Diagnostic Radiology
"Saber-Sheath" Trachea: A Clinical and Functional Study of Marked Coronal Narrowing of the Intrathoracic Trachea 1 Reginald Greene, M.D. and Gerhard L. Lechner, M.D. 2
Data are presented from thirteen patients with marked coronal narrowing of the intrathoracic trachea associated with widening of the corresponding sagittal diameter. The term "saber-sheath" trachea describes this deformity which can simulate a mediastinal mass. The absence of mediastinal masses and the presence of obstructive pulmonary abnormalities in each patient, either as a clinical diagnosis (11 of 13), functional abnormality (10 of 11) or both (8 of 11), suggest that the "saber-sheath" trachea may be a useful sign of chronic obstructive pulmonary disease. INDEX TERMS:
Lungs, chronic obstructive lung disease • Trachea
Radiology 115:265-268,
May 1975
• HERE HAVE BEEN many radiological descriptions of dynamic alterations in intrathoracic tracheal dimensions during respiratory maneuvers (1-3), but little attention has been paid to the marked variations in tracheal shape which are present in the resting state and have long been noted in postmortem studies (4-7). The trachea normally has an approximately circular cross section but may exhibit marked front to back or side to side narrowing falsely suggssting the presence of a mediastinal mass. Because the clinical significance and etiology of these deformities are not well understood and may be misinterpreted, we have begun to make clinical and functional correlations. This report concerns a group of patients with exceptional coronal narrowing and sagittal widening of the intrathoracic trachea which we have called "saber-sheath" trachea.
trathoracic trachea were based on the measured coronal and sagittal diameters using the formula of an ellipse (A = 7rab) where a is one-half the coronal diameter, b is one-half the sagittal diameter, and A is the area. Pulmonary function data, available in 11 of the 13 patients, included forced expiratory volume in one second (FEV 1), vital capacity (VC) and peak flow rate (PFR). Total lung capacity was determined by a radiographic method previously described (8). From these data FEV 11 VC and the ratio of residual volume (RV) to TLC (RV 1 TLC) was calculated. In some patients the maximum breathing capacity (MBC) was obtained.
T
MATERIAL AND METHODS
Each patient was included in this study on the basis of having an internal coronal diameter of the intrathoracic trachea which was one-half or less of the corresponding sagittal diameter. Cases were selected from the Thoracic Radiology Section of the Massachusetts General Hospital over a two-year period. Tracheal measurements were made 1 cm above the level of the aortic arch on chest teleoroentgenograms obtained at total lung capacity (TLC) with a film-focus distance of 3 meters at 150 peak kilovolts (Fig. 1). Thirteen patients fulfilled the requirement and none had evidence of a cervical mass, mediastinal mass, or previous tracheostomy. The ratio of the coronal to the sagittal diameter was computed and called the "tracheal index" (T.I.). The maximum coronal diameter of the extrathoracic trachea was also measured. Calculations of the cross-sectional area of the in-
RESULTS
Clinical, functional and anatomic data are shown in TABLE I. All patients were men ranging in age from 52 to 75 years. The primary clinical diagnoses were chronic obstructive pulmonary disease (7 patients), carcinoma of the lung (2 patients), and carcinoma of the lip, pulmonary emboli, chronic alcoholism and pericarditis. Associated clinical diagnoses included chronic bronchitis (10 patients), cor pulmonale (4 patients), asthma (2 patients), pulmonary emboli and Laennec's cirrhosis. The pulmonary function data showed that air flow rates (FEV 1, PFR) were significantly reduced in 8 of 11 patients. The vital capacity was reduced in 8 of 11 patients but to a lesser extent than the FEV 1 and PFR. Total lung capacity was increased in 7 of 11 and the RV and RV ITLC were increased in 10 of 11. In one patient with chronic bronchitis all these parameters of pulmonary function were normal. Arterial blood gases obtained in 6 patients showed hypoxemia and carbon dioxide retention in 3. The coronal intrathoracic tracheal diameters ranged from 7 to 13 mm (mean 10.5 mm). The calculated tra-
1 From the Department of Radiology, Harvard Medical School and Massachusetts General Hospital, Boston, Mass. Accepted for publication in January 1975. 2 On leave from the Radiology Department of the First Surgical University Clinic, Vienna, Austria. dk
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,
L. LECHNER
May 1975
Fig. 1. Posteroanterior (left) and lateral chest teleoroentgenograms (right) in patient NO.2. Arrows indicate the level at which internal diameter measurements of the trachea were taken in the coronal and sagittal planes. Note the overinflation of the lungs and central pulmonary artery enlargement. Fig. 2. Anteroposterior (left) and lateral tomographic sections (right) through the intrathoracic trachea in patient NO.2. Note the abrupt change from marked coronal narrowing to a more rounded cross-sectional shape at the thoracic outlet. Calcific densities are present in the tracheal rings.
