Operative Technique: Chiari Malformation and Syringomyelia
Creators: name: Shao, Belinda
Publisher: name: Zenodo
Year: 2024

description: Here we show a standard approach posterior fossa decompression with arachnoid opening and exploration of the obex, followed by expansile duraplasty.

EXPOSUREThe patient is positioned prone, in 3-point Mayfield fixation, with chin tuck and neck flexion, and posterior translation to open up the occipitocervical junction.  Pre- and post-flip SSEPs are obtained with intraop monitoring following.  An incision is made from 1 inch below the inion to the C2 spinous process.  A monopolar bovie is used to incise the midline nuchal fascia.  The avascular plane is respected by finding the midline raphe. The paraspinal muscles are dissected from bone. The muscles attached to C2 are left intact to help with pain and stability.  Some will also harvest pericranium for duraplasty. The muscles on the inion are left in place to help with closure. BONY DECOMPRESSIONWe perform bony decompression of a 2.5-by-4 centimeter wide suboccipital craniectomy to the foramen magnum, or similar, dependent on the patient’s suboccipital bone morphology on imaging.We use a 5mm burr and Kerrison rongeur, keeping in mind the irregularity of these abnormal posterior fossa in Chiari which may be extremely thin in irregular areas.  Bone wax and thrombin-soaked Gelfoam are useful in packing off vascular channels.Often the bone is held at the foramen magnum by the mydoural bridges and a thicker ridge of bone which may be removed. The foramen magnum is then defined further, and the lateral margins of bone are drilled away through the full diameter of the foramen magnum, which usually extends to the medial aspect of the condyles. A full and wide C1 laminectomy is performed.  In select cases, partial C2 laminectomy can be performed if the tonsils extend further to encompass that level. MYODURAL BRIDGESThe myodural bridges are sharply stripped from the dura.  Venous plexus bleeding is addressed after the myodural bridges are removed.DURAL OPENINGThe decision to perform dural opening may vary based on patient and institutional. Some perform dural opening for all Chiari patients, while others reserve this for patients with syringomyelia or specific intraoperative ultrasound findings after bony decompression.  If no dural opening is undertaken, some perform partial-thickness scoring. Here, we  open the dura in a Y-shape from the tonsils to the removed C1 lamina, using clips for any venous sinuses, and Nurolon tack-up sutures.

ARACHNOID OPENINGUnder the microscope, the arachnoid plane is examined. Some choose to leave the arachnoid intact if no arachnoid pathology is seen and free cerebrospinal fluid (CSF) flow is observed. Others open the arachnoid and attach it to the dural edges with clips or sutures to avoid contamination of the subarachnoid space with blood.   We widely open and excise the arachnoid, dissect any adhesions, and explore the foramen of Magendie. TONSILLAR COAGULATIONThe cerebellar tonsils are inspected.  If they appear gliotic or are obstructive of CSF circulation in the foramen magnum and the foramen of Magendie, they are partially reduced using low-power bipolar electric coagulation over the pia, shrinking the tonsils to let them ascended upward and outward, just to the point of relieving obstruction of the fourth ventricle and the basolateral cisterns.  OBEX AND BASOLATERAL CISTERN INSPECTIONThe obex is inspected by lifting the tonsils superolaterally. Any arachnoid adhesions, webs, veils, or other structures obstructing the foM and obex are lysed, until free egress of CSF is seen. Choroid plexus may also be seen. The basolateral cisterns are examined to visualize the dentate ligaments, vertebral arteries, spinal accessory nerve, and the first or second cervical nerve root.  OTHER INTRADURAL FINDINGSUnique intradural findings can be seen including arachnoid opacification, thickening, adhesions; tonsillar gliosis, hypertrophy, cysts, fixated tonsils, trilobed tonsils, or intertonsilar adhesions; PICA variations and branches, vascularity of the tonsils and spinal cord; and peculiarities at the obex and the basolateral cisterns. Compression and deformation of any of these structures can be addressed.EXPANSILE DURAPLASTYDural opening procedures involve expansile duraplasty to further enlarge the intradural cisterna magna. The duraplasty should provide a watertight seal, minimize scarring, and support the growth of the patient's own dura. Graft options include autologous pericranium, bovine pericardium, fascia lata, Gore-Tex, and Alloderm. Here, a triangular-shaped Alloderm graft is utilized with a running 5-0 Prolene suture, further secured with a Duragen and Tisseal plug.CLOSUREMeticulous hemostasis is achieved. The wound is closed in layers. The muscle should not be overtightened to avoid mass effect or constriction over the expansile duraplasty.  The fascia, deep dermal, and dermal layers are closed tightly to prevent CSF leak. The skin is closed with an absorbable running suture and dermabond skin glue.