Vascular anatomy of posterior fossa

Posterior fossa contains vital structures including cerebellum and brain stem and Vertebrobasilar vascular tree. Posterior fossa is supplied by AICA, PICA, SCA and PCA and their branches.

1. SURESH BISHOKARMA, MS MCH RESIDENT, NEUROSURGERY NINAS VASCULAR ANATOMY OF POSTERIOR FOSSA

2. In Posterior Fossa Rules of

3. Courses of three cerebellar arteries Segments of three cerebellar arteries Perforators R elationship with CNs and clinical significance P reoperative surgical strategy and safe surgery in the posterior cranial fossa. AIM OF PRESENTATION

4. SCA originates from the basilar artery (BA) near its apex. It dips caudally and encircles the brainstem near the pontomesencephalic sulcus. After passing CN V, SCA enters the cerebellomesencephalic fissure, where its branches supply the superior cerebellar peduncle and are finally distributed to the tentorial cerebellar surface. SCA usually arises as a single trunk, but may occasionally arise as a double trunk in 15–30 % of cases. SCA

5. SCA has four segments: ( 1) Anterior pontomesencephalic ( 2) Lateral pontomesen - cephalic ( 3) Cerebellomesencephalic ( 4) Cortical SEGMENTS OF SCA Hardy DG et al .

6. The anterior pontomesencephalic ( pontine ) segment begins at the origin of SCA, extending laterally below CNs III to the anterolateral margin of the brainstem The lateral pontomesencephalic (ambient) segment begins at the anterolateral margin of the brainstem and travels along the lateral part of the pons, parallel to CN IV

7. SCA BIFURCATION SCA often bifurcates into rostral and caudal trunks in this segment The rostral trunk supplies the cerebellar vermis and the medial part of the cerebellar hemisphere, whereas the caudal trunk supplies the lateral part of the cerebellar hemisphere lateral pontomesencephalic (ambient) segment

8. The cerebellomesencephalic ( quadrigeminal ) segment courses within the cerebellomesencephalic fissure. Bilateral SCAs become proximate in this segment. This segment gives rise to the precerebellar artery, which supplies the superior cerebellar peduncle. cerebellomesencephalic ( quadrigeminal ) segment

9. The cortical segment gives rise to branches distal to the cerebellomesencephalic fissure, which pass below the tentorial edge and are distributed to the tentorial cerebellar surface. The vermian branches ascend the culmen after dipping caudally at the level of the declive and anastomose with the vermian branches of PICA on the tentorial cerebellar surface. The marginal artery is the most proximal of the cortical branches. It usually arises from the pontomesencephalic segment and courses along the anterolateral margin of the cerebellum; therefore, it is also called the anterolateral marginal artery. cortical segment

10. CNs III, IV, and V SCA passes below CN III and above posterior cerebral artery ( PCA): IP cistern CN IV: passes superomedial to the lateral pontomesencephalic segment of SCA as it traverses forward: cerebellomesencephalic fissure. : penetrating the dura mater at the posterior edge of the oculomotor trigone to enter the cavernous sinus. CN V : lateral pontomesencephalic segment of SCA: Trigeminal neuralgia. RELATIONSHIP TO CRANIAL NERVES

11. D irect The direct type pursues a straight course to enter the brainstem in the interpeduncular fossa. C ircumflex types. The circumflex type winds around the brainstem and supplies the tegmentum , cerebellar peduncle, and inferior colliculus . SCA gives rise to the precerebellar arteries in the cerebello - mesencephalic fissure: inferior colliculus , superior medullary velum, and dentate nucleus. The superior colliculus is mainly supplied by PCA. PERFORATORS OF SCA

12. SCA may arise infrequently from the proximal PCA and pass above the oculomotor nerve

13. AICA originates from BA and courses along the pontomedullary sulcus laterally, forming a neurovascular com- plex with CNs VII and VIII near the porus acusticus . It travels in the cerebellopontine fissure, finally supplying the petrosal cerebellar surface The size of AICA is usually in inverse proportion to that of PICA. AICA exhibits several variations. A single AICA present on each side is usually found in most cases. However, two ipsilateral AICAs are evident in approximately 30 % of cases. AICA often bifurcates into rostral and caudal trunks in the cerebellopontine cistern, The rostral trunk usually travels above the flocculus laterally, giving rise to the perforating arteries to the middle cerebellar peduncle in the floor of the petrosal fissure, and terminates at the petrosal cerebellar surface . The caudal trunk travels below the flocculus laterally and supplies the lower part of the petrosal cerebellar surface. The peripheral branches of the rostral trunk anastomose with those of SCA, whereas the peripheral branches of the caudal trunk anastomose with those of PICA. AICA

14. AICA is divided into four segments: ( 1) Anterior pontine , ( 2) Lateral pontomedullary , ( 3) Flocculopeduncular , and ( 4) Cortica l SEGMENTS OF AICA

