1 Introduction
Pulmonary alveolar microlithiasis (PAM) is a rare disease characterized by intra-alveolar presence of microliths. It is an autosomal recessive disorder and runs in families. PAM occurs because of mutation in the sodium co-transporter (
SLC34A2) gene [
1]. Affected patients may range from early childhood to elderly and are usually asymptomatic during the early phase of the disease. This disease has a long and progressive course with deterioration of lung function [
2].
Malphighi from Italy (1686) was the first to describe the macroscopic features of the disease, and Puhr from Hungary is credited for naming the disease in 1933 [
2]. The first PAM case in India was reported by Viswanathan in 1961 [
3]. This disease is extremely rare; hence, a systematic review of published cases is helpful in understanding its epidemiology, etiopathogenesis, clinical features, and diagnosis. Two interesting national reviews have already been published from Turkey and Italy [
4,
5]. In this study, we present an interesting case of PAM and review the literature of 52 case reports (73 cases) from India [
3,
6–
56]. Majority of the case reports have been published in our national journals.
2 Our case report
A 23-year-old female, born of second-degree consanguineous marriage, presented with cough with expectoration, chest pain, and episodic wheeze for five months. The chest pain was associated with coughing. Sputum production was minimal and not foul smelling or blood tinged. No history of fever, hemoptysis, or loss of weight was present. She did not have any past history of pulmonary tuberculosis, and her personal history was unremarkable. She was not a smoker and had no history of exposure to toxic gases and fumes. Pedal edema, cyanosis, and clubbing were not present. On lung auscultation, vesicular breath sounds were heard bilaterally. Oxygen saturation was 96% on room air. Pulmonary function test revealed an obstructive pattern, with a forced vital capacity of 2.83 L (109% of predicted value), a forced expiratory volume in 1 s of 1.86 L (83% of predicted value), a FEV1/FVC ratio of 65.7 (78% of predicted value), and a PEF of 2.87 L/s (46% of predicted value).
Routine blood investigations were within normal limits, except for a mild eosinophilia. Chest X-ray showed diffuse fine micronodular opacities, predominantly involving the middle and lower zones of both lungs with relative sparing of the apices (Fig. 1). Miliary tuberculosis was initially suspected, but sputum smear and culture were negative for acid-fast bacilli. High resolution computed tomography (HRCT) of chest showed diffuse bilateral micronodules of 1–2 mm size, involving all segments of the lung (Fig. 2).
On bronchoscopy, the respiratory mucosa showed mild congestion without any significant abnormality. BAL was performed, and biopsy specimens were obtained by transbronchial lung biopsy (TBLB) from the right middle lobe. BAL did not reveal respiratory pathogens or any specific pathology. TBLB exhibited dilated alveolar spaces with laminated calcific concretions (Figs. 3 and 4). The calcospherites positively stained with von Kossa stain. Our final diagnosis was pulmonary alveolar microlithiasis with associated bronchial asthma.
We also assessed the sibling (elder brother) of our patient. He was in good health, and his chest X-ray was unremarkable. The patient was started on inhaled fluticasone propionate/formoterol combination therapy. She improved symptomatically and came regularly for follow-up visits for nine months. A repeat radiograph was performed eight months after her initial presentation, and significant change was not observed (Fig.5). Unfortunately, she was lost to follow-up thereafter.
3 Methods
3.1 Search strategy
A PubMed search was performed with the terms “pulmonary alveolar microlithiasis” OR “alveolar microlithiasis” OR “microlithiasis alveolaris pulmonale” AND “India.” The search was limited to articles published in the English language. Additional publications were also identified from the reference list of the published articles. The websites of national journals of India, such as Lung India and Indian Journal of Pathology and Microbiology were also searched to identify further citations. Cases published as a case report and subsequently in a case series or as an “interesting image” were not counted separately.
3.1.1 Inclusion criteria
We included only case reports and short case series from India published between 1961 and 2013. All cases diagnosed as PAM from India based on radiological investigations or biopsy were included in our study.
3.1.2 Exclusion criteria
Review studies were excluded. Cases without complete clinical and diagnostic details were also excluded.
Two authors independently screened the title and abstracts of the reports identified in the search. Full text articles were retrieved for relevant citations. The final list of articles was selected by consensus.
