REVIEW ARTICLE Mediastinal Assessment in Traumatic Rupture of the Aorta Abstract The precise method for measuring mediastinal widening in thoracic trauma is poorly understood and too much emphasis is placed on it as the sole criterion for performing aortography. This review article describes the established methods of measurement and their limitations, and also discusses the importance of other signs pointing to aortic rupture such as blurring of the aortic knuckle and a subjective impression of mediastinal abnormality which are more helpful in determining the need for angiography. Introduction CMYoung M88S(Lond) FRCS(Ed) FFRad(D)(SA) Department of Radi%gy, Groote Schuur Hospital and University of Cape Town The commonest site of aortic injury is at the isthmus i.e.between the left subclavian artery origin and the ligamentum arteriosum and is thought to be due to de- celeration affecting fixed and relatively mobile segments of the organ. Rupture causes immediate death in up to 90% of trauma victims. A small number of severely a SA JOURNAL OF RADIOLOGY· May 1996 injured patients however may be in the for- tunate position of having an increased chance of survival owing to the develop- ment of rapid evacuation transportation and regional trauma centres. Those who reach hospital have a contained rupture i.e. a localized false aneurysm. This feature is often not recognized on a chest radiograph as a much wider area of haem atom a ob- scures it'. Mediastinal widening in these cases is caused by bleeding from associated injuries to minor arteries and veins; never- theless the association has been shown to be useful as originally described by Marsh and Sturm.' If the mediastinal haematoma had been' caused by blood from the aortic rupture, then the patient would almost certainly have exsanguinated. The natural history of these false aneu- rysms is poorly understood. Clinical man- agement and also the modem radiological literature is still largely based on Parmley's 1958 post-mortem study of mortality data' as quoted in Pais' excellent Seminars review on angiography in the trauma patient," Pais states that in Parmley's paper there was no angiographic correlation and therefore pa- tients with aortic false aneurysm who had survived were not takén into consideration. A recent review on the subject in Clinical Radiology by White and Mirvis from Balti- more' states that 40% of untreated patients die within 24 hours of hospital admission. Although the evidence for this is not given, the percentage figure corresponds exactly with Parmley's data. Several other contem- porary authors suggest that the immediate natural history may be more benign than suggestedv" and this has also been our ex- perience" Cowley found that there was no statistical assodation between mortality and either transport time to the hospital or time from injury to operation." Richardson found that no patient with ruptured aorta or aortic branch injury reaching hospital died from that injury before its operative treatment, bearing in mind that other topage9 Mediastinal Assessment in Traumatic Rupture of the Aorta life-threatening injuries in these patients re- quired operation first. 7 Standard methods of mediastinal measurement It is common practice in our institution (both by radiologistsand trauma surgeons) to assessmediastinal width on chest X-ray post thoracic trauma, using 8 cm as a crite- rion for aortography. This measurement was described by Marsh and Sturm, radi- ologist and surgeon respectively in their landmark article in the Annals ofThoradc Surgery.2 They compared 47 severely in- jured patients with 100 controls using a 100 cm antero-posterior (AP) supine chest film. Five cases of aortic rupture were found in patients with a widened mediastinum, and they concluded that a mediastinal measurement more than 8 cm was a definite indication for aortography. The slightlymisleading phrase" at the level of the aortic knob" was used in the text, al- though their diagram clearly shows the measurement was made immediately above the aortic knuckle, the position rec- ommended by Milne et al in their descrip- tion of the "vascular pedicle".9 Figure 1.' Diagram of mediastinal structures seen on a supine chest radiograph to show method of transverse width measurement (ML+MR) (see text). Transverse mediastinal width (the "vascular pedicle") The distance from the midline (spinous process) to the left border of the left sub- clavian artery take-off (ML) is added to the distance from the midline to the intersec- tion of the right border of the superior vena cava (SVC) and right main bronchus (MR) (See Figure 1). Milne et al give an elegant explana- tion of the nature of the vascular pedicle in a series of three articles describing the normal subject, the patient with ac- quired heart disease and also the trau- matised patient," 10, II They point out that when the patient assumes the supine position, the pedicle width increases to a different extent on each side. The subclavian artery, forming the left side of the superior mediastinum, has a low compliance hence will change little in di- ameter. The venous structures making up the right border; superiorly the right brachiocephalic vein and inferiorly the SVC, will change markedly in size be- tween the erect and supine postures. It is important to note that the clarity of the right paratracheal stripe and the azygos vein is preserved. Milne et al also observed that increased mediastinal width occurs in fluid overload or cardiac decompensation, and this is also related to venous distension. When the vascu- lar pedicle width is increased because of extravascular fluid (as in aortic rupture), then the enlargement occurs principally on the left as the subclavian artery is posteriorly situated and haematoma re- lated to the rupture surrounds it. The clarity of the right side of the superior mediastinum, formed by the SVC, is pre- served as it is more anterior and hence remote from the haematoma. Note on the diagram how the right paratracheal stripe and azygos may be effaced and the paraspinal lines displaced (See Figure 2). 