Tau sign | Radiology Signs

Answer:

The tau sign describes the appearance on sagittal MR angiography or catheter angiography in which the flow void of the internal carotid artery, running as its own vertical limb, is joined posteriorly by a persistent primitive trigeminal artery coursing back to the basilar artery, so that the two vessels together trace the vertical stroke and horizontal crossbar of the Greek letter tau (τ). It marks the most common of the fetal carotid-vertebrobasilar anastomoses to persist into adult life, reported in roughly 0.1 to 0.6 per cent of angiographic studies, and is usually an incidental finding. The vessel is graded by the Saltzman classification: type I, in which it supplies the distal basilar artery while the proximal basilar segment and the posterior communicating arteries are hypoplastic or absent; type II, in which it supplies the superior cerebellar artery territory while the basilar artery still receives normal posterior communicating flow; and type III, in which it bypasses the basilar artery altogether and terminates directly in a cerebellar artery. Recognising the variant matters for planning, because the posterior circulation can depend substantially or entirely on the anastomosis, so inadvertent occlusion of the parent internal carotid artery during carotid intervention or skull base surgery risks posterior circulation ischaemia, and percutaneous procedures near Meckel’s cave carry a bleeding risk if the vessel is not recognised first. Older surgical series reported an increased prevalence of intracranial aneurysms with a persistent trigeminal artery, though more recent angiographic cohorts suggest the risk is closer to that of the general population; the vessel can still cause trigeminal neuralgia, hemifacial spasm or oculomotor palsy through neurovascular compression at the pons. The differential for a vessel crossing between the carotid and vertebrobasilar systems includes the rarer persistent hypoglossal, proatlantal and otic arteries, each with a different point of origin and course; the tau sign refers specifically to the trigeminal variant on the sagittal view and should not be confused with the analogous three-vessel trident, or bident, appearance of the same anomaly on lateral catheter angiography.

Why is it called so?

The vertical limb of the internal carotid artery and the artery running posteriorly to the basilar artery together trace the shape of the Greek letter tau.

Pathophysiology

In the early human embryo, before the posterior communicating arteries and the vertebrobasilar system mature, the developing carotid and hindbrain circulations are linked by four transient anastomoses: the trigeminal, otic, hypoglossal and proatlantal arteries. The trigeminal artery is the largest and most cranial of these and normally involutes once the posterior communicating arteries and the paired longitudinal neural arteries fuse into a competent basilar artery. Failure of this regression leaves a direct channel from the cavernous internal carotid artery, arising proximal to the origin of the posterior communicating artery, coursing posterolaterally alongside or through the trigeminal nerve root to join the basilar artery between the origins of the anterior inferior and superior cerebellar arteries. Because the vessel forms before the posterior circulation completes its own development, the segment of the basilar artery and the posterior communicating arteries proximal to the anastomosis often remain hypoplastic, which is why the pattern of flow, and the Saltzman type, follows directly from how much of the posterior circulation the persistent artery has taken over.

Alternative names: Persistent primitive trigeminal artery sign; PPTA

Other associated named signs: The trident (bident) sign of a persistent primitive trigeminal artery on lateral catheter angiography, and the rarer persistent hypoglossal, proatlantal and otic arteries, the other fetal carotid-vertebrobasilar anastomoses that can persist into adulthood

References

1. Goyal M. The tau sign. Radiology. 2001;220(3):618-9. PMID: 11526258.