cheal indices ranged from 0.50 to 0.25 mm (mean 0.40 mm). The narrow coronal diameters generally extended the entire length of the intrathoracic trachea from carina to thoracic outlet (Figs. 2, left and 3). Abrupt widening of the coronal diameter and narrowing of the sagittal diameter occurred at the junction of intrathoracic and extrathoracic trachea, resulting in a more circular configuration (Fig. 2). The consistently greater extrathoracic versus intrathoracic coronal diameters are shown in Table I. The mainstem bronchial dimensions appeared normal. As shown in Table I, the calculated cross-sectional areas varied from 148-306 rnrn'' (mean 229 rnrn-'). While no specific attempt was made to detect calcification or ossification in the tracheal rings with tomography, obvious tracheal ring calcific densities were seen in 10 of 13 patients. The lateral tracheal walls seemed relatively thick in comparison to normal dimensions. There was radiographic evidence of peripheral pulmonary vessel attenuation and hyperinflation in 9 of 13 patients. Pulmonary artery enlargement was obvious in 4 of 13 patients. During the two-year period of the study one patient (No.3) died of exsanguination from a necrotic metastatic squamous cell carcinoma in the neck. Postmortem examination of the lungs showed evidence of centrilo-
, bular emphysema and chronic bronchitis. The trachea had a rather rigid "saber-sheath" deformity with extensive ossification of the cartilaginous rings. There were no tracheal rings which lacked cartilage. DISCUSSION
That all of our patients were men over 50 years of age suggests that coronal narrowing with sagittal widening of the trachea in the absence of other mediastinal abnormalities primarily occurs in older males. This particular age and sex prevalence was also shown in a postmortem study reported by the pathologist Simmonds (4) who described a similar tracheal configuration which he called Atterssebetscnekientrechee (saber-
Table I: Patient Number Age (Years) Body Surface Area (M2) Cigarette Smoking (Pack/Years) FEV! (% Predicted) VC (% Predicted) FEVI/VC (%) PFR (% Predicted) M BC (% Predicted) TLC (% Predicted) RV/TLC (%) Extrathoracic Trachea: Coronal Diameter (mm) Intrathoracic Trachea: Coronal Diameter (mm) Sagittal Diameter (mm) Tracheal Index Tracheal Cross Section (mm-)
267
., SABER-SHEATH" TRACHEA
Vol. 115
Patient Data*
1 53 1.96
2 56 2.10
3 65 2.03
4 62 1.80
5 75 1.92
6 59 2.27
7 67 2.10
8 57 2.34
9 58
65 81 74 83 95 85 100 0.49
60 39 98 31 42 23 155 0.56
100 68 65
30 78 74 75 70 44 100 0.48
60 81 93 55 68 62 134 0.52
60 54 72 55 42 30 134 0.62
100 19 46 44 35 14 151 0.79
40
65 47 119 0.62
80 36 49 70 41 27 136 0.79
22
20
17
20
20
22
21
22
19
12 28 0.42 264
7 27 0.25 148
11 25 0.44 216
9 22 0.40 156
12 24 0.50 226
11 26 0.42 225
11 24 0.45 207
9 22 0.40 156
8 27 0.29 170
77
* FEV l = forced expired volume in one second; VC TLC = total lung capacity.
Diagnostic Radiology
= vital capacity;
sheath shaped trachea of the elderly). Campbell and Liddelow (9) in another postmortem study have drawn attention to marked variability in the cross-sectional shape of tracheas of older men. Some of their patients had more or less round tracheal cross sections while others showed flattening either of the coronal or sagittal diameter. In 3 of 53 cases, they reported marked coronal narrowing similar to the tracheal shape we are describing. In spite of the frequency of respiratory disease among older men in the general population and the small number of patients in our study, the striking incidence of chronic obstructive pulmonary disease (COPD) among our patients raises the question of what relationship this disease has to the cross-sectional shape of the trachea. All of our patients had histories of heavy smoking; 7 had primary diagnoses of COPO and 10 had associated diagnoses of chronic bronchitis. From the name that Simmonds gave to coronal narrowing of the trachea (Alterssabelscheidentrachea), it appears that this configuration was regarded as a consequence of aging (4). It is of interest, however, that in his report 45 of 61 patients had the pathologic diagnosis of emphysema. In another pathology study, Bryant et al. (3), contrary to the suggestion of other authors, found no definite relationship between COPD and any particular tracheal shape. Tracheas with wide coronal diameters and sagittal narrowing, (i.e., with flattening at 90° from the shape we are describing) have been regarded as less rigid than those with a rounded configuration, facilitating tracheal collapse during forced expiration or cough (10). It has been suggested that this shape which is accompanied by weakening of the posterior membranous wall of the trachea and sagittal diameter flattening is more common in chronic bronchitis (10, 11), but no such significant association was found in a postmortem study (7). The pulmonary function data from our study strongly supported the clinical diagnoses of COPD. Only one patient had normal function studies for age but he also carried a clinical diagnosis of chronic bronchitis. The re-
PFR
=
10
11 67 1. 78
12 68 2.08
75 19 44 33 26 12 130 0.76
20 28 57 36 24 161 0.75
74 0.36
20
22
22
21
20.6
8 27 0.29 170
13 27 0.29 276
13 30 0.43 306
12 26 0.46 245
10.5 23.5 0.40 229
71
90
peak flow rate; MBC
=
13 Mean 52 62 1. 70 30 93 92 76 89
61
17
maximal breathing capacity;
Fig. 3. Line drawings made from the posteroanterior (shaded) and left lateral teleoroentgenograms (unshaded) in the 13 patients. The location of the top of the aortic arch is indicated by the hatched lines.
