography for another reason, and the clinical manifestations develop years later. Gender differences have not been reliably proven, although in some samples, males are more common. [6]

Large reviews have compiled hundreds of case reports and highlighted the phenomenon of clinical and radiographic discordance: despite extensive deposits on imaging, patients may experience moderate symptoms for a long time. This complicates early referral to specialists. [7]

With the increasing availability of high-resolution CT scanning and genetic testing, the number of verified cases is increasing, allowing for a better understanding of the variability in the course and genetic variants of SLC34A2. [8]

Reasons

The etiologic basis is biallelic pathogenic variants in SLC34A2, which encodes the epithelial sodium-phosphate cotransporter Npt2b, expressed in type II alveolar cells. Disruption of this protein leads to accumulation of phosphate in the alveolar lumen and deposition of hydroxyapatite crystals. Inheritance is autosomal recessive. [9]

The spectrum of mutations includes nonsense, missense, in-frame, and splicing variants, described in both familial and sporadic cases. Expansion of the genetic spectrum continues thanks to next-generation sequencing and international rare disease registries. [10]

It is proposed that surfactant phospholipids, which are normally recycled with the participation of Npt2b, serve as the source of phosphate. When Npt2b is deficient, phosphate accumulates, binds with calcium, and forms microliths. This model is supported by experimental and molecular observations. [11]

No other obligatory causes other than a genetic defect in phosphate transport have been identified, although course modifiers, including respiratory tract infections and environmental factors that can precipitate symptoms, are discussed. Their role remains a subject of research. [12]

Risk factors

The main risk factor is family history and parental consanguinity, which increases the likelihood of homozygosity for the defective allele. In populations with a tradition of intermarriage, the risk is higher, as reflected in the geography of publications. [13]

Frequent respiratory infections and smoking are considered indirect factors of progression, which can increase symptoms and accelerate the decline in lung function in existing disease, although there is little evidence of a cause-and-effect relationship. [14]

Childhood age of onset and the presence of severe SLC34A2 variants are associated with a more rapid progression, development of respiratory failure and pulmonary hypertension. [15]

Lack of early referral to specialized centers and delay in diagnosis are also correlated with greater functional deficits at diagnosis.[16]

Pathogenesis

The central link is impaired phosphate clearance in the alveoli due to loss of Npt2b function. Excess phosphate binds to calcium, forming hydroxyapatite crystals that build up in layers and mechanically damage the alveolar surfaces, inducing inflammation and fibrosis. [17]

Microliths fill the alveoli, impair gas exchange and lung elasticity, and create a restrictive type of ventilation disorder. Over time, pulmonary vascular resistance increases, leading to pulmonary hypertension and right ventricular overload. [18]

Experimental data support the hypothesis that alveolar surfactant becomes a substrate for excess phosphate, and macrophages and epithelial cells trigger a cascade of inflammation and remodeling in the prolonged presence of crystals.[19]

Diversity of genetic variants likely determines the range of severity and rate of progression, explaining the differences between asymptomatic adult presentation and severe onset in early life.[20]

Symptoms

The early stages are often asymptomatic and are detected incidentally on chest X-rays. As microliths accumulate, shortness of breath develops, first with exertion and then at rest, followed by a dry cough, fatigue, sometimes chest pain, and rare episodes of hemoptysis. [21]

Physically, weakened vesicular breathing, fine-bubble crepitations, signs of restriction, and hypoxemia are possible. Some patients develop cyanosis and signs of cor pulmonale with the addition of pulmonary hypertension. [22]

The classic radiographic picture includes diffuse, symmetrical, small calcifications with a tendency toward basal-peripheral predominance and a pronounced pattern against a background of relatively few clinical symptoms. [23]

There is often a discrepancy between the severity of imaging changes and the mildness of complaints, which is misleading and delays referral for genetic testing and to a specialized center. [24]

Table 1. Common manifestations and clinical clues

Manifestation	What's alarming	Clinical clue
Gradual shortness of breath	It gets stronger over the years	Thinking about a rare interstitial disease
Dry cough	Not explained by infection	Requires high definition visualization
X-ray "sandstorm"	Dissonance with mild symptoms	Classic phenotype of alveolar microlithiasis
Hypoxemia, signs of pulmonary heart disease	Late stages	Pulmonary hypertension monitoring is needed.

