Surgery of the Cervical Spine

Chapter 40


Surgery of the Cervical Spine



General Principles and Techniques



Definitions


Weakness of all four limbs due to a cervical myelopathy is tetraparesis. When voluntary motor ability is lost to all four limbs, the term used is tetraplegia. Hemiparesis and hemiplegia are similar terms that refer to weakness of or inability to move the thoracic and pelvic limbs on one side of the body, respectively. The two most common surgical approaches to the cervical spinal cord are the ventral slot procedure (creation of a bony defect in the ventral aspect of an intervertebral space in addition to removing a section of the intervening disk) and dorsal laminectomy. The ventral slot procedure provides a small window to the ventral aspect of the spinal cord, but lateral access is limited by both the size of the defect and the large vertebral venous sinuses (internal vertebral venous plexus) on either side of the defect. Dorsal laminectomy provides a wide decompression and access to the entire dorsal surface of the cord. Further removal of the articular processes laterally (facetectomy) and/or the pedicles allows for exposure of the nerve roots/spinal nerve and lateral surface of the spinal cord, respectively. Removal of parts of the articular processes done to enlarge the intervertebral foramen is called foraminotomy. Removal of the dorsal lamina on one side is often performed, usually combined with facetectomy/foraminotomy; this is called dorsolateral hemilaminectomy. Hemilaminectomy is the unilateral removal of the lateral surface of the vertebral arch, including the articular processes. Although this can be done in the cervical region, the term is used more commonly when discussing surgery of the thoracolumbar spine (see Chapter 41). Fenestration is the creation of a defect (window) in the annulus fibrosus to the level of the nucleus pulposus.



General Considerations


The range of neurologic dysfunction possible with cervical myelopathies is broad, in terms of both severity and character. Many patients will present with severe neck pain as the primary or sole clinical sign of dysfunction. Neck pain typically is more severe with cranial cervical spinal cord lesions than with caudal lesions. Loss of motor function does occur with compressive cervical myelopathies but is less common than with thoracolumbar myelopathies, most likely owing to the larger volume of the vertebral canal in the cervical region compared with the thoracolumbar region. Ambulatory patients with caudal cervical myelopathy tend to have obvious pelvic limb weakness and ataxia, with less severe, sometimes even subtle, thoracic limb dysfunction. In many such cases, the thoracic limbs move with short, stilted steps, whereas the pelvic limbs display obvious ataxia. This odd gait is often referred to as a “two-engine gait” because of the disparate nature of the thoracic and pelvic limb movements. Patients with cranial cervical myelopathies typically have more obvious thoracic limb dysfunction. A phenomenon called central cord syndrome is sometimes appreciated in dogs and cats with caudal cervical spinal cord lesions that are confined to the central region of the cord. In this syndrome, the thoracic limbs display severe lower motor neuron (LMN) weakness, but the pelvic limbs are minimally affected or neurologically normal. This occurs because the centrally located lesion interferes with LMNs innervating the thoracic limbs, but it spares the more peripherally located white matter upper motor tracts (UMNs) that innervate the pelvic limb LMNs. Central cord syndrome is most commonly associated with intra-axial lesions like syringomyelia and neoplasia (see p. 1500).


Descending tracts from the medullary respiratory centers traverse the cervical spinal cord to innervate the phrenic nerve LMNs (C5-C7) and the LMNs of intercostal muscles in the thoracic spinal cord. Damage to these tracts can lead to respiratory compromise. Caudal cervical spinal cord lesions that spare innervation to the phrenic nerve LMNs (e.g., lesions caudal to the C5-C7 segments) but disrupt normal activation of intercostal LMNs can lead to the phenomenon of abdominal breathing. In this scenario, the diaphragm bears the sole burden of moving air through the lungs because of the denervated chest wall, and this motion causes the abdomen to visibly move back and forth during respiration. Tonic input to the sympathetic LMNs in the thoracolumbar spinal cord that maintain normal blood pressure is provided by neurons in the medulla (rostroventrolateral medulla). Because these medullary neurons send their processes through the cervical spinal cord, cervical myelopathies may be associated with severe hypotension.


