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Biology Articles » Anatomy & Physiology » Anatomy, Human » Microsurgical anatomy of the posterior circulation » Discussion
- Microsurgical anatomy of the posterior circulation
The microsurgical anatomy of the posterior circulation, especially the cisternal anatomy is of importance to the cerebrovascular surgeons in surgical approaches to the various pathologies in this region. Apart from aneurysms and arteriovenous malformations, the neurosurgeon may be called upon to operate other pathologies like tumors in this area, for epilepsy surgery, posterior temporal lobectomies and posterior cerebral revascularization procedures.
Vertebral artery (VA)
Many anomalies of the VA have been described by various authors. Fenestrations, duplications, complete atresia and ending of one of the VAs as the PICA and the other continuing as the BA are some of the variations reported. However, in our series we could not find any such variations. Several small branches to the anterolateral, lateral and posterolateral medulla have been reported.
The PICA has a variable site of origin and this might determine its course. Most commonly it arises 14-16 mm below the vertebrobasilar junction in the anterior medullary cistern. It forms a caudal loop around the cerebellar tonsil to enter the cisterna magna. It has been reported by Yasargil et al that in 35% of the cases the artery will extend a few mm lower than the tonsil or the foramen magnum. Hence, the position of the caudal loop as seen on the angiography is not an entirely reliable sign to assess tonsilar herniation. The PICA is present as a single vessel in 90% of the cases, duplicated in 6% and absent in 4% according to the studies of Fuji and Rhoton. No such variations were noted in our study.
The basilar artery (BA)
The BA begins in the area of the pontomedullary sulcus by the union of the two VAs and courses upwards in the prepontine cistern in a shallow groove on the surface of the pons. With increasing age the BA becomes more tortuous and elongated and the bifurcation may lie much more superiorly. In the series of Yasargil et al only 25% of the arteries were seen to run in a straight course. However, in our study 74% of the cases the artery was found to be in the midline in a straight course and in the rest it was seen to be deviated from the midline with a tortuous course. Various anomalies like fenestrations, duplications (persistence of the paired basilar arteries of the embryo), persistence of carotidobasilar anastomosis such as premature trigeminal, optic, hypoglossal or proatlantic arteries etc have all been described. In our series, however, we could not find any such anomalies. The BA gives off paramedian and circumferential perforating arteries that supply most of the pons and mesenchephalon in addition to the larger branches like the AICA, the SCA and the internal auditory artery. The same was also noted in our series. Two twig arteries arising from the proximal portion of the BA supplying and penetrating the anteromedial medulla were seen in the series of Yasargil et al . This could not be noted in our series.
The origin of the AICA from the BA is variable. Yasargil et al have found solitary AICA in 58%, duplicated in 20%, triplicated in 20% and rarely absent in 2%. In our series solitary was found to be in 78% and duplicated AICA was seen in 22%. The AICA arose from the lower third of the BA in 7.14%, from the middle third in 60.7% and from the upper third in 32.14%. This was in contrast to the findings of Yasargil et al who have noted 84% of the AICAs arising from the lower third of the BA and 16% from the middle third. In the cerebellopontine cistern the AICA bifurcates into two main trunks:The rostral and caudal trunks.The rostral trunk is generally also referred to as the nerve-related trunk due to its relation to the VII-VIII nerve complex. Rich anastomoses exist between the peripheral branches of the SCA, AICA and PICA. There is usually an inverse relationship between the AICA and the PICA in their diameter which was noted in our series., The AICA is commonly exposed during surgeries of the cerebellopontine angle. Aneurysms of the AICA are rare. The AICA may be displaced or may be involved in the vestibular schwannomas of the cerebellopontine angle. Great care must be taken to preserve the AICA. An injury or occlusion of the AICA may result in ischemia or infarctions of the brainstem and cerebellar peduncles, rather than cerebellar hemispheric infarctions. The recovery of patients from surgical injuries to the AICA is possible because of the adequacy of the collateral circulations from the SCA and PICA as mentioned above.
