The procedure of vessel occlusion has to be previously confirmed by an occlusion test. The test allows the interventionist to know whether it is possible to occlude the target vessel. A rough test could be done with a manual compression at the neck of the vessel one plans to occlude while carrying out angiographic runs of the other vessels. To obtain more complete information on collaterals and safety, it is necessary to perform a balloon test occlusion (BTO).

The BTO is done with the patient awake and under systemic anticoagulation with heparin (80 IU/kg). Two femoral sheaths must be placed: one for the guiding catheter and the other one for angiographic control from the other vessels during the occlusion test. A complete angiographic study of the cerebral circulation must always be obtained before starting the procedure. The angiographer has to carefully evaluate the function of the communicating arteries and the presence of anastomosis, particularly among the branches of the external carotid artery and internal carotid or vertebral artery. The second step is to perform BTO. The BTO is realized by placing an inflatable balloon into the target vessel under fluoroscopic guidance at the point where one wants to occlude the artery. The point of occlusion should be chosen very carefully, in light of the previous angiographic study, to avoid occluding the artery in the wrong location, i.e., a location where the occlusion may leave a persistent filling of the aneurysm sac through anastomosis. The balloon is inflated up to the point of occluding the lumen of the vessel; the occlusion is maintained for at least 15–30 min during which time, through angiographic runs; the intra- and extracranial collateralization is evaluated. Neuropsychological tests are performed to assess the possible occurrence of a neurological deficit. It is always possible to perform stress tests, provoking a drug-controlled hypotension, in such a way as to highlight subliminal neurological deficit.

The balloon test occlusion has two results: an angiographic result and a clinical one. The angiographic test is passed only when the vein opacification of the territory downstream from the occluded artery is contemporary to the rest of the brain up to, but not more than, two seconds later. The clinical test is passed if the occlusion does not provoke any neurological deficits. If the occlusion test is not tolerated, the balloon is deflated, the procedure is discontinued, and one may consider the realization of a surgical bypass. If the test occlusion is instead tolerated, one can finally occlude the artery, by detaching the balloon (in case of a detachable balloon) or by substituting the balloon with coils or a different occlusion device.

Arterial occlusion is simple and very effective, but it is certainly not riskless [14, 15]. Detachable balloons are not available anymore, mostly due to commercial reasons. For this reason in recent years, therapeutic occlusion is almost always performed using coils (Figs. 3, 4, and 5).

In the early 1990s a new device changed forever the treatment of cerebral aneurysms: the electrical detachable coil invented by Guido Guglielmi (Guglielmi detachable coil). Coils (metallic filaments that would curl up on themselves) were already being used for filling aneurysms, but the problem was that by pushing them through a microcatheter it was almost impossible to control their correct final position, with the risk of losing them in distal arteries (Fig. 6).

Guglielmi achieved the goal of controlling the detachment of the coils from the pusher wire. One could finally detach the coil only once there was the evidence of a correct positioning. The detachment is realized with an electrolytic mechanism. The aneurysm is then filled through the release of several consecutive coils, obtaining a small ball of thread. Initially the idea had been to cause the thrombosis of the aneurysm sac by coagulating the blood through an electric current necessary to detach the coil inside the aneurysm. It was soon evident that that approach would not work and that it was necessary to fill the sac with filaments rather than causing thrombosis [16, 17].

The first GDCs made their appearance on the world stage in 1991 and very quickly demonstrated their effectiveness. The first coils had a filament diameter of 0.010 in., and subsequently coils of 0.015 in. were produced, making more rapid filling of large aneurysms possible; the 0.015 in. versions were later replaced by those measuring 0.018 in. Platinum was chosen because it is a metal that does not undergo electrolysis and because it is spontaneously radiopaque, as well as being thrombogenic and inert.

The detachment of the coil takes place by the closing of an electric circuit powered by a battery, in which the positive pole is coupled to the tail of the coil, placed outside the microcatheter, and the negative pole is coupled to a needle inserted under the skin of the patient. The passage of a low-voltage current causes the electrolysis of the micro-welding causing the detachment of the platinum filament. The newest detachment systems do not need needle insertion anymore.