Two major surgical approaches to the cervical spinal cord have been identified: ventral and dorsal. A lateral approach to the cervical cord is rarely performed and is not discussed in this text. The ventral approach is technically less demanding than the dorsal approach, and considerably more bone is available for implants ventrally versus dorsally when vertebral stabilization is required. Exposure of the vertebral canal and the spinal cord is very limited with a ventral approach, and the likelihood of severe venous sinus hemorrhage is much greater than with a dorsal approach. The dorsal approach allows for wide decompression and access to dorsal and lateral aspects of the cervical spinal cord. This procedure is technically more difficult than the ventral approach, and patients with dorsal cervical laminectomies tend to have longer recuperative periods than those who undergo ventral slot procedures. The dorsal spinous processes in the cervical region are short (and are typically removed during decompression), and the articular processes afford a small amount of bone stock for implant placement. In some instances, both a ventral and a dorsal approach are needed to accomplish the goals of surgery. Dorsally directed (interarcuate) branches of the internal vertebral venous plexuses (venous sinuses) at the articular process levels may bleed during the dorsal approach or during facetectomy/foraminotomy. However, this hemorrhage is easier to control because of the visualization afforded by the dorsal approach. Venous sinus hemorrhage that is severe and difficult to visually pinpoint is addressed by placing Gelfoam over the site of hemorrhage with slight pressure, using Bishop-Harmon forceps. Suction is used to evacuate blood around the site of hemorrhage. If the hemorrhage is less severe and can be localized to a small area, Surgicel can be used over the hemorrhage site. This is less obtrusive and will allow the surgeon to continue working in areas that are not bleeding. This method of hemorrhage control is particularly useful with small approaches like the ventral slot approach.



Preoperative Management


Specifics of preoperative management depend largely on the disease process. In cases of suspected or known vertebral instability, the patient needs to be kept as still as possible and may need some external support (e.g., neck brace, secured to a board or other rigid surface). It is extremely important to emphasize the dangers of neck manipulation in such animals to everyone who may be handling the patient. Once the dog or cat is anesthetized, voluntary contraction of the cervical musculature will no longer be an available protective mechanism for that patient. If radiographs are obtained, overzealous neck flexion in an effort to demonstrate atlantoaxial instability may prove fatal to the patient. With increased availability of multislice computed tomography (CT) scanners and magnetic resonance imaging (MRI) units, safer and more accurate imaging methods are available for patients with cervical spine disorders.


A hallmark of many cervical myelopathies is neck pain, which is often severe. Oral prednisone (0.5 mg/kg every 12 hours) is often effective in relieving pain in some cervical myelopathies. In chronic compressive myelopathies, this dose of prednisone will also likely improve clinical signs through its effect on spinal cord edema. High doses of glucocorticoids should not be administered as an emergency or pre-emptive presurgical therapy for spinal cord injury. The “high-dose” methylprednisolone protocol has been ineffective for spinal cord injury and can have deleterious consequences. The hydrophilic polymer polyethylene glycol (PEG; 2 ml/kg of a 30% solution IV over 10 to 20 minutes, repeat dose in 6 hours) may be effective in treating patients with spinal cord trauma. This drug is not commercially available and needs to be prepared by a compounding pharmacy. PEG appears to be devoid of side effects (unlike glucocorticoids), but its efficacy is controversial. In animals with severe neck pain, it is preferable to initiate analgesic therapy as early as possible (see Table 12-3 on p. 141). Injectable opioid drugs (e.g., fentanyl) are usually effective in hospitalized patients. Gabapentin and pregabalin are oral calcium channel blocking drugs used as both antiseizure agents and antinociceptive agents (see p. 1440; Table 39-1). Pregabalin is the “next generation” of gabapentin. Although gabapentin is often an effective pain-relieving oral drug, the author has found pregabalin to be superior to gabapentin.



Anesthetic Considerations


Specific anesthetic concerns associated with cervical spinal cord surgery are hypotension, blood loss, cardiac arrhythmias, ventilatory compromise, and pain management. Because of the potential for venous sinus blood loss, blood should be available for transfusion. Severe hypotension—whether due to blood loss, lack of sympathetic tone, or a combination—can be treated with fluid support and/or positive inotropic drugs (see Table 39-2). Bradycardia may occur with vagal nerve stimulation during the ventral approach and can be treated with anticholinergic drugs (atropine or glycopyrrolate). Mechanical ventilation may be necessary both during and after surgery, depending on the severity of the myelopathy.


Taping the animal across the chest to maintain positioning at surgery can cause severe respiratory compromise if the tape is applied too tightly; it is best to avoid this practice and to rely on other methods (e.g., V-trough, towels, vacuum apparatus) to maintain positioning for surgery. Excessive neck extension should be avoided during positioning because this may exacerbate compression in some situations (e.g., disk protrusions, caudal cervical spondylomyelopathy). The endotracheal tube should extend beyond the thoracic inlet during ventral approaches to the cervical spine so that the trachea is not damaged during retraction at surgery.


Anesthetic protocols vary but are similar to those used for fracture repair surgery (see Chapter 32, Tables 32-5 and 32-6 on pp. 1045 and 1047). Premedication with an opioid drug is often performed, as many of the cervical myelopathies are characterized by severe neck pain. If myelography is to be performed before surgery, the potential for postmyelographic seizures exists, especially in larger dogs. Antiseizure drugs are discussed in Chapter 39 (see Table 39-1 on p. 1440).




