Six-minute walk test among silicosis patients: Its correlation with abnormal radiographic findings on chest radiograph and high resolution computed tomography scan

INTRODUCTION

Silicosis is an age-old pneumoconiosis that occurs following inhalation, retention, and subsequent pulmonary reaction to crystalline form of silica.¹ The “Chronic silicosis” is the most common type of silicosis that occurs following low silica dust exposure for more than 10 years. The “acute silicosis” develops following high silica concentrations exposure, usually over a period of weeks or months; on the other hand, the “accelerated silicosis” mostly develops about 5 to 10 years of high dust exposure to silica.² As per the recent International Labour Organization (ILO), guidelines, the standard method of silicosis diagnosis is X-ray chest studies and occupational history of exposure to the crystalline form of silica.³

Pulmonary function tests are not considered the standard diagnosis modality for this disease, however, these are commonly utilized for longitudinal studies among workers having silicosis.^4,5 The lung volumes measured on spirometry and “diffusing capacity for carbon monoxide” (DLCO) values are reduced among silicosis patients according to the extent of underlying pulmonary fibrosis.⁶ Six-minute walk test (6-MWT) is a commonly utilized tool to assess exercise capacity that essentially measures the distance walked by an individual over 6 minutes.⁷ This is cost-effective, easy to be performed by subjects, and indicates the functional assessment of patients therefore accepted as a useful and reproducible test specially for the “chronic respiratory disorders.”^8,9 This test helps to determine supplemental oxygen requirements among chronic pulmonary illnesses such as “chronic obstructive airway disease,” “idiopathic pulmonary fibrosis,” etc. It also serves as an important prognostic tool to predict morbidity and mortality in such conditions. This is now an important assessment parameter for lung transplantation too.^10,11

This study was an attempt to analyze the 6-MWT performance among subjects with silicosis and further to explore the correlation if any, between its parameters with radiologic and computed tomographic examination abnormalities.

MATERIAL & METHODS

RESULTS

One hundred twelve patients were found eligible as per the protocol and constituted the final study population. They were mainly between the third and fifth decades, with males constituting the most (male 94.6%, female 5.4%). More than one-third of subjects (37.5%; n = 42) were non-smokers, whereas 32.1% (n = 36) were ex-smokers, and 30.4% (n = 34) were current smokers. Most of these patients were engaged with stone cutting work (35.71%) followed by stone grinding (16.1%), well digging (15.17%), stone drilling (14.28%), stone slabs breaking (8.9%), stone fine cutting with chisel & hammer (0.89%), supervisory work at stone powder factory (0.89%) and stone powder packaging (0.89%).

The mean values among 6-MWT indicators, such as 6-Minute Walk Distance (6-MWD), fall in SpO₂ and Borg Rating of Perceived Exertion (BORG) rating changes, were 243.89 ± 108.94 (range 60–410) meter; 3.67 ± 2.64% (range 0%–10%) and 2.28 ± 1.57 (range 0.5–5.5) respectively [Table 1].

Chest X-ray grading was according to ILO categories in which distribution of study participants for small opacities was as follows: 28 (25%) in category “0”; 22 (19.69%) in category “1”; 22 (19.64%) category “2,” and 28 (25%) in category “3.” Large opacities were seen only in 28 patients; of them, type “A” opacity was seen in 10 (8.9%) patients, while type “B” and type “C” opacities were seen in 6 (5.3%) and 12 (10.7%) patients, respectively. As per the abnormal HRCT findings, the computed tomography (CT) grading pattern among the study subjects was as follows: 12 (10.71%) in category “0”; 16 (14.28%) in category “I”; 28 (25%) in category “II”; 24 (21.42%) in category “III,” and 32 (28.57%) in category “IV.” The emphysema findings were also notable in 32 (28.57%) individuals with different gradings on HRCT [Tables 2 and 3].

The 6-MWD, fall in SpO₂ and BORG dyspnea scale while performing 6-MWT in patients having grade “0” on chest X-ray was 362.79 ± 34.2 meters, 1.28 ± 0.91% and 0.96 ± 0.59 respectively and these values changed a lot with increasing ILO silicosis grading to an extent of 94.21 ± 29.4 meter, 7.11 ± 1.61% and 4.50 ± 0.93 in those individuals having ILO grading “C” with a clear difference in 6-MWD by 268 m, fall in SpO₂ by 5.83% and BORG dyspnea scale by 3.54; statistically these changes were highly significant (p < 0.01) [Table 4].

