Pre-op meds

50 g mannitol, 10 mg dexamethasone.

Monitoring

For all cases, Dr. Zenonos does basic 2-lead EEG in addition to the below monitoring.

For hemifacial spasm, monitor facial nerve, BSERs, and SSEPs. 

For trigeminal neuralgia, monitor BSERs and SSEPs.

Pinning

The head is always pinned. Zenonos will turn the bed, position the patient lateral, and support the head on a stack of blankets before pinning. Since he pins front-to-back, with pins on the occipital and frontal bones, it's easier to pin this way than to pin with the patient on their back. 

Double pins are occipital, in the sagittal plane, biased slightly off midline away from the side of the pathology. The single pin is in the forehead, at about the mid-pupillary line on the contralateral side from the pathology.

Be sure to review the preop CT for the frontal air sinus to be sure that the single pin is not going into the sinus. 

Positioning

First, turn the bed such that the patient's face is facing towards anesthesia after they are turned lateral. The bed should be parallel to anesthesia. 

An assistant should hold the head and endotracheal tube while the patient is positioned. Turn the patient lateral. Move the patient away from anesthesia until the back is flush with the edge of the bed.

If a patient has a higher BMI or if the surgery is likely to be long, the dependent arm should be in a sling. Otherwise, the dependent arm is placed on an arm board at a 90-degree angle to the bed. 

• No sling: move the patient north until the dependent arm is just shy of the break between the head and body of the bed. Place an arm board as high up on the bed as possible, without it being on the head of the bed. Position the arm board at a 90 degree angle to the bed. Put the dependent arm on the board and pad the underside with a thin purple foam, then wrap circumferentially in a thin purple foam and tape circumferentially around the arm board. 
• Sling: move the patient north until the dependent arm is on the head of the bed, ready to drop down into a sling when the head of bed is removed. Once the head of the bed is removed, have an assistant hold the dependent arm until the head is positioned and locked into the Mayfield. Wrap the dependent arm in purple foam and allow it to settle against the arm of the Mayfield. Fold a pillow in half width-wise and stuff into the remaining space between the dependent arm and the Mayfield. 

Place an axillary roll in such a way that it will stay under the patient when the head of bed is removed. 

Pad the legs with purple foam under the dependent knee and a pillow between the knees and ankles. 

Wrap the upper arm in a large purple foam and pull down towards the feet before securing to the patient's side by going around the bed with 3-inch tape. Above and below the elbow. 

Have someone hold the dependent arm before removing the head of the bed. Remove the head of the bed and attach the 360-degree Mayfield.

Positioning the head:Put one hand against the underside of the patient's neck

1. Flex the neck. Do not posteriorly translate the head. Allow the head to move forward, away from the back, when flexing. In essence, flexion should develop the angle described by the lime-green lines in the picture below. 
   
2. Laterally flex towards the floor
   
3. Translate towards the ceiling
4. Head rotation differs for trigeminal neuralgia (left) and hemifacial spasm (right)
   

Secure the Mayfield to the outside attachment of the head clamp. 

Pull the upper arm down towards the head of the bed with 3-inch tape. Tape goes on top of the shoulder and is pulled down to the foot of the bed nearer anesthesia. 

Image guidance attaches the guidance arm on the inside attachment of the Mayfield and positions it such that the tracker is on the same side as the patient's face.

Marking

Use image guidance to mark out the transverse and sigmoid sinuses and the transverse-sigmoid junction. Mark the inferior edge of the transverse and medial (posterior) edge of the sigmoid. 

Mark the mastoid tip and palpate the digastric groove. Mark the approximate location of the craniotomy itself.

For trigeminal neuralgia, the lateral, or anterior, border of the craniotomy overlies the sigmoid sinus. The superior border of the craniotomy overlies the transverse. It is approximately circle or oval shaped. The skin incision is a short 4-5 cm vertical incision centered on the craniotomy marking.

For hemifacial spasm, the incision is longer, the craniotomy is ellipsoid with the long axis in the sagittal plane and the short axis in the axial plane, and the epicenter of the craniotomy is lower, towards the foramen magnum. 

Prep

Chloraprep, lidocaine with epi, chloraprep again.

Draping

Four blue towels in a square. Image guidance cover. Crani drape. 

Place the Greenburg retractor like this: 

Approach

Skin

Use a 10-blade to incise through skin, subcutaneous fat, and galea. If you must use cautery, use bipolar only and below the level of the hair follicles. Do not burn the skin edge or follicles.

Develop the sub-galeal plane with the knife laterally, towards the ear. There is no need to develop this plane medially, towards the occiput. Make sure you stay in the plane below galea and above the fascia of the muscle. Be aware that as you move laterally, you will have to raise your hand away from the patient to keep following the plane as it curves down towards the ear. Err on the side of going deeper rather than more superficial. If you get into the muscle it's not a big deal, but if you poke out the skin it's very bad. 

