Endovascular neurosurgery, also known as interventional neuroradiology, uses thin tubes (catheters) inserted into blood vessels to treat conditions affecting the brain and spine. The catheters are directed under x-ray guidance and gain access to the circulation by puncturing an artery at the wrist, groin or occasionally the neck.
It is difficult to navigate a catheter across the convolutions and restrictions the carotid artery negotiates as in enters the cranial cavity through the base of the skull. The earliest technological innovations to accomplish that may be credited to a Russian neurosurgeon, Fedor Serbinenko. Watching the annual May Day parade in Moscow in 1959, he noticed children carrying helium filled balloon at the end of a string and how the balloons carried the string in different directions, guided by the breeze. This inspired him to develop a balloon guided catheter that eventually allowed more detailed angiography of the cerebral circulation as well as the therapeutic blockade of vessels. Serbinenko was using a detachable balloon device to occlude carotid-cavernous fistulae and even aneurysms in the 1970s.
Endovascular treatment was not adopted widely until later. In 1989 an Italian neurosurgeon Guido Guglielmi was continuing a research interest in treating aneurysm by electrolysis at the UCLA laboratory of Drs Fernando Vinuela and Ivan Sepetka. Fortuitously realising that the deposition platinum metal in the aneurysm when delivering the electrical current could promote thrombosis independent of the current, ultimately this led to the Guglielmi Detachable Coil (GDC coil).
Aneurysms
Endovascular treatments have become a mainstay of aneurysm treatment with the demonstration of its safety and efficacy in clinical trials. The International Subarachnoid Aneurysm Trial (ISAT) established coil embolisation as first line therapy in ruptured aneurysms for most specialists.
Coiling
The deposition of platinum coils in an aneurysm sac has proven to be an effective, durable and relatively safe treatment for most ruptured aneurysms since the ISAT study. The technology has continued to evolve, for example with coating on the coil to further encourage thrombosis of the blood in contact with it. Coils are also used to treat unruptured aneurysms when suitable although the documented incidence of recurrence will lead to more complex endovascular treatments with stents or endosaccular devices (see below) being considered in an attempt to "get it right first time" or microsurgical repair.
Endoluminal Stents
A stent is a metal device that is delivered in a collapsed state into a blood vessel then opened to conform to the inner surface of the vessel exerting a slight radial force to hold its position. If the base of an aneurysm is too wide for it to retain the coil ball within deployment of a stent in the parent artery will prevent the coils from falling onto the artery and blocking it. The skilful use of a stent may prevent or salvage the loss of a branch near the neck of aneurysm when coils threaten to occlude it. Stents may also be used to retrieve blood clots within arteries or to dilate a narrowed vessel (stenosis) in selected cases.
There are a large number of such devices entering the market all the time and each claiming unique benefits. Most of the comparative studies relating to safety and efficacy are led by industry.
Closed or covered stents are used relatively infrequently for neurosurgical indications. Open cell stents afford more flexibility in deployment and the opportunity to provide physical support to other devices without completely covering branches arising from an artery.
Open-cell, laser cut stents offers a great deal of flexibility and conformity to the arterial wall. Their length is the same before and after deployment but the design prevents them being retieved again once deployed. Braided stents expand and lengthen during deployment and can be collapsed back on themselves and recovered. Ask your treating specialist about the reasons particular devices are favoured in your case.
Flow Diverting Stents
Flow diverting stents merit mention in their own right. As the density of metal coverage in the stent wall increases a critical point is reached where it becomes easier for blood to flow in the stent or into normal anatomical branches than to enter an aneurysm. This reduction in blood flow into the aneurysm creates turbulent flow within it and promotes blood clot to form (thrombosis). This thrombosis may be sufficient to secure the aneurysm in its own right or it can assist coils deposited within the aneurysm in doing so. Crucially the flow into smaller, anatomically normal branches covered by the stent will usually be preserved.
