Operative Planning

Special Equipment

• Additional platinum-coated irrigating and nonirritating cauteries in multiple lengths
  
• Microligature clips
  
• Zeiss microscope (Carl Zeiss AG, Oberkochen, Germany) with bridge

Operating Room Set-up

• Extra headlight

Anesthetic Issues

• Hyperventilation to pCO₂ = 32 mm Hg
  
• Intravenous (IV) dexamethasone (10 mg) and antibiotics (i.e., cefazolin 2 g every 8 hours or vancomycin 1 g every 12 hours for adults) should be given 30 minutes prior to incision
  
• IV mannitol 1 g/kg is administered at the time of skin incision for brain relaxation, thereby facilitating elevation of the bone flap
  
• The patient’s maintenance anticonvulsant medications are continued, or they are loaded with the appropriate agent (usually phenytoin 1000 mg slow IV push)
  
• The ability to obtain intraoperative hypotension should be confirmed but is rarely if ever used since the advent of staged embolization for large AVMs

Positioning

• Depends on location of the lesion, but in any case the patient should be positioned in such a way as to facilitate simultaneous operating by two surgeons at all times; this generally requires that the microscope be brought in from above the patient’s head and a Mayo stand placed between the microscope stand and the patient’s head to allow both surgeons to rest the other hand
  
• Minimal shave
  
• Use disposable razor
  
• Three-cm wide strip along proposed incision
  
• Sterile scrub and prep

Incision

• Depends on location, but given the length of the operation and need for two surgeons to access the field, we have found that long, straight incisions retraced with fishhooks provide for excellent tissue health (potentially reducing the incidence of wound infection) and visual trajectory

Craniotomy

Dural Opening

• This should proceed as if it were part of the cortical resection with great care not to injure draining veins entering, or the nidus, which may be stuck to the dura as a result of the preoperative embolization
  
• Small dural: arterial adhesions are divided with the irrigating cauteries on low heat
  
• Important venous structures are sharply dissected out of the dura as necessary
  
• Microscopic instruments including variable pressure suction tips, micropinch scissors, small Cottonoids, and fine, well maintained cautery tips are used during this part of the operation

Cortical Incision

• Careful review of the angiogram and the MRI, together with inspection of the cortical surface, should be sufficient to identify the margins of the AVM; this is usually a gyrus-based disease (as the malformation forms at the time of neural development); therefore, it should be defined by sulci on each side
  
• Occasionally the malformation lies just beneath what appears to be normal cortex; in these cases, frameless stereotaxy is valuable
  
• The cortical incision is begun by cauterizing the pial surfaces between the large feeing vessels, providing ample exposure of these vessels prior to definitive cautery and division
  
• Once the malformation has been completely circumscribed, with the exception of the draining veins (all should be left undisturbed, at the least initially), the margins can be layered with Surgicel and Telfa and dissection can begin

Initial Dissection

Deep Dissection

• Once the dissection cavity is ~ 3 to 4 cm deep, lighting becomes a problem, and the microscope with a binocular bridge is brought into the field
  
• The co-surgeons approach continues with this instrument, but depending on the anatomy, a small surface draining vein is usually sacrificed at this point to allow greater manipulation of the AVM because of the reduced angles provided by the microscope
  
• The surgeons should take no vein without first performing a prolonged test occlusion with a temporary aneurysm clip
  
• As the dissection proceeds deeper, the angle becomes increasingly oblique, requiring a greater veering toward the AVM; it is not uncommon to encounter difficult-to-control bleeding at this point, a sign that the AVM has in part been violated; this is a signal to widen the dissection to obtain homeostasis
  
• When dealing with bleeding during the deep dissection, do not open new fronts of battle until the current one is secured

Securing the Ventricle

Final Removal of the Arteriovenous Malformation

• Once the test occlusion is safely completed, a permanent aneurysm clip is placed on the main draining vein, which is then divided
  
• At this point, the Cottonoid and Gelfoam are removed from the ventricle and the ventricular margin is reexamined under improved lighting; careful additional cauterization of the choroids and the ependymal interface is generally undertaken and a fresh piece of Gelfoam is wedged into base so that it cannot float into the ventricle (causing obstructive hydrocephalus)
  
• All the remaining walls are inspected for hemostasis and lined with Surgicel
  
• At this point all reddened veins should be blue; if this is not the case, search for small adjacent fistulae and cauterize

Testing the Resection Bed for Breakthrough Bleeding

• At this point the pressure is allowed to rise gradually to 140 mm Hg systolic, and the cavity is observed for breakthrough bleeding
  
• If this does not occur, the operation is terminated
  
• Breakthrough bleeding can be evidence of either retained AVM (more common) or simply disrupted autoregulation in surrounding tissues; in either case, the resection margin is extended to include this tissue to the extent that future pressure challenges are silent

Immediate Postoperative Angiography

• The incision is then closed in the usual fashion (close attention to watertight dural closure with ventricular entry, pressure lowered to 100 mm Hg systolic); the patient remains intubated on a propofol drip and is taken to the angiography suite
  
• Biplanar, high quality angiograms can be obtained
  
• Although dysplasia is seen in as many as 10% of cases, an early draining vein is rare, but must be considered a sign of retained AVM; under most circumstances this is an indication for returning immediately to the operating room for further resection; exceptions include very small residuals with uncertainty about the timing of vein-capillary phase

• Systolic blood pressure is maintained below 120 mm Hg for the first 24 to 48 hours
  
• Antibiotics continued for 24 hours
  
• Clips are removed on postoperative day 5
  
• Slow steroid taper if ventricular entry is wide or traumatic
  
• Occasionally leave postoperative externalized ventricular drain if bleeding into the ventricle is marked