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(Chest. 2004;125:737-743.)
© 2004 American College of Chest Physicians

Pneumoconiosis in Rush Mat Workers Exposed to Clay Dye "Sendo" Dust*

Clinical, Radiologic, and Histopathologic Features in Seven Patients

Kiminori Fujimoto, MD, PhD; Nestor L. Müller, MD, PhD; Seiya Kato, MD, PhD; Hiroshi Terasaki, MD; Junko Sadohara, MD; Toru Rikimaru, MD, PhD and Naofumi Hayabuchi, MD, PhD

* From the Departments of Radiology (Drs. Fujimoto, Terasaki, Sadohara, and Hayabuchi), Pathology (Dr. Kato), the First Internal Medicine (Dr. Rikimaru), Kurume University School of Medicine, Kurume, Japan; and the Department of Radiology, Vancouver General Hospital and the University of British Columbia (Dr. Müller), Vancouver, BC, Canada.

Correspondence to: Kiminori Fujimoto, MD, PhD, Department of Radiology, Kurume University School of Medicine, 67 Asahi-machi, Kurume, 830-0011, Japan; e-mail: kimichan{at}med.kurume-u.ac.jp


   Abstract
TOP
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Study objectives: The purpose of this study was to describe the clinical, chest radiographic, high-resolution CT, and histopathologic features of clay dye "sendo" dust pneumoconiosis in seven rush mat ("tatami") workers.

Patients: Seven patients with a history of occupational exposure to sendo dust and radiographic changes suggestive of pneumoconiosis were retrospectively reviewed.

Results: The duration of exposure ranged from 15 to 45 years (median, 30 years). Three patients had cough, and four patients had abnormal pulmonary function test results. Chest radiographs showed nodular opacities < 3 mm in diameter (types p and q) in all patients. The standard International Labor Office profusion score ranged from 0/1 to 1/1 (median, 1/0). High-resolution CT demonstrated small nodular opacities (types p and q) in all seven patients. In four patients, high-resolution CT demonstrated branching centrilobular structures, airway ectasia, airway wall thickening, and emphysematous changes. None of the patients had conglomerate nodules, large opacities, honeycombing, pleural effusion, or lymphadenopathy. Microscopic examination of the specimens obtained by open lung biopsy or transbronchial lung biopsy revealed nodular fibrosis with accumulation of dust-laden macrophages, but no silicotic nodules. Needle-like particles of 1 to 20 µm in length were evident among the dust deposits, and birefringent crystals were identified under polarizing microscopy. Four of seven patients showed intra-alveolar fibroblastic foci similar to Masson bodies, accompanied by dust deposition.

Conclusion: Rush mat workers’ sendo dust pneumoconiosis is caused by dust containing free silica. The radiographic and high-resolution CT findings consist of small nodular opacities < 3 mm in diameter and bronchial and bronchiolar abnormalities.

Key Words: clay • CT • dyes • lung • occupational diseases • pathology • pneumoconiosis • silicosis • thoracic radiography


   Introduction
TOP
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Tatami or "goza," a Japanese mat produced from "igusa" (rush), is one of the most important farm products in Japan. In recent years, igusa growth and tatami production has been extended to several other countries in Southeast Asia. In the process of tatami mat production, igusa farmers are exposed to a clay dye dust known as "sendo" dust, by using it to prevent fading of the tatami mat color and to increase its strength.1 2 3 Sendo dust contains 20 to 30% of free silica and therefore is a potential cause of pneumoconiosis.1 2 3

The CT features of various pneumoconioses, such as those resulting from inhalation of asbestos,4 5 6 silica,7 8 9 10 11 12 talc,13 hard metal, aluminum, and graphite dusts,14 have been well described. There have been some reports on pneumoconiosis in rush mat workers in the clinical Japanese-language literature.1 2 3 However, to our knowledge there has been no description of the high-resolution CT findings in the English-language literature. The purpose of this study was to describe the clinical, chest radiographic, high-resolution CT, and histopathologic features of sendo-dust pneumoconiosis.