9 SAJOURNAL OF RADIOLOGY. May 1996 Figure 2: Diagram of mediastinal GT scan showing azygos vein entering SVG, and effacement of the vein and the right paratracheal stripe by haematoma. Note how a haematoma that extends into the superior mediastinum would expand to left around the left subclavian artery, preserving the outline of the right border of SVG, a more anterior structure (see text). It is important to bear in mind that a focalmediastinal widening above the point of measurement of the vascularpedicle can be an indication of major vascular injury and is therefore an indication for aortography. Technical aspects The focus-film distance (FfD) of the radio- graph is of little consequence. A lordotic projection as is often obtained in the semi- erectICU patientmaymagnifythesupe- rior mediastinal structures with lossofdefi- nition of the aortic arch and hence may lead to unnecessary angiography" Figure 3: Diagram of chest radiograph to show delermination of mediaslinal width lo chest width ratio (MIG ratio) (see text). lapags 10 Mediastinal Assessrnent in Traurnatic Rupture of the Aorta Mediastinal width to chest width ratio (MIe ratio) Seltzer et al described the use of the MlC ratio in the assessment of traumatic aortic rupture in theAJRin 1981.13 Thegreatest width of the mediastinum is taken at the level of the aortic arch i.e.it includes the aor- tic knuckle and SVC (M) (SeeFigure3). It is divided by the intemal thoracic diameter from inner rib margin to inner rib margin (C) at the same level. The authors recom- mended that the maximum Mie ratio be taken as 0.25 and this would identify 95% of cases with aortic rupture and give a false positive rate of25%. Their study also found little relation between rib fracture and aortic tear. Technical aspects Seltzer et al found no statistical difference in Mie ratios comparing supine or sitting AP radiographs and postero-anterior (PA) erect films, although the overall trend was that supine AP films had the largest ratios. There is no significant change in Mie ratio with increase or decrease ofFFD.! Discussion Woodring and King discuss the limitations of transverse mediastinal width (TMW) and Mie ratio in an important paper' The overall object of their study was to exam- ine the usefulness of precise measurements as against subjective assessment of mediastinal abnormality. They concluded as a result of their own study that excessive mediastinal widening was helpful in deter- mining the need for aortography, but that a normal measurement was not. There was such an extreme overlap of values of mediastinal width and Mie ratio between normals and abnormals, that precise meas- urement could not reliably separate the two groups. They observed that using an Mie ratio ofO.25 or greater to define abnormal- ity as recommended by Seltzer et al13 would lead to a false positive rate as high as 99%. The main reason for this is that mediastinal width includes the aortic arch (as used in Mie ratio) and is related to normal unfold- ing of the aorta that occurs with age. There is only a modest correlation between in- creasing patient age and widening of the vascular pedicle.' InWoodring and King's experience there were a number of cases of ruptured aorta or brachiocephalic arter- ies that had a superior mediastinal width of less than 8 cm, the criterion for aortography quoted by Marsh and Sturm.' They suggested modifying the values of transverse mediastinal width and Mie Table I: Radiographic associations of aortic rupture Mediastinal widening Abnormality ol aortic knuckle or descending aorta Left apical pleural cap Displacement ol trachea to the right Displacement of nasa-gastric tube to the right Inferior displacement ol left main bronchus Filling-in of aortopulmonary window Widening ol right paratracheal stripe Effacement of vena azygos Left haemothorax R or L paraspinalline displacement Pulmonary contusions Rib Iractures (no association found by Seltzer et a~ ratio to 7.5 cm and 0.38 respectively to a give a more acceptable false positive rate than Marsh and Sturm's 8 cm,andSeltzeretal's Mie ratio ofO.25 would have given.Patients with values above these levels would have aortograms, but would also have angiography performed below these levels, in the presence of other signs of mediastinal abnormality (e.g.blurring of aortic arch or aorto-pulmonary window ete). Forty-one per cent of patients with proven arterial injury in their series and using their crite- ria had TMW within normal limits, and 69% had a normal Mie ratio. 