The SCA is the most consistent artery of the posterior circulation in terms of origin and location. Usually the SCA arises from just below the basilar bifurcation and sometimes directly from the P1 segment of the PCA [Figure - 1]. The SCA frequently has points of contact with the occulomotor, trochlear and trigeminal nerve. The bifurcation of SCA into two major trunks, rostral and caudal according to Rhoton et al is 0.6 to 34 mm [average 19 mm] from the origin. In our series it was at a mean of 8.6 mm from the origin on the right and at a mean of 7 mm on the left. Six cases of duplicate origin of the SCA could be seen and the rostral and the caudal trunks were formed by the duplications rather than the divisions of the same later. The perforating branches of the SCA are divided into direct and circumflex type. Direct perforators have a straight course to enter the brainstem while the circumflex perforators wind around the brainstem before terminating in it. Rhoton et al have noted two to five perforating branches. In our series none to ten perforators were noted arising from the SCA. The commonest perforators were the circumflex type in our series. Constant cortical supply of the SCA is to the tentorial surface. It supplies the majority of the tentorial surface and frequently the adjacent parts of the petrosal surface. The SCA is seen to have points of contacts with the occulomotor, trochlear and trigeminal nerves.
Occulomotor nerve: The proximal part of the SCA passes below and is separated from the PCA by the occulomotor nerve. This position of the occulomotor nerve between the PCA and SCA is nearly constant. In one of our cases one of the fascicles of the occulomotor nerve was seen to be entangled among the branches of the MPCA resulting in traction on the former [Figure - 8]. The authors hypothesize that this could be one of the causes of congenital anisocoria. Sunderland suggests that the occulomotor nerve may occasionally be constricted between the PCA and SCA. Marinkovic and Gibo have also noted the presence of vessels which penetrate the nerve trunk itself in addition to the above phenomenon.
Trochlear nerve: This nerve may come in contact with the SCA in almost all cases at the cerebellomesencephalic fissure.
Trigeminal nerve: The SCA encircles the brainstem above the trigeminal nerve making a shallow caudal loop on the lateral side of the pons. The contact between the SCA and the trigeminal nerve occurs at the most prominent caudally projected loops. Some of the SCAs may have a contact with the trigeminal nerve and may involve the main rostral or caudal or both the rostral and caudal trunks. The point of contact of the SCA is usually the superior or superomedial aspect of the nerve and often a few fascicles of the nerve are indented or distorted by the vessels. This was seen in 12% of cases in the Rhoton et al study. The significance of these contacts in trigeminal neuralgia has been studied by many authors.
The situation of the BA bifurcation is an important determinant of the initial course of the SCA. The level of the bifurcation of the BA is normal if the bifurcation occurs at the ponto mesenchephalic junction, high if it occurs above it and low if it occurs below this. In our study the bifurcation was at the normal position in 56% of the cases, in 44% of cases above it and in none of the cases below it. In Rhoton's study, 18 of 25 i.e., 72% were normal, 24% were high and 4% were low-lying BA bifurcations. A transsylvian approach is required to approach the high basilar artery bifurcation while for the low and normal bifurcations, a surgeon can adopt the sub temporal approach.
The PCA arises as the terminal branch of the bifurcation of the BA. It is joined by the PComA at the lateral margin of the interpeduncular cistern. It is distributed to the posterior part of the cerebral hemisphere and supplies the posterior part of the cerebral hemispheres, thalamus, midbrain and deeper structures including the choroid plexus of the lateral and third ventricles.
P1 segment: The P1 segment also called as the precommunicating segment extends from the basilar bifurcation to the junction of PCA with PComA. The branches of the P1 segment include the thalamoperforators, the circumflex perforators, the MPCA, etc. In one instance the SCA was seen to arise from the P1 segment. Embryologically, it arises from the ICA. With development, the P1 segment of PCA usually enlarges to form a major connection between the BA and PCA with subsequent reduction in the size of the PComA. Persistence of the fetal type of posterior cerebral circulation where the P1 was smaller than PComA, was seen by Saeki and Rhoton in 22% of cases (unilateral in 20% and bilateral in 2%). Yasargil et al reported 67.5% cases of the adult type and 24.5% of fetal type and an equal representation in 8% of the cases. Kameyama and Okinaka reported similar results. In our study the fetal type of circulation was seen in 10%, the adult type of circulation in 78% of cases with an equal representation in 12% of cases.
