Discussion
Pulmonary alveolar microlithiasis (PAM) is a rare genetic disease characterized by the accumulation of microliths in the pulmonary alveolar space.1, 3, 10, 18 These microliths induce a chronic inflammation of the alveolar septa, responsible for a chronic interstitial lung disease leading to respiratory failure and lung fibrosis. Common symptoms are dyspnea, dry cough, chest pain, hemoptysis, asthenia, and possible occurrence of pneumothorax. Children are always in the onset at the early stage of PAM and are often misdiagnosed because of nonspecific symptoms such as dry cough, acute respiratory failure and asthenia.19 As early symptoms are imperceptible, PAM has a low diagnostic rate in children aged ≤ 5 years, accounting for only 2%-3% of all cases (28 cases to 1022 cases in the most recent all-age cohort).19 In our study, we reported a case of PAM in a 7-year-old boy diagnosed by genetic testing and CT findings.
As reported in the literature, symptoms, signs, serological tests or imaging features of PAM are not typical at the early stage, especially in children. The diagnosis of PAM is often based on radiographic images at first, and a definitive diagnosis requires at least one additional clinical feature including genetic testing demonstrating a mutation in SLC34A2, microlith analysis or histopathology.15Genetic testing demonstrates pathogenic mutations in SLC34A2 are highly specific for PAM,15 and because of less invasive than lung biopsy or transbronchial biopsy, it is more frequently used to confirm PAM diagnosis in children ≤5 years of age than in the all-age cohort.1 Especially in families with unknown genetic backgrounds, genetic investigations are highly recommended to identify possible variants of SLC34A2. In the cases of suspected PAM with no prior family history, it is also preferred to perform genetic analysis. In our case, we tried to persuade the parents to perform a lung biopsy/transbronchial biopsy or whole-exome sequencing on the patient at age 2, but they rejected which caused delayed diagnosis, while finally diagnosed by gene analysis.
Up to date, there have been approximately 40 pathogenic variants in SLC34A2 reported. Based on the summary of SLC34A2 gene mutations by Bendstrup et al,16 we searched PubMed and Web of Science in these 3 years until Feb 1, 2023, and updated 7 novel pathogenic variants including this case inTable 1 , namely c.286 C>T ,20 c.448 G>A ,21 c.524-1 G>C ,22EXON 2-6 duplication, c.1218 C>A, c.1493 G>T ,23 c.1653_1660del.24 Types of DNA variants include 4 substitutions, 1 deletion, 1 splicing site and 1 duplication. According to the literature, there is no clear correlation between genotype/phenotype. Jönsson et al. demonstrated that the disease severity was associated with the pathogenicity of the variants,6 but this needs to be investigated in a larger patient population. In our case, we identified two heterozygous mutations in SLC34A2, EXON:2-6 duplication and c.1218C>A in EXON 11. The EXON:2-6 duplication was predicted to disrupt the reading frame and leaded to the transcription factor degradation. Compared to mutations of a single exon, 5 consecutive exons duplication in the coding region tended to cause loss of function. In monogenic autosomal recessive disease, duplications within one pathogenic gene could cause dysfunctions or correspond different phenotypes.25 The missense variant, c.1218C>A in EXON 11 was also predicted to be pathogenic by forecasting tools, like PROVEAN, SIFT, Polyphen2, Mutation Taster and Revel analyzing conservatism. And gene frequency in normal general was below 0.0005. The compound heterozygous mutations eventually leaded to dysfunction in SLC34A2.
The manifestations of PAM are not classical at the early stage, since the microliths have not caused obvious respiratory dysfunction. The present patient became symptomatic at age 2 and was diagnosed at age 7. His main symptoms were intermittent fever, cough, and expectoration which were consistent with the characteristics of children suffering from PAM. Intermittent fever and cough appear as first symptoms in children cohort and CCT is performed for other reasons such as a viral or bacterial lung infection. Combining with respiratory infections tends to be the first reason for children’s admissions, and it is also an opportunity to find abnormal chest images. Further genetic testing or biopsy confirm the diagnosis of PAM. We hypothesized that PAM combined with recurrent respiratory infections, which could be rational to explain why dry cough, fever, acute respiratory failure are frequent symptoms in PAM. Furthermore, recurrent infections are one of the factors resulting in bronchiectasis.
