Svistov D.V., Kandyba D.V., Antonov S.L.
Chair of Neurosurgery, Medicomilitary Academy, Saint Petersburg, Russia
Aneurysms of the posterior cerebral artery (PCA) make about 1% of all intracranial aneurysms [4, 7, 9, 10]. A surgical approach to PCA, exposure of the vessel and aneurysm are connected with considerable difficulties due to anatomic peculiarities, a great number of perforans arteries, close interrelations with the oculomotor nerve and midbrain. Selective catheterization of PCA and obliteration of aneurysm are much simpler from the technical point of view; that is why they are, to some extent, an alternative to surgical interventions. Favorable outcome of surgical treatment or other methods depends on knowledge of segmental structure of PCA and its branches, anastomotic connections, which is of particular importance in planning occlusion of carrying vessel.
PCA can be subdivided into four segments (fig.1) . P1 segment lies between the artery beginning (bifurcation of the basilar artery) and the beginning of the posterior communicating artery. P2 segment is between the posterior communicating artery and the midbrain back. P2 segment can be subdivided into 2 more segments: anterior (P2A) and posterior (P2P). The third segment (P3) is situated between lateral parts of lamina quadrigemina and an initial part of the calcarine fissure. The forth segment (P4) includes terminal branches of PCA: the parieto-occipital artery and artery of the calcarine fissure. Each of these segments has branches feeding various structures of the stem, temporal and occipital lobes, thalamus, ventricular system. [11, 14].
PCA branches have a lot of anastomotic ties with neighboring vascular regions:
Patient P., aged 35, was under our observation in June 2001. He was admitted to the neurosurgical department with large unruptured aneurysm of the right PCA, diagnosed at a prehospital stage. According to anamnesis, the patient had non-permanent minor headaches of uncertain periodicity and localization. MRI and MR-angiography (Magnetom Vision, Siemens) were used in order to prove absence of any organic lesion of the brain. According to MRI findings, there was a round structure of heterogenous intensity with artifacts, characteristic of low-velocity flow; it was watched on the right in the area of the notch of cerebellar tentorium in projection of hyppocampus (fig.2). MR-angiography showed change of a flow signal in PCA in projection of this structure; it was indicative of pathologic widening of PCA (fig.3). Dimensions of the structure were equal to 10´17 mm. Large, partially thrombosed aneurysm of the right PCA was suspected on the basis of these findings.
As the aneurysm borderlines exceeded 10 mm, a patient was offered surgical treatment. Cerebral angiograophy with one-stage total embolization of aneurysm by detachable microspirals was made on June 4, 2001. Preparation for an operation included taking aspirin per os (375 mg/day) and ticlopedin (250 mg/day) during two days before it. The patient was given proper premedication (1.0 ml of 2% promedol, 2.0 ml of 1% dimedrol, 2.0 ml of 0.5% dicam). Local anesthesia was used. The left vertebral artery was catheterized by IVASF introducer. Cerebral panangiography was carried out with 5F Vertebralis diagnostic catheter (Cordis). A version of separation of the left vertebral artery from the aortic arch was identified. Large aneurysm of the right PCA (fig.4) 14´10´8 mm (with taking into account magnification index). There was marked stenosis of P2 segment of the right PCA (an arrow) near the entrance into aneurysmal cavity; thus, contrast study of distal branches of the artery turned out to be insufficient for precise determination of a type of aneurysm (saccular or fusiform). According to data of right-sided carotid angiography, distal segments of cortical branches of the parietal trunk of the right middle cerebral artery were characterized by increased filling. It allowed to think of developed cortical collaterals with PCA branches.
Envoy Cerebral 5F guiding catheter (Cordis) was inserted into the left vertebral artery via 0.035 exchangeable guide. Vasco 10 (Balt) microcatheter was placed into a vascular bed against a background of total heparinization (8.000 units intermittently); then it was inserted into an intracranial part of the left vertebral artery, basilar artery, PCA with the help of SOR009 microguide. The patient complained of transient dizziness, when the opening of the right anteroinferior cerebellar artery was passed through. The microcatheter tip was inserted into the aneurysm cavity, aneurysmography was carried out (fig.5), Filled branches of PCA, going from the aneurysmal sac, were indicative of a fusiform type of aneurysm. Platinum SPID10´300P10; SPID7´300P10; SPID6´200P10 microspirals were inserted one after another in the order of reduction of their size and left within the aneurysm cavity. One of the loops of the first microspiral, placed in the aneurysm cupola, slipped out into the lumen of P2P segment of PCA without affecting its patency (fig.8c). Control aneurysmpgraphy, carried out after partial occlusion of the aneurysm cavity, confirmed its fusiform structure (fig.6).Four other microspirals were placed into the cavity subsequently (5´150, 5´100, 4´120, 4´80. It resulted in its complete obliteration (fig.7) with impaired patency of the right PCA at the borderline of P2A and P2P segments. Filling of proximal branches of PCA was normal. In spite of impaired patency of PCA there were no vision disorders.