Surgical Anatomy


Seven cervical vertebrae are present, each of which has a body, laminae, pedicles, transverse processes, and articular processes. All but the first cervical vertebra (C1) have a dorsal spinous process. From C3 to C7, these processes slope in a cranial direction and increase in size from cranial to caudal. No intervertebral disk is found between the first and second cervical vertebrae. Distinctive features of the first (C1), second (C2), and sixth (C6) cervical vertebrae provide important landmarks for the surgeon; similarly, the large dorsal spinous process of the first thoracic vertebra (T1) is a distinct surgical landmark (Fig. 40-1). The first cervical vertebra (C1), the atlas, has a shortened body (the ventral arch); wide, shelflike transverse processes (wings); and modified articular processes. The ventral tubercle of C1 feels like a distinctive “spike” when palpated during a ventral approach (Fig. 40-2), compared with the smooth “bump” of the ventral aspect of the intervertebral disk spaces. The dorsal spinous process of C2 (the axis) is a distinctive bladelike structure, the caudal aspect of which is very thick and partially overhangs the dorsal aspect of C3. The dorsal spinous process of C3 is extremely small and sometimes is nonapparent, especially in miniature and toy dog breeds. The dorsal lamina of C3 in these small breeds may be very short as well. It is important not to confuse C4 with C3 while performing a dorsal approach in the cranial cervical spine, especially in small dogs and cats. The articulation between C1 and C2 involves a cranial projection of the body of C2 (the dens) that rests on and is attached to (via several ligaments) the dorsal aspect of the C1 ventral arch (Fig. 40-3). The transverse processes of C6 are very prominent ventral structures that are easily palpated during the ventral approach. The dorsal aspect of the first rib as it articulates with T1 can also be palpated during the ventral approach; however, the surgeon must be cautious not to rupture the thin cupula pleura during palpation of this structure.





In addition to the venous sinuses on the floor of the vertebral canal, a vertebral artery is present on each side of the cervical spine. This large artery traverses the transverse foramina from C6 through C1 and is located ventral to the level of the articular processes from C2 through C6. The vertebral artery usually is not encountered during standard approaches to the cervical spine; disruption of this vessel during surgery can lead to rapid and extensive hemorrhage. Eight paired cervical spinal nerves are present. The paired first cervical spinal nerves exit from the lateral vertebral foramina of C1. The C2 through C7 spinal nerves exit through the intervertebral foramina cranial to the vertebra of the same number as the specific nerve (i.e., the C2 spinal nerve exits at the C1-C2 foramen). The C8 spinal nerve exits the foramen formed by the C7 and T1 vertebrae. The nuchal ligament extends from the T1 dorsal spinous process to the caudal aspect of the C2 dorsal spinous process (Fig. 40-4). This structure is helpful during the dorsal approach both in verifying anatomic location and as a tool for remaining on midline (the nuchal ligament is a paired structure that can be split on the midline).




Standard Surgical Approaches to the Cervical Spine


The main indication for performing a ventral approach to the cervical spine is to create a ventral slot for removal of ventrally located disk material. This approach is also used to stabilize cervical vertebral segments by placing implants in the vertebral bodies (e.g., atlantoaxial instability, caudal cervical spondylomyelopathy, traumatic fracture/luxation). The dorsal approach is used to decompress the spinal cord and to remove dorsally or laterally situated compressive lesions (e.g., lateralized disk, hypertrophied yellow ligament, spinal tumors). The dorsal approach can be extended laterally by removing articular joint capsules (facets) and adjacent pedicles to improve access to the lateral vertebral canal.



Ventral Approach to the Cervical Spine


Shave the skin and aseptically prepare the area from approximately the mid-mandible level cranially to several centimeters past the manubrium caudally. The length of the surgical incision depends on the specific area to be operated. Position the patient in dorsal recumbency with the head and neck in mild extension (Fig. 40-5). Secure the thoracic limbs by pulling them caudally and against the patient’s trunk. Use a V-trough or towels to position the neck area. Place a towel beneath the neck to facilitate extension. Secure the head with tape placed over the upper canine teeth.