Use fish hooks to hold the skin edges back. Two on the lateral edge and two on the medial one. 

Muscle

The first muscle incision is made on the mastoid process. Palpate with a finger or the tip of the Bovie to be sure you are on the bone of the mastoid. Start towards the tip of the mastoid and pull the Bovie back towards the transverse sinus. Go down to the bone. Make sure you pull the skin edge back and make the muscle incision under the edge of the skin incision. 

In the figure below, the skin incision is marked in red, the muscle and fascia incision in blue, and the tip of the mastoid in yellow. 

Elevate the muscle from the bone using a curved Adson periosteal dissector. Use a fishhook to hold the muscle down. 

Bone

Dura

Use a 4-0 Neurolon to tent up the dura before making the incision. Start at the lowest point of the incision, i.e. on the limb parallel to the transverse sinus nearest the foramen magnum.

Pass the needle through the dura then through the drape. Tie a single knot then secure to the drape with a snap. 

Use a 15-blade to make the dural incision then carry it through with the serrated microscissors. 

The dura is incised in a C-shape with one arm just shy of the transverse sinus, another shy of the sigmoid sinus, and the third at the inferior border of the exposure, parallel to the transverse cut. 

Tent up the dura along the venous sinuses with 4-0 Neurolon suture. Evert the dura by passing the needle first from the inside, through the dura, to the outside, then reverse the needle and pass it from outside through the dura to the inside. Zenonos secures the suture to the yellow plastic part of the fish hooks. 

Finally, place strips of BICOL^® collagen sponge on the venous sinuses and a larger piece over the dural flap, which is allowed to sit on top of the cerebellum.

Microvascular Decompression

First, open the cerebellomedullary cistern. Place a half-by-three Taka patty on the surface of the cerebellum pointed inferiorly. Use felt forceps and a Fukushima suction to push the patty down along the inside of the posterior fossa dura, following its curvature.

Keep going until you see the arachnoid covering the cistern. You will see cranial nerve 11 through the arachnoid at the 12 o'clock position of your exposure. Open the arachnoid by placing the closed tips of the felt forceps against arachnoid then allowing the tips to open, spreading open the arachnoid. Hold that position for a minute, allowing CSF to drain. 

Leave the half-by-three patty there, then point the scope superiorly towards the tentorial dura. Place another half-by-three patty towards the expected location of CN V. 

Superiorly, you will see the superior petrosal vein diving towards the tentorium (SPV in the image below). 

Inferiorly, you will encounter CN VIII in its course from the brain stem to the internal auditory meatus. The bony prominence at the superior margin of the meatus is called the suprameatal tubercle. You may also see a small artery entering the dura superficial to the CN VII/VIII complex called the subarcuate artery. 

Carry the arachnoid dissection to CN V. The area of vascular compression can usually be seen as an indentation in nerve from the underlying artery. 

Place Teflon felts between the nerve and artery. 

Sling technique

Cut a strip of Gore-Tex that is ~3 cm long and ~5 mm wide. Cut off one corner to create a beveled edge. Pass a 6-0 Prolene suture through the tip of the beveled end, tie a knot, and cut off the needle, leaving a small ~1 cm length of suture attached. 

Pass the suture behind the relevant vessel (in this case it was a dolichoectatic vertebral artery), and use the short length of suture to pull it out from behind the artery. 

Cut off the end with the suture tied through it. 

Suture to the dura overlying the petrous bone. First backhand the needle through the dura, then pass it through the sling, then tie it down. 

In the schematic on the right, the arrow marks the location of the dura that is sutured to. 

Take the free end of the Gore-Tex strip and use it to pull the artery away from the nerve. Once you've achieved the desired displacement, use a right angle mini aneurysm clip to clamp the two sides of the strip together. 

Monitoring

During the arachnoid dissection, you may get calls from neurophysiology regarding the BSERs. Bear in mind that the BSERs cycle once every 45 seconds or so. So information about changes usually lags behind the actions that caused the change. If you are told that the latency is increasing, stop what you are doing and fill the field with irrigation. Wait until the latency returns to baseline before proceeding. A change in latency and in amplitude is more concerning. If the changes do not return to baseline, consider moving or removing Teflon pieces to reduce stretch on CN VIII. 

Closure

Close the dura with 4-0 Neurolon in a watertight fashion. 

Fill the craniectomy defect with bone cement. 

Close the muscle layer with 3-0 Polys. 

Close the galea with 3-0 Polys. 

Close the skin with a subcuticular absorbable 4-0 Biosyn. Use a small amount of skin glue on top the running stitch. Wait for the glue to dry, then cover with a single Steri-strip lengthwise over the incision. 

Quiz Questions

What is the name of the bony prominence superior to the internal acoustic meatus? The suprameatal tubercle

What small dural artery is often seen superficial to the tubercle? The subarcuate artery