3.2 Data collection and statistical analysis
Clinical data and basic demographic details of individual cases were extracted and compiled using Microsoft Excel 2010. Categorical data and continuous variables were expressed as percentages and as mean and standard deviation, respectively.
4 Results
Our analysis included 73 PAM cases (52 case reports/short case series) reported from India between 1961 and 2013 [
3,
6–
56]. The details of familial screening were available in 38 patients. A familial preponderance was evident in 21 cases. All the familial cases were siblings, except for a familial occurrence in a mother and her two daughters. The proband (mother) was married to her maternal uncle who was not affected by the disease.
A total of 37 females and 36 males with a mean±SD age of 28.8±14.9 years (median age 27.0 years; range 4–60 years) were found. Maximum cases were reported in the age group of 21–30 years (Table 1). The cases were reported from all geographic locations of India. Although occupational history was not mentioned in all the cases, the majority were housewives, students, and farmers. Three cases had history of exposure to respiratory irritants, but the occupational exposure was probably not associated with the development of PAM. A 43-year-old male had worked for four years in a cement retail shop, 15 years before developing PAM [
3]. A 25-year-old housewife was employed part time in making biris (local hand-rolled cigarettes) [
45]. Both the cases were familial, and their siblings were not exposed to these occupational hazards. Similarly, radiological evidence of the disease was not observed in the coworkers of a 26-year-old machine operator employed in a Calico printing factory [
13].
4.1 Clinical features
Table 1 shows the frequency of common clinical signs and symptoms in various age groups. Twenty-two cases (30%) were misdiagnosed as pulmonary tuberculosis based on radiological features and started on ATT, but only one of them was sputum positive. This particular case was treated for pulmonary tuberculosis twice; the first treatment was based on sputum positivity for acid-fast bacilli, and the second treatment was based on radiological abnormalities. The patients did not improve symptomatically on ATT, and further investigations, such as lung biopsy, revealed PAM. Interestingly, only one case had developed superadded sputum positive pulmonary tuberculosis almost after 12 years of diagnosis.
Six patients aged between 24 and 60 years (mean age of 39.6±14.2 years) had died; five of them died within a week of admission, and one patient died within three months of admission. These patients had symptom duration that ranged from 3 months to 10 years. The 24-year-old patient developed pneumothorax following a bout of coughing and died of respiratory failure. Others died because of respiratory insufficiency associated with cor pulmonale. The 60-year-old female had worsening symptoms for five years and was treated at several centers with antibiotics and anti-tubercular drugs. The patient was finally diagnosed as PAM in a tertiary care center; unfortunately, she died within a week of admission. Two other patients, aged 50 years and 30 years, also had symptoms for 7 and 10 years, respectively; nevertheless, the diagnosis was established only before their death.
Long-term follow-up details were not available for majority of the patients in our series.
4.2 Imaging
Chest radiograph was performed in all patients except for one case. The characteristic finding in most of the cases were sand-like calcific micronodules (sandstorm pattern) diffusely involving both middle and lower lobes bilaterally with relative sparing of the apices. The calcific densities were confluent and denser at the bases. Other common roentgen findings included a dark radiolucent strip between the ribs, the markedly radiopaque lung parenchyma (black pleural line or negative pleural sign), and obscuration of the cardiac outline, diaphragm, and hilar vessels (the “drowned heart syndrome”). Twenty-five patients (34%) were evaluated using CT scan/high resolution CT scan. However, about 57% of the cases diagnosed after 1990 were evaluated using CT scan. Technetium-99m bone scan performed in nine patients showed diffuse increase in radiotracer uptake in both lungs.
PAM was associated with nephrolithiasis (one case), appendicitis (one case), and cholelithiasis (two cases). PAM was also associated with infertility (three males and two females).
Pulmonary function tests were performed in 40 patients. Restrictive pattern was evident in 34 patients (85.0%), and obstructive pattern and normal study in one patient (2.5%) and five patients (12.5%) were observed, respectively. Bronchoalveolar lavage was performed in nine patients and revealed the characteristic calcospherites in three cases. Lung biopsy proved to be confirmatory in 37 patients. CT-guided FNAC revealed the characteristic laminated microliths in a patient [
26]. Necropsy was carried out in only one case [
6].
Majority of the cases were treated conservatively, and none of the cases underwent lung transplantation. The cases in our series were predominantly reported from reputed tertiary care centers and probably represent the tip of the iceberg. A number of PAM cases are possibly undetected and even worse misdiagnosed as miliary tuberculosis in India.