mainder had abnormally high RV /TLC and most of them had other hallmarks of obstructive pulmonary disease. While it is conceivable that marked coronal narrowing of the intrathoracic trachea may contribute to increased airways resistance, it is unlikely that this phenomenon alone accounts for the high incidence of obstructive pulmonary function data (12). No normal cross-sectional areas have been established, to our knowledge, for this age group, but our values seem slightly lower than would be expected. If we assume that the trachea of an average man has a circular cross section with a diameter of 20 mm, then the expected normal cross-sectional area would be about 314 mm 2 which is somewhat larger than our mean value (229 rnm-'). Because of the small number of patients in our study, no definite association between depressed air flow rates, tracheal index and cross-sectional area can be established. It is interesting but difficult to speculate about the
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REGINALD GREENE AND GERHARD L. LECHNER
possible mechanisms for a relationship between COPD and the saber-sheath trachea. It is known, for instance, that normally during expiration there is a reduction in all dimensions of the intrathoracic airways and that expiratory tracheal collapse is markedly enhanced in patients with COPD. Under these dynamic conditions coronal narrowing is the result of the bending of the cartilaginous rings and the sagittal narrowing is primarily the result of an inward bulging of the posterior membranous wall (7). The saber-sheath trachea, therefore, cannot merely be a static reflection of a symmetrical dynamic reduction in the tracheal cross section which occurs during expiration or coughing unless the tracheal walls are asymmetrically involved by .some pathologic process which encourages collapse in the coronal plane. The postmortem findings in patient No. 3 indicate that there is apparently no local cartilaginous softening which promotes coronal collapse. Other investigators have recognized the presence of ossification of the tracheal cartilaginous rings in the sabelscheidentrachea (4, 13). Linzbach (13) formulated a rationale for the development of this tracheal shape based on degeneration, vascularization and ossification of the tracheal rings. He believed that ossification which occurred in the anterior part of the ring might be responsible for causing a fixed "saber-sheath" configuration. It is possible that one of the stimuli for the initial degeneration-vascularization-ossification sequence is chronic, tracheal collapse from repeated coughing. The change in the shape of the trachea at the thoracic outlet was not described by pathologists who considered the deformity to extend the entire length of the trachea (4, 13). Our observation of the sudden change from a saber-sheath shape to a more circular cross section at the thoracic inlet further supports the argument that somehow the intrathoracic tracheal shape reflects intrathoracic forces. Since the pressure acting on the intrathoracic trachea under static conditions is identical to the pressure acting on the lungs, i.e., transpulmonary pressure (pleural pressure minus intratracheal pressure), it is conceivable that the tracheal configuration also reflects the manner in which the trachea fits into the mediastinum of patients with increased lung volumes. This argument is not, however, supported by the fact that some of our patients had normal total lung capacity. While the mechanism for the transformation of rounded tracheas into "saber-sheath" tracheas is not entirely clear, the high incidence of chronic pulmonary disease, especially chronic bronchitis, in these patients suggests a significant association. To our knowledge, normal values for intrathoracic tracheal dimensions have not been established with respect to age and body size and no systematic correlation with pulmonary function data has been made. For this reason, we are in the process of carrying out a larger study to answer some of these questions. Preliminary results from our yet to be completed fol-
May 1975
low-up study of 90 men indicate that there is a tendency for those over 50 years of age with severe chronic obstructive lung disease to have more coronal narrowing of the intrathoracic trachea (T.\. 0.77 ± 0.15 S.D.) in comparison to controls (T.\. 0.84 ± 0.12 S.D.).
SUMMARY The saber-sheath trachea is a rather fixed deformity of the intrathoracic trachea. It is characterized by a coronal diameter one-half or less of the sagittal diameter, which abruptly changes to a rounded configuration at the thoracic outlet. It has thickened lateral walls and frequently shows evidence of ossification of the cartilaginous rings. Recognition of this saber-sheath shape can avoid the mistaken diagnosis of mediastinal mass and may help substantiate the presence of chronic obstructive pulmonary disease even in the absence of traditional radiographic signs. Department of Radiology Massachusetts General Hospital Boston, Mass. 02114
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