Vertebral artery (VA)
Only 15 VAs could be studied in the 25 cadavaric specimens acquired by the authors. In ten specimens the VA was cut during the separation of the specimen from the skull. The mean diameter of the VA was 3.4 mm on the left and 2.9 mm on the right. One specimen showed a left hypoplastic VA (less than 0.5 mm). Both the VAs were found to meet at the pontomedullary junction or up to 10 mm below it, at a mean distance of 2.7 mm below the pontomedullary junction. In no specimen was the junction of the VA above the pontomedullary sulcus. The angle of the fusion between the two VAs was found to be from 40o -80o (mean of 51o). No fenestration or duplications of the VA were seen in the specimens. The right and left VAs are usually different in caliber. The VAs on both the sides were equal in diameter in eight instances. The right was larger than the left in six and the left was larger than the right in one case. In our study, the anterior spinal artery originated from the VA 5-7 mm proximal to the vertebrobasilar junction on the right side (mean of 6 mm) and 4-8 mm (mean 6 mm) on the left side.
The posterior inferior cerebellar artery (PICA)
The PICAis the most distal branch of the VA. Only 11 PICAs could be studied on each side out of the 25 cadaveric specimens. The diameter of the PICA ranged from less than 0.5 mm (hypoplastic) to 2.5 mm (mean 1.23 mm) on the right side and less than 0.5 mm to 2 mm (mean 1.2 mm) on the left side. The site of origin of the PICA from the vertebral artery is variable and this to some extent determines its course. In our study the site of origin varied from 6 mm to 19 mm (mean 12 mm) proximal to the vertebrobasilar junction on the right side and 10-20 mm (mean 13 mm) on the left side.
Basilar artery (BA)
The BA begins at the junction of the two vertebral arteries, courses upward in the prepontine cistern in a shallow groove in the surface of the pons and it ends by bifurcating into the two PCAs. In 13 specimens (52%) the BA was curvaceous and elongated and in the rest it was straight. The diameter of the BA varied from 3-7 mm (mean of 4.3 mm). The length varied from 24-35 mm (mean of 24.9 mm). The origin of the BA was at the pontomedullary sulcus or up to 10 mm (mean 3.1 mm) below it. The bifurcation was 2-11 mm (mean 5.6 mm) below the mamillary bodies. No fenestrations, duplications or hypoplasia of the BA were noted. The BA gave off paramedian and circumferential perforating arteries that supply most of the pons and the mesenchephalon. The other larger branches included the anterior inferior cerebellar artery (AICA) and the superior cerebellar artery (SCA) and some times the internal auditory artery. The paramedian branches are the small branches that leave the BA and immediately pierce the pons. These were one to five (mean 3.5) in number on the right side and one to seven (mean 3.8) on the left side. The circumferential arteries go laterally and course around the pons and supply the lateral portion of the pons and the cerebellar peduncles. These range from one to five (mean 3.25) in numbers on the right side and one to seven (mean 3.4) in number on the left side. Thalamoperforators were seen to rarely arise from the basilar artery bifurcation. In one specimen a large thalamoperforator could be seen arising on the right side and in another from the left side of the midline and supplying the thalamo-geniculate area. Otherwise most of the thalamoperforators are seen to arise from the P1 portion of the PCA.
The origin of the AICA varied from 3-21 mm (mean 10.35 mm) from the vertebrobasilar junction on the right side and from 0-17 mm (mean 9.5 mm) from the vertebrobasilar junction on the left side. The diameter of the AICA varied from being hypoplastic i.e., less than 0.5 mm on the right side to 2 mm (mean 0.9 mm) and then again from less than 0.5 mm (hypoplastic) to 2 mm on the left (mean 1.11 mm). The AICA courses backwards from the pons and may cross the fourth cranial nerve dorsally or ventrally as it passes from the prepontine cistern to the cerebellopontine cistern. It passes along the flocculus of the middle cerebellar peduncle to supply the lips of the cerebellopontine fissure and petrosal surface. It commonly bifurcates near the faciovestibulocochlear complex to form the rostral and the caudal trunk. This bifurcation in our series was seen to occur from 8 mm from its origin to a very distal bifurcation on the right side (mean 7.8 mm) and on the left from 0-25 mm (mean 8.2 mm) from the origin. The rostral trunk follows the seventh and eighth cranial nerves towards the internal auditory meatus for a variable distance before looping beneath the cerebellum, while the caudal trunk traverses inferomedially near the brainstem to reach the inferior cerebellar surface. Duplication of AICA was noted in six instances - two on the right and four on the left. Whenever there was duplication the rostral portion of the AICA was seen to be nerve-related while the caudal portion was nerve unrelated. The AICA also gave rise to perforating arteries to the brainstem. We found one to two (mean 1.4) small perforators and 1.75 large circumflex perforators on the the right side and a mean of 2.3 small perforators and a mean of 2.0 large perforators on the left side.
