Principles of neurosurgery

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Competency in the neurosurgical management of animals with nervous system disease is based on an understanding of a wide variety of clinical and basic science topics, in addition to the neurosurgical procedures themselves. This chapters covers indications for neurosurgery, common neurosurgical procedures, considerations prior to neurosurgery, postoperative monitoring and care, vertebral column surgery, cranial surgery, stereotactic radiosurgery.

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22.1 Vertebral anatomy.
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22.2 Transverse T2-weighted, T1-weighted and fat suppressed MR images at the level of T11–T12 of a 10-year-old neutered Maltese cross-breed bitch showing compression of the spinal cord (arrowed) by fat. A hemilaminectomy had been performed at this site 2 years earlier. These images were taken 1 week after the progressive recurrence of clinical signs. The mass is hyperintense on the T1-weighted and T2-weighted images and hypointense on the fat suppressed image. Expansion of the surgical fat graft appeared to be caused by secondary revascularization and the active deposition of fat.
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22.3 Ventral slot at C5–C6, demonstrating the limited visualization of the ventral aspect of the spinal cord. Ventral lesions or lesions just lateral to the midline are best treated by this approach. T2-weighted MR images of a 4-year-old Cocker Spaniel with acute onset of cervical pain and tetraparesis. A ventral slot procedure was performed to treat this dog. Transverse image of the C5–C6 disc space, cranial to where the extruded disc is located. Transverse image of the C6–C7 disc space. Mineralized, extruded disc material is seen causing spinal cord compression ventrolateral to the spinal cord (arrowed). Sagittal image showing extruded, mineralized disc material ventral to the spinal cord at C6–C7.
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22.4 Sequential transverse T2-weighted MR images of a 5-month-old Great Dane with congenital spinal stenosis causing ataxia and tetraparesis. The vertebral canal is stenotic in all images but is most severe at the level of the C2–C3 articular processes. This dog was treated with a hemidorsal laminectomy, which allowed decompression of the spinal cord whilst preserving the attachment of the nuchal ligament and maintaining stability of the vertebral column. Cranial C2. Hyperintense epidural fat surrounds the spinal cord where compression is minimal. Cranial and caudal aspects of the C2–C3 articulation. Compression of the spinal cord is circumferential due to the bony encroachment (arrowed). Caudal C2. Epidural fat again surrounds the spinal cord.
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22.5 Lateral cervical myelogram of a 5-year-old Dobermann with a chronic history of cervical pain, ataxia, pelvic limb paresis and thoracic limb lameness. A narrowed disc space and ventral extradural compression are seen at C6–C7, consistent with chronic disc herniation. The compression resolved completely when traction was applied to the vertebral column. Lateral radiograph showing the pins and PMMA used to distract and stabilize C6–C7 and achieve decompression of the spinal cord.
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22.6 CT images of a 1-year-old Yorkshire Terrier that was referred for continued cervical pain 8 weeks after surgical stabilization for atlantoaxial instability. The pins and cement were removed. Arthrodesis had developed since the pins were placed at C1–C2 and the dog’s pain resolved. Transverse CT image showing that one of the pins is in the vertebral canal (arrowed) and the other pin is in the atlanto-occipital joint. 3D CT reconstruction showing the pin in the vertebral canal (arrowed).
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22.7 Thoracolumbar hemilaminectomy illustrating the visualization of the lateral aspect of the spinal cord. Lesions located ventrally, laterally and dorsolaterally are best treated by this approach. Intraoperative view of acute disc extrusion at L2–L3 (arrowed).
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22.8 Images from a 2-year-old Bulldog with back pain and pelvic limb paresis. The dog survived 15 months following surgical resection and radiation therapy before becoming non-ambulatory from presumed recurrence of a nephroblastoma (intradural–extramedullary tumour of young dogs). Sequential post-contrast T1-weighted MR images showing a contrast-enhanced mass occupying the majority of the cross-sectional diameter of the spinal cord, crossing over from the left side to the right side. A small crescent of spinal cord can be seen in (a) and (c) (arrowed). Sagittal post-contrast T1-weighted MR image of the mass. Postoperative transverse CT image following a dorsal laminectomy that was performed to remove the mass. Pins and PMMA were placed to help maintain stability of the vertebral column since the articulations were compromised bilaterally. The arrow denotes the spinal cord in the vertebral canal at the level of the dorsal laminectomy. The aorta can be seen at the tip of the arrowhead. Postoperative radiograph showing the position of pins placed in the thoracic vertebral column.