A flow diverting stent can reduce the likelihood of aneurysm recurrence after endvoascular treatment. As with other endoluminal devices they require treatment with antiplatelet medication to avoid clots developing within them that can occlude vessels causing stroke. There is considerable variability in the drugs used and duration of therapy advised so ask your treating specialist about what is recommended in your case.
Arteriovenous Malformations and Fistulae
Endovascular techniques (through the blood vessel) may be used to completely close an arteriovenous malformation or they may be deployed to complement another treatment such as microsurgery or stereotactic radiosurgery by either reducing the flow through the AVM or decreasing the nidal volume in hopes of rendering treatment safer and/or more likely to be curative.
Typically embolisation is carried out under a general anaesthetic which assures the patient remains completely still while the catheter is manoeuvred into deeper parts of the circulation and also affords tight control over the blood pressure and use rate to assure optimal conditions for the procedure. Tiny catheters are passed into the circulation and under x-ray guidance they are directed through the blood vessels of the brain to the malformation. Most often the arteries are used to deliver the embolic material (trans-arterial) although there are circumstances when material is delivered through veins (transvenous) or a combination of both.
Embolisation can be curative in AVMs with particularly favourable architectures. As the architecture increases in complexity and the nidus in size the likelihood of obliterating the arteriovenous shunt reduces and/or the risks of doing so increase. In our current practice embolisation is relied on more to complement microsurgical or radiosurgical treatment rather than to push for cure. Procedural risks are not less than other therapies and the occlusion rates are lower in the long term. On the other hand dural and pial arteriovenous fistulae lend themselves very well to endovascular treatment in most cases. As with any procedure case selection is the key to success.
Cyanoacrylate Glue
N-butyl cyanoacrylate (NCBA) glue is a relative of the original superglues applied to vascular disease. A related glue was first used in 1975. Because it adhered to the vessels, catheter and set very rapidly controlling its deployment could be challenging as this can lead to problems such as blockage of catheters, adhesion to the tissues or loss of access to the AVM through premature proximal occlusion as well as inadvertent blockage of non-AVM vessels. Diluting the glue with other chemicals made for a more pliable substance. Ultimately the glue induces an inflammatory reaction in the tissues with fibrosis of blood vessels in the long term.
Ethylene Vinyl Alcohol Copolymer Liquid Embolic Agents
More recently liquid embolic agents which do not adhere to the inner surface of the vessels have been developed. The first of these in use was Onyx , the trade name for ethylene vinyl alcohol copolymer. It is combined with tantalum powder to make it visible on x-ray and offers additional control and predictability as it solidifies compared to NBCA. For examples injections can stop and start again in a way not Possible with a rapidly setting material. Again it is available in a variety of concentrations. The catheters through which it is administered are first flushed with a chemical called dimethylsufoxide (DMSO). The patient and those around them may be aware of a characteristic odour that lingers for 24 hours or so after treatment, the result of DMSO being expelled throught the skin and mucus membranes.
SQUID is a newer EVOH agent with purportedly longer injection times and it is appears dense on x-ray allowing easier visualisation of vessels later in the procedure. PHIL (Precipitating Hydrophobic Injectable Liquid) is another with the benefit of prepreparation and less wastage. There are no high quality comparative studies between these agents to suggest any one is superior. Your treating interventional Neuroradiologist will be happy to discuss these agents further with you.
Mechanical Embolic Material- Coils and Particles
Aneurysms on arteries irrigating an AVM may be coiled as with aneurysms in other locations. If the the rest of the AVM remains in-the coils may be subject to excessive compaction and a higher likelihood of aneurysm recurrence than with non-AVM aneurysms. Coils (see above) may also be used to occlude larger vessels and selected nidal elements. Poly Vinyl Alcohol (PVA) particles can there to vessel walls as well as occluding vessels by aggregating and mechanically obstructing them. They have been largely supplanted by liquid embolic in brain AVM but may still be deployed in selected cases or tumour embolisations. Gelatin spheres provide another mechanical alternative. Many other temporary occlusive materials have been used in the past including Gel Foam, silk or Vicryl suture materials.
