Posterior Cerebral Artery Aneurysms: Parent Vessel Occlusion Being a Viable Option in the Era of Flowdivertors
Keywords: Flowdivertors, parent vessel occlusion, posterior cerebral artery aneurysmsKey Messages: Parent vessel occlusion in distal posterior cerebral artery aneurysm is still a viable option.
Posterior cerebral artery (PCA) aneurysms are not very common and account for ~1% of all intracranial aneurysms.,,, They most commonly involve the P1 or P2 segment of PCA. As compared with anterior circulation aneurysms, PCA aneurysms usually affect younger patients with a tendency to become giant aneurysms. Since the PCA is anatomically located in close proximity to the upper brain stem, cranial nerves, and gives origin to many crucial thalamo-perforating arteries, any neurosurgical approach is risky as compared with endovascular treatment.,
Embryologically, the PCA is a branch of the internal carotid artery (ICA); the connection with the basilar artery (pars basilaris) develops later. As it arises from the basilar artery, the PCA can be divided into four segments. The P1 (precommunicating segment) extends from its origin from the basilar artery to the junction with the posterior communicating artery (PCOM). The P2 (ambient segment) is located around the midbrain extending toward the quadrigeminal plate. The P3 (quadrigeminal segment) is located on the surface of the quadrigeminal plate. The P4 segment consists of the terminal cortical branches of the PCA, after the takeoff of the parieto-occipital and calcarine arteries.,,
Three types of branches arise from PCA, viz., 1) cortical branches to cerebrum such as posterior temporal artery and calcarine artery, 2) perforators to brainstem such as posterior thalamoperforators, and c) choroidal arteries such as medial and lateral posterior choroidal arteries. A precise knowledge of the segmental anatomy of the PCA and its branches is essential, before a treatment is planned more so when parent vessel occlusion is done.
We retrospectively reviewed our data between 2010 and 2017 of all PCA aneurysms that were managed endovascularly at our institute to evaluate the safety and efficacy of this technique along with anatomical details pertaining to the choice of treatment.
Our study included 11 patients with PCA aneurysms (we did not include basilar top aneurysms which were more toward the P1 segment of the PCA) from 2010 to 2017, between 8 and 60 years of age of which 5 patients had subarachnoid hemorrhage (SAH) and the other 6 patients had different degrees of headache and features due to mass effect. Three of them also had focal small thalamic infarct which led to the presentation probably embolic from partially thrombosed aneurysm. Most of them were located at the P2–P3 segments and one involved the P1–P2 junction and one involved PCOM–P2 junction. Of the five ruptured aneurysms, three were saccular in shape (of which one was giant aneurysm) and two were fusosaccular and of unruptured six aneurysms, two were partially thrombosed giant (diameter >25 mm), two were saccular in shape, and remaining two were fusosaccular large aneurysms (diameter 10-25 mm). Thus, in total we had three giant aneurysms (diameter >25 mm), three large (diameter between 10 and 25 mm), and five aneurysms were less than 10 mm in diameter.
All the patients underwent endovascular management of aneurysms under general anesthesia. Systemic anticoagulation was achieved using intravenous heparin infusion and monitoring of coagulation adequacy was done using activated clotting time. Through femoral artery route adequate size guiding catheter, mostly 6F either Envoy (Codman) or Chaperon (Microvention) was placed inside the vertebral artery. A micro-catheter, mostly Echelon 10 (EV3) with micro-guide wire (Traxcess, Microvention) or Expedion 10 (EV3), was then used to cannulate the aneurysmal sac. Detachable coils (GDC coils, Boston scientific or Axium, EV3 or Microplex, Microvention) were then used to pack the aneurysm. If the plan included the parent artery occlusion, then the dense packing of the sac was avoided and the parent vessel was packed with coils until no further filling of the aneurysm as well as feeding artery was seen on angiography.
To assess the adequate collateral flow, we did the balloon occlusion test in one patient before parent vessel occlusion. Even though it did not reveal any obvious collateral supply from ipsilateral middle cerebral artery, we went ahead with the coiling of aneurysm along with parent vessel occlusion. No major morbidity was seen in that patient on follow-up. In two cases of wide-neck unruptured saccular aneurysm, stent-assisted coiling of the aneurysmal sac was done in one case (LVIS Jr. of Microvention - 2.5-mm diameter) and in the other case flowdivertor FRED Jr (Microvention) was used.