15. A nterior pontine segment begins at the proximal part of BA, courses laterally on the dorsal pons parallel to pontomedullary sulcus, and terminates at the supraolivary fossette —a concavity in the lateral corner between the pons and the medulla oblongata. Large perforators arising from this segments supplies Brainstem Related to CN6 Anterior pontine segment

16. The lateral pontomedullary segment begins near the root exit zone of CN VII in the supraolivary fossette and passes above, below, or between CNs VII and VIII near the porus acusticus LATERAL PONTOMEDULLARY (MEATAL) SEGMENT Branches: Subarcuate , Internal auditory, and Recurrent perforating arteries, (related to hearing )

17. The flocculopeduncular segment begins where the artery passes CNs VII and VIII, traveling around the flocculus to reach the lateral part of the middle cerebellar peduncle. FLOCCULOPEDUNCULAR SEGMENT R oot exit zone of CN VII may cause hemifacial spasm Elongated segment can cause TN. In MVD for hemifacial spasm, great care should be taken in mobilizing AICA upwards because the perforators are extremely short.

18. The cortical segment predominantly supplies the petrosal cerebellar surface. Cortical segment

19. AICA also gives rise to the choroidal arteries, which pass near the foramen of Luschka and supply the lateral part of the fourth ventricle and choroid plexus. Note: Medial part of 4 th Ventricle is supplied by PICA.

20. Variation of AICA The common trunk type of AICA Duplicated AICAs ~ 30 % of cases

21. PICA PICA usually originates from intracranial VA at the level of the inferior olive or slightly inferiorly to it and passes near the rootlets of CN XII to reach near the foramen of Magendie . Then , it passes through the cerebellomedullary fissure between the tonsil and the posterior part of the roof of the fourth ventricle , reaches the cerebellar hemispheric and vermian surfaces, and finally supplies the suboccipital cerebellar surface

22. Anterior medullary Lateral medullary Tonsillomedullary Telovelotonsillar Cortical SEGMENTS OF PICA

23. The anterior medullary segment begins where the artery arises from VA, courses anterior to the pyramid and inferior olive, and passes near the nerve rootlets of CN XII in the preolivary sulcus—the boundary between the pyramid and inferior olive. Anterior medullary segment

24. The lateral medullary segment begins where the artery passes the most prominent point of the inferior olive and ends at the level of the rootlets of CNs IX, X, and XI, all of which arise in the retro- olivary sulcus Lateral medullary

25. The tonsillomedullary segment begins where PICA pos - teriorly passes the lower CNs and terminates on the posterior surface of the medulla oblongata near the caudal half of the tonsil. This segment courses in the cerebellomedullary cis- tern around the jugular tubercle and then makes a downward loop between the lower margin of the tonsil and the medulla oblongata before entering the cerebellomedullary fissure. This downward loop passing near the inferior pole of the tonsil is termed the caudal loop TONSILOMEDULLARY

26. The telovelotonsillar segment courses in the cerebello - medullary fissure, which forms the lower half of the roof of the fourth ventricle, and reaches the posterior aspect of the tonsil or the vermian surface. This segment never crosses the fastigium of the vermis . It often forms an upward loop, called the cranial loop, coursing around the superior part of the tonsil. PICA divides into the vermian (medial) and the tonsillohemispheric (lateral) trunks after it exits the cerebellomedullary fissure. TELOVELOTONSILLAR

27. Vermian (medial) and the T onsillohemispheric (lateral) trunks

28. The cortical segment is distributed on the suboccipital cerebellar surface, the inferior vermis , and the tonsil. CORTICAL SEGMENT

29. PICA exhibits a varied relationship with CNs VII–XII in the cerebellomedullary cistern. Lateral medullary segment : Root exit zone of CN VII: Hemifacial spasm Relation with CN X and XI 1/3: Pass between the nerve rootlets of CN X. 1/3: pass between CNs X and XI. 1/3: pass between the nerve rootlets of CN XI. Only Rare: Glossopharyngeal neuralgia: Tortous PICA. Realtionship with CNs

30. PICA gives rise to perforators, choroidal arteries, and cerebellar cortical branches. Arising from anterior and lateral medullary and tonsillomedullary segments: anastomose : posterolateral surface of the medulla oblongata. T elovelotonsillar Perforators: Dentate nucleus. C horoidal arteries in the cerebellomedullary fissure: Medial part of the fourth ventricle Note: Lateral part of fourth ventricle is contributed by AICA Perforators

31. Approximately 20 % of PICAs originate from the extracranial VA. PICAs are of high or low origin: the former arises approximately 20 mm above and the latter approximately 10 mm below the foramen magnum. Small or absent PICA ~ 16 % The size of PICA is usually in inverse proportion to that of AICA. Variation

32. Representative images

33. Thank you  References: Rhoton ; Neurosurgery:2002 Toshio Matsushima: Microsurgical Anatomy and Surgery of the Posterior Cranial Fossa