Carotid-basilar anastomoses develop during foetal life between the internal carotid and basilar arterial systems for the supply of the posterior cranial circulation. The channels include trigeminal, hypoglossal, otic and pro-atlantal intersegmental arteries. With the development of the posterior communicating artery, these channels get obliterated but may rarely persist into adult life. The persistent trigeminal artery is the most common anomaly among the four, followed by the hypoglossal artery. These channels are detected as incidental findings but may be associated with aneurysms, tumours, subarachnoid haemorrhage and arterio-venous malformations.6 We describe MR angiographic (MRA) appearances of a persistent hypoglossal artery in a patient with complaints of titubation.

Case Report
A 59 years old female patient presented with history of titubation and loss of hearing on the left side for the past six months.

Clinical examination revealed conductive deafness on the left side and right seventh nerve involvement (lower motor neuron). Patient was non-diabetic and non-hypertensive. She was further evaluated with MRI brain and MRA of the neck and Circle of Willis using 2D Time of Flight (TOF) and 3D Time of Flight sequences and additional thin high resolution T2 weighted sequences.

The MRI of the brain was normal.

The MRA showed the right vertebral artery to be hypoplastic.

Both vertebral arteries were hypoplastic, left more than right and the left was visualized only in its proximal one fourth segment. Incidentally detected was a persistent hypoglossal artery on the left arising from ICA, which was traversing through the hypoglossal canal.

Fig. 1a and 1b : T2 Weighted DRIVE sequence showing a persistent communication between the anterior and posterior circulation on the left side via the hypoglossal canal.
Fig. 2a and 2b : MR Angiography source images confirming the findings in Fig. 1.

Discussion
The persistence of a primitive hypoglossal artery is a rare anomaly with an estimated incidence of 0.027% to 0.2% of all cerebral angiograms.1-6 In the 4 mm stage embryo, two longitudinal arteries are formed along the basal surface of hind brain. These vessels are supplied by anastomotic channels that connect them to the internal carotid arteries. Craniocaudally, they are trigeminal, otic, hypoglossal and pro-atlantal intersegmental arteries. Normally, these anastomoses remain functional for about 7 to 10 days during the early stage of foetal development and then obliterate at the rate the posterior communicating arteries develop.7 With the development of the posterior communicating artery and vertebrobasilar system these channels regress. Otic is the first to disappear followed by the hypoglossal. The hypoglossal artery usually arises from the internal carotid artery and rarely from the external carotid artery. Arising from the internal carotid at the C1-C2 vertebral level it runs dorsally and lies lateral to the hypoglossal nerve to enter the hypoglossal canal and then into the posterior fossa to join the basilar artery. The artery widens the hypoglossal canal as noted on CT and tomograms. Both vertebral arteries are usually absent or hypoplastic and the ipsilateral posterior communicating artery may not be angiographically demonstrated as seen in our patient. In a review of 115 cases of persistent hypoglossal artery, the most common association was intracranial arterial aneurysms followed by ischaemic cerebrovascular attacks, brain tumours, subarachnoid haemorrhage, trauma and arteriovenous malformations.

When an aneurysm arises directly from the hypoglossal artery, ligation of the artery may be disastrous as it may be the only supply to the brainstem. Presence of this artery may cause confusion during evaluation of suspected cerebrovascular symptoms and it may complicate the performance of carotid endarterectomy.2 Angiography clearly demonstrates the anomaly but recognition of this anomaly on non-invasive imaging is important. Differentiation between the hypoglossal and the less common proatlantal intersegmental artery on angiographic examination requires careful analysis because these primitive arteries take a similar course.8 The proatlantal intersegmental artery enters the posterior fossa through the foramen magnum, following the course of the vertebral artery. Therefore, differentiation between these primitive arteries on MR will be easy by demonstrating the foramen through which it passes.9 Preoperative knowledge of these channels may suitably forewarn the surgeon in cases of skull base surgeries or carotid endarterectomy, helping them avoid disastrous results, as this vessel may be the sole supply to the brainstem. The present report has shown that MR angiography is useful in non-invasive identification of the anomalous artery.

References

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