Responsive Neurostimulation for Refractory Epilepsy: Right Hippocampal Depth Electrode, Left Temporal Cortical Paddle

Responsive Neurostimulation

• FDA approved in 2013 as adjunctive therapy for medically refractory epilepsy with focal onset seizures from 1 or 2 foci

• Programmable neurostimulator implanted into the skull via craniectomy and connected to depth electrodes or subdural cortical strip leads at pre-identified seizure foci

• Provides closed-loop neurostimulation upon detection of abnormal epileptiform activity on continuous ambulatory electrocorticography (ECoG)

• Long term ECoG data useful for identifying and refining seizure onset zones, as well as informing treatment alternatives such as resective or ablative surgery

Case Presentation

30 year old male presents with 12 year history of medically refractory epilepsy. No history of inciting injury or illness. Semiology: 1) Absence type, 5 minute episodes, 2-3 per day, 2) Convulsions, stiffening and eye rolling, occasionally in clusters of three.

Preoperative Workup

• Imaging unremarkable 

• EMU

• Frequent left temporal region epileptiform discharges

•  Occasional right temporal region epileptiform discharges

• Multiple seizures in left temporal region without clear evolution at times

•  sEEG

• Right mesial temporal lobe seizure onset

• Left lateral cortical temporal lobe seizure onset

• Wada

• Left injection: 9/16

•  Right injection: 15/16

Assessment: Medically refractory temporal epilepsy with bilateral foci (as above)

Recommendation: Implantation of RNS system with right hippocampal depth and left cortical paddle

Approach

RNS implantation of right hippocampal depth lead and left cortical paddle electrode using intraoperative CT, frame-based stereotaxy, and neuronavigation:

1. Preoperatively determine lead trajectories based on sEEG electrode trajectories with stereotactic targeting software

2. Obtain intraoperative CT Head

3. Coregister intraoperative CT with preoperative MRI; plan trajectories; obtain stereotactic coordinates for right hippocampal lead

4. Complete registration for left temporal cortical paddle placement

5. Plan incisions using neuronavigation

6. Perform placement of right stereotactic hippocampal RNS electrode

7. Perform placement of  left temporal cortical lead using neuronavigation

8. Perform left craniectomy and placement of the RNS device

9. Connect leads to RNS device

10. Irrigate and close

11. Obtain postoperative CT Head

Equipment and OR Layout

• Intraoperative CT
• Brainlab Curve and trajectory planning station
• Intraoperative electrocorticography system
• Stereotactic frame and localizer box placement
  • Head ring
  • Frame pin and screw set
  • Wrenches
  • Mayfield adapter
  • Localizer box
• Incision, Craniotomy, and Durotomy
  • 15 blade
  • Electric Drill with:
    • Craniotome
    • 4.5 mm drill bit
    • 6 mm round cutting burr
• Neuropace RNS components and equipment to assist placement, including:
  • Implantable neurostimulator device
  • Depth electrode
  • Paddle electrode
  • Protective sheath
  • Cranial plate
  • STarFix insertion tube and cannula
  • Penfield Dissector No. 3
• Misc: Hemostasis, Infection Prophylaxis, Anesthetic, Closure
  • Local anesthetic
  • Electrocautery
  • Floseal or other hemostatic agent
  • Bacitracin for frame pins
  • Vancomycin powder
  • 3-0 Vicryl
  • 4-0 Monocryl
  • Dermabond

Alternatives

• Deep brain stimulation of anterior nucleus of thalamus

• Vagal nerve stimulation

Positioning

• Supine for CT head, head ring and localizer box placement

• Sitting for lead placement, craniectomy, and device placement

Pre-Incision Set-Up

• Pre-plan trajectories based on sEEG electrode trajectories

• Place the frame and localizer box and perform intra-op CT head

• Line up the frame and identify 4 pin entry sites 

• Apply 2 posterior pins, finger tighten, lower the patient onto the pins, thus stabilizing the frame in place

• Apply 2 anterior pins, finger tighten 

• Tighten pins diagonally with wrenches 

• Attach Mayfield adapter to the frame

• Secure to the Mayfield base so that the patient is horizontal for the registration scan 

• Place localizer box over the frame

• Co-register with pre-op MRI, plan trajectories, get x,y,z coordinates for right hippocampal lead

• Example: RIGHT hippocampus: X = 83, Y = 109, Z = 131, Ring = 168, Arc = 86.5. 

