Subdural Grid Placement

Operative Planning

Ensure that lateralization has been clearly determined
Areas to be covered are discussed with epilepsy team prior to operation
Neurophysiologist or epileptologist scheduled for electrocorticography (ECoG) recording
Ensure epilepsy monitoring equipment is available for intraoperative use
Ensure epilepsy monitoring unit bed available for postoperative care
Frameless stereotaxy can be used to assist electrode placement, particularly if a neocortical lesion is present, and to facilitate central sulcus localization

Anesthetic Issues

Room should be cool for mild hypothermia
Intravenous antibiotics with skin flora coverage (cefazolin 2 g) should be given 30 min prior to incision
Dexamethasone 10 mg intravenously prior to incision
Mannitol (0.5 to 1 mg/kg) for brain relaxation and protection
Continue prior anticonvulsants: epileptologist may have tapered anticonvulsants in preparation for epilepsy monitoring unit (EMU)
Several anesthetic agents will alter ECoG, therefore 30 min prior to recording:
- Isoflurane is kept < 0.2%
- Propofol is discontinued
± N₂O (controversial)
- No barbiturates or benzodiazepines

Positioning

Foley catheter
Patient’s ipsilateral shoulder elevated
Mayfield head holder
- Leave room with draping for wires to exit through skin
- Alternatively, head may be placed on doughnut
Head turned 90 degrees to opposite side, parallel to floor (assess neck flexibility before surgery)
Neck is slightly extended, vertex down
Pressure points are padded

Planning of Shave and Incision

Elevate Scalp and Muscle Flap

Only scalp is incised
- Major bleeders are controlled with bipolar cautery
- Raney clips are applied
Superficial temporal artery is preserved to promote healing
Temporalis muscle is elevated with monopolar cautery
- Incision is undercut to palpate zygoma
- Sphenoid depression is visualized
- Anterior exposure is maximized
Flap roll is placed behind flap, which is retracted inferiorly
Wet sponge is placed over muscle
Two towel clips are placed at base of muscle and held with rubber bands to Leyla bar
Remaining muscle is everted over ear with suture to maximize inferior exposure

Bone Flap

Small drill burr is used to make a bony opening under muscle at inferior margin of bone flap (root of zygoma region)
B1 foot plate is used to free dura from overlying bone
Craniotomy is turned, extending as far anterior, posterior, and inferior as possible: medial extent is kept 1.5 cm off midline to avoid superior sagittal sinus and arachnoid granulations (or possibly across midline, if interhemispheric placement planned)
Bone flap is elevated with flap elevator and Penfield no. 3
Holes are drilled along superior temporal line to reapproximate temporalis muscle
Drill is used to shave down sphenoid bone
Temporal lobe exposure is maximized by removal of temporal-bone squamosa with Kerrison or Leksell rongeurs; air cells are good landmark to indicate adequate inferior exposure (image guidance also useful)
Bone wax is applied to bone edges, particularly if air cells are visible
Dural tack-up suture holes (optional) are drilled at bony margins and within bone flap
Screws and microplates are placed in the bone flap and then handed off to scrub nurse to store in antibiotic solution
FloSeal and strips of Surgicel are placed under bone edges
4–0 silk tenting sutures are placed (optional)

Dural Opening

An area as wide as needed (consulting with epileptologist) is covered
Grid is fashioned by removing electrodes with scissors
Grid is placed so as to avoid grid buckling; wires pierce edge of dura; “relaxing” cuts between electrode rows may be helpful
Strips are inserted under bone flap as needed; brain is depressed with Penfield no. 3 to avoid tearing bridging veins
Blood under grid can interfere with contact
Contacts are kept off large blood vessels (if possible)

Electrocorticography

A 5- to 10-min recording is obtained to ensure adequate coverage (optional; may decide on anatomic coverage preoperatively)
Significant interictal activity at edge of grid may indicate inadequate coverage
Grid location is documented with intraoperative photograph and schematic drawings

Closure