Korean Journal of Cerebrovascular Surgery 2001;3(1):5-10.
Published online March 1, 2001.
Microsurgical Anatomy of the Basilar Artery: Surgical Approaches to the Basilar Trunk and Vertebrobasilar Junction Aneurysms.
Sakata, Shuji
Department of Neurosurgery, Saga Prefectural Hospital, Saga, Japan. koseikan@bronze.ocn.ne.jp
Abstract
The success of treating basilar aneurysms lies in the preservation of all perforating arteries and best selection of the surgical approaches. The purpose of this study was to define the microsurgical anatomy of the basilar artery and surgical approaches to the basilar artery. The microsurgical anatomy of the basilar artery and its branches were evaluated in adult cadaveric brains using 3x to 20x magnification. The branches of the basilar artery can be divided into three main groups: the cerebellar, lateral pontine, and perforating arteries. The lateral pontine arteries arise from the basilar trunk and course laterally to the brachium pontis. The perforating arteries originated from the basilar trunk and penetrating near the midline of the pons on its basal surface. The basilar trunk was approached via subtemporal-transtentorial, anterior transpetrosal, posterior transpetrosal and transchondylar routes. Selection of approach for basilar aneurysms was discussed.
Key Words: Microsurgical anatomy, Basilar artery, Perforating artery, Surgical approach

Introduction


   Surgical management of basilar trunk and vertebrobasilar junction aneurysms remains a challenge for neurosurgeons.3)7)11) The aneurysm of this area are located deep in the surgical field and intimately related to the cranial nerves and the brain stem, and are blocked by petrous bone. The surgical exposure and obliteration of aneurysms in this region continues to be associated with high morbidity. Inappropriate approach and technical errors including perforator injury are important factors that influence postoperative high morbidity.1)
   It is obligatory to have a precise knowledge of the microsurgical anatomy of the basilar artery and to know via which route the aneurysm can be best approached in order to provide better result. Intensive studies of microsurgical anatomy of the cerebellar branches (SCA, AICA, PICA) have been published.5)8)10) However, The basilar artery is rarely the subject of the anatomical examination in recent literature.9)15)
   The purpose of this study was to define the microsurgical anatomy of the basilar artery and surgical approaches to the basilar artery.

Materials and Methods

   The Microsurgical anatomy of basilar artery and its branches were examined using 3x to 20x magnification after perfusing the vessels with a colored silicone to facilitate dissection in the ten adult cadaveric heads. The basilar trunk was approached via subtemporal-transtentorial, anterior transpetrosal, posterior transpetrosal (combined supra/infratentorial transpetrosal), and transchondylar routes.

Results

1. Microsurgical anatomy of the basilar artery (Fig. 1)

1) Basilar artery
  
The two vertebral arteries joined together forming the basilar artery (BA) in the area of the pontomedullary sulcus. The BA coursed upward in the prepontine cistern in a shallow groove in the mid sagittal line on the ventral surface of the pons, which was called the basilar sulcus. The BA usually reached the interpeduncular fossa at about the level of the pontomesencephalic junction where it divided into two posterior cerebral arteries. The basilar artery was frequent deviated from the midline, especially in older age group.

2) Branches of the BA
  
The branches of the basilar artery can be divided into three main groups£ºthe cerebellar, lateral pontine, and perforating arteries.

(1) Cerebellar arteries
Superior cerebellar artery (SCA)
   The SCA arose near the bifurcation of the basilar artery. The SCA usually arose as a single trunk, but may also arise as duplicate arteries. It encircled the midbrain near the pontomesencephalic junction, passing below the oculomotor and trochlear nerves and above the trigeminal nerve. After passing above the trigeminal nerve, it entered the precerebellar space. Upon leaving this space, its branches were distributed the superior parts of the cerebellar cortex.
Anterior inferior cerebellar artery (AICA)
   The AICA originated from the initial part of the basilar artery. The AICA usually arose as a single trunk, but it may also duplicate arteries or as triplicate arteries. From its origin, the AICA coursed backward around the pons near the abducent, facial, and vestibulocochlear nerves. After passing near the nerves, entering the acoustic meatus, it proceeded around the flocculus on the middle cerebellar peduncle.
Posterior inferior cerebellar artery (PICA)
   The PICA usually arose from the vertebral artery. If the PICA was defined as the cerebellar artery that supplied the posteroinferior part of the cerebellum and that generally arose from the vertebral artery, it may also arise from the basilar artery. In some cases, the PICA arose by a common trunk with the AICA. The PICA had the most complex relationship to the cranial nerves of any artery. The vertebral artery coursed anterior to the glossopharyngeal, vagus, accessory nerves and the proximal part of the PICA passed around or between the rootlets of these and adjacent nerves.

