Clinical History
A 68-year-old diabetic man presented with acute onset giddiness and slurred speech for the past 2 days. On examination the patient had reduced pain sensation in the right half of face and left half of the body. No definite motor deficit was seen.
Imaging Findings
A focal T1W hypointense and T2W/FLAIR hyperintense region showing restricted diffusion was seen in the right posterolateral aspect of medulla in keeping with a recent lateral medullary infarct. An enlarged right vertebral artery (RVA) was seen with absence of its normal flowvoid suggesting thrombosis. MR angiography of neck and intracranial arteries confirmed thrombosis of the intracranial portion of the RVA. A short segment of the distal RVA proximal to its union with the left vertebral artery (LVA) appeared spared.
An abnormal artery was seen originating from the external carotid artery at C2 vertebral level coursing posterosuperiorly in the left paraverterbral region before turning medially above the left transverse process of the atlas to enter the foramen magnum. This artery continued as the LVA. The characteristic origin and course of this artery confirmed the diagnosis of a persistent Type II left proatlantal artery. The LVA in neck was not visualised.
Discussion
A number of anastomotic channels exist between carotid and vertebrobasilar arterial systems in fetal life, namely the trigeminal, otic, hypoglossal and proatlantal arteries. Persistence of these primitive arteries into adult life may occur, the most common among these being the persistent trigeminal artery. A persistent proatlantal artery is considered rare [1].
Current knowledge about embryological development of cranial arteries is mainly derived from work done by Dorcas Padget. At around the 4 mm embryonic stage a pair of internal carotid arteries (ICA) and a pair of longitudinal neural arteries dorsal and parallel to the ICA are seen. The communicating branches between the ICA and longitudinal arteries on each side are named the trigeminal, otic and hypoglossal arteries. By the 4-5 mm stage the proatlantal arteries form another communicating channel. At 5-6 mm stage the definite posterior communicating arteries are formed with regression of the other communication channels. The proatlantal artery regresses last by around the 12-14 mm stage. Failure of regression results in a persistent proatlantal artery (PPA) [1, 2].
Two types of PPA have been described. Type I arises from the cervical ICA while Type II arises from the external carotid artery (ECA) usually at second cervical vertebral level. The type I artery also shows a more anteromedial course than type II and passes through the foramen transversarium of the atlas while the latter does not. Both types enter the foramen magnum and continue as the ipsilateral vertebral artery [1]. Rest of the vertebral artery on the same side is aplastic in most cases [3].
Based on patient symptoms the MRI study in our case was done primarily to rule out a cerebrovascular accident. MRI showed classic features of a recent lateral medullary infarct, namely T1W hypointense and T2W/FLAIR hyperintense signal and restricted diffusion along the right dorsolateral aspect of medulla with thrombosis of the right vertebral artery [4]. The vertebral artery was also the most commonly affected artery in other studies on lateral medullary infarction, followed by the posterior inferior cerebellar artery [5]. Our case also showed an anomalous artery originating from the ECA with a characteristic course in the left paravertebral region before entering the foramen magnum and continuing as the left vertebral artery confirming the diagnosis of a Type II left PPA. The PPA was likely an incidental finding in our case. No previous reports linking PPA and lateral medullary infarction have been made to the best of our knowledge.
Differential Diagnosis List
Right lateral medullary infarct with persistent left Type-II proatlantal artery.
Persistent Type I proatlantal artery
Persistent hypoglossal artery
Final Diagnosis
Right lateral medullary infarct with persistent left Type-II proatlantal artery.