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22.9 Dynamic compression plate used to treat a vertebral luxation at L2–L3.
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22.10 Suboptimal fixation in a dog referred following vertebral column stabilization using locking plates. Although locking plates have inherent biomechanical advantages over standard dynamic compression plates, the trajectories and positioning of the screws are fixed, and ideal placement in the vertebral bodies is technically challenging. This often results in the screws being biomechanically inferiorly placed in the transverse processes, disc spaces and, in extreme circumstances, into the vertebral canal. Postoperative ventrodorsal radiograph showing the bilateral placement of the locking plates. Screws in the L2–L3 and L3–L4 disc spaces on the left are unlikely to provide significant mechanical advantage. Transverse CT image of the most distal left screw entering the vertebral canal (arrowhead) and displacing the spinal cord (arrowed). Transverse CT image showing the placement of screws through the transverse processes rather than through the vertebral body on both plates. This also reduced the biomechanical stiffness of the fixation.
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22.11 Dorsal laminectomy at the lumbosacral junction. There is good visualization of the conus medullaris and nerve roots of the cauda equina. With gentle traction of these structures laterally, the dorsal annulus of the intervertebral disc can be seen and the L7–S1 intervertebral foramen explored. Sagittal T2-weighted MR image of a 4-year-old Pit Bull Terrier with degenerative lumbosacral stenosis. Lateral radiograph taken immediately prior to dorsal laminectomy, which was performed to decompress the cauda equina. Lateral radiograph taken when the dog was experiencing acute pain 4 days following surgery. Bilateral fracture of the L7 caudal articular processes was found (arrowed). This is a documented complication of dorsal laminectomy. Lateral radiograph showing surgical stabilization of L7–S1 with pins and PMMA.
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22.12 Surgical head-holding frame showing the versatility of head and neck positions into which the patient can be placed. An individualized bite mould is used to stabilize the upper jaw.
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22.13 Visualization of the olfactory/frontal lobes of the brain via a transfrontal craniotomy. The large air-filled frontal sinus lies between the frontal bone and the inner table of the cranial vault (arrowed). Post-contrast MR images from a 2-year-old Dalmatian presented for progressive mental deterioration. Sagittal and transverse images showing a well circumscribed, contrast-enhancing mass lesion in the right frontal lobe of the brain. Enhancement of the adjacent meninges is also visible. Postoperative sagittal and transverse images. The mass lesion, an intraparenchymal abscess, was removed via a transfrontal approach to the brain. was cultured from the abscess. A dural flap, using fascia from the temporalis muscle, was used to close the skull defect. The dog recovered well from surgery and had no recurrence of the infection.
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22.14 Visualization of the parietal/occipital lobes of the brain via a unilateral rostrotentorial craniotomy. The caudal extent of the craniotomy is limited by the transverse sinus (arrowhead), which receives venous blood from the dorsal sagittal sinus (arrowed). Transverse post-contrast T1-weighted MR images from a 7-year-old Boxer with a history of generalized seizures. Preoperative image at the level of the parietal lobe showing a heterogeneously contrast-enhancing mass, consistent with a glial cell neoplasm. Postoperative image at the same level after the tumour was removed via a rostrotentorial craniectomy. A grade III oligodroglioma was diagnosed. A PMMA skull cap was used to cover the craniotomy defect.
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22.15 Visualization of the caudolateral occipital lobe and cerebellum after occlusion of the transverse sinus. This approach is best used to treat mass lesions in the cerebellomedullary angle, caudal occipital lobe and lateral cerebellum. It is often combined with a unilateral rostrotentorial craniotomy and/or suboccipital craniotomy for removal of tentorial, brainstem and fourth ventricular masses. MR images from a 6-year-old male neutered Basenji cross-breed dog with a fourth ventricular mass. A combined approach, including occlusion of the transverse sinus, was used to remove the mass, which was diagnosed as a choroid plexus tumour. Transverse and sagittal post-contrast T1-weighted images showing a uniformly contrast-enhancing mass within the fourth ventricle, extending up to the caudal aspect of the midbrain. The transverse sinuses can be seen laterally on the transverse image (arrowed). Postoperative transverse and sagittal post-contrast T1-weighted images following tumour resection. Since complete removal of the mass required an extensive craniectomy, reconstruction using a PMMA skullcap was undertaken (arrowheads).