The AICA was found to be larger than the PICA in nine occasions; the AICA was equal to PICA in four occasions and the AICA was smaller than PICA in eight cases in our series.
The SCA is the most consistent branch of the BA in terms of origin and location among the arteries of the posterior circulation. The SCA arose very close to the basilar bifurcation in our series - 1 to 3 mm from the basilar artery bifurcation (mean 1.45 mm on the right side and 1.33 mm on the left side). In one instance the left SCA took origin from the P1 segment of the PCA [Figure - 1].
The diameter of the SCA varied from 0.5 mm to 2.5 mm on both sides (mean 1.4 mm on the right side and 1.6 mm on the left side). There were no instances of hypoplasia or aplasia of the SCA. The SCA origin lies within the interpeduncular cistern and it is separated from the PCA by the occulomotor nerve. It encircles the brainstem in a groove between the pons and the midbrain. It passes below the trochlear nerve and above the trigeminal nerve. The proximal portion courses medial to the free edge of the tentorium and the distal part passes below the tentorium making it the most rostral of the infratentorial arteries.
After passing above the trigeminal nerve it enters the cerebellomesenchephalic fissure. On leaving the cerebellomesenchephalic fissure it branches again medial to the tentorial edge and supplies the tentorial surface. Within the ambient cistern on the lateral side of the brainstem, the artery makes a shallow caudal loop and then divides into lateral and medial branches or the caudal and rostral trunks respectively. The SCA arose as a single trunk in 42 out of 50 specimens and as a double trunk in eight specimens [Figure - 2]. The bifurcation of SCA into the rostral (medial) and caudal (lateral) trunks occurred at a mean distance of 8.6 mm on the right side and 7 mm on the left side from the origin of the SCA [Figure - 3]. The bifurcation, however, can be very proximal, almost at the origin itself where it is duplicated or it can also be at a very distal point from the origin. The SCA gives off perforating branches to the brainstem and the cerebellar peduncles. We found two to nine thalamoperforators originating from the SCA on each side. In addition one to four circumflex perforators were seen on either side arising from the SCA which supply the lateral portion of the brainstem and cerebellar peduncles.
The SCA divides into two trunks i.e., the rostral and the caudal trunk. The rostral (medial) trunk supplies the vermis and paravermian area and the caudal trunk (lateral) supplies the hemispheres on the suboccipital surface.
The PCA arises at the basilar bifurcation and is joined by the posterior communicating artery (PComA) at the lateral margin of the interpeduncular cistern. It encircles the brainstem passing through the crural and ambient cisterns to reach the quadrigeminal cistern. It is divided into four segments P1 to P4.
The P1 segment: The P1 segment also called the precommunicating segment extends from the basilar bifurcation to the junction with the PComA. The average length of the P1 was 5-10 mm (mean 6.8 mm) on the right side and 6-11 mm (mean 7.5 mm) on the left side. The average diameter of the PCA was 2-3.5 mm (mean 2.76 mm) on the right side, 1-3.5 mm (mean 2.5 mm) on the left side. In 39 specimens the P1 segment was found to be larger than the PComA, in six specimens it was found to be equal to the diameter of the PComA and in five specimens it was found to be smaller than the PComA diameter. In the latter case which is referred to as the fetal pattern of circulation, the PCA arises predominantly from the internal carotid artery (ICA). A fetal pattern of circulation was seen on both sides in only one case [Figure - 4]. In the fetal pattern of circulation the P1 was generally longer than in the normal pattern of circulation. Two main types of perforators were seen to arise from the P1 segment.
- Thalamoperforating arteries: These arise from the superior and posterior surface of the P1 and course superiorly and posteriorly and they divide into numerous branches that terminate in the interpeduncular fossa, posterior perforating substance, cerebral peduncles, mamillary bodies and posterior midbrain [Figure - 5]. These perforators were seen to arise as a bunch of small arteries or as large stem arteries which would later divide into a tuft of smaller perforators. On an average one to four small thalamoperforators (mean 2.0) were seen to arise from both sides of P1 and one to four (mean 2.0) large thalamoperforators were seen to arise from either P1 segment.
- Circumflex perforating arteries: These encircle the brainstem for a variable distance before entering the diencephalon and mesenchephalon and are divided into long and short groups depending on how far they course around the brainstem. About one to two circumflex arteries were seen to arise from the P1 segment on either side. Sometimes the medial posterior choroidal artery (MPCA) was seen to arise from the P1 segment. In our specimens thalamoperforators were rarely seen to arise from the upper segment of the BA. However, the upper segment of the BA gave rise to pontine branches.