P2 segment: The P2 segment begins at the PComA and lies within the crural and ambient cisterns and terminates lateral to the posterior edge of the midbrain where the origin of the inferior temporal arteries takes place. The P2 segment is further divided into anterior (P2A) and posterior halves (P2P).,,, Rhoton et al have found P2A and P2P to be 25 mm long together, whereas in our study it has been around 19 mm in length. It helps in identifying the origins of the branches. The branches of these segments include:
- Peduncular perforating arteries: These peduncular branches penetrate the cerebral peduncles.
- Thalamogeniculate arteries.
- Apart from perforating branches the P2 segment also gives rise to branches to the ventricular and choroid plexus branches i.e., MPCA and LPCA in different cases.
- The common temporal origin gives rise to inferior temporal branches such as the anterior temporal artery, middle temporal artery and posterior temporal arteries.
P3 segment: The P3 segment or the quadrigeminal segment of the PCA which proceeds posteriorly from the origin of the inferior temporal arteries continues to curve around the mesenchephalic tectum until it pierces the quadrigeminal cistern and reaches the lateral geniculate body under the pulvinar thalami. Rhoton et al have noted the average length of the P3 segment to be 2 cm. The P3 segment in our series had a mean length of 22.4 mm on the right and 20.9 mm on the left.
The branches of the P3 segment include [a] Peduncular perforating arteries, [b] the thalamogeniculate arteries,
[c] the cortical branches which may be from none to four in number with a mean of 3.0 in our study which included the inferior temporal branches, the posterior callosal arteries, the calcarine and parietooccipital arteries etc.
P4 segment: The PCA, after the origin of the parietooccipital and calcarine arteries, is called the P4 segment.
Branches of the PCA:
The following are the branches of the PCA:
a] Perforating branches to the diencephalon and midbrain
b] Ventricular branches of the choroid plexus and walls of the lateral and third ventricle and adjacent structures and
c] Cerebral branches to the cerebral cortex and splenium of the corpus callosum.
a) The perforating branches
The perforating branches or the central branches of the PCA are divided into two groups: The direct perforating branches and the circumflex arteries. Direct perforating branches are those which originate from the PCA and directly go to the brainstem. They include thalamoperforating branches, thalamogeniculate branches and peduncular perforating arteries. The first one arises from the P1 segment; while the latter two arise from the P2 and P3 segments of the PCA.
The circumflex branches encircle the brainstem for a variable distance before entering the diencephalon and mesencephalon. They are divided into long and short, depending upon the course around the brainstem.
- The thalamoperforating arteries: These arise from the initial part of the PCA i.e., the P1 segment. They enter the interpeduncular fossa that is surrounded by cerebral peduncles laterally and to the rostral pons caudally and mamillary bodies rostrally. They then penetrate the posterior perforating substance and supply the portions of the midbrain, thalamus, pre-tectum and subthalamic regions. Marinkovic et al have studied the thalamoperforators (posterior thalamoperforating arteries or interpeduncular thalamoperforating arteries) in detail. They have found that the thalamoperforating arteries vary in number from none to ten with an average of 2.0, while in our study a mean of 2.0 was found on both sides which conform to the Western literature. Rhoton et al have reported that the majority of the thalamoperforating arteries originated from the middle third of the P1 segment of the PCA. However, such a disposition was not noted in our series and we found the perforators to be distributed all along the P1 segment. Marinkovic et al have also reported that most of the perforating branches arose from the P1 segment and only 5.8% of these perforating branches originated from the P2 segment. In our study no thalamoperforators were seen to arise from the P2 segment. Rhoton et al have noted that some of the thalamoperforators originated from the posterior and lateral surface of the upper 1 cm of the basilar artery. In our studies no specimen revealed any thalamoperforators from the BA bifurcation except in one case where a large thalamoperforator was seen arising from the basilar tip. In our study thalamoperforators (interpeduncular perforators) were seen to originate from the P1 segment and the SCA only. These perforators arose as either as a large stem artery or as very fine arteries. Marinkovic et al have also reported common stem origin of the perforating arteries in 88.4% of the cases. These stems were arc-like or S-shaped. Their proximal part as a rule was convex towards the opposite side and ran just above the bifurcation of the BA. The branching of these common stem arteries was usually complex. The thalamoperforating arteries always arose from the posterosuperior aspect of the P1 and very rarely from the anterior surface of the P1. The origin of thalamoperforators was not necessarily symmetrical on both sides, which was also noted by Rhoton et al . Thalamoperforators supply the anterior and part of the posterior thalamus, subthalamus and medial part of the upper midbrain including the substantia nigra, red nucleus, occulomotor and trochlear nuclei, the occulomotor nerve, mesencephalic reticular formation, pretectum, rostro medial part of the floor of the fourth ventricle and posterior portion of the internal capsule. Occlusion of the thalamoperforating arteries depending upon the size of infarction or ischemia may produce a variety of focal syndromes including contralateral hemiplegia, cerebellar ataxia and tremor along with ipsilateral occulomotor nerve paresis (Nothnagel's syndrome).