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22.16 Visualization of the caudal cerebellum and rostral brainstem via a suboccipital craniotomy. The location of the craniotomy is defined by the dorsal sagittal and transverse sinuses. MR images from an 8-year-old Golden Retriever with progressive generalized ataxia and difficulty going up and down stairs. Dorsal and sagittal post-contrast T1-weighted images showing a heterogeneously contrast-enhancing mass arising from the occipital bone. A suboccipital approach combined with a bilateral caudal fossa approach was used to remove this mass. Postoperative sagittal post-contrast T1-weighted image showing that the cerebellum and third ventricle have assumed their normal size and shape.
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22.17 Modified Pelorus biopsy system. (a) This CT-guided system requires the biopsy hardware to be fixed directly to the skull of the patient. (b) The biopsy needle can be seen targeting a ventral piriform location in the 3D reconstruction. The choice of trajectory is limited with this type of system. Center-of-arc biopsy system. This CT-guided system uses a mouth mould and ear bars to fix the head of the animal relative to the reference (fiducial) markers in the perspex arch. Once the lesion has been positioned in the centre of the arc, the biopsy needle can be rotated to any desired trajectory.
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22.18 A typical side-cutting Nashold brain biopsy needle. Biopsy openings are typically 8–10 mm in length. Intraoperative alcohol-fixed smear preparation confirming biopsy of the lesion, in this case a meningioma. Sample processing time is approximately 20–25 minutes. Two intra-axial lesions provisionally diagnosed as neoplastic disease on MR images. Analysis of biopsy tissue samples confirmed that the lesions were in fact a vascular infarct and an inflammatory lesion.
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22.19 Intraventricular shunt placement in a dog with a contrast-enhanced mass lesion in the third ventricle causing obstructive hydrocephalus and marked dilatation of the left lateral ventricle. Amelioration of progressive neurological signs (obtundation, circling and pacing) was necessary to allow for the delayed (several weeks) effects of stereotactic radiosurgery and treatment of the primary problem to occur. Dorsal plane post-contrast CT image prior to shunt placement. Postoperative image showing the shunt tip (hyperattenuating) in a more appropriately sized lateral ventricle. Placement of a Codman Hakimâ programmable shunt. 3D CT reconstruction showing shunt placement in the brain. The second burr hole (craniotomy) just rostral to the shunt was used to place an indwelling ICP monitoring catheter. Drainage of CSF into the peritoneal cavity was achieved following placement of a distal catheter. The opening pressure setting on the programmable valve can be confirmed by radiography and changed as necessary following placement. An opening pressure of 70 cmHO was chosen in this case.
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22.20 Peracute complication following craniectomy. Pre- and post-craniectomy MR images of a dog referred for lack of recovery from anaesthesia following resection of a frontal lobe mass. Preoperative transverse post-contrast T1-weighted image showing a uniformly contrast-enhanced extra-axial mass consistent with a meningioma. Postoperative transverse T2-weighted image obtained following referral showing an extensive mixed hyperintensity throughout the left cerebrum with cerebral herniation. Swelling and herniation appear to be secondary to ongoing haemorrhage, evidence of which can be seen as an area of extra-axial hyperintensity (arrowed).
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22.21 Chronic complication following craniectomy. Failure to reconstruct large craniectomy defects, particularly following a rostrotentorial approach, can result in compression of the neural tissue by the large overlying temporalis muscles. Resection of the tumour resulted in a large skull defect and continued compression of the brain by the overlying musculature. Resolution of the compression following subsequent skull reconstruction using PMMA.
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22.22 Linear accelerator based stereotactic radiosurgery system used to treat a dog with a caudal fossa mass. Transverse post-contrast T1-weighted MR images obtained (a) before, (b) 3 months following and (c) 6 months following radiosurgery. Patient undergoing stereotactic treatment. The restraining head frame and modified linear accelerator fitted with stereotactic cones can be seen. Stereotactic radiosurgery planning based on fused MR and CT images. Highly conformal delivery to the mass (pink) is achieved using multiple beam trajectories in a single treatment plan. Delivery of radiation to critical structures, such as the eyes (red and green) and middle/inner ear (yellow and cyan), is avoided. (Courtesy of Dr M Kent, University of California, Davis)

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