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Ventral Cervical Slot
222
CHAPTER 84
PA RT T H R E E
•
SURGICAL PROCEDURES FOR THE SPINE
Ventral Cervical Slot
INDICATIONS 1 Ventral decompression of the cervical spine is performed for intervertebral disc (IVD) disease of the cervical spine. It is often performed in combination with ventral fenestration. Surgical candidates for a ventral slot are those animals that have exhibited multiple bouts of cervical pain, that are unresponsive to previous conservative treatment, or that are nonambulatory tetraparetic or quadriplegic and that have been diagnosed with extradural compression of the spinal cord via myelography, computed tomography, or magnetic resonance imaging.
scalpel blade, Adson periosteal elevator, Senn retractors and/or Army-Navy retractors, DeBakey thumb forceps, Lempert rongeur, Kerrison rongeur, pneumatic drill system with burr guard and a variety of sizes of round and oval burrs, bone wax,* gelfoam,† or Avitene Sheets‡ (Microfibrillar Hemostat). Additional instrumentation that is useful for removing extruded disc material from the vertebral canal includes a Buck ear curette, Ball burnisher, tartar scraper, double-ended curette, Iris spatula, and small bone curette.
OBJECTIVES • To relieve compression of the cervical spine from extruded or protruded disc material
PREPARATION AND POSITIONING Prepare the patient from the mid-mandible to just past the manubrium. Position the animal in dorsal recumbency, crossing its forelimbs and securing them caudally. Pay careful attention to patient positioning, and make every effort to ensure that the animal is straight and in true dorsal recumbency. The head and neck can be stabilized with towels, sandbags, or a vacuumactivated surgical positioning system.§ Secure the head by taping the mandible to the table.
ANATOMIC CONSIDERATIONS 2–4 Cervical spinal cord compression accounts for 15% of the reported cases of IVD in the dog, with 80% occurring in the chondrodystrophic breeds. The most common site of disc protrusion is C2-C3, followed by C3-4 and C7-T1. Clinical signs associated with disc disease can vary from a nerve root signature and neck pain to quadriplegia, depending on the degree and location of spinal cord compression. Myelography or ancillary imaging of computed tomography and magnetic resonance imaging are important for both diagnostic confirmation and neuroanatomical localization of the lesion; they also determine lateralization of the disc extrusion and rule out the presence of multiple lesions. The midline location of the ventral slot must be precise to avoid the internal vertebral venous plexus. The internal vertebral venous plexus or sinuses comprise two valveless veins, which reside on the floor of the vertebral canal. These two thinwalled veins converge and diverge at the vertebral midbody and IVD space, respectively. To avoid damage of these vertebral sinuses and instability of the vertebral spine, the dimensions of the slot must be no greater than one third the width and length of the body of the vertebra, centering the slot slightly cranial to the IVD space (Plate 84A). EQUIPMENT • Standard surgical pack, two medium or large Gelpi retractors (depending on the size of dog), bipolar and unipolar cautery, suction hose and small Frazier suction tip, no. 11 Bard Parker
PROCEDURE Approach: Incise the skin and subcutaneous tissue on the ventral midline from the caudal aspect of the thyroid cartilage to the manubrium. Reflect the skin and subcutaneous tissue medially and laterally with blunt dissection. Bluntly separate the sternohyoideus and sternomastoideous muscles along the midline. Identify and retract the esophagus and trachea to the left with moistened laparotomy sponges and either self-retaining or Army-Navy retractors (Plate 84B). Identify the paired carotid sheaths, and gently maneuver them out of the surgical field. Palpate the ventral spinous processes of the vertebral column to locate the appropriate disc space(s). The large transverse processes of C6 and the wings of C1 are important landmarks that assist in anatomical orientation. Once the IVD space(s) has been located, cauterize the musculotendinous attachments of the longus colli muscle to the ventral spinous processes. The use of cautery will reduce the amount of muscular bleeding and improve visualization. Once it is free of its attachments, elevate and retract the longus colli muscle to expose the ventral annulus fibrosus at the affected disc space and the adjacent vertebral bodies (Plate 84C), and remove the ventral spinous process with a rongeur (Plate 84D). Continued
*Bone wax, Ethicon, Johnson & Johnson, Somerville, New Jersey. † Gelfoam, Pharmacia and Upjohn, Kalamazoo, Michigan. ‡ Avitene, Davol, Cranston, Rhode Island. § Hug-U-Vac, South Salem, Oregon.