Source: review articles and clinical series. [25]

Forms and stages

There is no single international staging scale, but clinicians describe conditional stages: an asymptomatic radiographic stage, a symptomatic stage with increasing restriction and hypoxemia, and then complicated by pulmonary hypertension and right ventricular failure. [26]

Based on age of onset, a distinction is made between childhood and adult variants, with the former often associated with a more severe course. Genetically, different classes of SLC34A2 mutations are distinguished, which aids family counseling. [27]

Based on imaging data, high-definition computed tomography allows assessment of the distribution and density of microliths and associated fibrosis, which is suitable for dynamic monitoring. [28]

In cases of severe symptoms, a phenotype with severe respiratory failure and a phenotype with dominant pulmonary hypertension are distinguished, which influences the choice of maintenance therapy and the timing of referral for transplantation. [29]

Complications and consequences

The main adverse outcomes are progressive respiratory failure, pulmonary hypertension, and right ventricular failure. These complications lead to a decrease in quality of life and an increased need for oxygen therapy. [30]

Repeated respiratory tract infections may worsen the course and accelerate the decline in lung function. Anecdotal reports describe severe infectious episodes in children with rapid deterioration. [31]

Delayed diagnosis leads to late routing, when options are limited, and increases the risk of emergency hospitalizations. Specialized centers can plan respiratory support and ensure timely placement on the transplant waiting list. [32]

Improvements in gas exchange and quality of life have been reported after transplantation, although long-term outcomes depend on general post-transplant monitoring factors. Recurrence of disease in the graft has been virtually unknown. [33]

Diagnostics

Initial imaging begins with radiography, but the standard of evaluation is high-definition computed tomography (HDCT), which reveals diffuse microcalcifications and their distribution. In complex cases, bone scintigraphy is additionally used, where the pleuropulmonary zones accumulate the isotope. [34]

The gold standard for confirming the etiology is molecular genetic testing of SLC34A2 with a search for biallelic pathogenic variants. This eliminates the need for invasive biopsy in most situations and allows for family counseling. [35]

Pulmonary function tests demonstrate a restrictive pattern of impairment with reduced carbon monoxide diffusing capacity and hypoxemia on exertion, which is useful for monitoring progression and selecting support. [36]

Echocardiography is indicated to assess pulmonary artery pressure and right ventricular load. Signs of pulmonary hypertension require further evaluation and management adjustments. [37]

Table 2. When to refer for SLC34A2 genetic testing

Situation	Arguments in favor
X-ray or CT scan showing a "sandstorm" appearance	High pretest probability of alveolar microlithiasis
Asymptomatic patient with characteristic findings	Allows to avoid biopsy and determine the prognosis
The presence of relatives with similar changes	Family counseling and prenatal options
Childhood onset and accelerated progression	Early monitoring and support tactics

Based on review publications and clinical series. [38]

Differential diagnosis

Alveolar microlithiasis must be distinguished from other causes of diffuse calcifications and ground-glass opacity: metastatic calcification of the lungs in uremia, silicosis, sarcoidosis with calcifications, previous infectious processes, and ossifying alveolar microlithiasis-like patterns. The distribution pattern, clinical context, and genetics play a key role. [39]

Metastatic calcification often presents with hypercalcemia or calcium-phosphorus metabolism disorders, whereas alveolar microlithiasis may not show systemic changes. Age, medical history, and laboratory data help narrow the diagnosis. [40]

If imaging is atypical or there is doubt, a radiologist with experience in interstitial lung diseases is consulted and molecular confirmation is performed. Biopsy is rarely required and is primarily used when differential diagnostic difficulties arise. [41]

