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Fissural thickening and exposure to asbestos Occurrence determinants and functional impact.

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American Journal of Industrial Medicine 20785-793 (1991)
Fissural Thickening and Exposure to Asbestos:
Occurrence, Determinants, and Functional Impact
Jan Zejda, MD, and Pierre Ernst, M D
The occurrence and determinants of thickened interlobar fissures were assessed in 147
insulators, in order to test the hypothesis that radiographic abnormalities in minor
(FMN) andlor major (FMJ) interlobar fissures are associated with restrictive respiratory
impairment, which may reflect early parenchymal lung fibrosis. According to the diagnostic criteria developed, definite interlobar thickening was found in 33 (FMN) and
in 73 (FMJ) men, and was frequently associated with the presence of classical asbestos
pleural disease, though not parenchymal abnormality. Multivariate analyses did not
demonstrate any association between either FMN or FMJ and lung function. Such an
association, however, was found in the case of the usual asbestos-related pleural change.
These findings suggest that fissural thickening seen on chest radiography represents a
component of asbestos pleural disease and not asbestosis.
Key words: asbestos, chest radiography, pleural disease, lung function, ILO classification
INTRODUCTION
Benign asbestos-related pleural abnormalities may result in lung function impairment [Corris et al., 1988; Cotes and King, 1988; Jarvholm and Sanden, 1986;
Oliver et al., 1988; Rosenstock et al., 19881. A restrictive ventilatory impairment is
independently associated with roentgenographic evidence of pleural fibrosis [Bourbeau et al., 19901 although the possibility that such impairment may reflect, in part,
interstitial lung fibrosis not visible on the chest roentgenographs cannot be unequivocally ruled out [Schwartz et al., 19901.
Thickening of interlobar fissures is known to occur in workers with long duration of exposure to asbestos dust. The relationship of this abnormality to either pleural
or parenchymal fibrosis is unclear [Solomon and Webster, 1976; Solomon et al.,
1979; Solomon et al., 1979a; Rockoff et al., 19871.
Kockoff et al. [1987] proposed that fissural thickening might be a marker of
early parenchymal lung fibrosis, i.e., asbestosis. If this were true one would expect
that, on average, subjects with fissural thickening would have lower lung function
Pulmonary Research Laboratory, Department of Epidemiology and Biostatistics, and School of Occupational Health, McGill University, Montreal, Quebec, Canada.
Address reprint requests to Dr. Pierre Ernst, Pulmonary Research Laboratory, Department of Epidemiology and Biostatistics, McGill University, 1 1 10 Pine Avenue West, Montreal, Quebec H3A 1A3,
Canada.
Accepted for publication April 22, 1991.
0 1991 Wiley-Liss, Inc.
786
Zejda and Ernst
than those without, after accounting for the effect of smoking and radiologically
apparent parenchymal fibrosis and pleural thickening along the chest wall and diaphragm.
The principal objective of the present study was to examine the relationship
between fissural thickening visible on the chest radiograph and level of lung function.
Our secondary objectives were to determine the prevalence and determinants of such
fissural thickening in an asbestos-exposed workforce.
MATERIAL AND METHODS
Subjects
The study group consisted of 147 construction insulators, members of Local 58
of the International Union of Frost and Heat Insulation and Asbestos Workers. The
subjects were men, all in the trade at the time of examination. They were selected
from a total of 644 members of Local 58, if they did not receive compensation for
asbestosis, were 50 years old or less, and lived within the Montreal area. The eligible
population consisted of 246 workers, 215 agreed to participate. The satisfactory and
complete examination data were available in 147 (60%) of the eligible population.
Lung Function Testing
The assessment of lung function was based on the measurements of spirometric
indices of ventilatory capacity and pulmonary diffusing capacity. Spirometry was
performed using a Ohio 827 volume displacement spirometer. The best forced vital
capacity (FVC) and forced expiratory volume in the first second (FEV,) were obtained according to the recommendations of the American Thoracic Society [ 19791,
expressed in both absolute (measured) values and as percentage of the reference
values (9% pred.) of Crapo et al. [1981a].
Diffusing capacity (DLCO) was measured by the single breath carbon monoxide
method, using Collins testing equipment. The results were accepted for analysis if at
least two measurements were reproducible within 10%. DLCO was calculated as an
arithmetic mean of the best two reproducible measurements, and analyzed both as
absolute value and % predicted, calculated from the reference equation of Crapo and
Morris [ 19811.