(2) Lateral pontine arteries (circumferential perforating arteries)
   The lateral pontine arteries (circumferential perforating arteries) arose from the basilar trunk and course laterally to the brachium pontis. These branches gave off small perforating vessels that penetrated a lateral surface of the pons and brachium pontis. These branches of the basilar artery can be divided into the pontomedullary, large lateral arteries, and posterolateral artery.
Pontomedullary artery
   The pontomedullary artery was located close to the pontomedullary sulcus. The artery was usually single, and rarely duplicated. The artery coursed close to the pontomedullary sulcus, and terminated in the rostrolateral part of the medulla.
Large lateral pontine artery
   The large lateral pontine arteries ranged in number from 1 to 3 on each side, but most often two vessels were present. They usually originated from the distal half of the basilar arteries. The artery coursed laterally the ventral surface of the pons and terminated on its lateral surface and brachium pontis. The artery often supplies the lateral pyramidal bundles, the central tegmental tract, the trigeminal nerve, and the trigeminal nuclei.
Posterolateral artery
   The posterolateral artery arose from the basilar artery just caudal to the origin of the superior cearebellar artery. It coursed laterally, parallel to the SCA, and terminated in the rostrolateral part of the pons.

(3) Perforating arteries (Paramedian perforating arteries)
   The perforating arteries originated from the basilar trunk and penetrating near the midline of the pons on its basal surface. They can be divided into three groups:the caudal, middle, rostral groups.
Caudal perforating arteries
   The caudal perforating arteries arose from the most proximal portion of the basilar artery and entered the foramen caecum. They ranged in number from 1 to 4. The arteries mainly supply the abducent nucleus, medial longitudinal fasciculus, and caudomedial pontine reticular formation.
Middle perforating arteries
   The middle perforating arteries originated from the middle part of the basilar artery and usually penetrated the pons along the edge of the basilar sulcus. The number of the perforators from each BA ranged from 5 to 9. The middle perforating arteries supply the pyramidal bundles, part of the medial lemniscus, the medial portion of the reticular formation, and the medial longitudinal fasciculus.
Rostral perforating arteries
   The rostral perforating arteries originated from the terminal portion of the basilar artery and entered the caudal part of the interpeduncular fossa. The vessels varied in number from 1 to 5.

2. Surgical approaches to the basilar trunk and vertebrobasilar junction
  
Three approaches to gain access to the basilar trunk and vertebrobasilar junction were examined.

1) Subtemporal transtentorial approach, Anterior transpetrosal approach (Kawase's approach) (Fig. 2)

(1) Subtemporal transtentorial approach
   A traditional middle subtemporal approach was performed and temporal lobe was retracted. The tentorium was opened as wide as possible£"the incision extended close to the transverse sinus laterally and to the pyramidal edge anteriorly. Direct retraction of the pons and trigeminal nerve are facilitated to expose basilar trunk. Exposure can be obtained as deep as the junction of the vertebral arteries by wide opening of the tentorium. Disadvantage of subtemporal approach is the possibility of postoperative edema or hemorrhage in the temporal lobe.

(2) Anterior transpetrosal approach (Kawase's approach)
   The temporal lobe was elevated by extradural approach. The middle meningeal artery was sacrificed at the foramen spinosum, and the petrous segment of the carotid artery was exposed in Glasscock's triangle. The area of drilling was surrounded by the trigeminal ganglion anteriorly, the cochlear organ posterioly, the shenopetrosal groove laterally, and the carotid canal and internal auditory canal inferiorly. The dural sleeve of the internal auditory canal can be identified by tracing the greater petrosal nerve posteriorly to the geniculate ganglion (lateral aspect of internal auditory canal). The location of the internal auditory canal can be also estimated by bisecting the 120-degree angle between the greater petrosal nerve and the arcuate eminence. The bone of Kawase's triangle was resected to the depth of the inferior petrosal sinus to expose the dura of the posterior fossa. The temporal lobe dura was opened and a second incision was made along the floor of the middle fossa toward the posterior fossa dura. The superior petrosal sinus was sectioned at the level of Kawase's triangle. The tentorium cerebelli was then sectioned into the incisura. The basilar trunk was approached between the fifth and seventh cranial nerves.

