CT of saber-sheath trachea

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1 Acta Radiologica ISSN: (Print) (Online) Journal homepage: CT of saber-sheath trachea Jean Paul Trigaux, G. Hermes, P. Dubois, B. Van Beers, L. Delaunois & J. Jamart To cite this article: Jean Paul Trigaux, G. Hermes, P. Dubois, B. Van Beers, L. Delaunois & J. Jamart (1994) CT of saber-sheath trachea, Acta Radiologica, 35:3, To link to this article: Published online: 04 Jan Submit your article to this journal Article views: 337 Full Terms & Conditions of access and use can be found at

2 Acto Rocliologico 35 (1994) Fuse. 3 Printcd in Denmork. All rights reserved Copyright 0 Arlo Rodiologica 1994 ACTA RADlOLOGlCA ISSN FROM SERVICES DE RADIOLOGIE, DE PNEUMOLOGIE ET UNITE DE STATISTIQUE. CLINIQUES UNIVERSITAIRES UCL MONT-GODINNE. YVOIR. BELGIUM. CT OF SABER-SHEATH TRACHEA Correlation with clinical, chest radiographic and functional findings J. P. TRIGAUX, G. HERMES, P. DUBOIS, B. VAN BEERS, L. DELAUNOIS and J. JAMART Abstract The diagnosis of saber-sheath trachea is easy at CT due to its cross-sectional imaging, but the significance of this CT sign has not been evaluated in the diagnosis of chronic obstructive pulmonary disease (COPD). Various signs of COPD were compared between a series of 20 patients with a saber-sheath trachea at CT (tracheal index 166%) and a group of 20 pneumologic control patients without saber-sheath trachea (tracheal index 2 70%). These signs included clinical and standard radiographic indices of COPD, sternum-spine distance and 3 functional tests of COPD: forced expiratory volume in one second, carbon monoxide diffusing lung capacity, and functional residual capacity (FRC). A significant difference was found between the 2 groups, concerning the values of FRC (p< and of sternum-spine distance (p< lo- ). The tracheal index was significantly correlated with the FRC values (r = I; p< lo- ) and with the sternum-spine distance (r= ; p< 0.05). No other significant difference was observed. It is concluded that saber-sheath trachea is basically a sign of hyperinflation. Key words: Trachea, comparison; -, CT: lung, ventilation; -, function. First described by GREENE & LECHNER in 1975 (5), sabersheath trachea is an intrathoracic narrowing of the trachea, with the coronal diameter equal to or less than two-thirds of the sagittal diameter, when measured 1.0 cm above the top of the aortic arch. Although.over 95% of patients with this deformity have clinical evidence of chronic obstructive pulmonary disease (COPD), only 55% have other evidence of it on chest radiographs (6). Saber-sheath trachea usually has a smooth inner margin but a nodular appearance, suggesting a malignant process, has been described (13); calcifications of the tracheal cartilaginous rings are frequently present (2, 3, 5). The degree of saber-sheath shape is gen- erally expressed as the ratio of the coronal and sagittal diameters, the tracheal index (5). Due to its cross-sectional ability, CT is an accurate method for the diagnosis of saber-sheath trachea (3, 4) and for measuring tracheal diameters (15). It is also accepted that CT is the procedure of choice for evaluating tracheal diseases, such as atypical appearances of saber-sheath trachea (3, 4). On the other hand, various papers have correlated quantitative CT data with pulmonary function tests in chronic bronchitis and emphysema (1, 8, 10, 1 I). Nevertheless, the sign of saber-sheath trachea has, to our knowledge, never been evaluated in CT series. The present study compares saber-sheath trachea of 20 patients selected from CT scans and a group of 20 unselected pneumologic patients without saber-sheath trachea. The aim of the study was to correlate the CT diagnosis of saber-sheath trachea with various functional, clinical and radiographic signs of COPD in an attempt to explain the significance of this tracheal deformity. Material and methods The saber-sheath configuration was defined as a trachea with an internal coronal diameter two-thirds or less than the internal sagittal diameter at the same level, i.e. one cm above the top of the aortic arch (6); the degree of sabersheath shape was expressed as the ratio of the coronal and sagittal diameters, the tracheal index (5). The study group consisted of 20 consecutive patients Accepted for publication 2 September