We did regular follow-up of these patients in our outpatient department at 6 weeks, at 6 months, and after 1 year of procedure. Modified Rankin scale was used to grade the patients on discharge and follow-up. There was no disabling visual complaint in any of the patient where parent artery occlusion was done, but on detailed visual field evaluation, two had partial cortical blindness. One patient developed hemiparesis which improved after 6 weeks. In three patients, only selective aneurysm coiling was done and none had any major morbidity on discharge or follow-up. Angiographic follow-up (with magnetic resonance angiography [MRA]) did show some neck residue but were stable, so no further treatment is done and they are kept on yearly time of flight (TOF) MRA follow-up. One patient of wide neck aneurysm, where stent-assisted coiling was done, did not develop any neurological deficits and angiographic follow-up after 6 months showed complete occlusion of the aneurysm with patent vessel. The Flowdivertor patient also had no complaints on follow-up; her complaints of vomiting, vertigo were relieved with time; repeat Digital Subtraction Angiography (DSA) done after 6 months showed complete occlusion of the aneurysm along with patent PCOM and P2 segment where FRED Jr was placed. Details of each case are tabulated in [Table 1] including their outcome using modified Rankin score.
One of the patients had a repeat SAH 7 years after the initial treatment in which parent vessel occlusion along with aneurysm coiling was done. However, diagnostic imagery revealed a fresh aneurysm in the posterior circulation possibly dissecting one, which got thrombosed on its own when patient was taken for therapeutic procedure.
A 53-year-old woman presented with severe headache and vomiting. Computed tomography (CT) scan showed Fischer grade III SAH. DSA showed a giant aneurysm located at P1–P2 segmental junction [Figure 1]a, [Figure 1]b, [Figure 1]c shows aneurysm filling from patent PCOM in ICA injection, and in [Figure 1]d, same aneurysm filling from vertebral angiogram]. The aneurysm was selectively coiled using detachable GDC coils [Figure 1]e and [Figure 1]f and finally the parent vessel was also occluded [Figure 1]e, arrow showing coils in parent vessel occluding the artery]. The patient developed hydrocephalus for which ventriculo-peritoneal shunting was done. The patient eventually recovered and was discharged at mRS scale 2. At the 6 years follow-up, the patient was totally asymptomatic with no neurological deficit.
A 50-year-old female patient suffered sudden onset of severe headache with vomiting and transient loss of consciousness for half an hour. CT head showed SAH in left ambient cistern. DSA showed large irregular fuso-saccular aneurysm possibly dissecting at P2 segment of left PCA [Figure 2]a. We did a balloon occlusion test [Figure 2]b, [Figure 2]c, [Figure 2]d with arrow in [Figure 2]c and [Figure 2]d pointing toward the compliant balloon hyperform 4 mm × 7 mm; ev3]; there were no significant vascular collaterals [Figure 2]e and [Figure 2]f shows no collaterals in PCA territory once it is occluded by the balloons and angiogram taken from ipsilateral ICA], but as no other viable option was there, we did coiling of the aneurysm along with parent vessel occlusion [Figure 3]a, [Figure 3]b, [Figure 3]c, [Figure 3]d; with arrow in [Figure 3]c pointing toward coils in parent vessel; [Figure 3]e, [Figure 3]f, [Figure 3]g shows no filling of the aneurysm and also no collaterals to distal PCA territory]. Post-procedure magnetic resonance imaging (MRI) showed left lateral thalamic and left PCA territory infarct possibly because of occlusion of lateral posterior choroidal artery, which arise from that segment. The patient developed mild hemiparesis and was discharged on MRS 4. On follow-up after 6 months, there was significant improvement in both upper and lower limb power and her MRS was 2 at that time.
A 55-year-female patient with headache and one episode of unconsciousness was evaluated with MRI head and was found to have unruptured PCA P2/3 segment wide neck aneurysm. DSA was done to assess the anatomy [Figure 4]a and [Figure 4]b. Discussion regarding the treatment was done and finally endovascular treatment was advised. As the aneurysm was unruptured wide neck, we planned for stent-assisted coiling and patient was started on aspirin 150 mg and clopidogrel 75 mg daily a week before the procedure. Stent used was Lvis jr 2.5 × 18 mm; Microvention, and coiling of the aneurysm done [Figure 4]c, [Figure 4]d, [Figure 4]e, [Figure 4]f: arrow in [Figure 4]c points toward the microcatheter placed to deploy stent and [Figure 4]f shows final coil mass with stent pointed by arrow in the artery] and patient was discharged on MRS of 1.
Follow-up angiogram after 1 year showed no filling of the aneurysm with patent distal PCA.
A 60-year-old female patient presented with headache, vomiting, vertigo, for which she was evaluated and found to have a partially thrombosed giant aneurysm in PCOM–P2 junction along with a small focal right thalamic infarct probably embolic [Figure 5]a and [Figure 5]b showing hyperdense lesion in interpeduncular cistern on noncontrast CT and similar lesion appear variegated on T2W and T1W MRI [Figure 5]c and [Figure 5]d, respectively]. On CTA [Figure 5]e and DSA [Figure 5]f and [Figure 5]g showed partially thrombosed aneurysm], P1 was absent (fetal PCOM); we planned this case with smaller size flowdivertor FRED Jr (Microvention), but it was challenging to place it so that it would not prolapse inside the aneurysm and come into the ICA. We placed few coils inside the aneurysm and then we placed the flowdivertor across it [Figure 5]h. On follow-up, DSA done after 6 months [Figure 5]i showed that there was complete occlusion of the aneurysm and MRI done at that time showed significant reduction in mass effect.