   Materials and Methods
TOP
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Patients
Seven tatami mat workers were reviewed retrospectively. All workers were referred to our hospital because abnormal opacities were identified on chest radiography. All of them were engaged in the production of tatami mats, were exposed to sendo dust throughout all seasons of the year, and were suspected clinically as having pneumoconiosis due to sendo dust. None of the patients had significant other medical histories. On reviewing the patient histories, there was no evidence of other exposure, other chronic lung diseases such as sarcoidosis, or other active process such as metastases. There were three male and four female patients (median age, 64 years; range, 54 to 75 years). The clinical assessment included recording the chief symptom, duration of exposure of sendo dust, smoking history, and dyspnea score. The dyspnea score ranged from 0 to 4: 0 = no dyspnea, 1 = dyspnea with strenuous exertion (ie, three flights of stairs), 2 = dyspnea with mild exertion (ie, one flight of stairs), 3 = dyspnea with minimal activity (ie, walking 20 to 50 feet), and 4 = dyspnea at rest or while eating or talking.15 Pulmonary function tests, arterial blood gas analysis, sputum analysis, chest radiographs, high-resolution CT scans, and lung biopsies were performed in all patients. All clinical and radiologic examinations and biopsy in each patient were done within 2 weeks (median, 9 days; range, 7 to 14 days). The hospital human research committee approved this study, and informed consent was obtained from each patient.

Pulmonary Function Tests
The pulmonary function tests were performed using a rolling-seal spirometer (Chestac-55V; Chest; Tokyo, Japan) within 2 weeks (median, 3 days) prior to high-resolution CT. At least three reproducible measurements were performed on all patients, and the best of the three curves was selected. Total lung capacity (TLC), vital capacity (VC), FVC, FEV1, maximal midexpiratory flow (MMF), forced expiratory flow rate at 25% of VC (FEF25), and residual volume (RV) were obtained in all patients. Single-breath diffusing capacity for carbon monoxide (DLCO), and alveolar volume (VA) were obtained in five patients. The results of VC, MMF, and DLCO were calculated as percentage of values predicted for age, sex, and height.16 17 18 FEV1/FVC, RV/TLC, FEF25/height, and DLCO/VA ratios were also calculated from these values.

Chest Radiography and High-Resolution CT Scanning
Chest radiographs were obtained using the standard technique, posteroanterior projection, 120 kilovolt peak, and 12:1 grid. The CT scans were obtained on a ProSeed scanner (Yokogawa Medical System; Tokyo, Japan) and a X-Force scanner (Toshiba; Tokyo, Japan). High-resolution scans with 1-mm (five patients) or 2-mm (two patients) collimation were obtained at 10-mm intervals through the chest. The scanning parameters included 120 kilovolt peak, 200 mA, and 1-s scan time. All images were obtained with the patient in supine position during breath-holding at end-inspiration. The scans were reconstructed using a high-spatial-resolution algorithm. The scans were viewed at window levels appropriate for parenchyma (mean, - 750 to - 650 Hounsfield units [HU]; width, 1,200 to 1,500 HU); and mediastinum (mean, 30 to 50 HU; width, 400 to 500 HU).

Analyses of Chest Radiographs and High-Resolution CT Scans
Two chest radiologists analyzed the chest radiographs and high-resolution CT scans retrospectively and reached a final decision on the findings by consensus. The chest radiographs were analyzed for patterns of opacities according to International Labor Office (ILO) classification of radiographs of pneumoconiosis.19 In addition, the lungs were divided into six zones (upper, middle, and lower zones of the right and left lungs), and each zone was graded for severity of disease based on the ILO grading system. The six independently assessed lung-zone profusion scores were converted to a 12-point scale, and an average profusion score was calculated by averaging these scores.15

The high-resolution CT scans were assessed for the presence or absence, extent, zonal predominance, and predominant location of the various parenchymal abnormalities. The findings of high-resolution CT scans were defined according to standard criteria.20 21 The extent of each CT finding was assessed based on the number of bronchopulmonary segments involved as follows: + = less than five segments, 2+ = five to nine segments, 3+ = more than 9 segments. The zonal predominance was assessed as being upper, lower, or random. Upper or lower predominance was considered present when the majority of the abnormal findings were above or below the level of the tracheal carina. The predominant location of each finding was also evaluated as being peripheral, central, or equal for each lung zone. Peripheral distribution was considered present if there was a predominance of findings in the outer one third of the lung parenchyma, central if there was predominance in the inner two thirds of the lung parenchyma, and equal if there was no predominance. The correlation between pulmonary function tests and the extent of each high-resolution CT finding was analyzed using Spearman rank correlation.