The microsurgical anatomy of this area is very important, especially in the surgical treatment of cerebral aneurysms of the posterior circle of Willis. It is well know that posterior circulation aneurysms usually arise from the basilar bifurcation or from the P1 segment of the PCA or from the PComA and P1 junction. Hence it is imperative that great care should be taken during surgery in this area to preserve the interpeduncular perforating arteries.
- Peduncular perforating arteries: These are one to seven in number (mean 3.0), usually arise from the P2 segment and pass directly from the P2 segment of the PCA into the cerebral peduncle. They supply the corticospinal and corticobulbar pathways as well as the substantia nigra, red nucleus and other structures of the tegmentum. They may also send branches to the occulomotor nerve.
- Circumflex branches: The circumflex groups of arteries arise from the P1 or P2 segment of the PCA. They are one to four in number with a mean of 2.0. They are divided into short or large circumflex arteries. The short circumflex branches reach as far as geniculate bodies while the long may reach the colliculi. These branches course medial to the P2 and MPCA and send branches to the cerebral peduncle. The short circumflex arteries send branches to the interpeduncular fossa and posterior perforating substance; while the long circumflex arteries also referred to as quadrigeminal artery or collicular artery pass around the brainstem to reach the quadrigeminal cistern and supply the quadrigeminal bodies. They encircle the midbrain medial to the PCA and send small rami to the cerebral peduncle and geniculate bodies and occasionally, to the tegmentum, pulvinar and quadrigeminal plate. They may arise from the P1 or P2. Duvernoy in 1978 consistently saw an accessory quadrigeminal artery paralleling the course of the quadrigeminal artery and supplying the lateral aspect of the superior colliculus. However in our study, this was not noted.
- Thalamogeniculate arteries: The thalamogeniculate arteries arise directly from the P2 segment or the P3 segment beneath the lateral thalamus and penetrate the part of the roof of ambient cistern formed by the geniculate bodies and the surrounding area. Rhoton et al reported most of the thalamogeniculate arteries to arise from the P2 segment. However, in our series we found that an equal number arose from the P2 segment as well as the P3 segment of the PCA. These were seen to arise individually unlike the thalamoperforators which usually were seen to arise as large stem perforators with complex division. Milisavljevic et al noted large stem perforators in only 26.67% of the cases. They have studied thalamogeniculate perforators of the PCA in great detail. They found the thalamogeniculate perforators to originate also from the anterior temporal, medial temporal, common temporal arteries and other cortical arteries like calcarine or parietooccipital arteries. According to the literature available the numbers of thalamogeniculate arteries can vary greatly. Zeal and Rhoton have reported one to seven (mean 2.4) in number, while Milisavljevic et al have reported two to 12 with a mean of 5.7. The number in our series too conformed to the latter ranging from two to 12 with a mean of 7.75. Aneurysms of the distal PCA may affect the thalamogeniculate arteries in several ways in the same ways as described for the thalamoperforators. Vasospasm of the thalamogeniculate arteries may also occur after SAH. Thalamogeniculate arteries may also be injured during surgery in this area for tumors and other lesions. All these may lead to ischemia of the area supplied by the thalamogeniculate arteries i.e. the medial geniculate body, portions of the subthalamus, a large region of the thalamus and part of the posterior limb of the internal capsule. This may result clinically in the Dejerine-Roussy thalamic syndrome characterized by disturbance in the sensation, thalamic pain, slight hemiparesis and homonymous hemianopia. Usually (66.67%), anastomosis may be present between thalamogeniculate arteries but may not connect all arteries. Hence it is important that the thalamogeniculate arteries are spared during operations in this area. It is also unlikely that occlusion of a single thalamogeniculate artery would produce the complete syndrome because of the anastomosis. Hence, usually this syndrome is caused by the occlusion of the PCA proximal to the origin of all these thalamogeniculate arteries.
b) Ventricular and choroid plexus branches
These constitute the MPCA and LPCA.