CHAPTER 84
223
VENTRAL CERVICAL SLOT
P L AT E 8 4
Slot is 1/3 the width of the vertebral body
B
A
Sternohyoideus muscle
Trachea
Slot is centered slightly cranial to interspace Carotid sheath
Sternocephalicus muscle
D
C Longus colli muscles
224
PA RT T H R E E
Ventral Cervical Slot: Drill the proposed ventral slot site with a pneumatic air drill using a burr of appropriate size (Plate 84E). Generally, select a burr that is equal to the width of the slot to promote even bone removal. Slowly trickle warm saline over the burr while drilling to prevent heat transfer from the burr to the hemilaminectomy site, and intermittently lavage the entire laminectomy site to remove bone debris and to keep the tissues moist. Once most of the bone has been removed, a smaller bit can be used to remove the edges of inner cortical bone plate. Any bone bleeding may be controlled with bone wax. Once the inner cortex has been removed, use a nerve root probe, ear loop, or tartar scraper to palpate the inner bone and periosteal shelf. When it is thin and pliable, use a probe to penetrate the site gently and create a long window to allow a Kerrison rongeur to remove any remaining bone in the oblong window. If necessary, excise the dorsal longitudinal ligament with a no. 11 blade to visualize and decompress the cord. Remove any remaining extruded disc with a Buck ear curette, ball burnisher, or iris scapula (Plate 84F). Avoid damaging the venous sinuses that circumferentially surround the slot ventral to the cord. If a venous sinus is damaged, control the bleeding with the gelfoam or Avitene Sheets. Wait 5 minutes for the cessation of bleeding before removing the hemostatic devices and reinitiating the exploratory surgery. Once the spinal cord has been fully decompressed, all sinus bleeding should cease. Closure: Lavage and close the longus colli in one layer with a simple continuous suture pattern. Remove the retractors, and return the trachea and esophagus to their normal position. Lavage the soft tissues, and close the sternohyoideus and sternomastoideus with a simple continuous suture pattern. Close the subcutaneous tissues and skin in a routine fashion.
CAUTIONS 1,5 Proper anatomic orientation is key to the identification of the correct surgical site. Postoperative or intraoperative radiographs should be taken if there is any doubt about lesion location. Severe hemorrhage can occur as a result of ventral sinus laceration. It is important to cross-match any animal that is a candidate for a coagulopathy before surgery. POSTOPERATIVE EVALUATION 1,5–8 The neurologic status of the patient should be serially evaluated upon recovery from anesthesia and surgery. Neurologic deterioration associated with ventral fenestration has been reported in the literature and is thought to be due to the herniation of residual disc material into the canal. Most animals experience a decrease in cervical pain associated with IVD disease; however, recovery times may vary depending on the severity of the neurologic dysfunction and the intraoperative complications. Postoperative radiographs or ancillary imaging (computed
•
SURGICAL PROCEDURES FOR THE SPINE
tomography or magnetic resonance imaging) of the cervical vertebrae is usually not indicated. POSTOPERATIVE CARE General supportive care and pain management are indicated in all neurosurgery patients. Clean, dry, soft padded bedding is important for patients that are nonambulatory or weakly ambulatory. Maintenance fluid therapy is routine for the first 24 to 48 hours, until the animal is able to eat and drink without assistance. Urine output and quantitation should be closely monitored for the first 24 to 48 hours, and bladder expression or catheterization should be provided for those patients that are not urinating on a voluntary basis. Rehabilitation may be implemented as soon as the acute postoperative pain has subsided (usually after 24 hours). Depending on the level of neurologic dysfunction, therapy may consist of massage, assisted standing, proprioceptive exercises, and controlled therapeutic exercise. Neck and buckle collars should be exchanged for a harness. EXPECTED OUTCOME 5,7 Most patients with cervical IVD have a fair to good prognosis; however, expected outcomes vary depending on the level of neurologic dysfunction. Most animals improve following surgical intervention, given a convalescence period of reasonable length (6 to 8 weeks). Recurrence of signs associated with IVD has been reported within the literature and should be discussed with the owner prior to surgery.