Table 3. Alveolar microlithiasis and mimics in images

State	What is in favor?	What does "against" say?
Alveolar microlithiasis	Symmetrical microcalcifications, sandstorm effect, normal calcium metabolism in the blood	No obvious hypercalcemia
Metastatic calcification	Systemic hypercalcemia, uremia	There is no characteristic diffuse granularity in the classical form
Pneumoconiosis	Occupational history, nodules and fibrosis	Absence of typical microliths
Sarcoidosis with calcification	Systemic symptoms, enlarged lymph nodes	A different distribution picture

Summarized from radiological reviews. [42]

Treatment: What really works today

There are no etiotropic drugs with proven effects on clinical outcomes. Attempts to use systemic bisphosphonates or other drugs have not yielded convincing benefits in controlled settings and remain experimental and individualized solutions. [43]

The mainstay of treatment is monitoring and support: vaccination against influenza and pneumococcus, pulmonary rehabilitation, oxygen therapy for hypoxemia, treatment of comorbid conditions, and smoking cessation. This slows functional deterioration and improves quality of life. [44]

In the development of pulmonary hypertension, general management principles are applied, focusing on symptoms and hemodynamics. The use of specific vasoactive agents should be discussed in experienced centers, as the evidence base for alveolar microlithiasis is limited. [45]

Lung transplantation is indicated for terminal respiratory failure. Successful cases of unilateral and bilateral transplantation have been described, with significant clinical improvement and no graft recurrence. Candidacy selection and decision-making require early referral to a specialized center. [46]

Table 4. Maintenance therapy by stages

Current stage	Measures
Early asymptomatic stage	Observation, vaccination, smoking cessation, basic rehabilitation
Symptomatic stage	Rehabilitation, oxygen if indicated, treatment of concomitant diseases, monitoring of function
With complications	Assessment of pulmonary hypertension, optimization of support, transplant planning
Terminal stage	Waiting list inclusion, pre-transplant preparation

Based on reviews and practical recommendations. [47]

Prevention

Primary prevention, i.e., preventing the onset of the disease, is impossible due to its genetic nature. However, family counseling, cascade testing of relatives, and information about the risks for future children are possible. This facilitates early recognition and monitoring. [48]

Secondary prevention aims to reduce the incidence of infections and slow down functional deterioration: vaccination, smoking cessation, treatment of exacerbations of bronchopulmonary diseases, rehabilitation program and monitoring of oxygen saturation during exercise. [49]

Forecast

Progression is usually slow but variable. Moderate symptoms may persist for decades despite significant radiographic findings, but some patients reach the stage of severe respiratory failure and require transplantation. [50]

Childhood onset, rapid decline in diffusing capacity, and development of pulmonary hypertension are unfavorable signs. Early referral to a specialized center improves care planning and outcomes. [51]

Table 5. Signs of an unfavorable course

Sign	Why is it important?
Children's debut	Associated with faster progression
Rapid decline in diffusion capacity	Marker of loss of gas exchange
Signs of pulmonary hypertension	Increase the risk of right ventricular failure
Frequent respiratory infections	Accelerates the deterioration of function

Summarized from clinical series and reviews. [52]

Frequently asked questions

• Is it possible to confirm the diagnosis without a biopsy?

Yes. In most cases, the combination of characteristic CT scan and detection of biallelic pathogenic variants in SLC34A2 is sufficient for definitive diagnosis, biopsy is rarely needed.[53]

• Are there any medications that dissolve microliths?

There are currently no drugs with proven clinical efficacy for reducing microliths and improving outcomes. Treatment is supportive, and in terminal cases, lung transplantation is considered. [54]

• Is the disease transmitted to children?

Inheritance is autosomal recessive. If both parents are carriers of the defective variant, the risk to the child is one in four. Family counseling and cascade testing are recommended. [55]