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Bronchoscopic Image

“Saber-Sheath” Trachea

Ismail, Saleh A. MD; Mehta, Atul C. MD

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The normal shape of the trachea is variable and can have different configurations at multiple levels. In a study by Gamsu and Webb, 1 looking at different tracheal shapes by computed tomography, it was documented that the most common configuration was round or oval. Less common shapes were horseshoe with flat posterior tracheal membrane, inverted pear, and almost a square. Many abnormal deformities can also exist, one of which is “saber-sheath” trachea.

“Saber,” literally, as stated in Encarta 2001, is “a heavy cavalry sword with a slightly curved blade that is sharp on one edge.” This term was first used in late 17th century, from the French sabre, from obsolete German Sabel, of uncertain origin: perhaps from Slavic or Polish szabla, Russian sablya, or from Hungarian szablya. “Sheath” is “a close-fitting covering or a case for the sward.”

The “saber-sheath” trachea is defined as intrathoracic narrowing of the trachea with internal coronal diameter two thirds or less than the sagittal diameter at the same level without any mediastinal pathology causing extrinsic compression. 2,3 Usually measurements are taken 1 cm above the aortic arch. Tracheal index is the ratio of the coronal and sagittal diameters, and it describes the degree of saber-sheath shape. This configuration usually involves the entire length of the intrathoracic part with an abrupt widening at the thoracic outlet. Also, the trachea exhibits ring-like ossification of the cartilages on computed tomography 1–3 (Fig. 1).

FIGURE 1.
FIGURE 1.:
View of the trachea. Note the tracheal index and the sudden widening of the trachea.

Simmonds first described the saber-sheath trachea in 1905 in cadavers. He named it “saber-sheath-shaped trachea of old age” because it was thought that the shape was secondary to aging. Although most of the patients had emphysema, no association was pointed out between the 2 conditions. 2,3 In another postmortem study by Campbell and Lindelow, 4 significant variability in the cross-sectional shape of the tracheas of older men was found, yet only 3 of 53 cases had coronal narrowing similar to that of a saber-sheath trachea. Most of the patients had more or less round tracheal cross-sections, whereas few showed flattening, either of the coronal or sagittal diameter. 3,4

In 1975, Greene and Lechner published the results of a study involving 13 patients selected to have a saber-sheath configuration. A tracheal index of 0.5 was used. All of the patients were men between the ages of 52 and 75 years. Eight of 11 patients for whom pulmonary function tests were available had significant reduction in forced expiratory volume in 1 second (FEV1) and peak flow rate. Similarly, the vital capacity was reduced, but to a lesser extent. The total lung capacity (TLC) was increased in 7, and the residual volume (RV) and RV/TLC were increased in 10. Only one patient had normal pulmonary function, but he had a clinical diagnosis of chronic bronchitis. Thus, the conclusion was that there might be an association between obstructive lung disease and the saber-sheath tracheal deformity.

In another study by Greene published in 1978, 2 a group of 60 patients with saber-sheath tracheas were compared with a control group of 60 individuals who were selected to be older than 50 years and had chest radiography performed during routine hospital admissions. Age was not significantly different, although it tended to be lower in the study group. The latter group consisted of men only. Seven percent of them were younger than 50 years. There was a strong correlation between saber-sheath trachea and the clinical diagnosis of chronic obstructive pulmonary disease (COPD), 93% in the study group compared with 18% in the control group. Although the tracheal index in patients with COPD was lower than normal values, it exhibited greater variability. Thus, it was concluded that patients with COPD do not necessarily have saber-sheath tracheas, but its presence highly suggests the diagnosis. 2

In an attempt to explain the significance of a saber-sheath trachea, Trigaux et al 5 conducted a study involving a group of 20 patients matched to a control group of 20 patients for age, sex, and clinical and radiographic diagnosis of COPD. The only significant differences were for functional residual capacity values and sternum–spine distances. Surprisingly, FEV1 was not significantly different between the 2 groups. Because forced vital capacity is a measurement of lung volumes and sternum–spine distance is an indicator of the expansibility of the thoracic cage, saber-sheath trachea was considered a morphologic sign of hyperinflation more than that of obstruction.