References 1. Macy NB, Stover SM, Kass PH: Effect of disk fenestration on sagittal kinematics of the canine C5-C6 intervertebral space. Vet Surg 28:171–179, 1999. 2. Tombs JP: Cervical intervertebral disk disease in dogs. Compend Contin Educ Pract Vet 14:1477–1488, 1992. 3. Lemarie RJ, Partington BP, Hosgood G: Vertebral subluxation following ventral cervical decompression in the dog. J Am Anim Hosp Assoc 36(4):348–358, 2000. 4. Bagley RS, Tucker R, Harrington ML: Lateral and foraminal disk extrusion in dogs. Compend Contin Educ Pract Vet 18:795–804, 1996. 5. Wheeler SJ, Sharp NJH: Cervical disc disease. In Small Animal Spinal Disorders. London, Mosby-Wolfe, 1994. 6. Dallman MJ, Giovannitti-Jensen A: Comparison of the width of the intervertebral disk space and radiographic changes before and after intervertebral disk fenestration in dogs. Am J Vet Res 52(1):140–145, 1991. 7. Nakama S, Tabaru H, Yasuda M: A retrospective study of ventral fenestration for disk diseases in dogs. J Vet Med Sci 55(5):781–784, 1993. 8. Tomlinson J: Tetraparesis following cervical disk fenestration in two dogs. J Am Vet Med Assoc 187(1):76–77, 1985.
CHAPTER 84
225
VENTRAL CERVICAL SLOT
P L AT E 8 4
E
F
Slot is centered slightly cranial to interspace
CHAPTER 84
PA RT T H R E E
•
SURGICAL PROCEDURES FOR THE SPINE
Ventral Cervical Slot
INDICATIONS 1 Ventral decompression of the cervical spine is performed for intervertebral disc (IVD) disease of the cervical spine. It is often performed in combination with ventral fenestration. Surgical candidates for a ventral slot are those animals that have exhibited multiple bouts of cervical pain, that are unresponsive to previous conservative treatment, or that are nonambulatory tetraparetic or quadriplegic and that have been diagnosed with extradural compression of the spinal cord via myelography, computed tomography, or magnetic resonance imaging.
scalpel blade, Adson periosteal elevator, Senn retractors and/or Army-Navy retractors, DeBakey thumb forceps, Lempert rongeur, Kerrison rongeur, pneumatic drill system with burr guard and a variety of sizes of round and oval burrs, bone wax,* gelfoam,† or Avitene Sheets‡ (Microfibrillar Hemostat). Additional instrumentation that is useful for removing extruded disc material from the vertebral canal includes a Buck ear curette, Ball burnisher, tartar scraper, double-ended curette, Iris spatula, and small bone curette.
OBJECTIVES • To relieve compression of the cervical spine from extruded or protruded disc material
PREPARATION AND POSITIONING Prepare the patient from the mid-mandible to just past the manubrium. Position the animal in dorsal recumbency, crossing its forelimbs and securing them caudally. Pay careful attention to patient positioning, and make every effort to ensure that the animal is straight and in true dorsal recumbency. The head and neck can be stabilized with towels, sandbags, or a vacuumactivated surgical positioning system.§ Secure the head by taping the mandible to the table.
