====== Posterior fossa decompression for Chiari malformation surgical technique ======

1. [[position]]: [[prone]]

2. [[equipment]]:

a) optional [[microscope]]

b) intra-op Doppler, if used (primarily in pediatrics)

3. consent:

a) procedure: surgery through the back of the neck to open the bone at the base of the skull and
to insert a “patch” to make more room for the [[brainstem]]

b) alternatives: non-surgical management is usually not effective

c) complications: CSF leak, brainstem injury/stroke, apnea, failure to improve syrinx (if present)

===== General information =====

The most frequently performed operation is [[posterior fossa decompression]] of the [[cerebellar tonsil]]s
using a [[suboccipital craniectomy]], with or without other procedures (usually combined with dural
patch grafting and [[cervical laminectomy]], which must be carried down to the bottom of the tonsillar
tip, which usually includes C1, and sometimes C2 or C3). Options for grafts: same incision (pericranium),
separate incision (e.g. fascia lata), and allograft (avoided by many authors because of dissatisfaction
with ability to provide water-tight closure and because of infectious risks).

Goals of surgery: decompress the brainstem and reestablish normal flow of CSF at the craniocervical
junction.
----



The patient is positioned [[prone]] on [[chest roll]]s with the [[head]] in a [[Mayfield]] head-holder or in a [[horseshoe headrest]]. Flex the [[neck]] to open the interspace between the [[occiput]] and posterior arch of [[C1]]. The [[shoulder]]s is retracted inferiorly with [[adhesive tape]]. If a [[fascia lata]] graft is to be taken, elevate one [[thigh]] on a [[sandbag]]. A midline [[skin incision]] from [[inion]] to ≈ [[C2]] [[spinous process]] is made. The removal of [[bone]] above the [[foramen magnum]] should be ≈ 3 cm high by ≈ 3 cm wide (keep the [[posterior fossa]] part of these operations small, the main thrust is to open the foramen magnum to decompress the [[tonsil]]s and an upper [[cervical laminectomy]]; the compression is not in the [[posterior fossa]]). Excessive removal of [[occipital bone]] may allow the [[cerebellar hemisphere]]s to herniate through the opening ("[[cerebellar ptosis]]”), and create additional problems. If a pericranial [[graft]] is to be taken, it should be harvested at this time to reduce the amount of blood entering the subsequent dural opening
((Stevens EA, Powers AK, Sweasey TA, Tatter SB, Ojemann RG. Simplified harvest of autologous pericranium for duraplasty in Chiari malformation Type I. Technical note. J Neurosurg Spine. 2009; 11:80–83)).

The pericranial graft can be procured without extending the [[incision]] about the [[inion]] using the technique of Dr. Robert Ojemann
((Stevens EA, Powers AK, Sweasey TA, Tatter SB, Ojemann RG. Simplified harvest of autologous pericranium for duraplasty in Chiari malformation Type I. Technical note. J Neurosurg Spine. 2009; 11:80–83)).
with subgaleal dissection and using a [[monopolar]] cautery with a bent tip to incise the periosteum and then a [[Penfield Dissector]] #1 to free it from the bone surface.

Open the [[dura]] in a “Y” shaped incision, and excise the triangular top flap. CAUTION: the [[transverse sinus]]es are usually abnormally low in Chiari malformations. Suture the patch graft to provide more room for the contents (tonsils+medulla).

An option that is sometimes used in pediatrics is to not initially open the dura but to lyse constricting bands over the dura at the foramen magnum and then and use [[intraoperative ultrasound]] to determine if there is adequate room for CSF flow, the dura is then opened only if there is not.

Historical procedures that have been appended to the above: plugging the [[obex]] (with [[muscle]] or [[teflon]]), drainage of [[syrinx]] if present ([[fenestration]], usually through [[dorsal root entry zone]], with or without [[stent]] or [[shunt]]), 4th ventricular shunting, terminal [[ventriculostomy]], and opening [[foramen of Magendie]] if obstructed.

Current [[recommendation]]s are that these or other additional procedures beyond [[dura]]l [[patch]] [[graft]]ing are usually not warranted.

Some authors repeatedly admonish not to attempt to remove adhesions binding the tonsils together (to avoid injuring vital structures, including [[PICA]]s). Others recommend cautiously separating the tonsils and even shrinking them down with bipolar cautery.

In cases with ventral [[brainstem]] compression, some authors advocate performing a transoral [[clivus]]-[[odontoid]] resection as they feel these patients may potentially deteriorate with [[posterior fossa decompression]] alone
((Dyste GN, Menezes AH, VanGilder JC. Symptomatic Chiari Malformations: An Analysis of Presentation, Management, and Long-Term Outcome. J Neurosurg. 1989; 71:159–168)).
Since this deterioration was reversible with [[odontoidectomy]], it may be reasonable to perform this procedure on patients who show signs of deterioration or progression of [[basilar impression]] on serial MRIs after [[posterior fossa decompression]].