Shunts


NB: this is adult shunting. peds has its own entire worldview about this topic that is very different.


primary placement


- set up your shunt. if valve is programmable, program it to your setting of choice before you take it out of sterile packaging. Once you're scrubbed, put it into sterile saline, pump until it fills up. Also inject your tubing full of sterile saline, clamp the distal end and attach proximal end to valve, tie to secure with silk ties. Also soak your proximal catheter in sterile saline.
- start with abdomen. measure using your tunneler from belly to retroauricular releasing incision that you plan to make. Make sure you will reach. Allow at least 3-5 cm of redundancy on both sides otherwise you will drive the strugglebus with the sheath getting sucked in on either end. incision lateral to rectus abdominus over rectus sheath, dealer's choice, go through subcutaneous tissue until you hit fascia, tunnel upwards until you hit somewhere behind the ear, then make an incision over your tunneler, pull it out, leave sheath there.
- then make your head incision per instructions below:
- for a virgin head, use the left diagram as your guide - red is kocher's point, blue is the shunt, green is the ideal incision. It allow you to enough exposure to drill the burr hole. If you have to come back for revision, you can extend the incision along the dotted green lines - discussion below.
- if you are doing a shunt after an EVD (i.e. post SAH hydro) and you are in the unfortunate situation of having to deal with a large vertical incision (dark red), most people will extend it anterior (purple line). But this may take you into the forehead if the incision is too anterior. Another alternative that I would do is extend it back, the grey line, and use generous retraction.

- this is why if you place a ventric, use a stab incision or a horizontal incision - it makes tunneling harder, but you save yourself a lot of grief if you have to do a shunt, or even a worse, a crani.
-Then you use some forceps to dissect a pocket for the valve (and you must dissect generously, make a big pocket).
- Then tunnel from head to retroauricular incision. Leave the sheath in place. .
- put distal tubing into the proximal sheath, pull it through, then through distal, pull it through. ideally you get it all the way through the sheath but I've noticed that if you just get it in at least 15-20 cm,  it'll stay in the sheath as you pull and won't pull out. you can try irrigation to help it along, sometimes it works. protip: when you are pulling the sheath+catheter out of abdomen, hold your hand at the belly to catch the catheter to prevent it from being sucked back in.
- If needed, drill burr hole, place ventricular catheter. If its an EVD, pull it out and either hard pass or soft pass the new catheter. Put the little plastic stopper at the depth you want it to be at. Screw in the anchor. NB: when you are draping for EVD to VPS, drape the exit site of ventric out
- trim ventricular catheter to desired length, then attach to valve and secure with silk tie. pull on distal end until it's well seated in the pocket. make sure CSF drips out of distal end. you can try pumping the shunt a few times to prime it
- put distal tubing into abdomen - can be done with either a trocar or old school (aka general surgery style) with picking up layers of fascia individually and cutting through them. some people pursestring the fascia around the tubing to keep it in. you don't have to, but in people who you think are at high risk of their catheter backing out (i.e. extremely overweight people), its probably a good idea.
- close. do not hit the catheter or the valve.


Revisions
- NB: you will need a white connector tubing, get it before the case
- when you come back, open the frontal incision just enough to access the point where the proximal catheter connects into the valve. Disconnect, and check for proximal flow (can use metal tip + manometer to check ICPs), and check for distal flow through both the valve and the distal catheter together simultaneously (syringe + manometer + metal tip + white connector tubing into valve. If the valve+distal tubing are patent, then you just do a proximal revision and you don't have to open the incision bigger.
- if either the valve or the distal catheter are failing, you have to open the incision big enough to access the entire valve so that you can pull it out and check the distal tubing independently from valve. If the valve is failing, replace it. If the distal tubing is failing, try to pull it out, if you can't, leave it in as an orphan catheter, tunnel a new one.
- the key concept is that once you pull peritoneal tubing out of belly, you cannot push it back in. It's physics - you are trying to push a long, soft, floppy tube down a scarred down (or even not-scarred down) path, it won't go. And if you try to shove the valve into the pocket and close over it, the tubing will kink. Therefore, you have to open the pocket bigger until the point where you see the distal tubing connect to the valve. Either anchor the distal tubing there and don't pull it out any more, and then put a new valve on, or if you do have to pull it out, then trim it down again such that the valve will be seated all the way back in the pocket.


more later, too tired to write more now.



Extradural Chiari


Before the case, look at the MRI to evaluate where the vertebral arteries course to make sure they are out of your intended bony decompression.


Pins - two on left, one on right. Pin bilat 1cm above pinna, and then the 3rd anterior on forehead behind hairline. Bias pins anterior as the patient is prone, also helps with being able to swing Mayfield forward.


Position prone, arms tucked. Translate and flex head (ie while prone and in pins, pull entire head upwards until neck and head are in line with superior aspect of back). Flex head as much as possible -- make sure there are still 2 fingerbreaths between chin and chest/table. Most people with chiari have a steeply sloped occiput so the more flexion you can get the easier the procedure.


Midline incision from inion to arch of C2. Knife through skin and dermis, bovie through midline. When dissecting, remember that some people have an incomplete ring of C1. Don't dig with the bovie, paint across and let things fall apart gently. Start at the occiput and go down, and when you get close to foramen magnum, use a penfield 1 or a curette to dissect the tissue off the bone and feel for the edge of the bone.


Nuchal fascia ends at superior nuchal line; Some people like to open fascia in a Y or T fashion, leaving cuff at superior nuchal line to sew to for closure. Some don't. If there is any chance of doing intradural decompression, harvest a pericranial graft.


feel for the posterior ring of C1, use curette/pen 1 to dissect tissues off bone. Generally speaking, you are safe approximately 10-12 mm on either side of midline. Go slowly, feel for the lip of vertebral sulcus. The vert classically runs up the transverse foramina, then curves medial at the posterior ring of C1, and enters dura. There should be a (thinner vs thicker) lip of bone between vert and posterior aspect of C1. Some people will thus bovie with abandon across posterior ring of C1, however this lip may be very thin vs absent, and you risk ending up in the vert.


Between C1 and foramen magnum there is some soft tissue that frequently contains an epidural venous plexus that can bleed a lot. carefully bipolar it, pull it apart, and cut. If it bleeds, don't panic, just put gelfoam and surgiflo


Different ways to do craniectomy
- can use fluted ball drill on occipital bone, starting at thumbprints (which are thinnest parts) and eggshell out the bone and figure out depth, then drill across through the keel, then use kerrison to remove the remainder of bone - lateral, inferior, and medial.
- can use perforator on thumbprints, and then use the B1 with a footplate to cut across to connect them, and then down to foramen magnum, then remove the bone en bloc.
- craniectomy goal is to decompress to lateral aspect of ring of C1. The most medial aspect will also be the most shallow; as you get more lateral, the bone will get thicker and will pinch the dural sac more; this is the scariest bone to take but also frequently missed and important to decompress


C1 laminectomy
- once you identify the safest lateral aspect of the C1 ring, you can drill troughs into it, stopping every so often to push down against the remaining bone to feel for its give, and then pull out the ring en bloc.


then use pickups, woodson, knife to remove the thick outer leaflet of dura at the craniocervical junction, leaving behind the soft inner dural layer


at this point, some people will ultrasound the craniocervical junction to ascertain the extent of compression, and if there is still compression, will open the dura and patch graft.