ANATOMIC CONSIDERATIONS 2–4 Cervical spinal cord compression accounts for 15% of the reported cases of IVD in the dog, with 80% occurring in the chondrodystrophic breeds. The most common site of disc protrusion is C2-C3, followed by C3-4 and C7-T1. Clinical signs associated with disc disease can vary from a nerve root signature and neck pain to quadriplegia, depending on the degree and location of spinal cord compression. Myelography or ancillary imaging of computed tomography and magnetic resonance imaging are important for both diagnostic confirmation and neuroanatomical localization of the lesion; they also determine lateralization of the disc extrusion and rule out the presence of multiple lesions. The midline location of the ventral slot must be precise to avoid the internal vertebral venous plexus. The internal vertebral venous plexus or sinuses comprise two valveless veins, which reside on the floor of the vertebral canal. These two thinwalled veins converge and diverge at the vertebral midbody and IVD space, respectively. To avoid damage of these vertebral sinuses and instability of the vertebral spine, the dimensions of the slot must be no greater than one third the width and length of the body of the vertebra, centering the slot slightly cranial to the IVD space (Plate 84A). EQUIPMENT • Standard surgical pack, two medium or large Gelpi retractors (depending on the size of dog), bipolar and unipolar cautery, suction hose and small Frazier suction tip, no. 11 Bard Parker
PROCEDURE Approach: Incise the skin and subcutaneous tissue on the ventral midline from the caudal aspect of the thyroid cartilage to the manubrium. Reflect the skin and subcutaneous tissue medially and laterally with blunt dissection. Bluntly separate the sternohyoideus and sternomastoideous muscles along the midline. Identify and retract the esophagus and trachea to the left with moistened laparotomy sponges and either self-retaining or Army-Navy retractors (Plate 84B). Identify the paired carotid sheaths, and gently maneuver them out of the surgical field. Palpate the ventral spinous processes of the vertebral column to locate the appropriate disc space(s). The large transverse processes of C6 and the wings of C1 are important landmarks that assist in anatomical orientation. Once the IVD space(s) has been located, cauterize the musculotendinous attachments of the longus colli muscle to the ventral spinous processes. The use of cautery will reduce the amount of muscular bleeding and improve visualization. Once it is free of its attachments, elevate and retract the longus colli muscle to expose the ventral annulus fibrosus at the affected disc space and the adjacent vertebral bodies (Plate 84C), and remove the ventral spinous process with a rongeur (Plate 84D). Continued
*Bone wax, Ethicon, Johnson & Johnson, Somerville, New Jersey. † Gelfoam, Pharmacia and Upjohn, Kalamazoo, Michigan. ‡ Avitene, Davol, Cranston, Rhode Island. § Hug-U-Vac, South Salem, Oregon.
CHAPTER 84
223
VENTRAL CERVICAL SLOT
P L AT E 8 4
Slot is 1/3 the width of the vertebral body
B
A
Sternohyoideus muscle
Trachea
Slot is centered slightly cranial to interspace Carotid sheath
Sternocephalicus muscle
D
C Longus colli muscles
224
PA RT T H R E E
Ventral Cervical Slot: Drill the proposed ventral slot site with a pneumatic air drill using a burr of appropriate size (Plate 84E). Generally, select a burr that is equal to the width of the slot to promote even bone removal. Slowly trickle warm saline over the burr while drilling to prevent heat transfer from the burr to the hemilaminectomy site, and intermittently lavage the entire laminectomy site to remove bone debris and to keep the tissues moist. Once most of the bone has been removed, a smaller bit can be used to remove the edges of inner cortical bone plate. Any bone bleeding may be controlled with bone wax. Once the inner cortex has been removed, use a nerve root probe, ear loop, or tartar scraper to palpate the inner bone and periosteal shelf. When it is thin and pliable, use a probe to penetrate the site gently and create a long window to allow a Kerrison rongeur to remove any remaining bone in the oblong window. If necessary, excise the dorsal longitudinal ligament with a no. 11 blade to visualize and decompress the cord. Remove any remaining extruded disc with a Buck ear curette, ball burnisher, or iris scapula (Plate 84F). Avoid damaging the venous sinuses that circumferentially surround the slot ventral to the cord. If a venous sinus is damaged, control the bleeding with the gelfoam or Avitene Sheets. Wait 5 minutes for the cessation of bleeding before removing the hemostatic devices and reinitiating the exploratory surgery. Once the spinal cord has been fully decompressed, all sinus bleeding should cease. Closure: Lavage and close the longus colli in one layer with a simple continuous suture pattern. Remove the retractors, and return the trachea and esophagus to their normal position. Lavage the soft tissues, and close the sternohyoideus and sternomastoideus with a simple continuous suture pattern. Close the subcutaneous tissues and skin in a routine fashion.
