C.W. Dewey, DVM, MS, DACVIM (Neurology), DACVS
There are a number of congenital disorders of the brain encountered in dogs which are amenable to surgical treatment. Recognition of these abnormalities has increased in recent years due to increased availability of MR imaging in our profession. Whereas surgical treatment options are well described for some of these abnormalities, others have only recently been reported in the veterinary literature. The purpose of this presentation is to briefly describe the surgical procedures associated with a number of disorders: congenital hydrocephalus, intracranial arachnoid cyst, and craniocervical junction abnormalities (e.g., chiari-like malformation and related disorders). Surgical management of other disorders (e.g., brain tumors and trauma) will also be briefly reviewed.
Ventriculoperitoneal shunt (VPS) placement is the typical method by which congenital hydrocephalus in dogs is surgically managed. There are a number of shunts available, all of which have the same basic design: a rostral end for placement into the lateral ventricle, a one-way valve, and a distal end for placement into the peritoneal cavity. There is some controversy regarding the appropriate opening valve pressure for shunts (i.e., low vs. high). For very small patients (e.g., Chihuahua, Yorkshire terrier), two incisions-one curvilinear incision over the caudodorsal region of the parietal bone and one vertical incision caudal to the last rib are made. For larger patients (e.g., English Bulldog), an additional incision approximately midway between these 2 incisions on the lateral thorax is often required. A high speed drill is used to drill two holes in the parietal bone: one for the shunt and one for an anchoring suture. After insertion of the rostral end of the shunt into the lateral ventricle, it is secured with a Chinese finger trap suture via the anchor hole using 3-0 or 4-0 prolene. A grid approach to the peritoneal cavity is performed and the distal end of the shunt is tunneled from the cranial incision to the caudal incision using long Carmalt or Doyen forceps. After placement of the distal end of the shunt tubing into the peritoneal cavity, it is secured to both the last rib and the abdominal musculature. Either post-operative radiographs or CT scans can be used to assess the accuracy of shunt placement postoperatively.
Intracranial Arachnoid Cyst
Surgical management of intracranial arachnoid cyst is generally accomplished via either fenestration or cystoperitoneal shunt (CPS) placement. The author prefers CPS placement after creating a wide craniotomy defect. With the exception of the placement of the rostral end of the shunt, the procedure is identical to that described for congenital hydrocephalus. For placement of the rostral end of the shunt in IAC cases, the author performs a wide caudolateral craniotomy incorporating the creation of both a rostrotentorial and suboccipital skull defect. The transverse sinus between these two defects is sacrificed and occluded with bone wax. As with congenital hydrocephalus, postoperative imaging is performed to assess accuracy of shunt placement. The success rate for surgical management of IAC appears to be high in humans and dogs, and whether fenestration or cystoperitoneal shunting is the preferred procedure remains controversial for both species.
Chiari-like malformation (CLM) is generally treated surgically via foramen magnum decompression (FMD). The author and colleagues developed a cranioplasty procedure to be used in conjunction with the FMD procedure. A dorsal midline incision is made extending from approximately 1 cm rostral to the external occipital protuberance cranially to the middle of the second cervical vertebra caudally. The superficial dorsal cervical musculature is separated at the median raphe, exposing the underlying biventercervicis muscles. The paired biventercervicis muscles are then separated on midline, exposing the rectus capitisdorsalis muscles. The caudal aspects of the rectus capitisdorsalis muscles are removed from the cranial half of C2 using sharp dissection and periosteal elevation and these muscles are then split on midline. The cranial aspects of the rectus capitisdorsalis muscles are sharply incised from the nuchal crest, exposing the caudal portion of the occiput and the arch of the atlas. Hemorrhage is controlled with bipolar electrocautery. A high-speed air drill with a 3- to 4-mm diameter round drill bit and Lempert rongeurs is used to remove a portion of the occiput and the dorsal aspect of the first cervical vertebra. Approximately 75% of the length of the dorsal arch of C1 is routinely removed. In cases in which there appears to be dorsal constriction at C1/C2, the entire arch of C1 as well as part of the dorsal lamina of C2 are also removed. Once the FMD is completed, the meninges (dura/arachnoid) are incised on midline and the meningeal tissue in the region of the FMD is resected. There is typically a fibrous band of tissue at the cervicomedullary junction that may be partially ossified. This band is transected carefully with a probe (Gross ear hook and spoon) and a sharp #11 blade. Four to 6 guide holes are drilled in the occipital bone around the periphery of the skull defect using a 1.1 mm drill bit. Self-tapping 6 mm length (1.5 mm width) screws are inserted into the guide holes for an approximate depth of 2–3 mm .Recently, the author has started using self-drilling screws that are directly screwed into the skull without the need for a predrilled guide hole. The dog's head is then released from the flexed position and repositioned at a normal resting angle. The skull plate is fashioned using titanium mesh (TM) and polymethylmethacrylate (PMMA) and fixed to the back of the skull, using the titanium screw heads as anchor posts for the PMMA. The plate is shaped somewhat like a guitar pick, the wide end of the pick toward the occiput. Only a thin layer of PMMA is applied to both surfaces of the plate, with some PMMA extending beyond the edges of the TM in order to adhere to the titanium screw heads. The caudal aspect of the plate is made to extend slightly over the dorsal defect of C1; the tail aspect of the plate is curved dorsally to avoid impinging on the medulla or cranial cervical spinal cord.
Other Craniocervical Junction Abnormalities
Atlanto-occipital overlap (AOO) and dorsal C1/C2 divot are two abnormalities that may occur alone or concurrently with Chiari-like malformation. Atlanto-axial (AA) instability may also be encountered with these abnormalities as well.
Brain tumor removal is usually chosen for those masses that are located over the cerebral or cerebellar convexities, though intra-axial masses are sometimes removed. Important principles in brain tumor removal include: creating a wide craniotomy defect; minimizing the time that the brain is exposed; using bipolar cautery and sharp excision, rather than retraction of parenchyma; and commitment to the procedure. When removing intra-axial neoplasms, intraoperative ultrasound is often helpful.
Although controversial, surgery is sometimes pursued in order to remove hematomas and/or depressed bone fragments in brain-injured dogs and cats and sometimes primarily as a decompressive maneuver. As with brain tumor surgery, wide craniotomies are recommended. Intraoperative ultrasound may be helpful in location and removal of intraparenchymal hemorrhage.