10 SA JOURNAL OF RADIOLOGY· May 1996 It has been suggested that an erect chest film be done ifpossible-" This may provide optimal assessment of the aortomediastinal contour compared with the supine view, but it is important to be aware that the mediastinal measurements as described in the literature are designed for the supine view. Angiographyvs cr Intra-arterial digital subtraction angiography (lA DSA) is recommended as the procedure of choice by Mirvis et al.!S It has the same accuracy as conventional arteriography with a considerable saving in time and film cost. At Groote Schuur Hos- pital we perform two views using lA DSA, frontal (postero-anterior) and left anterior oblique, similar to Mirvis et aI's technique. We prefer angiography to computed tomography (CT) in our institution for sev- eral reasons: cr is prone to misregistration artefacts, and can be difficult to interpret, although these problerns may be overcome with the advent of spiral CT; the patients are often unco-operative leading to major motion image degradation; and our tho- racic surgeons request angiography to pre- cisely define the abnormality. If CT was performed first some patients would then require two investigations both with a large contrast load, i.e. a patient whose CT showed a mediastinal haematoma would then go on to angiography. We consider that an arch aortogram is a minimally invasive, quick and relatively easy proce- dure to perform and to interpret, bear- ing in mind the usual pitfalls caused by the ductus which are beyond the scope of this article. The necessity for angiography inpatients with a normal chest radiograph Savastano et al state that angiography is not necessary in patients with blunt chest trauma whose chest radiographs show no to page 11 Mediastinal Assessrnent in Traumatic Rupture of the Aorta signs ofleft haemornediastinum." White and Mirvis state that a normal radiograph excludes traumatic aortic rupture (TAl) unless there is a compelling clinical reason to suspect the diagnosis, and usually obvi- ates further studies." They were unaware of any chest film falsely negative for TAl in their institution. Woodring and King how- ever did have two cases with small false aneurysms of the isthmic region with nor- mal radiographs, and four cases with no mediastinal haemorrhage but large false aneurysms visible on the film.' They con- cluded that chest radiography has limi- tations in detecting vascular injury but Table II: Learning points 1. Mediastinal measurements are made on a supine film 2. Transverse mediastinal width (TMW) is measured above the aortic knuckle 3. Maximum TMW can usefully be modified to 7.5 cm 4. MIC ratio is of little value 5. Mediastinal measurements within normal limits do not exclude major vascular injury 6. Pay close attention lo mediastinal abnormalities, ask advice of a senior colleague if unsure 7. It is acceptable to have a low threshold for performing aortography when there are mediastinal abnormalities 8. A large number of false positives suggesting aortic rupture are to be expected on chest radiography 9. Angiography is not necessary in patients with a normal chest radiograph 10. If aortic rupture is suspected perform aortography not CT nevertheless have adopted a policy of per- forming aortography based on specific mediastinal abnormalities and no longer perform precise measurements ofTMW andMiCratio. It seems reasonable therefore to accept that a normal chest radiograph precludes the need for aortography, but it is impor- tant to be aware that mediastinal widening may progress whilst the patient is in hospi- tal. We have observed this phenomenon on occasion at our institution. Timing of angiography Does the aortograrn have to be performed immediately, or can there be a delay? This is an ethical problem even though current evidence points to a benign immediate course of aortic false aneurysms. These data have been gathered retrospectively,and as stated above more critically ill patients who formerlywould have died may present to trauma units, i.e. the potential for early rupture of a false aneurysm still exists. I consider that urgent angiography should still be done, for although the potential for early rupture is small, the consequences of missing such an injury are catastrophic. The plain radiographic features that suggest or are associated with aortic rup- ture are well known, and I shall not elabo- rate on them further. (See Table 1). Note the learning points as described in Table II. Acknowledgement I am most grateful to Dr Hillel Goodman for his helpful comments. References I. Woodring JH, and King Je. Determination of Nor- mal Transverse Mediastinal Width and Mediastinal- width to Chest-width (M/C) Ratio in Control Subjects: Implications for Subjects with Aortic or BrachiocephalicArteriallnjury. The Journal of Trauma I 989;29:No 9:1268-1272. 2. Marsh DG and Sturm Jr. Traumatic Aortic Rup- ture: Roentgenographic Indications of Angiography. TheAnnals of Thoracic Surgery ApriI1976;21:No 4:337- 340. 3. Parmley LF, Mattingley TW, Manion WC et al. Nonpenetrating traumatic injury of the aorta. Circu- lation 1958;17:1006-1101. 4. Pais SO. Diagnostic and Therapeutic Angiography in the Trauma Patient. Seminars in Roentgenology July 1992;XXVll:N03:211-232. 5. 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