CAUTIONS 1,5 Proper anatomic orientation is key to the identification of the correct surgical site. Postoperative or intraoperative radiographs should be taken if there is any doubt about lesion location. Severe hemorrhage can occur as a result of ventral sinus laceration. It is important to cross-match any animal that is a candidate for a coagulopathy before surgery. POSTOPERATIVE EVALUATION 1,5–8 The neurologic status of the patient should be serially evaluated upon recovery from anesthesia and surgery. Neurologic deterioration associated with ventral fenestration has been reported in the literature and is thought to be due to the herniation of residual disc material into the canal. Most animals experience a decrease in cervical pain associated with IVD disease; however, recovery times may vary depending on the severity of the neurologic dysfunction and the intraoperative complications. Postoperative radiographs or ancillary imaging (computed
•
SURGICAL PROCEDURES FOR THE SPINE
tomography or magnetic resonance imaging) of the cervical vertebrae is usually not indicated. POSTOPERATIVE CARE General supportive care and pain management are indicated in all neurosurgery patients. Clean, dry, soft padded bedding is important for patients that are nonambulatory or weakly ambulatory. Maintenance fluid therapy is routine for the first 24 to 48 hours, until the animal is able to eat and drink without assistance. Urine output and quantitation should be closely monitored for the first 24 to 48 hours, and bladder expression or catheterization should be provided for those patients that are not urinating on a voluntary basis. Rehabilitation may be implemented as soon as the acute postoperative pain has subsided (usually after 24 hours). Depending on the level of neurologic dysfunction, therapy may consist of massage, assisted standing, proprioceptive exercises, and controlled therapeutic exercise. Neck and buckle collars should be exchanged for a harness. EXPECTED OUTCOME 5,7 Most patients with cervical IVD have a fair to good prognosis; however, expected outcomes vary depending on the level of neurologic dysfunction. Most animals improve following surgical intervention, given a convalescence period of reasonable length (6 to 8 weeks). Recurrence of signs associated with IVD has been reported within the literature and should be discussed with the owner prior to surgery.
References 1. Macy NB, Stover SM, Kass PH: Effect of disk fenestration on sagittal kinematics of the canine C5-C6 intervertebral space. Vet Surg 28:171–179, 1999. 2. Tombs JP: Cervical intervertebral disk disease in dogs. Compend Contin Educ Pract Vet 14:1477–1488, 1992. 3. Lemarie RJ, Partington BP, Hosgood G: Vertebral subluxation following ventral cervical decompression in the dog. J Am Anim Hosp Assoc 36(4):348–358, 2000. 4. Bagley RS, Tucker R, Harrington ML: Lateral and foraminal disk extrusion in dogs. Compend Contin Educ Pract Vet 18:795–804, 1996. 5. Wheeler SJ, Sharp NJH: Cervical disc disease. In Small Animal Spinal Disorders. London, Mosby-Wolfe, 1994. 6. Dallman MJ, Giovannitti-Jensen A: Comparison of the width of the intervertebral disk space and radiographic changes before and after intervertebral disk fenestration in dogs. Am J Vet Res 52(1):140–145, 1991. 7. Nakama S, Tabaru H, Yasuda M: A retrospective study of ventral fenestration for disk diseases in dogs. J Vet Med Sci 55(5):781–784, 1993. 8. Tomlinson J: Tetraparesis following cervical disk fenestration in two dogs. J Am Vet Med Assoc 187(1):76–77, 1985.
CHAPTER 84
225
VENTRAL CERVICAL SLOT
P L AT E 8 4
E
F
Slot is centered slightly cranial to interspace