5 Discussion
PAM has been reported worldwide and has no specific geographic or racial distribution. It is widely prevalent in Turkey, Italy, Japan, India, and USA. Recently, PAM is also being reported frequently from other Asian countries, such as China. PAM has no obvious sex predilection. Although PAM affects all ages, it has been most frequently discovered from birth up to 40 years old [
2]. The youngest reported cases were premature twins, who died soon after birth [
57]. PAM occurs in families, as well as sporadically [
2,
4,
5,
58]. Familial cases account for about half of the reported cases [
4]. PAM usually affects siblings and less frequently cousins in a horizontal pattern. Parents and children are rarely affected in a vertical pattern [
2]. PAM is a recessively inherited disease with complete penetrance, and no role for non-genetic mechanisms seems to exist [
59,
60]. Familial PAM is common in our series, and all the familial cases were siblings, except for a rare involvement of a mother and her two daughters [
11]. Although the male:female ratio was equal in our series, a female predominance was noted in our familial cases (1:2 ratio). History of significant exposure to specific environmental or occupational risk factors was not observed in almost all our cases.
Recent studies have shown that PAM resulted from the mutation of the
SLC34A2 gene, a type IIb sodium phosphate co-transporter gene involved in phosphate homeostasis in several organs.
SLC34A2 is expressed in the apical membrane of type II alveolar cells in the lungs. These cells synthesize surfactants using phospholipids and are also involved in recycling and degrading the outdated surfactant, thereby releasing phosphates in the process. The clearance of phosphate from the alveolar space is possibly reduced because of SLC34A2 dysfunction, hence leading to the formation of microliths [
1,
60]. Dogan
et al. analyzed three siblings with PAM in an inbred family. The genetic analysis revealed homozygous mutation in the
SLC34A2 gene in all three siblings diagnosed with PAM. Both their parents (consanguineous marriage) were carriers, with heterozygous mutation of
SLC34A2 gene. Thus, the impaired activity of the
SLC34A2 gene may be responsible for familial PAM [
61].
Autopsy studies revealed that the lungs in PAM are very hard, heavy, and incompressible. The outer surfaces of the lung appear irregular and granular because of microlith protrusion under the visceral pleura. The lungs are gritty on sectioning, and the cut surface has a sandpaper texture. The sandy microliths can be squeezed out [
6,
62–
64]. Bullous emphysematous areas can be found in the anterior margin and at the apex of the lungs [
64]. Moreover, the microliths were also found within the bronchial walls in an autopsy case study [
65]. Histologically, concentrically laminated calcospherites are observed within dilated alveolar spaces. The pulmonary interstitium shows varying degrees of fibrosis. Recently, morphometric analysis of CD34 immunostained sections in an autopsy case demonstrated a significant reduction in the pulmonary capillary beds, which probably leads to pulmonary hypertension [
62]. Chemically, the calcospherites are composed of calcium and phosphorus salts. Traces of potassium, iron, copper, and magnesium can also be found [
66].
5.1 Clinical course
PAM occurs in different age groups, but is frequently diagnosed in the second and third decades. Microliths are probably formed early in childhood, but the manifestations occur much later. The youngest patient in our series was four years old. Many patients were asymptomatic at presentation and were diagnosed incidentally. Breathlessness and cough are the most common symptoms (Tables 1 and 2). Cough probably results from the direct stimulation of the C fibers or other receptors through the microliths [
58]. Clubbing is also detected frequently, and wheeze and chest pain are present in less than 10% of our cases. The disease worsens over time and usually progresses into respiratory failure associated with cor pulmonale. Death can also occur because of tension pneumothorax. In our series, the mean age at death is 39.6 years. Mascie-Taylor estimated the mean age at death to be 41.7 years [
67]. Mean age at death is not mentioned in the Turkish and Italian series [
4,
5].
About one-third of the cases from India were misdiagnosed and treated for pulmonary tuberculosis. PAM was also misdiagnosed as miliary tuberculosis in 29% of the Italian cases [
5].