- MPCA: The MPCAhas a variable origin. Lang and Kapplinger have described that the artery originated from the P2 segment in 83.3% of cases and from P1 in 9.4%, from both P1 and P2 in 1.2% cases and from the distal PCA segments in 7.1% cases. Zeal and Rhoton also confirmed these findings. In their series the MPCA arose from the P2 in 71% of cases, from P1 in 12% of cases, from P3 in 4% and P4 in 13%. The same authors report a single MPCA in 54%, duplication in 32% and triplication in 14%. In our series the MPCA arose from P1 in 38.6% and from P2 in 61.4% of the cases. It was seen as a single artery in 86% and seen to be duplicate in 14%.
- The LPCA: The LPCA too has a variable origin. Zeal and Rhoton have found the origin of the LPCA to be from P2 in 51%, P3 in 30%, P4 in 15% and rarely from the MPCA (in 4%). Herthem et al have noted that the origin of the LPCA was from the P2 segment in 24%, P3 segment in 41%, the parietooccipital artery in 13%, the splenial artery in 12%, posterior temporal artery in 6%, the common inferior temporal trunk in 2% and hippocampal artery in 2%. In our series the LPCA was seen to arise from the P2 in 62%, from P3 in 34% and in 4% from the common temporal origin. The number of LPCAs usually vary from one to nine with a mean of three to four according to Galloway and Greitz. In our series too we found one to two LPCAs in the specimens with a mean of 1.5.
c) Cortical branches
The cortical branches of the PCA were not studied in detail as it was beyond the purview of this study. However, the cortical branches include the inferior temporal arteries, which include the hippocampal arteries, anterior, middle, posterior and common temporal arteries; the posterior callosal or splenial artery; the parietooccipital arteries and the calcarine artery.
PComA: The PComA which forms the lateral boundary of the Circle of Willis is seen to arise from the posteromedial surface of the supraclinoid portion of the ICA within a few mm of the anterior portion of the carotid cistern, approximately midway between the origin of the ophthalmic artery and the ICA bifurcation., In the embryo the PComA is continuous with the PCA, but in the adult the latter artery is seen to be included by the basilar system. When the PComA remains the major origin of the PCA the configuration is said to be fetal. If the PComA is normal or small in size, it courses posteromedially to join the PCA but if it is a fetal type it courses further laterally above or lateral to the occulomotor nerve i.e., if it is normal it is medial to the occulomotor nerve, if it is fetal type it is lateral to the occulomotor nerve. The caliber of the PComA is highly variable. Dilenge in 1962 found the PComA diameter to be greater than 2 mm in 38.7%, between 1-2 mm in 41.2% and less than 1 mm in 18.9%. De Vriese (1905) and Paget (1944) suggest that the mean caliber of PComA in children is larger and is seen to decrease with advancing age. Hypoplasia, aplasia, duplication of the PComA, fenestration etc. have all been reported. In our series hypoplasia of the PComA was seen in eight cases. However, no duplication or aplasia was noted. An average of eight, ranging from 4-14 perforating branches arise from the PComA, mostly from the superior and lateral surfaces. In our series a mean of 7.8 branches on the right side and 5.7 on the left were seen to arise from the superolateral surface of PComA. Hence it is advisable while dissecting the PComA to remain on the medial aspect of the artery in order to avoid injury to these perforators. These perforators are generally divided into the anterior group and the posterior group.,, These branches are also quite variable in size and may originate as large stem branches with ramifications or small thin branches. Even when the PComA is small, the perforators may be seen to be stout and large. The mamillary bodies too are supplied by the branches from the PComA or by the branches from the proximal PCA i.e., P1.
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