Radiographic Examination
The standard posteroanterior (PA) chest radiographs were available in 163
subjects and lateral films in 158 subjects. They had been read previously by two
certified B readers, who blindly assessed parenchymal and pleural abnormalities,
according to the ILO International Classification of Radiographs of Pneumoconioses
with the standard set of radiographs used for comparison [International Labour Office, 19801. Fissural changes were assessed subsequently by two different readers
who did not know the results of the readings of parenchymal and peripheral pleural
abnormalities.
Parenchymal abnormalities were read according to the 12-point profusion scale
described in the ILO classification, and this score (1-12) was used to calculate means.
A profusion of 110 or greater was chosen to represent radiographically apparent
parenchymal fibrosis.
Asbestos-Related Fissural Thickening and Lung Function
787
Pleural abnormalities (excluding interlobar fissures) were expressed as a score
obtained by summing up the numeric scores for (1) costophrenic angle, (2) diaphragmatic pleura, and ( 3 ) chest wall pleura as described previously by BCgin and colleagues [ 19841.
Obliteration of one costophrenic angle was given 1 point, of two angles 2
points. Similarly, the thickening of diaphragmatic pleura was assigned 1 or 2 points,
depending on whether it was visible on one or both sides.
Chest wall scores were also computed for each side independently and then
added together. Each (left or right) score included the assessmentface on, expressed
as 1, 2, or 3 points for the respective extent of changes of 1, 2, or 3. This index was
added to the assessment in proJle, expressed as a product of extent category (1,2, or
3) and width category (1, 2, or 3 substituted for the original notation of a, b, or c ) .
The final score (1 2 3), with a possible range from 0 to 24 points, was used as a
linear variable for multivariate analysis. Subjects were also divided into two groups
on the basis of this score as follows: a value of 0 was accepted as category without
pleural changes versus the category with any abnormalities (score 1 or greater). No
attempt was made to distinguish between discrete pleural plaques and diffuse thickening. The validity of this score is suggested by the previously demonstrated relationship between this score and the degree of lung function impairment [Bourbeau et
al., 19901.
Thickening of interlobar pleura (fissural thickening) was assessed from the
postero-anterior and lateral chest radiographs independently for the minor (horizontal)
and major (oblique) fissures, without specifying whether the latter was located in the
left or right lung. The following scale, modified from that proposed by Rockoff et al.
[1987], was agreed upon by the authors of this paper and used throughout the readings:
+ +
0
1
2
3
4
5
6
7
fissure not visible;
normal (visible) fissure: width ‘‘hairline”;
questionable thickening: width <0.5 mm;
minimal localized thickening: width 0.5-1 .O mm; length of thickening < 1/2
total length of fissure;
minimal diffuse thickening: width 0.5-1.0 mm; length of thickening >1/2
total length of fissure;
moderate localized thickening: width >1.O mm; length of thickening <1/2
total length of thickening;
moderate diffuse thickening: width >1.O mm; length of thickening > 1/2
total length of thickening;
marked thickening.
A value of 3 or greater on the above scale was used as a cut-off value for
dichotomization of fissural changes into normal and abnormal classes. The linear
regression analysis and assessment of between-reader variability were also performed
with the above classification rescaled to 5 points instead of 8, in order to increase the
numbers of subjects in higher classes. This was achieved by grouping together the
categories of minimal localized thickening and minimal diffuse thickening and, separately, moderate localized thickening and moderate diffuse thickening.
788
Zejda and Ernst
TABLE I. Between-Reader Agreement in Assessment of Radiographic
Changes in Interlobar Fissures*
No. of fissures read as
Crude
agreement
K-
(%)
Value
0.499
Fissure
Reader
0
1
2
3
4
Minor
A
76
64
46
49
39
39
I
10
1
B
1
66.3
A
B
13
29
57
40
78
63
10
24
0
2
55,7
Major
0.355
*Fissural thickening measured according to the modified scale: 0, nonvisible fissure;
1, visible but normal fissure or questionable thickening; 2, minimal thickening; 3,
moderate thickening; 4, marked thickening. For details concerning modification of
the scale, see under Methods.
Analysis
Statistical analysis was performed using SAS software [SAS Institute, Inc.,
19881. Linear regression was used to analyze linear dependent variables and logistic
regression for categorical outcomes, both with the level of statistical significance
p<0.05. Between-reader variability was assessed by means of the crude percent
agreement and the Kappa statistic [Musch et al., 19841.
