LAUSR

Dear Editor, Both arterial spin labeling (ASL) and 3D time-of-flight MR angiography (TOF MRA) techniques visualize blood that flows in the caudocranial direction into the head, which in normal circumstances is limited to arterial structures. High signal on these sequences in the dural venous sinuses is often attributed to shunting lesions like dural arteriovenous fistulae, but venous reflux is increasingly recognized as an important mimic[1–4]. Surprisingly, Iwamura and colleagues reported that venous reflux to the cavernous sinus is exclusively observed on MRA and not on ASL [2]. They observed ASL signal in the cavernous sinus only in cases of dural arteriovenous fistula and concluded that this finding had perfect specificity for a fistula. A limitation of their study was that ASL analysis was not reported for cases without high MRA signal in the cavernous sinus. Therefore, they may have missed cases of venous reflux that resulted in high signal in the cavernous sinus on ASL but not MRA. We report such a case from our institution. A 69-year-old man presented to the emergency department for an episode of dizziness. MRI/MRA was performed to exclude a vascular etiology (Fig. 1). High signal on TOF MRA and ASL colocalized to the jugular bulb, sigmoid sinus, and inferior petrosal sinus (Fig. 1a–d). ASL also demonstrated high signal in the transverse, cavernous, and intercavernous sinuses, but no signal abnormality at these sites was apparent on MRA. Contrast-enhanced MRA of the neck revealed a right-sided aortic arch with aberrant left subclavian artery and apparent aortosternal compression of the left brachiocephalic vein (Fig. 1e). Further evaluation was conducted (Fig. 2). CT angiography of the head showed no asymmetrically enlarged external carotid artery branches, engorged venous structures, or early venous contrast opacification to suggest arteriovenous shunting (Fig. 2a–d). Rather, on the venographic phase, the left sigmoid sinus and jugular bulb showed delayed, mild enhancement without evidence of venous thrombosis. Cerebral digital subtraction angiography (DSA) was performed after selective catheterization of the bilateral vertebral and right carotid arteries; the left common carotid artery was not selected owing to the technically challenging configuration of the right aortic arch. There was no evidence of a dural arteriovenous fistula, but the left sigmoid sinus did show stagnation and reflux on venous phase (Fig. 2e). Various mediastinal and aortic pathologies can compress the brachiocephalic vein and cause jugular venous reflux and hemostasis, with the most common being atherosclerosis of the aorta and its proximal branches [5]. Compression almost always occurs on the left due the vein’s long course between the aortic arch and sternum. The current case corroborates this physiology and, to our knowledge, is the first reported in the setting of a right-sided aortic arch variant. Jugular venous reflux can extend to the intracranial venous structures when patients are supine for imaging, as flow is unimpeded by gravity. Additionally, given a lack of valves in the intracranial veins, retrograde flow may easily spread to contiguous dural venous sinuses. The most common pathway visualized on MRI/MRA involves the sigmoid and transverse sinuses, as these are large caliber and dependent sinuses. This case shows that venous reflux can also extend through smaller, anti-dependent intracranial pathways including the inferior petrosal, cavernous, and intercavernous sinuses. This letter is a response to the article 10.1007/s00234-020-02588-5