2) Posterior transpetorosal approach, Combined supra/infratentorial approach (Fig. 3)
   A L-shaped supra/infratentorial craniotomy was performed to complete the bone exposure. A partial mastoidectomy skeletonizing the labyrinth, posterior fossa dura, sigmoid sinus, and superior petrosal sinus was accomplished. This radical posterior petrosectomy facilitated to gain sufficient exposure of the presigmoid dura from the superior petrosal sinus to the level of the juglar bulb. The temporal dura was incised parallel to the transverse sinus. The presigmoid posterior fossa dura in the Trautmann's triangle inised up to the superior petrosal sinus with ligation of the superior petrosal sinus. The temporal lobe was slightly elevated and the tentorium was transected parallel to the petrous bone to the tentorial incisura. After the tentorium had been cut completely, the sigmoid sinus and the remaining portion of the tentorium were retracted, exposing the basilar artery from the upper basilar region down to the level of the vertebrobasilar junction and ipsilateral vertebral artery.

3) Far lateral approach, Transchondylar approach (Fig. 4)
   After the muscle and soft tissue were reflected, a retromastoid craniectomy was extended through the foramen magnum, and the inferior portion of the mastoid process was drilled away to expose the distal sigmoid sinus and juglar bulb. The vertebral artery was isolated above the posterior arch of C-1. The remaining anterolateral rim of the foramen magnum and the lateral portion of the C-1 were removed to include the posterior one- third of the occipital condyle. Removal of the condyle is the key step giving a more direct approach to the distal intradural vertebral artery and vertebrobasilar junction without the need for retraction of the neural structural.

Discussion

   Aneurysms of the lower basilar trunk and vertebrobasialr junction are located in sort of "no man's" land. Numerous approaches have been attempted to gain access to aneurysms of the basilar trunk and vertebrobasilar junction.1-7)11)12)15) Classical routine approaches are subtemporal transtentorial and suboccipital approaches.
   In 1944, Dandy2) described an approach via the suboccipital route. This exposure has been associated with significant morbidity, especially for aneurysms locating at the midline. In 1965, a subtemporal-transtentorial approach was described by Drake.4) Opening the tentorium facilitates visualization of basilar artery trunk aneurysm. Sometimes, direct retraction of the pons and trigeminal nerve are required to expose aneurysms. Disadvantage of subtemporal approach, however, is the possibility of postoperative edema or hemorrhage in the temporal lobe.6)7)12) In this region, the subtemporal and suboccipital approaches can only applied with considerable difficulty and risk of damage to the neighboring neurovascular structures of the brainstem and cranial nerves.
   A number of cranial base approaches such as the anterior petrosal,1)6) the combined supra/infratentorial posterior transpetrosal,3)11) and extended far lateral approaches3)7) have been reported. A guiding principle in cranial base techniques has been to create maximum surgical exposure by removing bone rather than retracting brain.1)3)6)7)11) A direct approach to the basilar trunk aneurysms often blocked by the petrous bone and the main component of many of these cranial base approaches to the basilar artery is the removal of petrous bone.
   Selection of the approach for the basilar trunk aneurysms
   Upper clival basilar aneurysms are relatively easily accessible by either the traditional pterional or the subtemporal approach.11) Exposure of the aneurysms in this region can be enhanced by addition of posterior clinoidectomy, transcavernous approach, or transtentorial approach.12) Extended orbitozygomatic approach was proposed for large upper clival aneurysms.3)
   Midclival basilar aneurysms are located at the middle of the clivus anterior to the pons. This area is exposed best by the anterior1)6) or posterior7)11) transpetrosal approaches. Midclival basilar aneurysms located between the floor of the sella turcica and the internal auditory canal are successfully exposed by anterior transpetrosal approach. Midclival basilar aneurysms at or below the internal auditory canal and above the upper margin of the jugular tubercle can be best exposed by posterior petrosectomy.1)
   Lower clival basilar aneurysms below the jugular tubercle and most aneurysms of the ventral artery can be satisfactory reached by addition of the retrochondylar farlateral, transchondylar, or suboccipital approach.7)13)14)

Conclusion

   The success of treating basilar aneurysms lies in the preservation of all perforating arteries and best selection of the surgical approach. The Microsurgical anatomy of basilar artery and its perforating branches were examined. The upper clival basilar aneurysms can be relatively easily accessible by either the traditional pterional or the subtemporal approach. Midclival basilar aneurysms ca be exposed best by the anterior or posterior transpetrosal approaches. Lower clival basilar aneurysms can be satisfactory reached transchondylar approach.


REFERENCES


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