GDCs were initially used for the treatment of ruptured aneurysms which neurosurgeons considered technically impossible – or at least very difficult – to address. It could be because of their location (e.g., the tip of the basilar) or of their characteristics (Fig. 7). The selected patients were mostly elderly or in poor clinical condition [18]. The range of measurements and spatial conformations of the initially available GDCs, defined as the diameter of a single loop of the coil and the length of the entire filament, was very limited. Nevertheless, the idea had an immediate extraordinary success. Endovascular treatment with coils showed clear advantages compared to neurosurgical clipping: in many “difficult” cases the treatment was relatively easy, fast and, especially, less aggressive than a neurosurgical intervention, at the same time ensuring a more-than-satisfactory result in excluding the aneurysm. At the beginning of the coiling experience, there was no scientific proof of the truth of these observations, but the efficacy of the method was simply evident [19–22]. Over time different companies provided a more comprehensive range of sizes of coils with different degrees of stiffness (normal, soft, ultrasoft, etc.) and with different spatial conformations of the loops (2D, 3D, complex, etc.).

Treatment with coils and the balloon remodeling technique aroused great enthusiasm and interest, and many interventionists tried new ways to advance the techniques. From the mid-1990s onward, some of them experimented sporadically with intracranial stenting procedures – the idea being that a stent might produce the cure of an aneurysm by changing the flow inside the sac and by being a scaffold for endothelial regrowth. They used coronary balloon-mounted stents, but the maneuvers were extremely difficult and cumbersome due to the high rigidity of the systems [28, 29]. The idea of an intracranial stent subsequently considered the possibility to better fill the difficult wide-neck aneurysms, being a support to the coils, in an even more effective way than did the remodeling technique. The need of a stent dedicated to the specificity of intracranial vessels became apparent: the new stent should be softer and easily trackable along curves and tortuosities and should be self-expandable, in order to avoid possible ruptures of vessels by balloon inflation (Fig. 12).

The first prototype of a self-expandable intracranial stent, the Smart, appeared in 2001: it was developed by Dr Peter Kim Nelson and his team at Smart Therapeutics (San Leandro, California), and renamed Neuroform in 2002 when the patent was acquired by Boston Scientific and approved by the US FDA [30, 31]. The Smart system was composed of three parts, a self-expandable stent, a microcatheter, and a stabilizer over the wire. The stent, preloaded in the microcatheter, was an open-cell stent, formed by radiolucent filaments made of nitinol. The diameters of the devices ranged from 3 to 4.5 mm, while the lengths ran from 15 to 20 mm. At the two extremities of the stent, four radiopaque markers ensured visibility and controllability. The delivery catheter was 3 Fr in outer diameter. The stabilizer was a second microcatheter of smaller diameter (0.25 in.) with a radiopaque marker at the distal end. The whole system was navigated by an exchange 0.014″ wire, needed to reach the landing zone. Both the catheter and the stabilizer were equipped with “Y” valve connectors. The system was assembled on the sterile surgical field by inserting the stabilizer in the proximal end of the microcatheter containing a preloaded stent. At the time of the release of the stent, the distal end of the stabilizer was placed next to the proximal end of the stent. Upon reaching the area of release, the microcatheter was withdrawn, keeping the stent in position thanks to the stabilizer; so doing, the stent was released at the target point, across the neck of the lesion. The Neuroform (Smart) was followed by other competing devices: Leo (Balt, Montmorency, France), Enterprise (Cordis, Neurovascular J&J), Solitaire (Ev3, Covidien, Paris, France), and LVIS (MicroVention, Tustin, California, USA), characterized by different design philosophies [32–35] (Fig. 13).

Intracranial stents used for assisted coiling are tubular structures in cells, made of a metal alloy with shape memory, typically nitinol or cobalt-chromium. The stents are released into the parent vessel of an aneurysm, across the aneurysm sac, usually before filling the aneurysm with coils. The self-expandable stents do not need a balloon to open, and hence need an intrinsic radial force, produced in part by the material of which they consist and in part by the geometry with which the cells that constitute them are joined. The memory metal alloys, at a predetermined temperature (usually 37 °C), tend to shape themselves, expanding and bending in space, allowing the stents to achieve a preprogrammed diameter once unconstrained by the catheter. It is mainly the different types of construction – open cell (Smart, Neuroform I, II, and III), braided stent (Leo and LVIS), closed cell (Enterprise), or folded sheet (Solitaire) – which influence the characteristics of different stents: their radial force, their capacity in adapting to vessel tortuosity, their opening and closing resistances, and their ability to retrieve the stent. Every stent has its unique combination of these characteristics. The Solitaire adds the specificity of being fully retrievable.