Cerebral protection devices can be divided into three groups : distal occlusion balloons, filters and proximal protection systems. A Cerebral Protection Device (CPD) should be selected on the basis of the patient’s anatomical characteristics and of the plaque morphology.

Distal Occlusion Balloons

Distal Occlusion Balloons have been the first devices routinely used in clinical practice.
They work blocking the blood flow within the ICA. Emboli are aspirated before the catheter is removed. A 0.014" guide-wire is equipped with a distal balloon that is inflated through a small channel present within the wire.
After crossing the lesion with the guide-wire, the balloon is positioned above the lesion itself and inflated to completely occlude the ICA so as to avoid the passage of microemboli.
After treatment of the lesion with angioplasty and stenting, a catheter is advanced up to the balloon.
The blood proximal to the balloon, possibly containing the debris dislodged from the atheroma, is aspirated, the balloon completely deflated and the guide-wire removed.
The very low profile of the balloon crossing the lesion (<3 FR) and the good flexibility and torquability of the system are important elements in favour of this type of CPD but the complete occlusion of the ICA - an emodynamic condition that 6-10% of the patients are unable to tolerate- and the impossibility to visualize the lesion constantly during the procedure since no contrast media injection is possible, are clear disadvantages.

Filters

When using filter systems the flow is maintained during all the procedure and emboli are captured and removed together with the device by retraction of the system.
Filters have a metallic structure coated with a membrane made of polyethylene. The membrane may have different shapes and presents several holes with a diameter ranging from 80 to 220 µm.
Filters are mounted on a 0.014" guide-wire, generally 30 mm proximal to a flexible tip and are delivered through a very small profile catheter (=3 Fr).
Once the lesion is crossed, the filter is opened in the ICA lumen by withdrawing the delivery catheter.
When the procedure is over a retrieval catheter is advanced into the wire to capture and remove the filter.
If a very tight stenosis obstructs the passage of the delivery catheter through the lesion a pre-dilation of the stenosis with a 2-3 mm low-profile balloon catheter is recommended.
Several types of distal filters are now available on the market and they differ for the rigidity of the metallic structure, the diameters of the holes and the stiffness of the wire.
As filters cause a sharp reduction of the flow within the ICA, they should not be left in place longer than 15 minutes.

Proximal protection systems

A stenotic lesion is always to be crossed when distal occlusion balloons and filters are conveniently placed but this manoeuvre is risky because it may provoke complete lumen occlusion and distal embolization in particular when instable and complex plaques are present.
Instead proximal protection systems provide cerebral protection with no device advancing through the stenosis because they are based on the inflation of an occlusion balloon at the level of the common carotid artery (CCA) and at the origin of the external carotid artery (ECA) causing the inversion of the flow or the complete stop of the flow within the ICA.
These systems take advantage of the vascular anastomosis of the circle of Willis.
After occlusion of the common and external carotid arteries, the collateral flow through the circle of Willis creates the so-called "back pressure" which will prevent antegrade flow into the ICA. After stenting of the lesion, the stagnant blood present within the ICA, possibly containing embolic material, is aspirated and removed or filtrated.
This procedure has the advantage of being entirely performed under protection but the complete flow occlusion - not well tolerated by 6-10% of the patients - sets a limit to its use.
In addition, the systems available on the market are not user-friendly and require a large introducer sheath (8-10 Fr) making the cannulation of the CCA a challenging task when the arch anatomy is complex.

Complications:

Placing cerebral protection devices in a convenient position does't exclude possible complications, the most frequent being: inability to cross the lesion, failure to capture the emboli and vasospasm or injury of the vessel wall.

Failure to cross a lesion.
CPD deployment isn’t a problem in straight anatomy, but crossing an ICA lesion in case of very tortuous vessels and tight stenoses is a technical challenge not exempt from complications.
The crossing profile of the device is of basic importance in presence of tight strictures and a pre-dilation with a very low profile balloon (2-3 mm) is generally used to overcome this problem. Obviously this manoeuvre makes the procedure more complicated and increases the risk of distal embolization.
Another failure in crossing the lesion may be caused by the abrupt change in stiffness between the floppy guidewire attached to the tip of the device and the portion where the device, protection balloon or filter, is placed.
As a consequence the catheterization of a tortuous ICA may be negatively affected but the problem is generally solved with a "buddy wire technique" deploying a second wire, parallel to the first one, into the same artery to straighten the vessel and make catheterization with the CPD possible.

