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].