RESULTS
Between-Reader Variability in Assessing Fissural Thickening
In order to avoid arithmetical distortion of the Kappa statistics resulting from
small numbers in some, particularly higher classes, the between-reader variability
was assessed in relation to a 5-point instead of the original 8-point scale. Table I
presents the number of subjects with different degrees of fissural thickening, as
diagnosed by two readers, the crude agreement, and the Kappa values. According to
a verbal scale [Musch et al., 19841, the between-reader agreement was “fair” (major
fissures) and “good” (minor fissure).
Descriptive Statistics
The mean values and range of age, smoking status, duration of exposure, the
results of lung function testing, and radiographic status are shown in Table 11.
Univariate Analysis
The differences between subjects with and without each category of chest radiographic abnormality considered alone are shown in Table 111.
Small irregular opacities of category 1/0 or greater were found in 12 subjects.
They were significantly older than those with normal parenchymal appearance on the
chest radiograph (p=O.OOl), had more advanced pleural disease (p = 0.02), but in
terms of duration of exposure, smoking habit, fissural changes, and lung function,
subjects with small irregular opacities appeared to be no different from those without.
Classical pleural abnormalities (thickening of the chest wall pleura and/or diaphragmatic pleura and/or obliteration of the costophrenic angle) were found in 58
subjects. In comparison with those without abnormalities, the men with peripheral
pleural changes were significantly older (p = O.OOOl), had longer exposure (p = 0.01),
Asbestos-Related Fissural Thickening and Lung Function
789
TABLE 11. Age, Smoking Status, Duration of Exposure to Asbestos
and Lung Function in 147 Asbestos-Exposed Insulators
Age (years)
Amount of smoking (pack-years)
Duration of exposure (in thousand hours)
Fvc 1
% pred
FEV, 1
% pred
DLCO ml/min/mmHg
% pred
Mean
SD
Range
36.9
15.7
28.2
4.78
100.7
3.85
103.7
28.6
82.1
8.2
12.1
40.36
0.77
11.2
0.67
12.9
5.3
12.8
18-50
0-58
0.37-453.9
2.83-7.35
63.9-135.0
1.97-5.95
61.6-140.7
14.7-41.3
45.3-124.5
TABLE 111. Unadjusted Comparisons of Asbestos-Exposed Subjects According to
TvDe of Abnormalitv*
Age
Category
SIO +
SIOWALL+
WALLFMN
FMN-
+
FMJ+
FMJ-
N
12
135
58
89
33
114
73
74
(yr)
43.4
36.4
41.9
33.6
40.1
36.0
39.6
34.3
Radiography scorea
Percent predicted
Exposure
(h)
SIO
WALL
FMN
FMJ
FVC
FEV,
DLCO
38,288
27,347
-
37,139
22,410
47,514
22,486
38,405
18,010
0.4
0.2
2.5
1.o
2.3
0.8
1.6
0.7
1.5
1.5
1.9
1.2
2.7
2.7
3.3
2.3
-
-
-
-
97
101
96
103
96
101
98
102
99
104
100
105
99
105
102
104
80
82
84
80
80
83
80
84
0.2
0.3
0.3
0.2
-
-
-
* S O , small irregular opacities; WALL, thickening of chest wall pleura; FMN, thickening of minor
fissure; FMJ, thickening of major fissure.
aSIO+ if profusion 2 1/0; otherwise SIO-; WALL+ if WALL>O, otherwise WALL-; FMN+ or
FMJ if FMN>2 or FMJ>2. otherwise FMN- or FMJ-.
+
higher scores for fissural thickening (minor: p = 0.01, major: p = O.OOOl), and lower
mean FVC (p=0.0005) and FEV, (p=0.04).
In order to simplify comparisons, the score for interlobar fissures was calculated
as the arithmetic mean of the scores reported by two readers. Thickening of the minor
fissure, defined by the score of 3 or more on the 8 point scale, was observed in 33
men. They were significantly older (p = 0.009), had more advanced thickening of the
chest wall pleura (p=0.02), lower FVC (p=0.02) and FEV, (p=O.Ol) than the
remaining subjects. Thickening of the major fissure, observed in 73 subjects, was
associated with older age (p = O.OOOl), significantly longer duration of exposure
(p = 0.003), higher score for pleural disease (p = 0.01) and diminished FVC
(p=O.O49). Note that subjects with or without fissural thickening had a similar
prevalence of small irregular opacities.
