Indication: lateral posterior fossa structures; with a big retrosig you can reach all the way from foramen magnum below (ie to open cisterna magna for CSF drainage) to the back of the sella above, and to lateral clivus anterior (maybe even further, depends on how big the pre-pontine cistern is and how tight the posterior fossa) but the optimal working window is gonna be around the CPA. Workhorse approach for CPA pathology, MVDs to access CN 5-10, and tumors that are either very lateral and/or very deep - i.e. where you would rather retract cerebellum medially than punch through it.
Before the case, look at imaging:
- location of transverse and sigmoid sinus relative to EAM/mastoid
- thickness of mastoid temporal bone
- amount of air cells and how far posterior they extend
- slope of petrous bone (to make tiny adjustments to head position)
Positioning: dealer's choice
- supine + head turn: advantages are that it's a lot easier to set up, so if you are trying to do a lot of cases in one room and don't want to lose an hour per case on positioning, it's a good approach. The other mostly hypothetical advantage is that gravity pulls the cerebellum down and out of your field, but in reality most people have a pretty full posterior fossa, even with generous CSF drainage you end up having to do a lot of retraction and the gravity assist thing doesn't make a big difference. The main disadvantage is that for all the intradural work, you have to operate UP - which means sitting down with your arms up high in front of you. This is a hard position to hold, and most people will develop muscle fatigue in their hands and forearms pretty quickly, even with a chair where you can rest your arms. This works great for a procedure you can do in 15-20 minutes (i.e. MVDs if you're good at them) but this is not gonna work for a big tumor where you may need to cusa or drill for hours and hours. And since the microscope is often positioned very angled or lateral, the observer scope can be in a difficult position, which means the first assist often has to assume an incredibly awkward half-stoop. NB: Asking your ENT colleagues to drill the petrous in this position is a fast way to make enemies.
- 3/4 prone - tedious to set up, but much easier ergonomically for long cases. The goal/final position is head facing halfway between lateral and prone - i.e. 3/4 prone. You stand behind the patient's head, so that when you are moving in the corridor between petrous and cerebellum, you are going essentially straight in, which is nice and ergonomic for microscope neurosurgery.
How to position someone in 3/4 prone - there are many variations
- reversed OR bed, put the head of the bed on.
- put beanbag on OR bed before you have patient get on it.
- anesthesia intubates and gets all their lines. tape the eyes shut. if you are using neuromonitoring, they should get their needles in now and tape them in well
- put the adaptor on the bed that secures the upper-arm holder device
- pin - the goal is to have the bar of the mayfield roughly parallel to the floor in final position. Pinning you put two pins low on the occiput, and contralateral goes high on forehead behind hairline if you can. The further your incision from pinna, the further towards contralateral ear you will have to move your posterior pins and the closer towards ipsilateral ear you move your front pin.
- first, position patient in lateral. The idea is that both shoulders/arms will be off the bed in the end, so you need to slide the patient up in the bed until both their axillae are above the head of the bed, which you will remove and replace with the mayfield holder. Then you slide the patient over to one side and push into lateral.
- put an axillary roll under their chest at the edge of bed to create space in the axilla
- suck air out of beanbag to hold them in lateral.
- now all at once (and you need multiple people to help you on this step) - you remove the head of the bed to exchange for the mayfield holder, at which point both the patient's arms/shoulders will no longer be supported by the bed - and thus require people to hold them. The dependent arm, cradled, goes into a soft foam sling that secures around the contralateral shoulder. Put foam padding under the straps. The superior arm goes into some sort of arm support device (we use a metal frame with a stockinette over it) that secures to the bed.
- turn the head to 3/4 prone, and lock the mayfield.
- the superior shoulder should be tape-pulled inferiorly; when someone pulls with the tape, another person should provide counter-traction proximally so you don't york on the brachial plexus.
- if you are doing BAERs, no xeroform in ears.
- if you are doing facial nerve monitoring, now is the time to put it in -- blue into orbicularis oculi (stick into eyebrows), red into orbiculars oris (stick into skin next to lips), and the other two as ground into the shoulder. NB: when you put in needes, put them in at a very shallow angle-- if you stick straight in you risk the mucosa.