The P2 segment: The P2 segment of the PCA extends from the junction of the PComA to the point where the common temporal origin takes place. The length of the P2 segment varied from 12-28 mm (mean 19.9 mm) on the right side and 10-28 mm (mean 18.44 mm) on the left side. The P2 anterior begins at the PComA and courses between the cerebral peduncle and uncus that forms the medial and lateral wall of the crural cistern inferior to the optic tract and basal vein that crosses the roof of the cistern to enter the proximal portion of the ambient cistern. The P2 posterior commences at the posterior edge of the cerebral peduncle at the junction of the crural and ambient cistern and courses between the lateral midbrain and parahippocampal and dentate gyrii which forms the medial and lateral wall of the ambient cistern below the optic tract and basal vein and geniculate bodies and inferolateral part of the pulvinar in the roof of the cistern superomedial to the trochlear nerve. Basically, the different branches of the P2 segment are as following.
- Thalamogeniculate arteries: These arise from the P2 beneath the lateral thalamus and penetrate the part of the roof of the ambient cistern formed by the geniculate body and surrounding area. About three to five (mean 3.5) were seen on the right side and one to seven (mean 4.0) on the left side arising from the P2 segment.
- Peduncular perforating arteries: These were one to six in number on the right side and one to seven on the left side with a mean of 3.0 on both the sides. These arose from the P2 segment and pass directly from the PCA into the cerebral peduncle. They supply the corticospinal, corticobulbar pathways as well as the substantia nigra, red nucleus and other structures of the tegmentum.
- Circumflex branches: As previously described these run parallel and medial to the PCA and may be short or long. About one to two circumflex branches could be seen arising from the P2 segment apart from one to two circumflex branches from the P1 segment on both the left and right side.
- MPCA: One to two were seen to arise from the P2 segment on both the sides.
- Lateral posterior choroidal artery (LPCA): One to two could be seen arising from either side from the P2 segment.
The common temporal origin, which forms the distal limit of the P2 segment, consists of the inferior temporal group of arteries, which arise from the PCA. Detailed dissection of these cortical arteries was not done as the authors concentrated on the cisternal anatomy of the posterior circulation.
The P3 segment: The P3 segment of the PCA extends from the inferior temporal artery or common temporal artery origin to the origin of the parieto-occipital and calcarine arteries. It is also termed the quadrigeminal segment. The segment of the PCA beyond this is termed as P4 segment and was not dissected.
The average length of the P3 segment varied from 13-38 mm (mean 22.4 mm on the right and 20.9 mm on the left). A single LPCA was usually seen to arise from the P3 segment. Peduncular arteries vary from none to four on the right side and two to five on the left side (mean 3.5 on the right and 3.8 on the left). Thalamogeniculate perforators ranged from none to six on the right side and one to eight on the left side with a mean of 4.0 on either side. Cortical branches too ranged from none to four on both sides with a mean of 3.0 cortical branches from the P3 segment.
The MPCA has a variable origin. It arose from the P1 segment in 38.6% cases and from P2 in 61.4% cases. There was a single MPCA in 43 cases and duplications were noted in seven cases [Figure - 6]. No cases of triplication or aplasia were seen. After arising from the posteromedial aspect of the PCA the MPCA encircles the midbrain medial to the PCA distributing branches to the peduncle, tegmentum, geniculate body and colliculi. It passes the pulvinar and turns forwards lateral to the pineal gland to enter the roof of the third ventricle between the thalami and ends at the foramen of Monroe in the choroid plexus.
The LPCA too has a variable origin. It was seen to arise from P 2 in 62% of cases, from the P3 segment of the PCA in 34% of the cases and from the common temporal artery origin in 4% cases. The number of LPCAs in each hemisphere ranged from one to two. Single LPCA was seen in 32 cases and double LPCA was seen in nine cases. From its origin the LPCA courses laterally into the lateral choroid fissure and to supply the choroid plexus of the temporal horn.
Posterior communication artery (PComA)
The PComA forms the lateral boundary of the circle of Willis. The diameter of the PComA varied from hypoplastic (less than 0.5 mm) to 4 mm (fetal circulation) on the right side (mean 1.25 mm) and 0.5 mm to 3 mm (mean 1.125 mm) on the left side. Length of the PComA ranged from 8 mm to 18 mm on both sides with a mean of 12.13 mm on the right side and 11.88 mm on the left side. The main branches from the PComA were the perforators which arose from the superior and lateral surfaces of the PComA and coursed superiorly to penetrate the tuber cinereum, premamillary part of the floor of the third ventricle and the posterior perforating substance of the interpeduncular fossa [Figure - 7]. The PComA branches ranged from two to 15 on both sides with a mean of 7.8 on the right side and 5.7 on the left side. As in the P1 segment there are basically two types of perforators: Small, fine delicate perforators and stem artery which would then divide into tuft of perforators.