Analysis of Histopathologic Findings
All histopathologic examinations were reviewed by an experienced lung pathologist. In six patients, specimens were obtained by transbronchial biopsy and in one patient by open lung biopsy within 2 weeks (median, 5 days) after CT. All pathologic specimens were fixed with 10% formalin, embedded in paraffin wax, and stained with hematoxylin-eosin (HE), azan, or elastica-van Gieson methods for conventional microscopy. Polarizing microscopy was performed using a polarizing plate (Nikon; Tokyo, Japan), and images were obtained through a digital microscope camera (Polaroid PPMC Iii; Polaroid Japan; Tokyo, Japan).


   Results
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Patient characteristics and high-resolution CT and histologic findings are summarized in Tables 1 2 3 , respectively.


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  		Table 1.. Patient Information*

 

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  		Table 2.. Relation of High-Resolution CT Findings With Extent and Distribution*

 

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  		Table 3.. Histopathologic Findings*

 
Clinical Data
All of the patients had > 15 years of exposure to sendo dust (median, 30 years; range, 15 to 45 years). None of them had significant other medical histories. One patient, a 75-year-old man, had a smoking history (current smoker; one pack per day for 50 years). Three of the patients had productive cough, and one patient had breathlessness on exertion. The other three patients were asymptomatic. The dyspnea score ranged from 0 to 2 (score 0 in three patients, score 1 in two patients, and score 2 in two patients). In two patients with productive cough and dyspnea score 2, sputum cultures revealed Haemophilus influenzae.

Pulmonary Function Tests
Six of the seven patients had a decrease in FEF25, and four patients had a decrease in the MMF consistent with airway obstruction involving mainly the small airways. Four patients had increased RV/TLC ratio indicative of air trapping. DLCO/VA was decreased in three of the five patients in whom it was obtained. Blood gas analysis revealed slight hypoxemia in these three patients.

Chest Radiographs
Chest radiographs showed bilateral nodular opacities < 3 mm in diameter (types p and q) randomly distributed throughout the lungs (Fig 1 , top, A). The standard ILO profusion scores ranged from 0/1 to 1/1 (median, 1/0). The average profusion scores obtained by averaging the profusion scores of the six lung zones ranged from 0/0 to 1/1 (median, 0/1). Thickening of bronchovascular bundles was detected in three patients. None of the patients had large nodules, conglomerate nodules, honeycombing, lymphadenopathy, or pleural effusion.



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  		Figure 1.. Patient 6, a 63-year-old woman with 35 years of sendo-dust exposure. Top, A: Chest radiograph shows small nodular opacities < 3 mm in diameter in the periphery of bilateral lungs. The ILO profusion score was 1/0. Bottom, B: High-resolution CT images reveal centrilobular small nodular opacities < 3 mm in diameter (arrowheads) and branching structures (thin arrows). Also noted are small areas of decreased attenuation and vascularity (air trapping is shown by thick arrow).

 
High-Resolution CT Scans
On high-resolution CT, all patients had nodular opacities < 3 mm in diameter (types p and q) in a predominately centrilobular distribution (Figs 1 , bottom, B, 2 ). The nodules had an equal distribution in the upper and lower lung zones, peripheral, and central lung regions. Only one patient had three large nodular opacities (>= 10 mm) that showed smooth round area of attenuation without parenchymal distortion. Focal areas of decreased lung attenuation consistent with air trapping was observed in all patients: extensive in two patients, moderate in two patients, and mild in two patients. Six patients had centrilobular branching structures, and four of these six patients had centrilobular small nodular opacities, airway ectasia, airway wall thickening, and emphysematous changes in the lung parenchyma (Fig 2) . Small areas of ground-glass attenuation and small areas of airspace consolidation were detected in three patients. Two of these three patients had positive sputum culture findings for H influenzae. Pleural irregularities were detected in three patients. None of them had honeycombing, pleural and/or pericardial effusion, or lymphadenopathy.



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  		Figure 2.. Patient 1, a 66-year-old woman with 40 years of sendo-dust exposure. High-resolution CT images reveal bronchiectasis and bronchial wall thickening (thin arrows) in the middle lobe and lingula. Also noted are centrilobular small nodular opacities with branching structures (arrowheads) particularly in the lower lobes.

 
There was good correlation between extent of abnormalities evident on high-resolution CT and functional impairment. There was a positive correlation between the overall extent of areas, with decrease in attenuation and the RV/TLC ratio (Spearman r = 0.902, p < 0.05), a negative correlation between the extent of centrilobular branching structures and the MMF (r = - 0.866, p < 0.05), and a negative correlation between extent of airway ectasia and airway wall thickening with MMF (r = - 0.92, p < 0.05, and r = - 0.866, p < 0.05, respectively).