The development of a saber-sheath trachea is not well understood, yet the following evidence is known. Postmortem examination of one patient with this deformity showed a fixed and rigid-shaped trachea with heavily calcified tracheal rings and no evidence of tracheomalacia. 3 Obvious tracheal calcific densities were seen in 10 of 13 patients having a saber-sheath trachea on computed tomography. 3 Thus, a hypothesis based on degeneration, vascularization, and ossification that occur in the anterior part of the tracheal rings resulting in a fixed shape has been postulated. 3 Ossification is considered a manifestation of recurrent injury to the cartilaginous rings. The cartilage-like bone has the tendency to remodel and degenerate in response to injury. The injury is a result of recurrent coughing seen in patients with COPD. The observation of the limitation of a saber-sheath trachea to the thoracic cage with sudden widening at the thoracic outlet also suggests that intrathoracic forces or factors have a role in its development. 3 Pulmonary hyperinflation leads to an increase of the anteroposterior diameter of the chest and subsequently elongation of the sagittal dimension of the trachea. Because of their “U” shape, the cartilaginous rings resist such elongation, which results in excessive strain from both lateral sides. This strain will cause the cartilage to weaken, degenerate, and calcify, especially in the anterior part. 5 This effect is accentuated by recurrent coughing seen with patients with COPD. 6

Patients with a saber-sheath trachea are shown to have only smooth tracheal walls. However, atypical appearance of the tracheal walls has been also reported. Rubenstein et al. reported 2 rare cases with saber-sheath tracheas having irregular nodular inner margins on computed tomography unrelated to any underlying pathology. 7

In conclusion, “saber-sheath” is a deformity of the intrathoracic portion of the trachea that highly correlates with pulmonary hyperinflation and male gender. The tracheal rings are heavily calcified as a result of recurrent injury caused by repetitive vigorous coughing in patients with COPD, which causes excessive strain on the lateral sides of the cartilaginous rings, resulting in remodeling. The saber-sheath trachea has a rigid shape with no tendency to collapse during coughing and expiration. The clinical significance of a saber-sheath trachea when seen on chest radiographs is to rule out mediastinal pathology. Also, if a patient with such a deformity requires intubation, air leaking around the cuff of the endotracheal tube can be a problem for maintaining adequate ventilation. 7,8,9

REFERENCES

1. Gamsu G, Webb WR. Computed tomography of the trachea: normal and ab normal. AJR Am J Roentgenol. 1982; 139:321–326.
2. Greene R. Saber-sheath trachea: relation to chronic obstructive pulmonary disease. AJR Am J Roentgenol. 1978; 130:441–445.
3. Greene R, Lechner GL. Saber-sheath trachea: a clinical and functional study of marked coronal narrowing of the intrathoracic trachea. Radiology. 1975; 115:265–268.
4. Campbell AH, Lindelow AG. Significant variations in the shape of the trachea and large bronchi. Med J Aust. 1967; 1:1017–1020.
5. Trigaux JP, Hermes G, Dubois P, et al. CT of saber-sheath trachea. Acta Radiol. 1994; 35:247–250.
6. Callan E, Karandy EJ, Hilsinger Jr. RL Saber-sheath trachea. Ann Otol Rhinol Laryngol. 1998; 97:512–515.
7. Rubenstein J, Weisbord G, Steinhadt M, et al. Atypical appearances of saber-sheath trachea. Radiology. 1978; 127:41–42.
8. Wallace E, Chung F. General anesthesia in a patient with an enlarged saber-sheath trachea. Anesthesiology. 1998; 88:527–529.
9. Bayes J, Slater E, Hedberg P, et al. Obstruction of a double-lumen endotracheal tube by saber-sheath trachea. Anesth Analg. 1994; 79:186–188.
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