- if you are using navigation (i.e. to find the sinuses) now is the time to register
Incision
- for a tiny quarter-sized retrosig (i.e for MVD) two fingerbreaths behind pinna, approx from digastric notch inferiorly to top of pinna superiorly
- for a big retrosig for tumor, four fingerbreaths behind pinna, from skull base to four fingerbreaths above
Prep, drape the ear out of the field.
Knife through skin, bovie to bone with impunity. you can do this part fast, there's nothing at risk
When you dissect periosteum off the bone, you can do so with impunity posterior, but watch out anterior - you dont want to dissect through skin into EAM. Poor form. So when you move anterior just go slow and feel with your instrument (pen 1 or joker or whatever) - you will feel the bone drop off into the EAM before you hit it. If you feel that, stop. its Ok to find that ridge as long as you don't violate the overlying skin.
When you retract the skin, if you are pulling skin anterior - warn your neuromonitoring techs, because you may disloge the ear needles they need for BAERs.
Now you have to figure out where to drill. Where are the sinuses? At this point, the head is in 3/4 prone and buried underneath yards of drape. you cannot rely on typical craniometric landmarks - you won't be able to feel inion or zygoma or tragus.
The sigmoid sinus will run essentially right behind the EAM. Remember, deep to the sigmoid sinus you will find petrous bone, just as deep to transverse sinus you will find tentorium. The transverse sinus you can find by the curve of the skull - as the skull curves from the flatness of temporal bone towards the curve of the sub-occiput, that junction lies the transverse sinus. There also lies the attachments of the cervical musculature, and thus the junction between cervical fascia and galea.
Or you can use navigation.
blue - transverse and then sigmoid sinuses
pink - where you drill for small crani
dotted pink - how you expand your drilling for a bigger crani
black - pinna and EAM
green- incision for small crani
Drilling
- beforehand, you looked at the CT scan to see how thick the bone is, and also to see how far posterior the mastoid air cells are. When you are drilling, if you run into something that looks like dura really early on, either they have a super thin mastoid temporal bone and you're running into dura, or its a mastoid air cell.
- you want to eggshell out over dura and sinus -- sometimes, the sinus can push upwards and will actually be more superficial than the dura. So if you've found the dura and there's bone over the sinus, you cannot just drill with impunity to the same depth over sinus. You might run into sinus before you expect. When you've eggshelled over dura, peel off remainder of bone with curette and kerrison (slowly and carefully anterior and superior, quickly and with reckless abandon posterior and inferior). When you get close to sinus, use a woodson, point it upwards flush along the bone, and sweep side to side carefully. You will feel the sinus. Don't jab or poke it with the instrument.
Wax the mastoid! Wax in and wax out, as one of my wiser chief residents once said. You cannot wax too much! if your patient leaks postop you will regret not having waxed more!
What to do if you hit the sinus
- step 1: do not panic.
- step 2: tell anesthesia that you hit the sinus. A decent anesthesiologist will understand that this means (potential) massive blood loss and air embolus risk.
- step 3: if lots of blood is coming out, stick your finger over the hole to stop the bleeding
- step 4: obtain a very large (very large!) gelfoam, and put the gelfoam over the sinus injury, put a patty over the gelfoam. Do not use a small gelfoam, it could get sucked into the sinus and cause a sinus thrombosis or a PE
- step 5: wait, and reassess. sometimes, waiting it out with gelfoam + pressure is enough for it to stop. If not, you may need to think about a dural flap closure, so you may need to do more drilling - sometimes people even extend the crani supertentorial to obtain a patch of dura to flap down over the sinus.
- step 6: call for help early from a more experienced surgeon
Open the dura in a c-shape with the flap side towards the sinus. Some people like knife through outer leaflets of dura, then woodson + knife, some people like tenotomy scissors with geralds to pick up dural edge. Use a patty to protect the brain no matter what you do.
Tack up the dura with neurolon sutures. Put pattys or telfas on the brain, and with gentle pressure from your retractor, suction out CSF slowly to relax the brain. if you have a lumbar drain, pull off 15-20 cc of CSF from it. In a big crani for tumor, direct these efforts southward - you don't need to do a crani all the way to foramen magnum, just far enough to reach with gentle pressure to open the cistern.
In an MVD for TN, direct your efforts towards the junction of petrous bone and tentorium ("the tunnel of love"). FYI you will often run into some sort of superior petrosal vein superficially. sometimes there is one, sometimes many branches. Try really, really hard not to sacrifice it - the complication rate described is small but not insignificant.
