POSTERIOR FOSSA STROKE vascular syndromes with MRI correlation Sangam Kanekar, MD Assistant Professor Dept of Radiology and Neurology Penn State Milton S Hershey Medical Center Hershey, PA, USA

INTRODUCTION The posterior fossa is home for the brainstem and cerebellum. The brainstem contains all the cranial nerve nuclei and many efferent and afferent fiber tracts that connect the brain with the rest of the body while the cerebellum is the major organ of coordination for all motor functions, as well as mental activities of the brain. Any lesion in this region can cause multiple cranial palsy in addition to the sensory and motor deficit. Anatomy of this region is complex and more so the vascular anatomy. There is wide congenital variation of the posterior fossa vessels. Collaterals are extensive and boundaries are not well defined. In this exhibit we present the normal arterial anatomy of the posterior fossa and various territorial infarctions. We also describe the various syndromes associated with these infarctions which are significant for our neurology colleagues. In the era where we get very limited clinical information and examination findings it becomes more and more important for radiologist to get familiarize with these lesions.

VERTEBRAL ARTERY VA originates from 1st part of subclavian artery (*). In 5% of population, left VA directly arises from aortic arch. V1 segment extends from its origin to its entry into C6 foramen transversorium (FT). V2 segment continues in FT till atlas (C1). At C2 vertebra (+), it deviates laterally to adjust to the widest diameter of C1. It emerges out of C1 TF (@) to become V3 segment, which courses over the posterior arch of atlas (#). Next it pierces the dura at foramen magnum to become intracranial / V4 segment. Finally it joins with contralateral VA to form basilar artery, in front of pontomedullary junction V4 V3 V2  V4 V3   V1   V2  V3  V4 V2 @ V3 + V2 # Variations Normally L VA is dominant – 50% RVA dominance – 25% Codominance – 25%

VERTEBRAL ARTERY Branches V1- Muscular branches V2- Radiculomedullary artery to reinforce to ASA Muscular branches V3- Muscular branches Occasionally PICA V4 –Posterior spinal artery (PSA) () (Very rarely visualized on angiography) PICA () Anterior spinal artery (ASA) () (Not commonly visualized on angiography) V4 V3 V2  V4 V3   V1   V2  V3  V4 Blood supply to brainstem Supplies ventral & lateral medulla through anteromedial & anterolateral perforators from V4 segment ASA also gives rise to anteromedial & anterolateral perforators [Check the next few slides for territorial representation on MRI slices] V2 @ V3 + V2 #

Posterior inferior cerebellar artery (PICA) PICA most commonly originates from V4 segment of vertebral artery. In neurosurgical practice, it is divided it into anteromedullary (), lateral medullary (), tonsillomedullary (), telovelotonsillar () segments. PICA is closely related to the rootlets of lower cranial nerves (IX, X & XI) and passes posterior to the medulla to course medial to cerebellar tonsils (tonsillomeduallry and telovelotonsillar segments). Finally it divides into lateral hemispheric branch (@) and medial inferior vermian branch ().           @    @  @ Blood supply (Medulla & Cerebellum) Perforators to lateral and posterior medulla. Posterior spinal artery may originate from PICA directly to supply posterior medulla Also supplies cerebellar tonsils, inferior cerebellar hemisphere, inferior vermis and dentate nucleus [Check the next few slides for territorial representation on MRI slices] 

Posterior inferior cerebellar artery (PICA)    @  @ PICA most commonly originates from V4 segment of vertebral artery. In neurosurgical practice, it is divided it into anteromedullary (), lateral medullary (), tonsillomedullary (), telovelotonsillar () segments. PICA is closely related to the rootlets of lower cranial nerves (IX, X & XI) and passes posterior to the medulla to course medial to cerebellar tonsils (tonsillomeduallry and telovelotonsillar segments). Finally it divides into lateral hemispheric branch (@) and medial inferior vermian branch ().

BASILAR ARTERY (BA)     V4 V4 V4 V4 BA () originates from confluence of both vertebral arteries at pontomedullary junction and courses in the ventral sulcus of pons. In the interpeduncular fossa, it terminates below mamillary bodies by bifurcating into two posterior cerebral arteries (). It also gives rise to anterior inferior cerebellar artery () and superior cerebellar artery () along with many perforators.

BASILAR ARTERY (BA) BA Perforators ()       V4 V4 BA Perforators () Paramedian perforators are short & directly enter pons Circumflex perforators course laterally to reach the lateral surface and tectal region of pons Rarely thalamoperforators may originate from basilar top to supply thalamus Blood supply Pons, cerebellar peduncles, mid brain & occasionally thalamus from thalamoperforators

Anterior inferior cerebellar artery (AICA) AICA originates from proximal basilar artery () and courses laterally in close relationship with 7th and 8th cranial nerves. It takes a characteristic loop () near the internal auditory canal. It gives rise to labyrinthine artery and finally divides into caudal and cranial branches. It shows variations frequently in origin and may arise in a common trunk with PICA () . When PICA is dominant (#), AICA is often hypoplastic(@) and vice versa      # @ Blood supply AICA supplies pons, middle cerebellar peduncle, 7th & 8th cranial nerves, labyrinth, flocculus, petrosal surface of cerebellum and choroid plexus in foramen of Luschka

Superior cerebellar artery (SCA) SCA is the most consistent cerebellar artery and originates just proximal to the bifurcation of basilar artery in the interpeduncular fossa (). It courses laterally around the brainstem and reaches posteriorly to come close to its counter part from right side. It is closely related to 3rd and 5th cranial nerves. It gives rise to perforators and 2 main branches, the lateral trunk () and medial trunk ()                   Blood Supply Cerebral peduncle, midbrain, superior vermis and superior cerebellum

Posterior cerebral artery (PCA) PCA originates as terminal branches of basilar artery in the interpeduncular fossa. It winds round the cerebral peduncle and courses in the ambient cistern to reach the quadrigeminal cistern. PCA is divided into 4 segments namely P1 extending from origin to posterior communicating artery (Pcom) (), P2 from Pcom junction with PCA to anterolateral corner of cerebral peduncle ($) and P3 from ending of P2 to posterior surface of quadrigeminal bodies ().  @   @   @  @    $  + +   @ @ Variations Fetal PCA () - originates directly from internal carotid artery with absent P1 segment Dominant Pcom - Very hypoplastic P1 with predominant blood supply to PCA from ICA via Pcom  @ 