Although pleural thickening is a common finding on routine chest X-rays, its radiological and clinical features remain poorly characterized. Our investigation of 28, 727 chest X-rays obtained from annual health examinations confirmed that pleural thickening was the most common abnormal radiological finding. In most cases (92.2%), pleural thickening involved the apex of the lung, particularly on the right side; thus, it was defined as a pulmonary apical cap. Pleural thickening was more common in males than in females and in current smokers or ex-smokers than in never smokers. The prevalence increased with age, ranging from 1.8% in teenagers to 9.8% in adults aged 60 years and older. Moreover, pleural thickening was clearly associated with greater height and lower body weight and body mass index, suggesting that a tall, thin body shape may predispose to pleural thickening. These observations allowed us to speculate about the causative mechanisms of pleural thickening that are attributable to disproportionate perfusion, ventilation, or mechanical forces in the lungs.
Pleural thickening is a common finding on routine chest X-rays. It typically involves the apex of the lung, which is called ‘pulmonary apical cap’. On chest X-rays, the apical cap is an irregular density located at the extreme apex and is less than 5 mm in width [1]. In the early twentieth century, a pulmonary apical cap was thought to be a tubercular lesion; however, detailed pathological studies conducted in the 1970s found no evidence of tuberculosis [2, 3, 4]. The apical cap is a fibroelastic scar involving the visceral pleura and lung parenchyma at the apex and is occasionally observed in healthy and asymptomatic individuals [5].
In 1974, Renner et al. [3] identified unilateral or bilateral apical cap shadows in 22.1% (n = 57) of 258 routine chest X-rays. This was a pioneering radiological study; however, the sample size was small. Surprisingly, no subsequent studies have investigated the prevalence of an apical cap on chest X-ray examination [3, 6] or its association with various subject characteristics.
A) Ct Through The Lung Apices Demonstrating Classical Features Of Ppfe...
Pleural thickening may be a manifestation of several pulmonary diseases, including mycobacterial infection, lung cancer, and idiopathic interstitial pneumonia. Of particular note, pleuroparenchymal fibroelastosis is increasingly recognized as a rare form of idiopathic interstitial pneumonia characterized by pleural and subjacent parenchymal fibrosis predominantly in the upper lobes, which may mimic an apical cap. Therefore, clinicians must rule out these pathological conditions when evaluating pleural thickening found on a chest X-ray. Moreover, it is necessary to characterize the radiological and clinical features of pleural thickening or an apical cap that is not disease-related in and of itself.
We reviewed 28, 727 chest X-rays obtained from annual health examinations performed between April 2017 and March 2018 in a large population. We confirmed that pleural thickening, typically located at the apex of the lung, was the most common abnormal finding. Furthermore, we investigated the prevalence and laterality of pleural thickening and its association with subject characteristics, including sex, age, smoking status, height, body weight, and body mass index (BMI). Our findings suggest that individuals with taller and thinner body shapes may be prone to pleural thickening. Given these findings, we speculate that an apical cap may be the result of disproportionate perfusion, ventilation, or mechanical forces in the lungs.
We conducted a cross-sectional analysis of chest X-rays obtained from the annual health examinations of 28, 727 employees and students at the University of Tokyo between April 2017 and March 2018. The chest X-rays were assessed independently by two physicians. Abnormal findings were referred to board-certified pulmonologists (M.S., M.T., Y.M., A.M., and A.S.) for further evaluation using available clinical information and previous X-ray images.
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Four areas of the lung (apical, upper, middle, and lower portions) were examined bilaterally for the presence of pleural thickening. Pleural thickening that involved the apex of either lung was defined as an apical cap.
Between-group comparisons were made using two-tailed Student’s t-tests, and the chi-square test was used to assess the associations of pleural thickening with sex and smoking status. The binary logistic regression model was used to evaluate the effects of sex, age, smoking status, and BMI on pleural thickening. All statistical tests were performed using the Statistical Package for the Social Sciences (SPSS Inc., Chicago, IL, USA).
We investigated the prevalence and distribution of pleural thickening and its association with subject characteristics, including sex, age, smoking status, height, body weight, and BMI.
