Fenestration is a rare but well-known arterial anatomic variation in which a segment of artery divides into two distinct lumens that reunite distally. This type of variation is closely related to aneurysm formation, and the incidence of aneurysms in double-origin PICA ranges from 50 - 71% 17. The reported prevalence of this variation is about 1.45% 16. Double origin of the PICA is a rare variation different from duplication, in those cases two distinct PICAs with separate origins converge distally.

Duplication of the PICA occurs in 2 - 6% of hemispheres 1, 10, 15. If one PICA is hypoplastic or absent, the ipsilateral anterior inferior cerebellar artery (AICA) or the contralateral PICA is larger and supplies the area normally nourished by the ipsilateral PICA 10. Unilateral absence of PICA is reported in 6 - 26% of cases, absence bilaterally of PICA is reported in 2 - 3.6% of cases 1, 11 - 14. The PICA is observed to be hypoplastic in 15 - 32% of the cases 9, 10. The PICA shows a high frequency of variations. Furthermore, PICA mostly supplied the choroid plexus on the roof and the median opening of the fourth ventricle and gave the majority of branches of choroid plexus 8. The PICA supplies the medulla, the choroid plexus and tela choroidea of the fourth ventricle, the tonsils, the inferior vermis, and the inferior aspects of the cerebellar hemispheres 1, 7. The trunk of PICA gives rise to perforating, choroidal, and cortical branches 1. The trunk of PICA is divided into five segments (Figure вЂ‹ (Figure1): 1): (1) the anterior medullary segment, which begins at the origin of the PICA and ends at the level of a rostrocaudal line that passing through the most prominent part of the inferior olive (2) the lateral medullary segment, which extends from the level of the most prominent point of the olive to the level of the origin of the glossopharyngeal (CN в…Ё), vagus (CN в…©), and accessory (CN в…Є) rootlets (3) the tonsillomedullary segment, which begins where the PICA passes posterior to the CN IX-XI and ends at the midpoint of the PICA's ascent toward the roof of the fourth ventricle along the medial surface of the tonsil (4) the telovelotonsillar segment, which begins where the PICA ascends to the mid-level of the medial surface of the tonsil and ends where the artery exits the fissures between the tonsil, vermis, and hemisphere to reach the suboccipital surface (5) the cortical segment, this segment begins where the PICA leaves the groove between the vermis, tonsil and hemisphere, and includes the terminal cortical branches 1, 6. It is the fourth segment of VA that gives off the PICA, which is the largest branch of VA 3 - 5. The first three segments were extracranial and the fourth segment is entirely intracranial and terminates with forming the basilar artery (BA). The VA, arising from the subclavian artery, is classically divided into 4 segments. The posterior inferior cerebellar artery (PICA) usually originates from the vertebral artery (VA) at an average distance of approximately 16 or 17 mm below the vertebrobasilar junction 1, 2.

In conclusion, PICA is very important in clinical practice. The PICA can be exposed and injured during surgeries especially in telovelar approach, and it also plays an important role in bypass surgeries, hinting the surgical importance of PICA. The PICA supplies many brain tumors and can be used in intracerebellar chemotherapy. VA terminating in PICA probably cause Bow hunter's syndrome (BHS). PICA injured by head trauma can cause fatal SAH.

Arteriovenous malformation (AVM) fed by PICA are associated with aneurysm and dissection which have high risk of rupture and worse outcome. The PICA frequently compresses the medulla and the cranial nerves resulting in various neurovascular compression syndromes (NVCS). The PICA infarct typically manifests lateral medullary syndrome (LMS) and is more likely to cause mass effects. Various aneurysms involving PICA were not uncommon, of which the treatment is challenging. The PICA has tortuous and variable course and territory, divided into 5 segments. In this study, we perform a literature review of PICA by searching all the associated papers in the PUBMED database hoping to provide a better understanding of the artery. However, a comprehensive systematic review of the importance of the PICA is currently lacking. The posterior inferior cerebellar artery (PICA), with its unique anatomical complexity, is of great clinical importance and involved in many diseases including aneurysm, ischemic stroke, neurovascular compression syndrome (NVCS), arteriovenous malformation (AVM), and brain tumor.

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