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Chiari Decompression: Closed Technique (Posterior Fossa/C1 Bone Removal, Partial Durectomy)

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In this video, we present a posterior fossa decompression with C1 laminectomy and partial durectomy (outer dural leaflet removed) under the guidance of intraoperative ultrasound in a pediatric patient (22-month female) with symptomatic Chiari Malformation type 1.5.
The indications for surgery, particularly in the young child, are heavily debated. Clear and less controversial indications include: 1) intractable occipital headaches, often exacerbated by Valsalva maneuvers, 2) neurological deficits attributed to brain stem compression, and the 3) presence of a large syrinx with or without scoliosis. Our patient presented with progressive neurological findings of right foot drop leading to frequent falls, gait instability, increased tone in right lower extremity with ankle clonus, bilateral esotropia and neck discomfort with mild limitation in hyperextension. MRI of the brain revealed a Chiari malformation type 1.5 with significant tonsillar descent (17mm on the left and 16mm on the right) and mild basilar invagination. The tonsils were wedge-shaped and there was significant crowding at the craniocervical junction. CSF flow study demonstrated slight to minimal CSF pulsation at the anterior and posterior cervical medullary junction respectively. MRI of the spine did not reveal any evidence of syringomyelia or tethered cord. We preferred a surgical treatment (instead of observation) in light of the neurological findings and significant crowding at the craniocervical junction. Following extensive discussions with the family, including the risks and benefits of an intradural versus extradural approach, a simple closed technique (PFD/C1 laminectomy and partial durectomy) was chosen.
PFD is contraindicated when the tonsillar herniation is caused by etiologies other than Chiari malformations, such as, a posterior fossa mass or CSF hypotension syndrome.
The patient is placed in a prone position on well-padded bolsters, taking care to avoid pressure points, especially around the face and eyes. Pins may be used in the older child (>3 years old). The neck should be flexed adequately to allow placement of two fingers between the chin and chest. Adequate flexion is crucial; limited flexion fails to open the craniocervical junction while extreme flexion risks airway compromise and/or postoperative macroglossia (from congestion at the oropharyngeal compartment). A 3-cm microshave from the inion to C2 spinous process is performed and the area is prepped with standard chloroprep solution. Multiple intravenous lines and a Foley catheter are placed prior to the start surgery. Prophylactic antibiotic is given to all patients. Steroids are administered if an intradural exploration is anticipated. 5-10 cc of 0.5% lidocaine with 1/200,00 parts epinephrine is injected into the planned incision to minimize bleeding.
MRI is the test of choice for evaluation of Chiari malformation (to evaluate tonsillar descent and morphology) and associated syringomyelia. Cine-MRI is helpful to evaluate CSF flow at the craniocervical junction. Plain XR and CT are of limited utility but can be useful to evaluate bony deformities such as platybasia, atlas assimilation, congenital cervical fusion or scoliosis.
The inion and spinous process of cervical segments serve as important anatomic landmarks to define the length of the incision. The incision is taken down to the occipital bone and posterior elements of C1/C2. Care must be taken to avoid any potential bony defects at the foramen magnum or arch of C1. In our patient, we observed a somewhat atypical appearing C2 with a very small spinous process as well as flattening of the occipital shelf. When removing the posterior arch of C1, it is important to extend this lateral to the point where the C1 arch begins to curve away at the lateral portion of the spinal canal. Dissection beyond the lateral canal does not offer any further advantage but increases the chance of an inadvertent discovery of the vertebral artery. We were able to identify a large and prominent left vertebral artery with modest degree of medial extension on intraoperative ultrasound and subsequent dissection in that area was performed carefully.
The advantage of the closed technique (extradural approach) is a lower risk of complications, particularly, CSF leak while providing adequate decompression as evidenced on intraoperative ultrasound. Early utilization of the intraoperative ultrasound helps establish baseline motion/position of cerebellar tonsils, CSF flow and helps identify any abnormal anatomy (anomalous vertebral artery). Periodic use of the intraoperative ultrasound at critical stages of the surgery can guide subsequent intervention (dural opening, tonsillar manipulation).
Potential risks include CSF leak, bleeding, infection, neurologic injury and lack of efficacy
None
We would like to thank the Board of Visitors of Children’s National Medical Center for the generous grant
Massimi l, Novegno F, Di Rocco C. Chiari Type I Malformation in Children. Adv Tech Stand Neurosurg. 2011;37:143-211.

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