Patients
For the purposes of this study, patients referring to a third level centre for the diagnosis and treatment of RA were consecutively included from June 2018 to January 2020. Patients with RA diagnosed in accordance with the 2010 American College of Rheumatology/European League Against Rheumatism criteria were included [11]. Alongside the diagnosis of RA, additional inclusion criteria defining a category of patients at risk of RA-ILD have been added. In detail, the risk of ILD was defined by clinical criteria, i.e. the presence of dyspnea and/or velcro sounds, instrumental criteria, i.e. the presence of suggestive abnormalities in the chest X-ray and/or a reduction in carbon monoxide diffusion lung capacity (DLco), or the presence of at least two risk factors for RA-ILD, i.e. smoking habit, male sex, advanced age (>65 years), presence of anti-citrullinated peptide antibodies (ACPA). None of the enrolled patients had a previous diagnosis of RA-ILD. Patients with a positive history of fibrosing lung diseases other than suspected RA-ILD, severe congenital or acquired thoracic deformities, previous lung surgery, heart failure, and a recent or current history of low respiratory tract infections were excluded.
Patients at risk of RA-ILD underwent a clinical examination performed by an experienced rheumatologist (MDC), and on the same day LUS was performed by an expert rheumatologist with 10 years of experience in chest ultrasound (MT). Within two weeks of the clinical evaluation and LUS, patients underwent pulmonary function tests (PFTs) with DLco evaluation and chest HRCT.
Patients signed informed consent for the procedures performed in this study and the study protocol was approved by the local ethics committee.
Lung ultrasound examination
LUS examination was conducted by an operator who was aware of the diagnosis of RA, but blinded to the clinical (e.g. disease duration, dyspnea, serological status, velcro sounds, current therapy), chest HRCT, and PFTs data. In this study a simplified protocol including 14 lung intercostal spaces (LIS) already used for CTD-ILD was adopted [12]. This protocol, initially described by our group [12], was also used by other researchers to assess pulmonary involvement in patients with SSc [13]. Simplified LUS protocols, i.e. with a reduced number of evaluated LIS, have also been shown to correlate with HRCT scores for idiopathic pulmonary fibrosis [14].Starting from the posterior thoracic region, the 14 LIS studied were the eighth in the paravertebral, subscapular, and posterior axillary lines, the fourth in the middle axillary, anterior axillary, and hemiclavicular lines, and the second in the parasternal line. All the LIS were bilaterally examined through longitudinal scans. The B-lines were counted and added up. When more B-lines appeared confluent, the semi-quantitative rule already proposed was applied for which in each single space the percentage of white screen was divided by 10, so for example a 40% of white screen corresponds to 4 B-lines, 50% to 5 B-lines and so on [15]. In Figure 1 is depicted an illustrative B-line. The examination was carried out with a linear probe from 4 to 13 MHz of a MyLab Class C, Esaote S.p.A., Genoa, Italy. Chest high-resolution computed tomography interpretationChest HRCT images were acquired and evaluated through OsiriX MD 7. This software is a DICOM viewer for Mac operating systems. Using this computer-aided method (CaM), for each chest HRCT a semi-automatic lung segmentation was obtained. First of all it was calculated in total lung volume, then a second lung volume was calculated by applying -700 Hounsfield units (HU) as cut-off for the presence of normal lung tissue (the attenuation between -800 and -900 HU is that of the normal lung parenchyma, while values between -500 and -700 are considered in the ILD range). It was therefore possible to calculate the percentage of pulmonary parenchyma affected by ILD. This method has demonstrated good reliability in the evaluation of SSc-ILD [6, 16], and it is also valid for RA-ILD [3].
At this point, the cut-off of the percentage of fibrosis indicative of significant RA-ILD was deduced from a previously published case history in which the chest HRCT of patients with RA was evaluated using both a conventional visual method (Warrick’s score) and the CaM [3]. The definition of significant RA-ILD has therefore been exclusively related to the chest HRCT findings. Briefly describing the results of that work, it was found that 29 out of 151 patients (19.2%) showed a significant RA-ILD estimated through Warrick's score. Comparing Warrick’s score to CaM for the detection of a significant RA-ILD, the analysis of the receiver operating characteristic (ROC) curve showed that a significant RA-ILD measured by the Warrick’s score (Warrick’s score >7 applied as dichotomous criterion) [17], corresponds to 10.7% (Youden index) at the CaM. This cut-off identifies a significant RA-ILD with a sensitivity of 96.55%, a specificity of 80.33%, and a positive likelihood ratio (LR+) of 4.91 (Supplementary material). In the present study, reference was therefore made to percentages above 10.7% as indicative of a significant RA-ILD at chest HRCT.
Statistical analysis
Data are presented as mean and standard deviation (SD) and as median and interquartile range. First of all, a correlation analysis (Pearson’s rank correlation test) was made between the number of B-lines, the percentage of fibrosis evaluated with the CaM method at HRCT, and the PFTs measurements, specifically the first second forced expiratory volume (FEV1), forced vital capacity (FVC), and DLco.
Therefore, in order to answer the main objective of the study, i.e. to establish the number of B-lines at the LUS that identifies the presence of significant RA-ILD, an analysis of the receiver operating characteristic (ROC) curve was conducted, applying as a dichotomous criterion the presence of 10.7% of fibrosis evaluated with the CaM at the chest HRCT. The area under the ROC curve (AUC-ROC) was evaluated considering that an AUC-ROC between 0.50 and 0.70 has a poor accuracy, between 0-70 and 0.90 is considered "useful for some purposes”, values above 0.90 identify a high accuracy [18]. The cut-off of the number of B-lines for significant RA-ILD was evaluated in the Youden index.
In addition, through one-way analysis of variance (ANOVA), the number of B-lines in relation to gender and in relation to current therapy were compared (grouping patients treated with csDMARDs vs those treated with bDMARDs or tsDMARDs). Statistical analyses were performed using MedCalc 18.0.0.