Multivariate Analysis
This analysis was carried out to determine whether the lower lung function seen
in subjects with fissural changes was actually due to the associated greater prevalence
790
Zejda and Ernst
TABLE IV. Linear Regression Models With Lung Function as Dependent Variable and
Parenchymal Changes (SIO), Peripheral (WALL), and Fissural (FISS) Pleural Thickening as
Independent Variables*
Covariates
Independent variables
Age
(yr)
Height
(cm)
Smoking
(pack-years)
SIO
(score: 0-15)
WALL
(score: 0-10)
FVC
Standard error
Probability
-0.032
0.006
0.060
0.007
c
-0.019
0.046
NS
-0.059
0.020
c
-0.003
0.002
NS
FEV,
Standard error
Probability
-0.033
0.005
0.042
0.006
-0.005
0.002
c
c
a
-0.023
0.041
NS
DLCO
Standard error
Probability
-0.086
0.129
NS
0.251
0.115
-0.007
0.033
NS
-0.357
0.556
NS
Dependent
variabie
a
FISS
(score: 0-8)
Intercept
-0.019
0.019
NS
-4.103
1.300
-0.044
0.018
-0.015
0.017
NS
- 1.940
1.155
NS
-0.325
0.262
NS
-0.418
0.279
NS
-8.702
20.717
NS
b
b
*NS, not significant.
"p<0.05.
hp<O.Ol.
'p<O.OOl.
of pleural plaques and costophrenic angle blunting in these subjects. The contribution
of fissural thickening to the observed FEV,, FVC, and DLCO was examined by
multiple regression analysis after adjusting for age, height, smoking in pack-years,
the grade of small irregular opacities and the degree of pleural thickening. Duration
of exposure in the insulating trade was not retained in the models since it did not
contribute significantly and its removal did not affect the regression coefficients for
the other variables. In the models shown in Table IV, fissural thickening (FISS)
represents the sum of the scores for thickening of the major (RMJ) and minor (RMN)
fissures. There was no significant relationship between any of the lung function
parameters examined and thickening of fissures whether these were considered together or separately (the latter models are not shown). In the models shown, the mean
of the scores obtained from the two readers is utilized. The results were similar using
either score alone. The only category of radiographic abnormality significantly associated with a decrement in lung function was peripheral pleural thickening which
caused a decrease in FEV, and FVC but not DLCO.
In order to avoid the masking of an effect by inclusion of nonsignificant fissural
changes, the analyses were repeated in subjects with a more obvious thickening of
minor fissure and the major fissure (score 3 or more) for each fissure. In neither
subgroup did any radiographic variables other than peripheral pleural thickening
significantly contribute to the regression model after controlling for age, height and
smoking.
DISCUSSION
In healthy individuals, interlobar fissures are either not visible on standard chest
radiographs or are visible but of hairline thickness [Fraser et al., 19881. The scale
designed by Rockoff et al. [1987], which was adopted for the purpose of this study
used this definition to distinguish normal from thickened interlobar fissures. The
Asbestos-Related Fissural Thickening and Lung Function
791
between-reader agreement was better in relation to minor than to major fissures, This
can be attributed to the greater difficulty in identifying and quantifying abnormality
of the major fissures that may be superimposed as well as have varying degrees of
thickening along their considerable length, which is difficult to describe by a summary score. The degree of agreement (crude 55.7-66.3%; K-value 0.355-0.499) is
difficult to interpret, since there are no data for comparison, except for the crude
agreement of dichotomous classification of changes in the minor fissure (36-46%)
reported by Solomon et al. [1979]. The obtained level of agreement, however, compares favorably with between-reader variability obtained in assessing the classic signs
of asbestos pleural disease [Solomon et al. , 1979; Bourbeau and Ernst, 1988; Robins
and Green, 19881.
The complete validation of the scale would also require the measurement of
within-reader variability and comparison of the results obtained by different teams
using the same diagnostic criteria. This is, however, impossible without standardized
radiographs demonstrating various degrees of fissural thickening. On the other hand,
the assessment of classical pleural changes, characterized by similar variability, has
been widely used to investigate the impact of asbsetos pleural disease on lung function. Measurement of thickening of only 0.5 mm on the chest radiograph may be
difficult. However, the scale used provided significant correlations between fissural
and peripheral changes read by two different readers on different occasions and
correlated with lung function. Therefore, the preliminary results concerning the diagnostic feasibility of assessing thickenings of interlobar fissures seems to justify
addressing the main objectives of the study.