5.2 Extra pulmonary manifestations
PAM can also be presented with extra-pulmonary manifestations and co-morbidities. Approximately 8% of males in our series were infertile. Semen analysis showed azoospermia in a 40-year-old male [
36]. A detailed work up of infertility in a 27-year-old male revealed calcific deposit in the seminal vesicles [
28]. Epididymal and periurethral calcifications causing azoospermia and punctate calcification in the seminal vesicles are associated with PAM [
68,
69]. Calcification of the sympathetic nervous system and possible involvement of the testes in a case of PAM is also reported [
70]. Moreover, mutations in SLC34A2 are possibly associated with testicular microlithiasis [
60]. These observations make an interesting aspect of PAM, which requires further research.
Co-morbidities, such as mitral stenosis [
71], autosomal recessive Waardenburg anophthalmia syndrome [
72], lymphocytic interstitial pneumonitis [
73], milk-alkali syndrome [
74], and diaphyseal aclasia [
75], are rarely reported. A case of PAM with concomitant valve calcification, cholelithiasis, and idiopathic medullary nephrocalcinosis is also reported [
76]. PAM is rarely associated with pulmonary tuberculosis.
5.3 Investigations
Chest X-ray, CT, and high-resolution CT findings are often diagnostic of PAM. Chest radiographs reveal diffuse areas of micronodular calcifications involving both lungs symmetrically. Tomographic evaluation confirms chest X-ray features. Minimal structural changes of the lung parenchyma can also be detected using high resolution CT scans. Most commonly, the CT scan reveals diffuse ground-glass opacities, which are associated with confluent and diffuse calcified nodules [
63,
77]. Ground-glass attenuation occurs because of small calculi in the alveoli. The calcifications can be observed along the bronchovascular bundles and interlobular septae. The diffuse ground glass attenuation and septal thickening with calcified nodules is referred to as the “crazy-paving” pattern in HRCT scans [
77]. Meanwhile, the occurrence of calcification along the interlobular septa is attributed to the accumulation of calcospherites within the peripheral lobular parenchyma adjacent to the septa. HRCT scans can also reveal small sub-pleural cysts and linear sub-pleural calcifications. The small thin-walled sub-pleural cysts form a peripheral translucent band between the calcified lung parenchyma and the ribs, probably explaining the black pleural sign observed in chest radiographs [
77–
79]. These small sub-pleural cysts represent dilated alveolar ducts histologically [
80]. Apical bullae can also be detected by chest X-ray or CT scans. Multiple calcified pleural plaques [
49] and extensive interstitial fibrosis with traction bronchiectasis are observed rarely in our series [
45].
The micronodular pattern observed in PAM can mimic diseases, such as miliary tuberculosis. However, PAM involves the middle and lower lobes with relative sparing of the apices, in contrast to pulmonary tuberculosis. The characteristic dissociation between the extensive radiographic alterations and minimal symptoms also helps in diagnosing PAM. Biopsy may be needed to confirm the disease, specifically in regions with a high incidence of pulmonary tuberculosis. In the youngest patient in our series aged four, the lesions were exudative in nature during the earliest phase of the disease [
13]. The exudative nodules later got calcified. Discerning these exudative lesions from miliary tuberculosis in young children who presented at an early phase of the disease can be difficult. Small calcifications in children can be difficult to detect on a chest X-ray [
73].
Pulmonary function tests usually demonstrate a restrictive pattern, although patients may have a normal lung function initially. An obstructive pattern is rarely observed (Table 2). Our case showed an obstructive pattern probably due to associated bronchial asthma. Brown
et al. demonstrated that the vital capacity level correctly predicts the development of pulmonary hypertension with exercise in PAM. This relationship suggests the presence of a restricted pulmonary vascular bed [
81], as demonstrated recently by morphometric analysis [
62].
BAL has proved to be a useful tool in the diagnosis of PAM. In our series, BAL revealed the diagnosis in 3/9 cases and in 4/6 cases in the Turkish series. Occasional cases have been diagnosed by sputum examination [
36].
5.4 Treatment
Many authors emphasize that lung transplantation is the only effective treatment for PAM. Cases from India have generally been treated supportively. However, our case showed a good clinical response to steroid therapy. The role of steroids or other drugs in the management of PAM is not clear. Literature review clearly reveals that steroid cannot cure PAM. Several cases from India were treated with steroids before the diagnosis, but did not show any improvement. By contrast, a few authors have reported clinical improvement with steroids. A 56-year-old male in our series showed minimal improvement in the follow-up chest X-ray following steroid therapy [
55]. Another patient initially showed some improvement of dyspnea and cyanosis, but later deteriorated [
7].