Histopathologic Findings
Microscopic examination demonstrated that nodular fibrotic foci containing abundant dust-laden macrophages were present around the bronchiolovascular structures distal to the terminal bronchioles and extended into adjacent pulmonary parenchyma. Fibrosis of the adjacent interstitium and the thickening of the alveolar walls replaced normal structures resulting in the collapse of the air spaces; however, typical silicotic nodules consisting of hyaline collagen arranged in a whorled pattern were not identified (Fig 3 , top left, A). Four of seven patients showed intra-alveolar fibrotic foci similar to Masson bodies, accompanied by dust deposition (Fig 3 , bottom left, B). Needle-like particles 1 to 20 µm in length were visible under conventional microscopy, and birefringent crystals were evident among the dust deposits under polarizing microscopy (Fig 3 , top right, C, and bottom right, D).



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  		Figure 3.. Histopathologic findings of sendo-dust pneumoconiosis. Top left, A: Patchy interstitial fibrosis with dust deposition (asterisk) associated with emphysema is detected (patient 5, by transbronchial biopsy; HE, original x 100). Bottom left, B: Representative example of intra-alveolar fibrotic foci similar to Masson bodies (asterisks) is shown (patient 1, by open lung biopsy; HE, original x 200). Top right, C: Needle-like particles (arrows) 5 to 20 µm in diameter are present among the dust-cell depositions, and some of them (bottom right, D) [arrows] show birefringence under polarizing microscopy (patient 1, top right, C, HE; bottom right, D, same area with a polarizing filter, bar = 50 µm).

 

   Discussion
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Sendo-dust pneumoconiosis is a pneumoconiosis caused by dust containing free silica in a very low concentration and is seen in igusa mat (tatami) workers. It has been found that the workers tend to acquire a relatively early stage of pneumoconiosis after approximately 20 years of dust exposure. In the Japanese-language literature, it has been estimated that the incidence of pneumoconiosis in igusa mat workers is > 30%.1 2

Igusa is a type of rush used in the manufacture of tatami mats. It takes approximately 1 month for the harvesting process of igusa and 8 to 10 months for the weaving process in producing tatami mat. The harvesting process includes reaping, dyeing, drying, carrying, and storing the igusa. Igusa is dyed into mud using sendo (a type of clay dye) in order to prevent fading of the color and to provide strength. The dyed igusa is dried and carried into the storehouse. It has been shown that in the process of drying and storing, approximately 70 to 80% of particles of floating dusts measure < 5 µm in diameter.1 In this process, the workers are exposed to dense concentrations of sendo dust (50 to 100 mg/m3) for 1 to 3 h/d.1 The stores of igusa are weaved into tatami mat domestically throughout the year. The workers are continually exposed to low-density (1 to 5 mg/m3) sendo dust during the weaving process and relatively dense concentrated dust (15 to 25 mg/m3) in the tatami storing process.1 The sendo dust is clay dust mixed with quartz, chlorite, kaolin, sericite, and other minerals.3 The dust contains 20 to 30% of free silica.

The upper respiratory tract is a remarkably efficient filter, removing upwards of 90% of particles > 7 µm in diameter.22 Smaller particles may remain in suspension and be exhaled or may be deposited on the respiratory epithelium.23 The dust accumulates predominately in the respiratory bronchioles and alveolar ducts.

In the present study, the patients were asymptomatic or had mild symptoms. Two patients with productive cough and dyspnea score 2 showed chronic infection of the lower respiratory tract with H influenzae. These two patients had no consolidation and no apparent change in the radiographic findings over the previous 1 year. Pulmonary function revealed mild functional impairment consistent with small airway obstruction and mild air trapping.

The radiographic findings have been reported as consisting of small, rounded type-p and irregular type-s opacities, the latter type being generally dominant involving mainly the middle and lower lung zones.2 Large opacities, lymphadenopathy, and pleural abnormalities have not been reported on radiography, and marked emphysema is rare.3 In our small series, chest radiographic findings consisted of small nodular opacities randomly distributed throughout the lungs. In addition, thickening of bronchovascular bundles and bilateral basilar infiltrates such as chronic inflammation of lower respiratory tract were seen in four patients. Large opacities, reticular opacities, or hilar lymphadenopathy were not detected.

