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118 Section II: Intracranial Procedures

Figure 12.11 Tenotomy scissors are used to extend the incision through the
meninges after the initial opening is created using a #12 scalpel blade.

below the occipital protuberance dorsally, and ventrally to the fora-
men magnum. These limits avoid the dorsal sagittal sinus, conflu-
ens sinum, and the transverse sinus. Ventrolaterally, the occipital
condyles are the border. This much exposure may not be necessary
Figure 12.9 Three‐dimensional CT reconstruction of severe occipital dys- and the surgeon should plan the dissection and craniectomy
plasia in a Chihuahua with Chiari‐like malformation. The surgeon must
exercise extreme care in dissecting the musculature from the occipital area accordingly. For instance, if the procedure is addressing drainage of
to avoid damaging the neural structures in this region when this defect is a quadrigeminal cyst (usually dorsal to the cerebellum), this much
present. ventral exposure is unnecessary.
If the surgeon ventures too far in the ventrolateral bony
dissection, the condylar vein that drains the sigmoid sinus is
encountered. Bleeding here can be substantial and must be
controlled by packing with an absorbable gelatin sponge or
neurosurgical patties.
With the initial exposure completed, any continued hemor-
rhage from surrounding structures is controlled with bipolar
cautery and the tissues are lavaged with sterile saline before con-
tinuing. The exposed meninges are incised longitudinally with a
#12 scalpel blade. After the initial opening, the incision is con-
tinued rostrally and caudally with Potts or tenotomy scissors
(Figure 12.11). This includes incising through the dorsal atlan-
toaxial membrane that may be vascular. When the desired length
of the meningeal incision is achieved, the incision is extended at
each end from medial to lateral on both sides, making the com-
pleted incision (viewed laterally) in the shape of the letter H.
This allows reflection of the meninges laterally for improved
exposure. Stay sutures (6‐0) can be placed in the meninges to
maintain retraction (Figure 12.12).
Figure 12.10 Kerrison rongeurs being used to perform the craniectomy. The remainder of the procedure is dictated by the original
indication for the surgery. Any handling of the nervous system
malformation. Incising this structure and the underlying menin- tissue must be done with extreme care and generally involves
geal layers exposes the neuropil and results in a copious flow of the use of lint‐free cellulose spears. The obex and caudal aspect
cerebrospinal fluid. of the fourth ventricle are exposed with gentle dorsal traction of
After completing the exposure, the craniectomy is performed. the vermis and carefully incising the thin transparent tissue
The occipital bone has irregular undulations and therefore varying covering the obex. Adhesions are often present with chronic
thickness of the bone. In patients with severe occipital dysplasia or Chiari‐like malformation; such adhesions are cleared using the
a very thin occipital bone, only the use of Kerrison and Lempert cellulose spears.
rongeurs is necessary to perform the craniectomy and dorsal lami- Quadrigeminal diverticula, if not immediately apparent over the
nectomy of C1 (Figure 12.10). Thicker bone requires the use of a dorsal aspect of the cerebellum, can be visualized by gently retract-
high‐powered nitrogen drill to begin the process [4]. One or more ing the cerebellum ventrally, sometimes with slight medial retrac-
burr‐holes are made lateral to the midline. The opening is expanded tion. Well‐encapsulated tumors arising from the region of the lateral
using the aforementioned rongeurs. The boundaries of the bony cerebellomedullary junction are visualized with medial retraction
defect can extend from just ventral to the nuchal line laterally, just of one cerebellar hemisphere. These masses can be biopsied and