Zenonos Microvascular Decompression

Last modified by Ali Alattar on 2023/07/16 13:06

Pre-op meds

25 mg mannitol, 10 dex


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

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

For trigeminal neuralgia, monitor SSEP, and BSERs.


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. 



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:

  1. Put one hand against the underside of the patient's neck
  2. Translate towards the ceiling
  3. Flex the neck
  4. Laterally flex towards the floor

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.



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. 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.



Chloraprep, lidocaine with epi, chloraprep again.


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

Place the Greenburg retractor like this: 




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 both medially, towards the occiput, and laterally, towards the ear. 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. 


Pull the skin edge back and make an incision through the fascia and muscle to the bone using the Bovie. 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. Clip it to the drape over the shoulder. 



Use the M8 to perform a craniectomy, exposing the bottom-most part of the craniectomy first, then moving to the transverse, transverse-sigmoid junction, then sigmoid. You only need to expose the edge of the sinuses and junction. 

In the image below, make the red cut first, then blue, and finish with orange. 

There is often an emissary vein connecting to the sigmoid sinus. When you encounter this vein, wax it to stop the bleeding, then skeletonize the vein in its bony canal on its way to the sigmoid sinus. Once it's sufficiently skeletonized, use a bipolar to cauterize the emissary. 

Be sure to wax the mastoid air cells well at this point. Wax them off before irrigating because irrigation within the mastoid air cells can cause loss of BSERs. 




Use a 15-blade to make the dural incisions. 

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. 

The benefit of making a C-shaped dural opening as opposed to the L-shaped opening is that the dural flap can then be folded medially and tucked between the cerebellum and medial dura. This protects it from drying out and makes it easier to close without needing a patch graft. 


Tent up the dura along the venous sinuses with 4-0 Neurolon suture. Zenonos secures the suture to the yellow plastic part of the fish hooks. 

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. 


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. 


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