An 8-year old female child had headache with loss of consciousness. She was evaluated and found to have SAH along with intraventricular hemorrhage; on angiogram, there was a large saccular aneurysm arising from right P3 segment with distal filling of the PCA branches [Figure 6]a, [Figure 6]b, [Figure 6]c, [Figure 6]d. Trapping of the aneurysm was done with coiling [Figure 6]e, [Figure 6]f, [Figure 6]g with arrows in [Figure 6]e and [Figure 6]f pointing toward the coil mass in distal and proximal vessels used for trapping] and postembolization, distal PCA branches were seen filling from the MCA collaterals [Figure 6]h and [Figure 6]i with arrow in [Figure 6]i pointing toward delayed MCA collaterals]. Steroids and low molecular weight heparin was given to avoid immediate thrombosis and mass effect due to aneurysm thrombosis. Follow-up MRI done after 6 months showed no parenchymal changes in PCA territory with reduction in size of thrombosed aneurysm [Figure 7]a, [Figure 7]b, [Figure 7]c, [Figure 7]d, [Figure 7]e, [Figure 7]f, [Figure 7]g, [Figure 7]h and no neurological deficit.
A 12-year-old male child with long-standing headache developed suddenly right hemiparesis was evaluated with NCCT head and MRI brain, which revealed acute infarct in left anterior thalamic region [Figure 8]a DWI and b T2W MRI] with a partially thrombosed large aneurysm in left perimesencephalic cistern [Figure 8]c T2W and d TOF MRA]. DSA done confirmed that the aneurysm was arising from the left P2–P3 segment and distal PCA branches were not filling even on delayed angiograms [Figure 9]a. Selective cannulation of the aneurysm with the microcatheter done and partial coiling of the aneurysm along with occlusion of the parent vessel was done [Figure 9]b. Postcoiling collaterals were seen from the left ICA; MCA–PCA Collaterals [Figure 9]c. Follow-up MRI done after 3 days showed no cortical infarct in PCA territory [Figure 8]e, [Figure 8]f, [Figure 8]g, [Figure 8]h.
Aneurysms arising in PCA are rare and most of them have saccular morphology. Dissecting aneurysms are common in posterior circulation and almost 80%–90% of dissecting intracranial aneurysm occurs in posterior circulation with more than three-fourth being in vertebral artery V4 segment. Dissecting aneurysms on angiography present with typical findings of either a fusiform aneurysm, pearl and string sign, delayed filling of parent vessel beyond aneurysm in which the double lumen can rarely be seen. We had two cases of dissecting aneurysms which were managed by aneurysm and parent vessel occlusion. Aneurysms have predilection for P1 and P2 segments of PCA and affect relatively younger patients.
Most of the PCA aneurysms are usually giant aneurysms (almost 23% of PCA aneurysms versus 3%–5% at other anatomic sites)., We had three giant aneurysms (27%). Large and giant aneurysms generally present with signs of mass effect on the surrounding brain parenchyma, resulting in hydrocephalus, seizures, or other neurological deficits. One of our patients, who had a giant saccular aneurysm of the P3 segment compressing the brain stem and hippocampus, presented with memory loss. One patient had a serpentine aneurysm with parieto-occipital and calcarine arteries arising from the neck of the aneurysm.
Patients with ruptured aneurysms present with sudden onset of severe headache related to SAH. In our series, five (50%) patients presented with SAH of which two were dissecting aneurysms. Treatment strategy differ when treating in acute phase after SAH in dissecting aneurysm, its parent artery occlusion which is generally recommended in this setting, whereas in chronic phase stent-assisted coiling with long-term antiplatelets can be undertaken.
Selective endovascular coil obliteration of the aneurysmal sac with or without parent artery occlusion is the treatment of choice for PCA aneurysms. Berry aneurysms with narrow neck (<4 mm) are effectively treated with endovascular obliteration using GDC coils. However, giant aneurysms with wide necks need balloon or stent-assisted procedure to avoid poor packing of the coils and/or encroachment on the parent artery. Though preservation of antegrade flow in the PCA is one of the prime objectives, but fusiform or giant serpentine aneurysms may need parent artery occlusion. There are several reports which showed that parent vessel occlusion is equally effective as selective aneurismal coiling and this was mainly due to microvasculature of PCA.,,,,, One of our previous case report where parent vessel occlusion was done in a fusiform aneurysm of PCA also stated the effectivity of the technique.
