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.