Lung ultrasound limitations in rare cystic lung diseases

Lung ultrasound (LUS) is a fast and non-invasive examination procedure to diagnose and monitor conditions in the lungs. It is used to identify interstitial syndrome (IS) and pleural thickening related to diffuse parenchymal lung disease (DPLD) and has shown significant correlations with ground glass opacity (GGO) on high-resolution computed tomography (HRCT). However, the applicability of LUS in patients with rare cystic lung diseases, a DPLD subtype, has not been addressed. In their new study, Davidsen et al. (2017) investigate if distinctive LUS findings could be found in patients with lymphangioleiomyomatosis (LAM), pulmonary Langerhans cell histiocytosis (PLCH), and Birt-Hogg-Dubé syndrome (BHD). LUS findings were compared to findings on HRCT and the results revealed that LUS has limitations as a diagnostic tool in patients with LAM, PLCH, and BHD.

LUS is a radiation-free procedure that can be performed relatively rapidly, with minimal discomfort and is a procedure which is easy to assimilate. Therefore, LUS may be preferable to diagnose and monitor DPLDs in some settings compared to HRCT. LUS was performed by two experienced physicians. The LUS records for each patient were independently reviewed by two pulmonologists. The latest available HRCT prior to examination date for each patient was evaluated and scored according to a modified version of Belmaati et al. (2009) which specifically aims to identify dichotomised presence of findings such as GGO, among others.

In total 12 patients with rare cystic lung diseases participated, six were diagnosed with LAM, three with PLCH, two with BHD, and one with uncharacteristic cystic lung disease. All LAM patients were females and two patients had tuberous sclerosis complex (TSC) associated LAM. The two patients with BHD were men and they also presented renal cysts. The three patients with PLCH were smokers. BHD patients and the patient with unclassified cystic lung disease had normal ventilation and diffusion parameters. Patients with LAM and PLCH exhibited a moderately reduced obstructive ventilation and diffusion capacity.

In general, all patients had normal LUS findings. In three (two with LAM and one with PLCH) patients pleural thickening was found in one zone, and observed only in the inferior lateral zones. Lung consolidation was observed in two patients. One had sonomorphologic characteristics of bilateral pneumonia, and the other had uncharacteristic findings. HRCTs prior to individual examination date were performed between 2005–2015. HRCT findings consistent with interstitial syndrome (IS) and pleural thickening showed low scores coherent with no or almost no observation of these findings. High mean scores for GGO were present in nine patients (75%) and in only two of the patients’ HRCTs were no GGOs observed.

In summary, this is the first study to investigate the applicability of LUS in a population of patients with rare cystic lung diseases. IS and pleural thickening, which are characteristic LUS findings in other subtypes of DPLD, were not significantly present in these patients with rare cystic lung diseases. Despite severe cystic formation on HRCTs (showed by the high GGO scores) the LUS results were normal. One possible explanation for the contradicting HRCT-GGO and LUS-IS findings is a possible time-bias, since recently performed HRCTs were not available. However, the chronic nature of these cystic lung diseases reduces the impact of time bias, and it is unlikely that the results of the study would have changed if more recent HRCT examinations were available. The applicability and diagnostic value of LUS in patients with rare cystic lung diseases such as LAM, PLCH, and BHD seems limited as normal LUS findings did not rule out severe cystic lung disease.

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