Notably, our patient showed unusual bronchiectasis at age 2. And CCT at age 7 showed that central bronchiectasis had contracted. However, Deniz and his partners found a different observation that peripheral bronchiectasis was seen at a high incidence rate of 60% (6/10) and none of the group (mean age: 22±3.2) had central bronchiectasis.26 Pathophysiological mechanisms of bronchiectasis include persistent bacterial infections, dysregulated immune responses, airway obstruction and impaired mucociliary clearance.27 Most common pathogenic causes associated with development of bronchiectasis in children are idiopathic factors, post-infection, congenital immunodeficiency or associated with dysplastic syndromes. 28 For our patient, the hereditary factor could be the primary reason when bronchiectasis was noticed in early childhood since his grandmother and his father had respiratory diseases. Following hereditary factor, airway epithelium was destroyed due to respiratory infections. Especially complicated with persistent infections, central bronchiectasis could be more severe at young age.
In addition, our patient’s PFTs showed a restrictive syndrome with FVC of 1.21L (60.5% of predicted), a mild obstructive ventilation dysfunction with FEV1/FVC of 69.8%, and a positive BDT with FEV1 improvement ratio at 33.6%. PAM caused by SLC34A2 mutations lead to the accumulation of calcium phosphate in the alveoli, restrict alveolar dilatation, and then progress to a restrictive lung function impairment. Our case also showed a mild obstructive ventilation dysfunction possibly because of excessive sputum. Furthermore, his positive BDT suggested airway spasm, possibly associated with asthma orMycoplasma pneumoniae . The patient denied history of allergies or asthma and lung auscultation had no sonorous rhonchi or sibilant wheezes. There was no insufficient evidence to diagnose asthma. Mycoplasma pneumoniaeis considered as a factor resulting in a trigger in recurrent wheezing and exacerbations of asthma in children.29 Besides, there may be bias in this data of BDT, especially under the condition of a reduced vital capacity, which needs multiple measurements.
As PFTs showed a restrictive ventilation dysfunction and CCT showed mosaic sign, bronchitis obliterans was diagnosed by bronchoscopy.30, 31Histologically, bronchiolitis obliterans is defined by obliteration of the lumen of bronchioles owning to inflammation, granulation tissue or scarring.32 Bronchial obliteration presents as a complete obliteration of the bronchus by a smooth-surfaced membrane.33 Typically, such changes are associated with chronic inflammation of the bronchial walls and cartilage destructions, resulting in structural shifts such as thickening, bronchiectasis, and fibrosis.34-36 Then the bronchial or bronchiolar lumina may be contracted or dilated and filled with mucopurulent debris34, with partial or complete luminal obliterans.35-37 In addition, infections also play an important role in the development of bronchitis obliterans and the most common post-infection pathogens occurrence of bronchiolitis obliterans in children are Mycoplasma pneumoniae , adenovirus ,respiratory syncytial virus , influenza , measles andtuberculosis. 38 Mycoplasma pneumoniaadheres to the ciliated columnar epithelium of the respiratory tract and produces local cytotoxic.39 And P1-adhesin, a transmembrane protein, helps Mycoplasma pneumonia in cell-to-cell transfer which eventually increases the infective surface area and results in extensive airway epithelium damage.40Mycoplasma pneumoniae was detected in our case through BALF-NGS, therefore, bronchitis obliterans in our case was considered to be the results of bronchiectasis and Mycoplasma pneumoniae infection. Currently, there has been no evidence to contact PAM with infections, but it has been predicted that environmental factors such as exposure to passive smoking and infections may accelerate the process of PAM.
As for PAM, we should distinguish between miliary tuberculosis (PAM occurs frequently in countries where Mycobacterium tuberculosisis common), hemosiderosis, silicosis, carcinomatosis, or sarcoidosis.19 When there are extensive calcifications in both lungs on mediastinal windows, we should especially distinguish them from miliary tuberculosis (TB).41 Miliary tuberculosis is a potentially fatal form of disseminated disease due to the hematogenous spread of tubercle bacilli to the lungs, and other organs. It results in the formation of millet seed-sized (1 to 2 mm) tuberculous foci.42However, the size (3 to 6 mm) of calcifications in our child’s imaging was bigger than that of miliary TB and calcifications were distributed in the middle and lower lobes. While in miliary TB, innumerable micronodular (1 mm) infiltrates, diffusely scattered in both lungs, especially the lung apices.43 Besides, there was no obvious lymphadenopathy and no associated cavities to spread satellite lesions. No evidence of tuberculosis was found in BALF-NGS. Though the patient’s TST was positive, probably because of vaccination. To summarize, TB infection is not considered at this time and calcifications in PAM are quite different from TB in location and size.