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.