The 6-MWD, fall in SpO₂, and BORG dyspnea scale while performing 6-MWT in patients having grade “0” on HRCT chest was 370.33 ± 32.91 m, 1 ± 1.09% and 0.75 ± 0.61, respectively and this also drastically fall with increasing HRCT silicosis grading to 131.00 ± 65.51 m, 6.60 ± 1.56% and 3.97 ± 1.27 in those with grade IV of HRCT abnormality with a change in 6-MWD to 239.33 m, fall in SpO₂ to 5.60% and BORG dyspnea scale to 3.22. Changes in these parameters were statistically significant (p < 0.01). Parameters of 6-MWT significantly changed from patients having “progressive massive fibrosis” (PMF) to those with “simple silicosis” as per the ILO from grade 0 to III and IV (p value < 0.01) [Table 5].

This study also analyzed correlations between 6-MWT and different scores observed with abnormal findings on high-resolution computed tomography and chest skiagram. Distance covered during 6-MWT by patient was found to be inversely proportionate to the chest X-ray grading (r = –0.82) (p < 0.000) as well as HRCT chest grading (r = –0.87) (p < 0.001) while O₂ desaturation and change in BORG rating were directly proportionate to chest X-ray (r = 0.78; p < 0.000 and r = 0.71; p < 0.000, respectively) as well as HRCT chest grading abnormality according to the profusion (r = 0.93; p < 0.000 and r = 0.7; p < 0.000) [Table 6].

DISCUSSION

The “simple silicosis” (category 0, 1, 2) was not observed to produce significant impairment in pulmonary functions in this study; however, this was evident among those having advanced simple silicosis (category 3). At the end of 6-MWT, Izabela Cozza et al.¹³ noticed a significant rise in “BORG rating perceived exertion scale” (Borg scale) as well as a drop in SpO₂ by 5% (n = 18, 28%; p < 0.01), however, 6-MWT was not correlated with larger opacities in radiological images. In a series of 83 coal workers’ pneumoconiosis, a good correlation of the 6-MWT to lung function was found by Noh,¹⁴ but its significant correlation with abnormal radiological grading in silicosis was lacking (p-value = 0.706). These observations are different from our study and may be explained by the variable selection criteria of study subjects. Deepak et al.¹⁵ observed a gradual decrease in distance walked with increased dyspnea rating and fall in SpO₂ when compared to abnormal radiographic grading on ILO classification. A mean difference of 188 meters in 6-MWD was seen among patients with grade “C” i.e., “pulmonary massive fibrosis.” Blanco Pérez et al.¹⁶ also observed abnormal 6-MWT in patients having complicated silicosis with grade “B” or “C” abnormality, and this was also associated with an increase in desaturation and a decrease in distance of walking during the test. Karhate Andaloussi et al.¹⁷ also observed abnormal 6-MWT among 147 patients with silicosis with a statistically significant correlation between dyspnea, quality of life, anxiety level, and distance walked. The overall observations in most studies are similar to those with the present study and reflects compromised lung functions and poor functional reserve in patients having extensive radiological lesions.

There is scantly literature addressing the relationship between the profusion of parenchymal opacities detected on “HRCT” and 6-MWT. While correlating the HRCT abnormalities of silicosis patients with 6-MWT, we observed significant changes in 6-MWT parameters among patients having “pulmonary massive fibrosis” (PMF) to “simple silicosis” with HRCT grade 0, I, II, and III. Further, increased profusion on HRCT was significantly associated with increased dyspnea rating with a fall in SpO₂ that reflects the increased degree of parenchymal destruction to be associated with increased grading on HRCT.

There is a paucity of data in radiology on a description of the computed tomography abnormalities in silicosis patients.¹⁸ Computed tomography is definitely superior to standard chest radiographs as far as early detection of pulmonary nodules and interstitial changes in silicosis patients is concerned. Thin CT slices in the “trans-axial plane” decrease the superimposition of parenchymal structures and permits more clearer visualization of the distribution and severity of parenchymal abnormalities.¹⁹

The main strength of this study lies in conducting 6-MWT among fairly good number of silicosis patients and correlating 6-MWT parameters with various grades of HRCT abnormality first time in literature. However, there were a few limitations also that include a lack of inclusion of patients with baseline SpO₂ < 88% with supplement oxygen to perform 6-MWT, which could have further increased the sample size. We also could not compare 6-MWT parameters among smokers versus nonsmokers.

This is to be noted that restriction in 6-MWT may also occur due to other co-founding factors like muscular strength, nutritional status of a patient, and ability to comprehend instructions etc. and this part was also not addressed in this study. But despite such limitations, this study suggests that compromised lung function in the form of decreased exercise capacity, decreased oxygen saturation, and increased BORG rating, dyspnea score, etc. correlates well with the extent of radiological abnormalities.

CONCLUSION

6-MWT is a simple and non- invasive investigation, this can be used as a complementary test to evaluate disease severity and monitor disease progression in patients having silicosis at frequent intervals during follow-up visits. It could also be utilized as a prognostic factor to predict functional status, quality of life and need for hospitalization and rehabilitation among such patients.