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|Year : 2014 | Volume
| Issue : 5 | Page : 580-581
Angiographic "Tau (τ)" sign in persistent trigeminal artery
Krishnan Prasad1, Eswararao Thamatapu1, Manash Saha2, Sayan Das2
1 Department of Neurosurgery, National Neurosciences Centre, Kolkata, West Bengal, India
2 Department of Radiodiagnosis, Peerless Hospital and Bhudeb Kanti Roy Research Centre, Kolkata, West Bengal, India
|Date of Web Publication||12-Nov-2014|
Flat 3B, 9 Southend Park, Kolkata - 700 029, West Bengal
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Prasad K, Thamatapu E, Saha M, Das S. Angiographic "Tau (τ)" sign in persistent trigeminal artery. Neurol India 2014;62:580-1
Brain magnetic resonance imaging (MRI) of a 44-year-old lady done as part of the evaluation of non-specific headache, revealed a left side vascular anomaly. Neurologic examination was normal. MR-angiogram with Time Of Flight (TOF) sequence showed a persistent trigeminal artery on the left side extending backwards by the side of the sella from the internal carotid artery to the basilar artery [Figure 1]. Coronal T2 and reconstructedimages showed that the artery was joining the basilar trunk below the origin of the superior cerebellar artery [Figure 2]. The vertebrobasilar complex below the union was hypoplastic. Sagittal reconstructions showed that both posterior communicating arteries were absent and "tau" sign was present at the site of origin of the vessel from the internal carotid artery [Figure 3].
|Figure 1: MR angiogram [Time of Flight sequence] showing a persistent trigeminal artery on the left side extending backwards by the side of the sella from the internal carotid artery to the basilar artery|
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|Figure 2: Coronal T2 and reconstructed MR angiogram showing that the artery was joining the basilar trunk below the origin of the superior cerebellar artery. The vertebrobasilar complex below the union is hypoplastic. Both superior and posterior cerebellar arteries are seen originating from the distal basilar artery|
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|Figure 3: Sagittal reconstructions showing that both posterior communicating arteries are absent and "Tau" sign is present at the site of origin of the vessel from the internal carotid artery|
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Persistent trigeminal artery is an embryological remnant of the anastamosis that existed in fetal life between the developing carotid arteries and the longitudinal neural arteries that go on to make the vertebrobasilar system. , Its incidence is reported to be 0.1% to 1%  and it isthe commonest (accounting for 85%) , of all the fetal carotid basilar anastamosis that may persist- the others being the persistent otic, hypoglossal and proatlantal arteries.  Its termination may be in one of 3 ways - anastamosing with the basilar artery below the superior cerebellar artery and supplying both posterior cerebral and superior cerebellar arteries (Saltzman Type 1), anastamosing with the basilar artery below the superior cerebellar artery but supplying only both superior cerebellar arteries and one or both posterior cerebral arteries being supplied by the posterior communicating arteries (Saltzman Type 2) or it may end in one of the cerebellar arteries (Saltzman Type 3). ,,, Our case was one of Saltzman Type 1.
The "Tau (τ)" sign is a classic angiographic sign of a persistent trigeminal artery.  It is so called because of the visual similarity on sagittal images to the Greek letter "Tau" - the vertical and anterior horizontal limbs of which are formed by the internal carotid arteries and the posterior limb by the persistent trigeminal artery. The only other vessel that may arise in this vicinity from the ICA and course backwards to meet the basilar complex is the otic artery but its origin is lower and also the vertical limb of the ICA can be seen above its the point of origin thereby precluding any cause of confusion.
The existence of the persistent trigeminal artery is often underdiagnosed and underreported . Usually diagnosed incidentally, , it may rarely cause cranial neuropathies (like trigeminal neuralgia)  and this anomaly must be kept in mind during any surgeries involving this area to prevent iatrogenic injury. ,
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[Figure 1], [Figure 2], [Figure 3]