Pseudopathologic Brain Parenchymal Enhancement due to Venous Reflux from Left-Sided Injection and Brachiocephalic Vein Narrowing

Discussion

Both original CT and MR angiograms were obtained with left-sided venous injections. Further review of the initial CT angiogram revealed narrowing of the retrosternal distance (Fig 2) to 0.2 cm and narrowing of the brachiocephalic vein ipsilateral to the side of injection, which was redemonstrated on MR angiography.

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Fig 2.

Axial source image from the CT angiogram of the neck demonstrates narrowing of the brachiocephalic vein between the brachiocephalic artery and the sternum, with a wire from a prior sternotomy. Retrosternal distance measures 2 mm at this level.

With normal conventional angiography and CT angiography by using a right-sided injection, the most likely explanation for the abnormal pattern of enhancement on the initial studies was functional and/or structural narrowing of the left brachiocephalic vein related to a narrowed substernal distance. We propose that this resulted in preferential retrograde flow into the left jugular vein and intracranial venous system with reflux through the transverse and sigmoid sinuses to the right intracranial and cervical venous system, in this case overwhelming vascular autoregulation such that parenchymal enhancement occurred. The mechanism for the left thyroid gland enhancement is less clear, though elevated venous pressures may be an explanation.

Artifacts related to left-sided venous injection for cross-sectional angiographic studies are well-documented in the literature. These may include attenuated contrast in the brachiocephalic vein obscuring adjacent arteries,¹ reflux of contrast into the jugular veins simulating filling defects, and simple reflux into the internal jugular vein and/or vertebral veins or vertebral venous plexus² reaching the dural venous sinuses.^3,4 None, however, have demonstrated retrograde brain parenchymal enhancement.

There are multiple suggested etiologies for the left-sided predominance of venous reflux. Anatomic differences in the course of the brachiocephalic veins may be a factor.⁵ The right brachiocephalic vein is more parallel to the ascending aorta, whereas the course of the left crosses the aorta and is, therefore, more susceptible to anatomic compression. An aberrant right subclavian artery may compress the left brachiocephalic vein.⁴ Hypertension or age-related ectasia and tortuosity of the aorta may also compress the brachiocephalic vein.

Retrosternal narrowing was evident in this case, which has been found to predispose to jugular venous reflux with left-sided injections. Tseng et al³ found that patients who were scanned serially with injections on both sides had no reflux when the retrosternal distance was 1.3 cm but did reflux with retrosternal distances of 0.65 cm. Our patient's retrosternal distance was 0.2 cm. The phase of respiration during contrast injection may also affect the retrosternal distance inversely with thoracic volume.⁴ Also, the jugular venous valves may be absent, more often on the left than the right in autopsy series.⁶ When present, venous valves may become incompetent, especially in patients with long-standing severe tricuspid regurgitation⁷ or after thoracoabdominal trauma.⁸ In addition to left-sided reflux, general venous return from injection of the upper extremities can be decreased by increased intrathoracic pressure, mediastinal masses, occlusion or stenosis of the superior vena cava, arm positioning, or congestive cardiac failure.^9–12 Any of these concomitant conditions may contribute to venous reflux or delay of the arterial phase of enhancement.

The best explanation for this unusual pattern of enhancement on our patient's initial CT and MR angiographic studies is retrograde flow into the cervicocranial venous system, due to left-sided injection with impaired venous flow at the level of the left brachiocephalic vein related to narrowing of the retrosternal distance. This explanation confirms and reinforces existing recommendations that CT or MR angiography be performed with right-sided injection when possible, to minimize venous reflux.