Control vertebral angiography (fig.8) was indicative of complete obliteration of aneurysm; distal segments and branches of the right PCA did not fill. Control carotid angiography was carried out for estimation of a distal vascular bed in a region of the right PCA (fig.9). It demonstrated retrograde filling of branches of the right PCA via cortical anastomoses with an angular branch of the right MCA.
There were no complications in the postoperative period. Focal neurologic symptoms and visual field defects were absent. Symptomatic therapy was carried out; aspirin (375 mg/day) was prescribed. The patient was discharged on the 3rd day after the operation. He returned to his job in a week.
This case is characterized by some peculiarities. They are a combination of large unruptured aneurysm of a vertebrobasilar region with a structural version of the left vertebral artery; presence of stenosis of the artery carrying aneurysm; absence of vision disturbances in spite of thrombosis of the vessel carrying aneurysm.
According to our experience in the field of angiography, the left vertebral artery goes from the aortic arch in 0.2% of cases. Clinical manifestations of this anatomic version vary from absence of any symptoms up to circulatory insufficiency in a vertebrobasilar region, recurring subarachnoid hemorrhages. In our case, version of the anatomic structure of vertebral artery was of no importance. The only exception was a rectilinear path of the vessel, favoring its catheterization with the above-mentioned guiding catheter. It had a positive effect on manipulating the microcatheter in the intracranial vascular bed.
Stenosis of major arteries in large aneurysms has already been described, but it concerns mainly extradural segments of ICA. As a rule, it is connected with deformity of the artery by a large, partially thrombosed aneurysmal sac. As for PCA, such vascular changes are watched by us for the first time. Probably, hemodynamically significant (more than 70%) stenosis of the main trunk of the right PCA was a "training" factor for development of collaterals between distal branches of PCA and MCA in a zone of adjacent circulation. Thanks to these connections, the artery occlusion did not cause vision impairment. Control angiography showed, that filling of P2P segment spread as far as a knot of microspirals in the aneurysm cavity. Preoperative use of desaggregants and intervention performed against a background of total heparinization allowed to avoid thromboembolic complications, resulting from thrombus destruction in a cavity of large aneurysms.
It appears, that it is necessary to discuss some problems of treatment, arising in situations similar to that, mentioned above:
A number of reports, devoted to diagnosis and surgical treatment of unruptured aneurysms, is growing constantly. Achievements in this field are indisputable. Screening of potential patients from a risk group, diagnosis of family aneurysms allowed to reduce the rate of primary subarachnoid hemorrhages in some regions of Japan and Finland. Brennan J.W. and Schwartz M.L.  analyzed medical information, presented in the Internet, and worked out recommendations (using criteria of "conclusive" medicine) on treatment of patients with unruptured aneurysms. They are given in table 1.
Thus, our case is referred to a group of patients, who can be subject to surgical intervention, recommended as a measure of preventing subarachnoid hemorrhage. A choice of a method of aneurysm exclusion from blood flow is conditioned by a considerable risk of operative trauma in microsurgical direct intervention and ischemic and infectious complications. If to take into account a fusiform structure of aneurysm, it becomes quite clear, that it was impossible to perform reconstructive clipping. Thus, only trapping of aneurysm could be used; however, it would result in necessity to prepare P2P and P3 segments of the artery, localized medially from the aneurysm cupola, and, thus, a risk of discirculatory disorders in the ipsilateral cerebral peduncle. Refusal from detachable balloons for occlusion of the aneurysm cavity was justified, as the carrying artery had marked narrowing near the entrance to the aneurysm cavity; thus, it was almost impassible even for a rigid microcatheter with a diameter of 0.6 mm. Possibility of inserting a latex balloon (home-made or, all the more, manufactured) via the lumen of the vessel with such a degree of lesion seemed doubtful.
|STANDARDS||There is insutficient data to support a standard regarding whether to surgically repair unruptured intracranial aneurysms|
* - patients with no prior history of subarachnoid hemorrhage.
** - patients who have experienced previous subarachnoid hemorrhage from rupture of different aneurysm.
In the long run, the operation and its nearest result has proved rightness of our choice of tactics. A final result of it will be estimated in six months during carrying out control examination.