3 248 J. P. TRIGAUX ET AL. Table Comparison between the study and control groups Study group Control group n=20 n=20 Wilcoxon Mean f SD Range Mean f SD Range test Age, years Weight, kg Height, cm FEVI, % DLCO, 9'0 FRC, % Clinical index Radiologic index Sternum-spine distance, c :m 61.3f f k f24 14 f f f f f I f l72f f f f47 l.9f O.Of I p< lo4 p< 102 FEW - forced expiratory volume in 1 s. DLCO - carbon monoxide diffusing lung capacity. FRC - functional residual capacity. - nonsignificant. from our Department of Pneumology, in whom a sabersheath trachea configuration (tracheal index < 66%) was identified on the basis of chest CT (male 20; female 0; age years; mean 63 years). The control group consisted of 20 consecutive male patients matched for age, issued from the same Department of Pneumology with a tracheal index >70%, on the basis of chest CT (age years; mean 65 years). For the study and control groups, the exclusion criteria were a parenchymal tumor classified above TINI, a pleural disease, parenchymal consolidation or infiltrate involving more than one pulmonary segment, no pulmonary function tests and/or no p.a. and lateral chest radiographs obtained within 10 days of the referring chest CT and previous lung surgery. CT examinations were made during breathholding at endinspiration on a Somatom DRH (Siemens), with 4- or 8- mm collimation at one-cm intervals. All images were photographed at window levels and widths appropriate for lung parenchyma (level HU; width HU) and mediastinum (level 50 HU; width 400 HU); 8-mm sections were part of the standard program for chest CT and 4-mm sections were part of the specific program for evaluation of bronchiectasis. Diagnostic criteria and statistical analysis. A clinical COPD index for each control and study group patient was obtained by reviewing clinical records (6). A maximum of 4 was possible, with points given for each of the following: 1) a positive cigarette smoking history between one- to 20 pack-years yields one point; 2) a positive cigarette smoking history more than 20 pack-years 2. points; 3) an explicit clinical diagnosis of COPD one point; 4) a chronic productive cough one point. A radiographic COPD index was calculated by reviewing the chest radiographs (6, 14). A 5-point score was possible, with one point given for each of the following observations: 1) depressed diaphragmatic level; 2) anterior bowing of the sternum; 3) aerated lung visible beneath the cardiac silhouette on the frontal view; 4) focal or multifocal pulmonary vascular attenuation; and 5) definite bullous lung disease. The chest radiographs were analyzed independently by 2 radiologists. In the instances where they disagreed, a score was given by consensus. For each control and study group patient, the results of pulmonary function tests obtained within 10 days of CT evaluation were reviewed, with special focus on forced expiratory volume in one second (FEVl, sign of obstruction), carbon monoxide diffusing lung capacity (DLCO, sign of destruction), and functional residual capacity (FRC, sign of hyperinflation) determined by body plethysmography (body test Jaeger, Wurtzburg, Germany). All these values were expressed as a percentage of the predicted values for patient age, height, weight and sex by using the prediction equations and the recommendations of the European Community for Coal and Steel (12). FEVl was considered definitely abnormal if <70%, DLCO if <70% and FRC if > 150%. Finally, for each patient, the distance in cm between the anterior border of the spine and the posterior border of the sternum (called the sternum-spine distance) was calculated on the CT images at the level of the carina. Differences in values between the study and control groups were tested for significance by the Wilcoxon rank sum test; correlations between tracheal indices versus sternum-spine distance and pulmonary function test values were evaluated with Spearman rank correlation coefficient. Results The mean value of the tracheal index in the study group was 45.5% (f 5% SD) and in the control group 101% (k 9% SD). No significant difference was observed between the study and control groups concerning the age, weight, height, the clinical and radiographic index of COPD, or the values of FEVl and DLCO, but a significant difference was observed between the study and control groups concerning