Failure to capture emboli:
The main purpose of using cerebral protection devices is to capture all emboli and to prevent adverse neurological events.
CPD capture efficiency depends on which type of device is used and can be hindered by a number of different mechanisms.
The distal balloon systems accomplish brain protection by occluding the ICA.
A rare but possible event is the gradual deflation of the balloon resulting in inadvertent reestablishment of blood flow toward the brain.
To prevent this complication the size of the balloon must be carefully monitored under fluoroscopy or some contrast medium must be injected during the procedure bearing in mind that an excessive inflation of the balloon may increase the risk of vessel spasm or dissection.
Neurological effects can derive from the embolization of the ECA territory.
When balloon occlusion is performed all blood, except the volume in the ICA proximal to the occlusion site, is directed to the ECA.
As balloon occlusion causes turbulence within the occluded ICA segment, some emboli may pass in the ECA before post-procedure aspiration thus determining ECA territory embolization.
This event increases the risk of embolization via ECA-to-ICA collaterals which open up during ICA occlusion.
An identical situation may occur when a filter, which supposedly should not block the ICA flow, behaves like a balloon and occludes the ICA lumen.
Placement of balloon occlusion closer to the bifurcation reduces the volume of blood containing emboli but may also lead to even more frequent embolization into the ECA.
Both distal occlusion balloons and filters present the disadvantage that the lesion must be crossed to place them in the right position and the risk of distal embolization increases especially in case of instable and complex plaques. Particles are usually released during the initial phase of the procedure when no protection has been provided yet.
Filters differ from balloons because filtration devices preserve flow in the ICA during the procedure.
This means that the flow to the brain is maintained and angiography can be performed during the procedure but sometimes blood flow preservation may be a good chance for the emboli to travel to the brain.
The selection of the pore size is a compromise between risk of filter thrombosis and risk of microembolization.
Despite an adequate filtering efficiency, embolization may as well occur when the filter adherence to the vessel wall is incomplete.
This often happens if vessels are tortuous and the filter can’t be aligned with the long axis of the ICA and particles may flow around the device.
Consequently the filter should be placed in the most straight portion of the vessel whenever possible.
An incorrect size of the filter, smaller than the arterial diameter, may lead to the same bad situation.
In selected cases, the use of two neuroprotection devices is suggested because they can achieve, working together, what a single system can’t and drawbacks diminish.
A 12% rate of embolic events is associated with the use of distal protection devices (balloons and filters) as a consequence of the emboli generated when crossing the lesion.
To avoid this complication in patients with a very unstable plaque the "seat belt and air bag technique" has proven useful: distal embolization is prevented reversing the flow through the ICA and placing a filter immediately after.
Using a filter, embolization can also occur during the retrieval phase either because some filters have a limited volume or because the material filling the filter can be squeezed when the filter is encapsulated into the retrieval catheter. The problem can be overcome partially recapturing the filter into the catheter.

Arterial Damages:
ICA is a relatively delicate vessel with regard to dissection and spasm.
Its wall may be irritated by any distal device that exerts some force on the vessel wall to achieve complete apposition.
The degree of irritation increases with the frequent movements of the protection device even if the procedure is performed by experienced hands.
Although most episodes of spasm are self-limiting and do not result in clinical sequelae, the long-term effect of such a spasm in the development of intimal hyperplasia is unknown. In most situations spasm can be easily resolved by intra-arterial injection of vasodilators.
Damage to the intimal layer, resulting in ICA dissection, may happen as a consequence of an unexpected movement of the inflated balloon or of the filter basket deployed by the operator or as a consequence of the patient’s neck sudden movement.

Retrieval phase:
Complications may arise during the retrieval phase of the distal filter when the recovery catheter proves unable to cross the stent.
The filter capture requires several different manoeuvres such as external neck compression, advancing the guiding catheter into the proximal stent, redilation of the stent, use of a "buddy-wire" or moving the patient’s head in different positions to straighten the stent.

Technical Complications:
Although extremely rare, detachment of the filter from the guidewire has also been reported when the filter is caught on the stent during retrieval.
Similar events may happen using also other components of the CPDs system, including retrieval catheters.

References