PCA aneurysms are more typical of its proximal segments [5, 7, 9]. They have some specific morphologic signs, which differ them from aneurysms of other localizations. They are more frequent in young people (38 years old), whereas an average age of patients with aneurysms of anterior areas of the arterial circle is 50-60 years . One more typical feature is a high rate of large and giant aneurysms (23% among PCA aneurysms in comparison with 3-5% among aneurysms with other localizations) [4, 8]. According to Drake C. , the rate of gigantic aneurysms is 42%. Yasargil M.  and Ciceri E., Pia H.  watched them in 50% and 24% of cases respectively.
Subarachnoid hemorrhage is one of the most frequent manifestations of PCA aneurysms and is watched in 50-80% of observations [5, 7, 9]. Almost a half of PCA aneurysms is diagnosed accidentally; it especially concerns large, gigantic and "serpentine" aneurysms. They are not accompanied by neurologic disorders.
In general, serpentine aneurysms are more typical of a vertebrobasilar region and young patients [5, 6, 7, 9]. They can manifest themselves as mass-effect, leading to epileptic fits or focal symptoms . Ciceri E.  observed mnestic disorders in one patient and insomnia and memory loss due to compression of the brain stem and hippocampus in another.
Pia H.  reports a high frequency (27%) of vision impairment in PCA aneurysms; oculomotor disorders and hemianopsia are most frequent. Ciceri E.  watched 5 patients (25%) with oculomotor and vision disturbances due to compression of the oculomotor nerve (3) and chiasm (1). A combination of PCA aneurysm and AVM of the occipital lobe with homonymous hemianopsia took place in one patient.
PCA aneurysms combunes with various vascular abnormalities are considered to be a rather typical phenomenon. Accompanying AVM, multiple aneurysms, Moya-Moya disease, stratifying aneurysm of ICA are observed in 43% of cases , it is indicative of hemodynamic nature of saccular aneurysms.
According to Ciceri E.  a method of choice in treatment of saccular aneurysms of PCA is selective intravascular obliteration with Gugliemi detachable microspirals with preservation of patency of the carrying artery. Embolization of small aneurysms with a narrow neck by microspirals is a safe procedure with a persistent angiographic effect. Large aneurysms of PCA with a wide neck (it concerns aneurysms of other localization as well) are far from being an ideal object for embolization, though technical difficulties (spiral dislocation into the carrying artery, bad filling of aneurysm with embolizing material) can be overcome by using assisting balloon-occlusion, implantation of a net stent, application of 3D spirals.
Preservation of antegrade blood flow in PCA is one of the most important tasks in surgical treatment of aneurysms. However, it cannot be achieved in fusiform or serpentine aneurysms, when the artery occlusion is the main condition of aneurysm exclusion. Circulation disorders in the PCA region with persistent vision impairment is one of the most threatening complications of this tactics. It should be noted, that a rate of such complications is rather low and equals 14.2% according to Ciceri E. , It is conditioned by rich collateral blood supply of the PCA region. Unfortunately, it is impossible to estimate functional efficacy of these collaterals before an operation. Pre- and postoperative angiograms and MRI of a patient with giant serpentine aneurysm of the left PCA are given in fig.10. In spite of retrograde filling of branches of the right PCA from the middle cerebral artery, occlusion of the carrying artery led to development of homonymous hemianopsia and hemiparesis due to ischemic infarction in the PCA region (Ciceri E. ).
The best statistical indices on intravascular treatment of PCA aneurysms are given by Ciceri E. , 14 out of 20 aneurysms were excluded from blood flow with preservation of patency of the carrying artery. Aneurysm was occluded together with the artery in all the rest observations. Patients had fusiform or serpentine aneurysms. There was perforation of saccular aneurysm in one case, but it did not result in any unfavorable consequences. Control angiographic examinations of patients operated with application of "reconstructive" technique demonstrated absence of aneurysm recanalization. When aneurysms are excluded from blood flow with the carrying artery, control MRI is indicative of thrombosis and shrinkage of aneurysm. Clinical outcomes are favourable; 95% of patients were discharged in state I (Glasgo scale); there were no fatal outcomes. Complications were watched in 15% of cases. They included aneurysm perforation during the operation (no neurologic disorders), two cases with persistent neurologic disorders after occlusion of the carrying artery.
In conclusion it should be noted that selective intravascular treatment of PCA aneurysms with application of detachable microspirals is an available, relatively safe and effective method of treatment. It concerns patients with unruptured large fusiform aneurysms as well.