It was once believed that if endovascular embolisation of an AVM could not cure the lesion completely then it would convert the nidus into an inert mass that could be more easily removed or the residual treated with radiosurgery. As so often in the neurosciences reality proved to be more complicated and these days pre-surgical embolisation is recognised as a technique that can bring significant benefits during an operation but if not planned well may add to the technical difficulty of an operation. In our practice the neurosurgeon and interventional neuroradiologist work extremely closely together to plan a pre-operative embolisation of an AVM.
Frequently a procedure called a microcatheter angiogram will be carried out. This procedure done under anaesthetic allows the interventional neuroradiologist to define the internal architecture of the AVM in tremendous detail. In discussion with their surgical colleague a decision will be made as to
- The goal of embolisation- what volume of the nidus should be closed to optimally enhance a subsequent surgical procedure.
- Which of the arterial pedicles irrigating will be addressed first
- What the timeframe for the endovascular treatment should be to optimise the result with the greatest safety
This last decision is very important as overly aggressive embolisation that does not close the malformation complete may produce changes to the stresses on vessels within the AVM and in the surrounding brain. The effect can be to precipitate bleeding or swelling of the brain. Embolisation of an AVM is every bit as technically demanding a prociedre as surgical removal. The treatment may be staged over several procedures to minimise the risk of such changes while creating the optimal enviroment for the last stage of definitive removal.
It is perhaps counter-intuitive to argue that filling the AVM nidus with embolic material can make surgery more difficult. While blood is flowing through an AVM the vessels may still be gently compressed by the surgeon which allows them to access its different faces with minimal disturbace to the adjacent brain tissue. If the AVM has become solid and immovable then more retraction becomes necessary on the surrounding cortex.
Embolisation and Radiosurgery
Embolisation of nidus of an AVM before radiosurgical treatment also requires planning and ideally liason with the radiosurgical unit before any endvascular treatment takes place. With larger volume AVMs which are unsuitable for surgical removal but which are too large a volume to be confident that radiosurgery will be effective, volume reduction of the nidus by embolisation may be considered. However certain architectural characteristics of the AVM require consideration first as in some cases embolisation will reduce the prospect for a curative treatment. Larger volume AVMs may still be treated by volume or dose fractionation. This treats the AVM over multiple sessions and may be more effective than attempting to shrink it by endovascular means in selected cases. Securing selected bleeding points e.g. assosciated aneurysms without embolisation of the nidus is unlikely to adversely affect subbsequent radiosurgical treatment.
The Future- Radiosurgery as a Prelude to Surgery?
A potentially exciting therapeutic option for AVMs that were not previously safely operable and too large to expect a radiosurgical cure is to treat elements of them with stereotactic radiosurgery and slowly alter the nidus over time such that it is more amenable to surgical, endovascular or mutimodality cure. This strategy should be distinguished from situations where unsuccessful radiosurgical treatment has resulted in conditions that allow further curative treatment as a result of happy accident. The challenge is to ensure predictable changes in the AVM that allow for deliberate staged treatments. At the moment there are only a few case reports of such treatments deliberately carried out.