Chest X Ray Abnormalities
Chest X-rays obtained from the annual health examinations of 28, 727 individuals between April 2017 and March 2018 were independently reviewed by two physicians. The study included 10, 012 females and 18, 715 males, and the mean (± standard deviation) age was 30.5 ± 12.1 (range, 17–83) years. Four areas of the lung (apical, upper, middle, and lower portions) were examined bilaterally for the presence of abnormal findings. There could be more than one finding in a subset of individuals. In total, 4041 abnormal findings were observed on the chest X-rays of 3113 individuals by at least one physician (Additional file 1: Table S1). Pleural thickening was the most common finding (35.2%; n = 1423) that was identified in 911 individuals. A representative X-ray image of pleural thickening is shown in Fig. 1. The prevalence of pleural thickening was 3.2% (n = 911/28, 727) in our sample.
Representative chest X-ray image of pleural thickening. a. Chest X-ray image of a healthy 47-year-old male. The right apical cap appears as an irregular, wedge-shaped density. b. Enlarged image of the right apex. The white triangles indicate pleural thickening
The apex of the lung was the most frequently affected area (Additional file 1: Table S2). Pleural thickening involving the apical area of either lung was defined as an apical cap, which accounted for 92.2% (n = 836/907) of the cases (Fig. 2a). More than half of the cases were bilateral and 35.7% involved thickening on the right side only. Together, the bilateral and right-sided cases comprised nearly 90% of the 907 cases (Fig. 2b). These findings indicate that the pleural thickening and apical cap cases overlapped, and that pleural thickening occurred predominantly in the right lung (Fig. 2).
Chest X Ray Showing Pleural Thickening
Pleural thickening distribution. a. Pie chart showing the percentages of pleural thickening cases with and without an apical cap. b. Pie chart showing the percentages of bilateral and unilateral pleural thickening cases
Pleural thickening was more common in males (3.4%) than in females (2.7%; p < 0.01) (Additional file 1: Table S3), and the incidence increased with age, ranging from 1.8% in teenagers (17–19 years) to 9.8% in adults aged 60–83 years (Additional file 1: Table S4). It is worth noting that pleural thickening occurred in individuals as young as 18 years, and the prevalence increased markedly after the age of 40 years (Fig. 3).
We further investigated whether smoking history was associated with pleural thickening. Information about smoking status was available for 25, 291 individuals (16, 043 males and 9248 females). The smoking rates of those with and without pleural thickening were 6.3 and 4.7%, respectively. The percentages of individuals with a smoking history (i.e., current smokers and ex-smokers) with and without pleural thickening were 14.6 and 10.2%, respectively. The prevalence of pleural thickening was higher in current smokers (4.4%) and ex-smokers (5.0%) than in never smokers (3.2%; p < 0.01) (Additional file 1: Table S5). Moreover, these trends were confirmed when males and females were analyzed separately.
Pleural Fibrosis And Calcification
Next, we investigated the association between pleural thickening and body shape. Height, body weight, and BMI were compared in individuals with and without pleural thickening (Fig. 4a). Males and females with pleural thickening were taller, weighed less, and had lower BMI than those without pleural thickening (p < 0.01 for each comparison). Further comparisons of sex, BMI, and pleural thickening revealed that underweight individuals (BMI < 18.5) had the highest percentage of pleural thickening, which was approximately 4% in both males and females (Additional file 1: Table S6). Conversely, the frequency of pleural thickening was lowest among overweight or obese individuals (25 ≤ BMI). The tendency toward an association between higher BMI and lower frequency of pleural thickening was more prominent in females (Fig. 4b). These findings indicate that greater height and lower body weight and BMI predispose to pleural thickening, suggesting that the causative mechanism is related to a tall, thin body shape.
Associations of pleural thickening with height, body weight, and BMI. a. Box plot showing the height (cm), body weight (kg), and BMI in individuals with and without pleural thickening. The line in the middle of the box indicates the median with the top and bottom ends of the box indicating the 75th and 25th percentiles, respectively. The circles and asterisks indicate outliers and extreme outliers, respectively. b. Frequencies of pleural thickening according to BMI values
Finally, a binary logistic regression analysis was performed to evaluate the effects of sex, age, smoking status, and BMI on the likelihood of pleural thickening. We analyzed 25, 286 cases with available information of all these characteristics, and odds ratios (OR) with 95% confidence intervals (CI) were calculated. As a result, male sex (OR = 2.042, 95% CI 1.738–2.399), age (OR = 1.051, 95% CI 1.045–1.057), and BMI (OR = 0.836, 95% CI 0.814–0.860) were found to be related to pleural thickening with statistical significance.
Pleural Thickening: Causes, Symptoms & Treatment
Although a pulmonary apical cap has been recognized as a non-specific fibrotic change at the apex of the
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