Collateral supply to PCA territory exists at multiple levels, viz, 1) collaterals through the perforating branches and leptomeningeal collaterals, 2) anterior and posterior choroidal arteries, 3) pericallosal–splenial arteries, and 4) cortical branch collaterals between PCA and MCA. However, the degree of collateralization is difficult to assess. Many reports have shown the safety of parent vessel occlusion in PCA aneurysms. Two main techniques of balloon test occlusion are utilized: 1) clinical assessment during balloon occlusion and 2) assessment of retrograde opacification of PCA branches during balloon occlusion. We use angiographic assessment of collaterals opacifying PCA branches. These techniques of balloon test occlusion are elaborated in article by Piotin and Moret ; nevertheless, these technique have both false-positive and false-negative rates and is also technically demanding. Balloon occlusion itself carries a risk of vessel rupture.
Parent vessel occlusion in proximal PCA aneurysm (P1 segment) is not always safe; it might lead to occlusion of the perforator resulting in thalamic infarct. Preservation of visualized perforator is mandatory during embolization/parent vessel occlusion. Parent vessel occlusion beyond P3 segment is relatively safe, but then also, sometimes it may lead to some inevitable neurological deficits or cortical blindness. In one of our cases where PAO was done, the patient had developed initial hemiparesis which was due to occlusion of posterior choroidal artery which was arising at that level. Balloon occlusion test in that case had showed poor collateral supply from MCA branches, but we went with parent vessel occlusion as flowdivertors was out of question due to cost. This patient developed neurological deficits but recovered on follow-up.
In one of our patients, no distal PCA branches beyond aneurysm were seen and no collaterals from the ICA side were noted, but after aneurysm coiling along with parent vessel occlusion, collaterals from the MCA were noted. Thus, if a patient is tolerating and no collaterals are seen before occlusion, it does not mean collaterals are not there and they become angiographically visible after aneurysm and parent vessel occlusion. Parent artery occlusion is an accepted treatment as much of the literature supports it, but there is no literature suggesting where other treatment than parent vessel occlusion is to be done. We suggest that in those cases where the distal vessel is large and is filling in early phase of angiogram, it needs to be protected either by bypass surgery or by endovascular means such as stent-assisted coiling or flowdivertor. There is significance of critical evaluation of the angiogram and to decide the distal vessel lumen and when it is filling.
PCA to occipital bypass surgery has limited experience and a appraisal of this procedure done in one of the large series by Chang et al. who have stated that it carries higher risk of complications than once thought and reported by earlier small series.
With the advances in endovascular tools, such as smaller size stents and flowdivertors, in some of the selected cases, coiling of the aneurysm with preservation of the parent vessel should be done, but this requires long-term antiplatelet intake. These can specifically done in unruptured aneurysms with no collateral or poor collateral supply. There is limited literature available regarding use of braided stent or flowdivertor in distal PCA; however, we used them in two of our cases, and on follow-up, there was total occlusion of the aneurysm with patent distal vessel. A stent/flowdivertor is likely to avoid immediate ischemic consequences, the price being antiplatelet therapy. Delayed stent occlusion, if it happens, may be better tolerated due to slow occlusion and chance to develop collaterals as we have seen in anterior circulation. On the contrary if delayed aneurysm recurrence is seen, further treatment or parent vessel occlusion is still an option.
No procedure is without complications; we had one major ischemic complication where hemiparesis occurred which recovered over a period of time and no other major disabling ischemic complication occurred. Recurrence rate of endovascular coiling is reported to be as high as 22%, but for parent vessel occlusion, recurrences are not known. None of our patient had recurrence even on follow-up, which we did with both MRA and DSA.
Obstructive hydrocephalus was noted in two of our patients, for which ventriculo-peritoneal shunting was done. Both these patients did well on follow-up study with mRS scores of 2. Some of our patients experienced transient headache for 6 weeks that was presumed to be because of initial rise in the aneurysmal volume when aneurismal thrombosis begins which causes surrounding brain compression and mass effect. This headache resolved with time.
In cases where aneurysm coiling along with parent vessel occlusion is done post-procedure steroids and low molecular weight heparin is given to avoid edema and sudden thrombosis of the aneurysm. Keeping the blood pressure on higher side initially for few days also helps in developing functional collaterals in such cases of PAO; these small things should be well emphasized while treating these aneurysms.
PCA aneurysms are usually large aneurysms that affect younger population with predilection to female gender. Endovascular procedures are safe, and as PCA has good collateralizations, parent artery occlusion is a viable option. In the era of flowdivertors, parent vessel occlusion is a low-cost option giving long lasting results in country like us with no significant procedural morbidity.
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Conflicts of interest
There are no conflicts of interest.
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