The radiographic and high-resolution CT findings of sendo-dust pneumoconiosis are distinct from those of silicosis by the absence of nodules > 3 mm in diameter (type-r opacities), progressive massive fibrosis, and enlarged or calcified lymph nodes. The high-resolution CT manifestations of sendo-dust pneumoconiosis consist mainly of bilateral pulmonary micronodular opacities, in a predominately centrilobular distribution but otherwise randomly distributed in the lungs. None of the patients had progressive massive fibrosis, lymphadenopathy, or lymph node calcification.

Kinsella et al24 suggested that silicosis, in the absence of progressive massive fibrosis, does not result in emphysema. In the present study, mild emphysema was found in two patients, one of whom was a smoker, in the absence of progressive massive fibrosis. However, focal areas of decreased lung attenuation consistent with air trapping were seen in all patients. The histologic findings of sendo-dust pneumoconiosis consist of fibrotic foci with needle-like particles in the bronchiolovascular interstitium, including the wall of respiratory bronchioles and alveolar walls with associated emphysematous change. Small airway obstruction resulting from the narrowing of the respiratory bronchioles and alveolar ducts presumably accounts for the areas of air trapping seen on high-resolution CT. Small airway obstruction and air trapping were also evident on pulmonary function testing by the presence of decreased airflows at low lung volume (FEF25 and MMF) and increased RV/TLC ratio, respectively. The extent of abnormalities on high-resolution CT correlated with the severity in functional impairment. The extent of decrease in lung attenuation correlated with the presence of air trapping as reflected by an increase in the RV/TLC ratio. High-resolution CT findings of airway disease, including centrilobular branching structures, airway ectasia, and airway wall thickening correlated with presence of airway obstruction as reflected by a reduction in MMF.

High-resolution CT finding of small areas of airspace consolidation detected in three patients presumably reflected the presence of mild bronchopneumonia, two of these patients having sputum culture findings positive for H influenzae. In sendo-dust pneumoconiosis, the prevalence of chronic infection of lower respiratory tract is higher than in other pneumoconiosis.2 The reason for this is not clear. It is possible that it is related to the small airway abnormalities seen in sendo-dust pneumoconiosis. Love et al25 described that chronic bronchitis was seen in 14% of the workforce in the heavy clay industry, and both chronic bronchitis and breathlessness were significantly related to dust exposure.

Histopathologic characteristics of typical silicosis include combined pattern of classic or incomplete silicotic nodules, interstitial fibrosis, massive fibrotic lesion, or mixed-dust fibrosis associated with emphysematous change and/or honeycombing. In contrast histopathologic characteristics of sendo-dust pneumoconiosis appear to be consistent with pulmonary damage by inhalation of the dust containing low percentages of fibrogenic silica particles. Our patients showed a nodular fibrosis arising from the bronchioloalveolar interstitium, which is further extending to the pulmonary parenchyma and resulting in focal emphysematous change or atelectasis. None of seven patients revealed typical silicotic nodules, massive fibrotic lesion, or honeycombing. Needle-like particles 5 to 20 µm in diameter were present among the dust-cell depositions. Some of them showed birefringent under polarizing microscopy, which are mainly considered to be silicates inhaled with the silica.26 27

It is not clear why workers exposed to sendo dust do not acquire typical silicotic nodules histologically and radiologically. Because the dust contains 20 to 30% silica, it seems reasonable to speculate that the different appearance is related to the lower doses and concentrations of silica as compared to the concentrations of silica in patients with typical findings of silicosis.

In conclusion, sendo-dust pneumoconiosis is a distinct form of pneumoconiosis seen in tatami workers. Although sendo contains free silica, typical silicotic nodules are not seen histologically, and progressive massive fibrosis, lymph node calcification, and lymphadenopathy are not seen on the radiograph or high-resolution CT. The radiographic and high-resolution CT findings consist of small nodular opacities < 3 mm in diameter and bronchial and bronchiolar abnormalities.


   Footnotes
 
Abbreviations: DLCO = single-breath diffusing capacity for carbon monoxide; FEF25 = forced expiratory flow rate at 25% of VC; HE = hematoxylin-eosin; HU = Hounsfield unit; ILO = International Labor Office; MMF = maximal midexpiratory flow; RV = residual volume; TLC = total lung capacity; VA = alveolar volume; VC = vital capacity

Received for publication March 7, 2003. Accepted for publication July 29, 2003.


   References
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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