Spontaneous Delayed Migration/Shortening of the Pipeline Embolization Device: Report of 5 Cases

Discussion

Several groups have recently reported their experience with the PED. In most series, remarkably high rates of aneurysm occlusion were achieved at follow-up with an acceptable risk for major complications.^{1⇓⇓⇓–5} Some authors, based on favorable results from noncontrolled series, have even recommended the PED as a first-line option for unruptured aneurysms.³ Long-term safety results, however, are lacking and new complications have been emerging as neurointerventionalists accumulate experience with the device. The phenomenon of delayed migration or shortening of the PED has not been described in previously published series.

In this study, we have reported 5 cases of patients with spontaneous delayed migration or shortening of the PED (Table). In most cases, the PED migrated proximally (patients 2–5), and there was a significant difference in luminal diameter between inflow and outflow vessels. This may have produced a constant retrograde squeezing force at the distal end of the PED causing it to “watermelon-seed” backward into the aneurysm sac. Also, given the vertical position of the ICA, the weight of the device may have dragged the construct downward. The PED migration may also be at least in part the result of an “accordion effect,” a phenomenon of foreshortening of a device that was possibly stretched during deployment. This is particularly true in patients 1, 2, and 5 where the PED moved from its initial position because of device shortening rather than migration. In patient 1, the PED moved distally due to shortening because it was probably better anchored distally than it was proximally.

The incidence of spontaneous migration seems to be higher with the PED (5/155, 3.2%) than other self-expanding stents such as the Neuroform (Stryker Neurovascular, Fremont, California) (no cases previously reported) or Enterprise (Codman & Shurtleff, Raynham, Massachusetts) (only 6 cases previously reported).⁸ The PED is particularly vulnerable to the phenomenon of spontaneous migration because it is a low-porosity stent, which facilitates the transmission of a force exerted at one end of the device to the other end.^9–10 The migration of the PED is somewhat similar to the migration of Enterprise stents. Previous cases have involved Enterprise stents deployed from the midbasilar artery to the P1 segment of the posterior cerebral artery during treatment of basilar tip aneurysms.^{10⇓⇓⇓⇓–15} The migration was primarily attributed to the diameter difference between the basilar artery and the P1 segment of the posterior cerebral artery. Lubicz et al¹⁶ previously described the delayed migration of a Silk stent (Balt Extrusion, Montmorency, France) in a patient with a giant saccular carotid ophthalmic aneurysm. The patient had a massive SAH as a result of the migration and died.

Our present series shows that PED migration can occur both early and late after treatment. In patient 2, the migration occurred within 72 hours. Conversely, in patient 4 the migration likely occurred several months after the initial intervention, as suggested by the recurrence of the patient's symptoms at 5 months.

Although PED migration was uneventful in 3 patients, it was associated with fatal or severely disabling complications in 2 patients. In patient 2, the distal end of the PED migrated proximally into the aneurysm sac, creating a direct jet of blood against the aneurysm wall and precipitating aneurysm rupture. This occurrence is further supported by the observation that the rupture site on the aneurysmal dome was directly facing the distal end of the device. In patient 1, the patient presented with complete occlusion of the MCA, but it is not clear whether this event was related to the distal migration of the PED. Still, the device migration into the aneurysmal sac precluded any attempt at mechanical thrombolysis, and the procedure had to be aborted because of the high risk for iatrogenic aneurysm perforation (thus preventing potential neurologic recovery).

The following precautions should be taken to minimize the occurrence of spontaneous delayed PED migration or shortening, especially if there is a mismatch in diameter between inflow and outflow vessels:

1. Obtaining complete expansion of the PED with optimal vessel wall apposition. Accurate sizing of the PED is of utmost importance. DynaCT may be useful to confirm adequate apposition of the device to the vessel wall.

2. Using longer PEDs to prevent excessive foreshortening. In addition, the conveyance of retrograde forces is lower with longer stents.^9,10 Increasing vessel coverage proximally and/or distally to the aneurysm may also be helpful. A second stent can be deployed to increase vessel coverage when needed. One could argue in retrospect that PED deployment was too proximal in patients 3 and 5 and was too distal in patient 1.

3. Avoidance of dragging of the PED to prevent foreshortening. Instead of going in distally and dragging the device proximally into the landing zone, it would be preferable to go directly into the landing zone to detach and deploy the device.

4. Avoidance of stretching of the PED during deployment to prevent the “accordion effect.”

5. Adjunctive aneurysm coiling, as this may prevent prolapse of the PED into the aneurysm for larger aneurysms.

We have recently applied these precautions, and we will assess in a future study whether they can lead to a significant decrease in the incidence of PED migration or shortening.

PED migration is best managed by placing additional PEDs to achieve complete coverage of the aneurysm and divert the blood flow away from the aneurysmal sac. A J-shaped guidewire is passed through the migrated stent and is navigated directly into the distal ICA. This is followed by deployment of 1 or more PEDs as necessary until the parent vessel is reconstructed. If the proximal part of the device has migrated into the aneurysmal sac and attempts to recatheterize the PED have failed, retrograde access of the PED is gained from the distal ICA through a microwire advanced from the basilar artery through the posterior communicating artery or from the contralateral ICA through the anterior communicating artery.¹⁷ Endovascular parent vessel deconstruction or surgical ligation, when tolerated, is an alternative strategy to manage PED migration. In the setting of subarachnoid hemorrhage, as in patient 2, the goal of treatment is to obtain immediate occlusion of the aneurysmal sac and/or the parent vessel.