Investigations
Inherited surfactant dysfunction has recently been classified under the
diffuse lung disease (DLD) group of disorders by the American Thoracic
Society.(Figure 1) In the clinical setting, chILD should be considered
in any child under the age of two that presents with respiratory
symptoms and signs, diffuse parenchymal changes on computerized
tomography (CT) or X-ray, and hypoxemia[61]. Prior to genetic
testing or lung biopsy, it is important to rule out other causes of DLD
like illness including infection, recurrent aspiration and congenital
heart disease or pulmonary hypertension. The latter should warrant an
echocardiogram prior to the initialisation of investigations for
surfactant protein deficiencies.
Chest x-ray is usually the first and most accessible imaging that is
performed, although it will not specifically lead to the diagnosis of
chILD, it may help to identify causes of DLD. Similarly, a chest high
resolution computerized tomography (HRCT) may further characterise the
radiological abnormality in chILD but again may not point towards a
specific diagnosis. The usual features seen on CT in SPDS include ground
glass opacification and thickening of the inter and intra-lobular
septae. Over time, there may be resolution of the ground-glass
opacification, but progress to develop parenchymal cysts. In the
evaluation of chILD, it is essential to obtain high quality images which
is often difficult in younger children due to motion and respiratory
artefacts[65, 66]. These images generally should be acquired
following deep inspiration when lung volumes as closest to total lung
capacity. In cooperative school age children, a HRCT with breath-hold
manoeuvres provides the best quality images while in younger children
and infants where volitional breath-hold manoeuvres are not practicable,
controlled ventilation under general anaesthesia with the help of the
anaesthetist may be employed to provide good inspiratory images. These
techniques that provide greater lung inflation will enhance the
contrasts between lung tissue and air. Children undergoing anaesthetic
induction for the HRCT are prone to atelectasis and may also require
images in both supine and prone positions if there are regions that
shows dependent atelectasis.[66] (Figure 2)
Bronchoscopy and broncho-alveolar lavage can help with the diagnosis of
SPDS although there is no clear biomarker to assist in the diagnosis.
There is qualitative difference in the expression of SP-B and SP-C in
the broncho-alveolar lavage fluid[67]; in patients with SP-B
deficiency, the levels of SP-B may be low in the BALF while patients
with low SP-C may have mutations in TTF1, SFTPC, ABCA3 and other
genes associated with surfactant metabolism. Studies have also shown
raised SP-A and SP-D levels in SPDS although the precise correlation to
SPDS is not yet known[68]. The main utility of broncho-alveolar
lavage in the context of diagnosing SPDS is in identifying other causes
of DLD like infection, pulmonary haemorrhage syndrome and alveolar
proteinosis. Lipid laden macrophages are also important to be looked for
as recurrent aspiration may also mimic DLD.
Lung biopsy may facilitate a histopathological diagnosis fairly quickly
in cases where disease progression is rapid or there is insufficient
time for genetic testing. Lung biopsy tissue can be obtained through
video-assisted thoracoscopy, open
thoracotomy, or transbronchial biopsy depending on the expertise
available in the institution and the stability of the patient.
Video-assisted thoracoscopy and open thoracotomy provide the largest
yield of diagnosis (about 50% of cases) but is generally more invasive
and have longer recovery periods, while it has been well known that
transbronchial and endobronchial biopsy produce samples that are often
inadequate to facilitate a diagnosis[69].The choice of biopsy
technique is largely expertise dependent, although video-assisted
thoracoscopy is a more preferred technique[70, 71].
Histological examination of lung tissue from patients with SPDS may show
various differing patterns. The common characteristics include
hyperplasia of the type 2 alveolar epithelial cells, interstitial
widening, foamy macrophages and proteinaceous material in the distal
airspaces.
Although there is significant overlapping in terms of the histological
features, there can be certain features that show predilection to the
different types of SPDS. In ABCA3 deficiency the most common
histopathological finding is pulmonary alveolar proteinosis (PAP),
desquamative interstitial pneumonitis (DIP) and non-specific
interstitial pneumonia (NSIP). PAP appears to be more common in
symptomatic, younger children with
ABCA3 deficiency while NSIP is
seen in older children presenting with the same deficiency[61].
SP-B deficiency may have congenital alveolar proteinosis or infantile
desquamative interstitial pneumonitis changes on histology[72] while
SP-C deficiency may show changes of chronic pneumonitis of infancy,
particularly in children under 2 years of age[71]. SP-C deficiency
and ABCA3 deficiency can also present with non-specific interstitial
pneumonia on histology.
The study of the biopsy sample via electron microscopy may also help
discern the genetic causes of surfactant dysfunction. In ABCA3
deficiency the lamellar body may appear small and dense or fusion of two
or more lamellar bodies with some normal lamellar bodies[73] may be
seen while SFTPB deficiency may show more disorganized and poorly
lamellated lamellar bodies[74].
Genetic testing, when available in a timely fashion, may obviate the
need to perform invasive lung biopsies and may assist in the
prognostication of the disease. Genetics testing should be considered in
children with a positive family history of SPDS, unexplained respiratory
symptoms in either infancy or childhood [34, 75]. The choice of
genetic tests can be guided by the age of presentation. In term newborns
over the 36 weeks gestation period with unexplained DLD testing for
mutations of SFTPB and ABCA3 should be considered, while
an older child DLD should be screened for SPTPC or ABCA3mutations[75]. Children with thyroid, neurological symptoms such as
chorea, hypotonia or developmental delay associated with DLD features
should be tested for NKX2.1 mutation as well. Genetic testing
should also be considered in cases of diffuse lung disease on CT scan,
histopathological findings of alveolar proteinosis or absence in
lamellar bodies in lung biopsy. Nevertheless, despite advances in
detecting a large array of mutations, there has been reports of patients
with no identifiable genetic mutations despite the presence of clinical,
familial and histological evidence of SPDS[76, 77].
Genetic counselling and testing of family members is also required once
the genetic diagnosis is made and is particularly important in the
context of SP-C deficiency which may developed into adult ILD.
Serum examination is not particularly helpful in the diagnosis of SPDS
given the lack of any biomarkers at present, however there has been a
small study that showed elevated levels of glycoprotein KL-6 in children
with SFTPC and ABCA3 mutations[78].