• Complete Brainlab registration for left temporal cortical paddle placement

• Plan the incisions using neuronavigation

Anatomy

Incision

1. Right Hippocampal Lead Placement: Incision determined by preplanned trajectory coordinates [X,Y,Z] obtained using neuronavigation
2. Left Temporal Cortical Paddle Placement: linear incision above left ear
3. Craniectomy for RNS Device Placement: left parietal bone incision with skin flap

Operation

Right Hippocampal Lead Placement

• Place patient in the right seated position

• Set Leksell coordinates on arc and posts

• Mark scalp entry and skin incision surrounding the entry point; inject local anesthetic

• Incision, hemostasis, dissect pericranium, place a self-holding retractor

• Use a frame to mark skull entry point, place a single burr hole with a 4.5 mm drill bit on a handheld drill

• Irrigate bone chips, coagulate the dura, epidural hemostasis with Gelfoam

• Set up STarFix insertion tube and pass FHC cannula to 20mm from target

• Place RIGHT Neuropace lead with the bottom of Contact 1 at target depth

• Remove the stylet, mark the lead and secure in position with a protecting sleeve and a cranial plate

• Tunnel the lead to the pre-planned internal pulse generator (IPG) incision

Left Temporal Cortical Paddle Placement

• Linear incision above left ear, achieve hemostasis

• Verify craniotomy site and trajectory with Brainlab neuronavigation

• Make a small burr hole craniotomy with a 6 mm round cutting burr 

• Coagulate the dura

• Open dura with 15 blade

• Advance Neuropace cortical strip lead over a Penfield 3 in pre-planned trajectory

• Secure the lead with a protective sheath and a cranial plate

Craniectomy for RNS Device Placement

• Inject pre-planned scalp flap over left parietal bone with local anesthetic

• Incise and achieve hemostasis

• Reflect flap 

• Using the device template, trace out the craniectomy on the bone

• Complete craniectomy with 6mm cutting burr and a craniotome

• Achieve dural hemostasis with bipolar and Floseal

• Fit the ferrule (device carrier) into the craniectomy and secure to the skull with 4mm screws

RNS Device Placement

• Tunnel the leads to the IPG incision

• Remove electrocautery from the field after adequate hemostasis

• Position and secure the RNS device in place

• Attach leads to device, with left lead in the anterior position; mark anterior lead with silk tie

• Perform intraoperative device interrogation of the device noting epileptiform discharges

• Irrigate incision, dress with vancomycin powder 

• Close with inverted interrupted 3-0 Vicryl sutures for the galea and pericranium, and a running 4-0 monocryl and Dermabond on the skin

Post Op

• In the immediate postoperative period, the patient experienced a very brief, self-limited episode of global weakness and aphasia with full spontaneous resolution, consistent with a seizure

• Post-op recovery was otherwise uneventful without complications

Outcome

• No seizure activity reported in the weeks immediately following discharge from hospital
• Fully intact sensorimotor and cranial nerve function at two week postoperative follow-up examination

Pearls and Pitfalls

• Frame Placement
  • There are several methods to place the frame, including manual fixation, ear plug assisted fixation, and a head-device supported fixation.
  • Regardless of the method used, strive for a symmetrically placed frame, fairly parallel with AC-PC plane (a line between upper border of tragus and lateral canthus is roughly parallel to AC-PC).
• RNS lead length and spacing 
  • Choice of lead length and spacing depends on location and depth of the targeted brain region

Discussion

• This case demonstrates successful implantation of a NeuroPace responsive neurostimulation device with a right hippocampal depth electrode and left cortical paddle.

• The operation was facilitated by use of pre-operative sEEG to plan target trajectories, intraoperative CT and ECoG, frame-based stereotaxy, and neuronavigation.

• RNS is recommended for medically refractory epilepsy with focal onset seizures originating from 1 or 2 foci. The system provides closed-loop neurostimulation upon detection of abnormal epileptiform changes, and long-term ambulatory ECoG data can be used to further inform management.

References

1. Matias CM, Sharan A, Wu C. Responsive Neurostimulation for the Treatment of Epilepsy. Neurosurg Clin N Am. 2019 Apr;30(2):231-242. doi: 10.1016/j.nec.2018.12.006. Epub 2019 Feb 18. PMID: 30898274

2. Ma BB, Rao VR. Responsive neurostimulation: Candidates and considerations. Epilepsy Behav. 2018;88:388-395. doi:10.1016/j.yebeh.2018.09.032