Thickened minor fissures were reported in 25% (reader A) or 30.7% (reader B)
of subjects. The respective prevalences for major fissures were 53.9% and 54.6%.
Since both fissures were scored using the same numerical scale it cannot be excluded
that higher numbers of definite changes in major fissures resulted from the magnification factor inherent in a lateral chest projection.
Using a similar scale, Rockoff et al. [1987] found definite thickening in 54.5%
of 220 active and retired asbestos workers. In a large epidemiologic study reported by
Solomon et al. [1979] in 1,962 asbestos miners, thickening of the minor fissure, as
assessed from the PA chest radiograph, was present in 5.1% to 9.4%, depending on
the reader. The chest radiographs were read against a “standard” radiograph, showing evident thickening of the minor interlobar fissure. The clearly lower prevalence
in asbestos miners in comparison with the results of our study or those of Rockoff et
al. [1987] is likely attributed to the different definitions of fissural thickening employed in the studies. On the other hand, it could be due to differences in occupational
exposure. The prevalence of classical pleural abnormalities varies from 1.5% in
asbestos miners [Enarson et al., 19881, 14.6% in textile workers [Berry et al., 19791
to 17.5-29.4% in plumbers [Rosenstock et al., 19881, and 43.7% in insulators [Lilis
et al., 19861.
The similarity of the prevalences reported by Rockoff et al. and ourselves must
be interpreted with caution, given the different age range of the two populations (our
subjects were younger) and their report of a prevalence for both fissures combined
only. It does appear, however, that thickening of interlobar fissures is not infrequently observed in asbestos-exposed subjects, particularly thickening of the major
fissure which, when assessed from both PA and lateral chest radiographs, may occur
in up to 50% of currently employed subjects.
792
Zejda and Ernst
Subjects with thickening of interlobar pleura were on average older and had
been exposed for approximately twice as long as those without fissural thickening.
The two groups did not differ in terms of the presence of parenchymal abnormalities.
Subjects with fissural thickening also had more chest wall pleural abnormalities (and
vice versa), and these had similar determinants. The close association between interlobar and peripheral thickening of the pleura suggests that they belong to the same
category of asbestos pleural disease. That fissural thickening can occur without radiographic changes in the lung parenchyma has been known since the first case
reports describing this abnormality in asbestos-exposed subjects [Solomon and Webster, 19761 and was confirmed in the previously cited epidemiologic study in asbestos
miners and the case series in asbestos workers. Thus, the present study, as well as the
little information available from the literature, suggest that the thickening of interlobar pleura can develop independent of radiographic signs of asbestosis and seems to
be as readily detected as the classical appearance of asbestos pleural disease. Computed tomography (CT) scanning could add useful information about the nature of
these fissural changes.
Fissural thickening has been proposed as an early manifestation of parenchymal
fibrosis due to asbestos exposure. If this were the case one would expect subjects with
such radiographic changes to have, on average, some deficit in lung function as
compared to other exposed subjects without fissural changes or more obvious radiographic evidence of asbestosis. To test this hypothesis, we examined spirometric lung
volumes and single-breath diffusing capacity in a workforce with approximately
15-35 years of asbestos exposure but in an age range in which frank asbestosis is
unusual today. When examining the relationship between lung function impairment
and fissural thickening in isolation by a univariate analysis, subjects with this abnormality had, on average, a lower FVC. However, this association could be completely
accounted for by the lung function deficit that resulted from the accompanying classic
asbestos pleural disease, as demonstrated in a multivariate analysis.
CONCLUSION
It appears that fissural thickening is a not infrequent radiographic finding in
subjects exposed to asbestos and represents a component of asbestos pleural disease
and not asbestosis. Furthermore, any effect on lung function can be explained by the
associated pleural disease and fissural thickening does not therefore appear to have an
independent effect on spirometric lung volumes or diffusing capacity.
ACKNOWLEDGMENTS
We would like to thank the members of Local 58 of the International Association of Heat and Frost Insulators and Asbestos Workers. This work was supported
by a grant from the Environmental Research Fund of the School of Occupational
Health. Dr. Jan Zejda is in receipt of The Romer Foundation Scholarship. Dr. Pierre
Ernst is a Chercheur-Boursier, Fonds de Recherche en Santk du Quebec (FRSQ).
Asbestos-Related Fissural Thickening and Lung Function
793
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