The approach may be from the front of the spine in selected cases but in most the malformations is aproached from the back. The commonest procedure is called laminectomy where a window is fashioned in the bone covering the spinal canal. Occasionally additional support will be required to the spinal column after laminectomy by the placement of screws and rods in the bones but this is usally not the case. An alternative is to fashion a door in the bone of the spine which may be secured back in place at the close of the operation. This is termed laminoplasty.
The risks of the procedure vary according to the type of malformation being dealt with. The commonest type I spinal dural arteriovenous fistula is very ameanable to surgical treatment with a favourable safety profile making microsurgical division of the fistula our favoured treatment in most cases. The prospects of obliterating the fistula permanently is >95% with a safety record comparable to other types of relatively routine spinal surgery.
Unfortunately type 1 sDAVFs often present with very established disability because it has take a long time for the diagnosis to be considered. The MR appearences, although characteristic, may mimic inflammatory processes of the spinal cord and patients may be investigated at length for the various causes of such inflammation. In general following sucessful obliteration of a spinal dural arteriovenous fistula sensory symptoms such as pain or uncomfortable pins and needles in the legs are the most likely to improve. it is unusual for function in the legs to return completely to normal. If the patient's bladder or bowel function has been affected only about 30% report any significant improvement over time.
Type IV perimedullary fistulae are then next most common lesion referred for surgical treatment. Most are considered first for endovascular treatment especially when the fistula is formed from the anterior spinal artery. Where endovascular treatment is not possible it is our our experience that most of these can be successfully treated microsurgically. There are different subtypes of perimedullary fistula as described by Merland (Types A ,B and C). Type A lesions are "simple' fistulae usually formed by the anterior spinal artery joining onto the ventral coronal venous plexus. Type C lesions are larger, more complex polyfistuous lesions and the distinction from arteriovenous malformations can become more difficult.
Symptomatic and compact type II AVMs within the cord may be considered for microsurgical removal. Endovascular embolisation may be utilised to complement the procedure for higher flow lesions. If intramedullary AVMs are asymptomatic conservative management may be appropriate in selected cases.
The risks assosciated with treatment of type II AVMs are greater than with type I and IV. They are rare lesions and while it is generally agreed that they carry a worrying prognosis, proceeding with treatment merits careful discussion between patient and doctor to arrive at the right plan for the individual. A treatment recommendation is always made after multidisciplinary discussion between specialist neurosurgeons and interventional neuroradiologists. Every effort will be made to individualise the risk profile of the procedure and balance against the expected benefits of the treatment.