4 CT OF SABER-SHEATH TRACHEA 249 FRC ( % of the predictive valuer) " :I :......i...i: P~O.0001 STUDY QROUP CONTROL QROUP n.40 Fig. 1. Comparison of individual FRC values between the study and control groups (Wilcoxon test). FRC is considered definitely abnormal if > 150%. STERNUM-SPINE DISTANCE (cm) I pco.01 5' I STUDY GROUP. CONTROL GROUP n-40 Fig. 2. Comparison of individual sternum-spine distances between the study and control groups (Wilcoxon test). Fig. 3. Schematic drawing explaining the mechanical stress exercised on the trachea by chest hyperinflation (T = trachea). the values of FRC (p< and of sternum-spine distance (p< (Table, Figs 1 and 2). Among the 40 patients of the study and control groups overwhelmed, the tracheal index was significantly correlated only with the FRC values (r= ; p< and with the sternum-spine distance (r = ; p < 0.05). The sensitivity and specificity of a tracheal index I 60% for a FRC value 2 150"h was 78% and 88'94 respectively. Discussion Saber-sheath trachea is usually considered a valuable sensitive and specific sign for COPD on standard chest radiography (5, 6). Various authors agree that CT is the method of choice for the study of tracheal morphology and pathology (3, 4, IS), but the impact of CT to the analysis of saber-sheath trachea has not been precisely evaluated. The pathogenesis of this deformity is unknown. Our 2 groups were similar concerning age, sex, weight, height, clinical and radiographic COPD indices and the values of DLCO and FEVl (Table). These 40 men were obtained through the same Department of Pneumology, with a high frequency of smokers and bronchitic patients. Between the study and control group, the only significant differences were FRC values (p< and sternum-spine distances (p< lo-*). FRC is a reliable pulmonary function measurement of lung volume (1 1) and we think that the sternum-spine distance is a good indicator of the expansibility of the thoracic cage in the a.p. plane. Saber-sheath trachea is thus a morphological sign of hyperinflation. In our experience, saber-sheath trachea is not correlated with FEVl, nor with DLCO and thus is not a sign of airflow obstruction, nor a sign of parenchymal destruction. Our results suggest a pathogenic hypothesis about the development of saber-sheath trachea. Pulmonary hyperinflation results in an increase in the sagittal diameter of the chest: the chest distends easily in the sagittal plane, as shown by the sternal bulging and dorsal hyperkyphosis clinically observed in COPD (14). This increase in the sagittal diameter of the chest results in an elongation of the sagittal dimensions of the trachea. The U-shaped arrangement of the tracheal cartilaginous rings does not permit a considerable extension of the trachea in the sagittal plane. However, the strain on the cartilaginous rings might suffice to make them weak, calcified and secondary malacic. This acquired chondromalacia results in an uncontrolled decrease in the coronal diameter of the trachea, which is submitted, in case of hyperinflation, to a considerable mechanical stress in the coronal plane as well. The result is a saber-sheath trachea deformity (Fig. 3). In this retrospective study, our images were obtained in planes perpendicular to the long axis of the body; such scans falsely elongate the sagittal dimensions of the trachea, because the trachea is angled with respect to this axis (9). Ideally, the gantry should have been angled with a cranial tilt of 11' to 20" (7), or - more accurately - the best angula-

5 250 J. P. TRIGAUX ET AL. tion should have been selected by examining the angle of the trachea on a lateral scan view. The specificity of this sign of saber-sheath trachea may consequently have been slightly biased in this CT study. ACKNOWLEDGMENTS The authors thank J. Burion for translation assistance and M. P. Heylens for help in the preparation of the manuscript. Request for reprints: Dr. Jean Paul Trigaux, Service de Radiologie, Cliniques Universitaires UCL Mont-Godinne, B-5530 Yvoir, Belgium. REFERENCES I. BIERNACKI W., GOULD G. A., WHYTE K. F. & FLENLEY D. C.: Pulmonary hemodynamics, gas exchange, and the severity of emphysema as assessed by quantitative CT scan in chronic bronchitis and emphysema. Am. Rev. Respir. Dis. 139 (1989), CHOPLIN R. H., WEHUNT W. D. & THEROS E. G.: Diffuse lesions of the trachea. Semin. Roentgenol. 18 (1983), GAMSU G. & WEBB W. R.: Computed tomography of the trachea. Normal and abnormal. AJR 139 (1982), GAMSU G. & WEBB W. R.: Computed tomography of the trachea and mainstem bronchi. Semin. Roentgenol. 18 (1983), GREENE R. & LECHNER G. L.: Saber-sheath trachea. A clinical and functional study of marked coronal narrowing of the intrathoracic trachea. Radiology 115 (1975), GREENE R.: Saber-sheath trachea. Relation to chronic obstructive pulmonary disease. AJR 130 (1978) GRISCOM N. T.: Computed tomographic determination of tracheal dimensions in children and adolescents. Radiology 145 (1982), GURNEY J. W., JONES K. K., ROBBINS R. A. et al.: Regional distribution of emphysema. Correlation of high-resolution CT with pulmonary function tests in unselected smokers. Radiology 183 (1992), HEMMINGSSON A. & LINDGREN P. G.: Roentgenologic examination of tracheal stenosis. Acta Radiol. Diagnosis 19 (1978), KINSELLA M., MULLER N. L., STAPLFS C., VEDAL S. & CHAN- TEUNG M.: Hyperinflation in asthma and emphysema. Assessment by pulmonary function testing and computed tomography. Chest 94 (1988), KINSELLA M., MULLER N. L., ABBOUD R. T., MORRISON. J. & DYBUNCIO A.: Quantitation of emphysema by computed tomography using a density mask program and correlation with pulmonary function tests. Chest 97 (1990), QUANJER P. H.: Standardized lung function testing. Report Working Party from the European Community for Coal and Steel. Bull. Eur. Physiopathol. Respir. 19 (1983), RUBENSTEIN J., WEISBROD G. & STEINHARDT M. I.: Atypical appearances of saber-sheath trachea. Radiology 127 (1978), THURLBECK W. M. & SIMON G.: Radiographic appearance of the chest in emphysema. AJR 130 (1978), VOCK P., SPIEGEL T., FRAM E. K. & EFFMANN E. L.: CT assessment of the adult intrathoracic cross section of the trachea. J. Comput. Assist. Tomogr. 8 (1984), 1076.