A 10-year-old girl who presented with associated lymphocytic interstitial pneumonitis showed a rapid improvement in pulmonary function and exercise tolerance in response to steroid therapy [
73]. Improvement was also evident in an eight-month-old infant treated with corticosteroid and hydroxychloroquine [
82]. A 45-year-old woman who developed PAM after a varicella zoster virus infection with concomitant antiphospholipid syndrome and discoid lupus erythematosus responded to inhaled steroids (200 µg of budesonide per day) [
83]. Pulmonary function tests revealed an obstructive pattern in our patient. Bronchial asthma has probably resulted from the chronic irritation caused by the microliths. PAM can also involve the central airways in addition to the alveoli as previously mentioned. Thus, literature review shows that only few patients have benefited from steroid therapy. Steroids cannot cure PAM, but they have provided symptomatic relief in some cases. Moreover, steroid therapy is unlikely to be effective in the late stages. Further research is needed to identify patients who are likely to benefit from steroids.
Few authors have also described the utility of disodiumetidronate, a bisphosphonate inhibiting the precipitation of hydroxyapatite microcrystals. Ozcelik
et al. [
84] reported clinical and radiological improvement of two patients with PAM who were treated with disodium etidronate for 9 and 11 years, respectively. The patients receiving this drug have to be closely monitored for possible side effects, such as rickets, bone fractures, and hyperphosphatemia. The drug was not effective in an earlier case treated for 18 months [
85]. The effectiveness of disodium etidronate for patients who present in the late phase of the disease should be evaluated.
Lung transplantation is the only effective treatment for end-stage pulmonary disease and can be performed safely even in elderly patients. Shigemura
et al. [
86] reported a case of successful double lung transplantation in a 63-year-old recipient with end-stage PAM. Recently, a 64-year-old woman underwent a right-lung transplant and is able to lead an active life without any complications over a 5-year follow-up period [
87]. Although complications related to transplantation have occurred in some cases, recurrence has not been reported to date [
86–
89].
5.5 Recent advances
Recent studies, specifically from Turkey, China, and Japan, have helped us better elucidate the genetic basis of PAM [
1,
60,
61,
90]. The mutation of
SLC34A2 gene is responsible for PAM, as discussed earlier. Eleven different homozygous mutations of
SLC34A2 gene involving exons 1, 2, 3, 4, 6, 7, 8, 11, 12, and 13 have been identified. The predominant result of these mutations was protein truncation [
90]. Interestingly, mutations seem to vary among races. Mutations in Chinese and Japanese are more likely to affect exons 7 and 8 [
90]. Yin
et al. [
90] suggested that exon 8 may be the screen target for Chinese patients. No disease-causing mutations or single nucleotide polymorphisms in the
SLC34A2 gene have also been reported [
90,
91]. Genes other than the
SLC34A2 gene could possibly be involved in the pathogenesis of PAM. Advanced molecular research tools, such as genome-wide single nucleotide polymorphism analysis, will certainly help us gain better insights and devise novel therapeutic strategies.
Takahashi
et al. [
92] showed that surfactant protein-A and surfactant protein-D are potential serum markers to monitor the disease activity and progression of PAM. Dysfunction of
SLC34A2 gene reduces the clearance of phosphate, hence leading to the formation of microliths. Thus, drugs targeting phosphate metabolism rather than calcium metabolism could prove to be beneficial [
1].
6 Conclusions
Physicians should have a high degree of suspicion for this rare disorder in regions with a high incidence of pulmonary tuberculosis. Siblings of a patient with PAM should be screened for PAM even if they are asymptomatic. The clinical manifestations vary with the disease stage. PAM has a prolonged course and can be associated with extra pulmonary manifestations. Long-term follow-up and genetic analysis will help us to better understand the etiopathogenesis. Cost-effective treatment should also be established for PAM in developing nations.
Compliance with ethics guidelines
Nidhya Ganesan, Marie Moses Ambroise, Anita Ramdas, King Herald Kisku, Kulwant Singh, and Renu G’ Boy Varghese have no conflicts of interest to declare. This manuscript is a systematic review article and does not involve a research protocol requiring ethics committee approval.
Higher Education Press and Springer-Verlag Berlin Heidelberg
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