Assuming that the entire cervical region is to be exposed, make a ventral midline incision from the level of the larynx cranially to the level of the manubrium caudally (Fig. 40-6). Divide the sternocephalicus muscles to the level of the manubrium with Metzenbaum scissors or unipolar cautery if access to the caudal cervical spine is required. Divide the paired sternohyoideus muscles on midline (median raphe) with Metzenbaum scissors (Fig. 40-7). During this division, use bipolar cautery to address branches of the caudal thyroid veins that are in the dissection plane. Digitally separate the deep neck fascia, and visually identify then retract the trachea, esophagus, and left carotid sheath to the left and the right carotid sheath to the right. Once these structures are retracted, visualize the longus colli musculature (Fig. 40-8) and perforate the overlying fascial covering with Metzenbaum scissors; remove this fascia digitally using anatomic reference points discussed previously to identify the intervertebral space of interest. Bluntly separate the longus colli musculature on midline at the cranial and caudal aspects of the disk space (for spaces C2-C3 and caudally), using a straight Kelly or mosquito forceps, while exposing the tendons of insertion of the longus colli muscles on the ventral tubercles of the vertebrae. Dissect these tendon insertions from the tubercles using Freer elevators (small dogs and cats), or cut them with Mayo scissors (large dogs). Continue the dissection of the longus colli musculature off the ventral aspect of the vertebral bodies to be exposed using Freer elevators or narrow Army/Navy osteotomes (large dogs). Once this dissection is complete, place Gelpi retractors in the cranial and caudal aspects of the area of interest, with the tips of the retractors beneath the longus colli musculature (Fig. 40-9). Control hemorrhage during this approach using bipolar cautery. When approaching the C1-C2 intervertebral space, transect the belly of the sternothyroideus muscle to facilitate exposure of this joint space. Using a No. 11 blade, incise the small muscle (rectus capitis ventralis) over the ventral surface of C1 and reflect it laterally with Freer elevators to expose the C1 ventral arch. To perform the ventral slot procedure, first fenestrate the disk of interest by resecting a rectangular section of the ventral annulus, and remove this piece of annulus with Lempert rongeurs, exposing the nucleus pulposus (Fig. 40-10, A and B). Remove the ventral tubercle from the caudal aspect of the cervical vertebra forming the cranial aspect of the intended slot. Center the slot toward the vertebral body cranial to the disk because of the caudal-to-cranial direction of the intervertebral space from ventral to dorsal (Fig. 40-11). Do not exceed one-third the width or length of the vertebral bodies when creating the ventral slot. Using a high-speed air drill with a 4- to 5-mm burr, remove the outer cortical and inner cancellous bone layers, noting the spongy red nature of the cancellous layer during drilling. Stay on midline throughout creation of the slot to minimize the chance of rupturing the venous sinuses when entering the vertebral canal (Fig. 40-12, A and B). If the cancellous bone bleeding is enough to interfere with visualization, use bone wax to stop the hemorrhage. When the bone again turns white, the inner cortical layer has been reached. Use a smaller burr (e.g., 2 to 3 mm) to remove this final bone layer. Be cautious to avoid abruptly breaking through this layer. The inner cortical bone takes on a flaky appearance—much like filo dough—just before the vertebral canal is penetrated. Use a probe (e.g., Gross ear hook and spoon) to get under the dorsal annulus, and incise this last layer of the disk with a No. 11 blade. The adjacent periosteal layer is often incised concurrently if the inner cortical layer is thin enough. Once the inner cortical bone layer is thin enough, or if there is a window into the vertebral canal, use a 4-0 bone curette to enlarge the slot defect. In small breed dogs, a distinct dorsal longitudinal ligament may not be apparent (may be removed with the periosteal layer); in larger dogs, this dull white fibrous structure may be distinct (Fig. 40-13). Carefully incise the dorsal longitudinal ligament with a No. 11 blade, if necessary, to gain entry into the vertebral canal (Fig. 40-14). Fibrous (annular) material at the sides of the slot defect will have a frayed appearance, like irregularly cut rope. Calcified nucleus pulposus will appear white and granular, like miniaturized cottage cheese. The dura mater will be smooth and glistening and may be a bright white color or bluish-white (bruising from disk rupture). Remove disk material within the vertebral canal using a probe and Bishop-Harmon forceps so that the dura is visualized. Use the probe to “sweep” underneath the edges of the slot to remove as much disk material as possible. During disk removal and sweeping around the slot defect, it is common to lacerate a venous sinus. Control venous sinus hemorrhage as previously described. After copious lavage of the surgical site, close the longus colli muscles with simple interrupted sutures. If a caudal exposure was performed, reappose the sternocephalicus muscles with simple interrupted sutures. The sternothyroideus muscle does not need to be reattached if this muscle was severed during C1-C2 exposure. Subcutaneous and skin closure is routine.












Dorsal Approach to the Cervical Spine


The specific dorsal approach chosen will differ depending on what part of the dorsal cervical spine is operated. For all variations of the dorsal approach, it is extremely important to stay on midline and to divide paired muscular and tendinous structures as much as possible; doing so will minimize hemorrhage and postoperative discomfort. Carefully identify dorsal cervical musculature layer by layer, using Gelpi retractors in the cranial and caudal limits of the surgical exposure as successive layers are reached.


For all of the dorsal approaches, place the patient in sternal recumbency with the neck gently flexed in a neutral position (Fig. 40-15). When the approach to the cranial cervical spinal cord is combined with suboccipital craniotomy (see p. 1446), flex the neck more acutely. The lateral limits of the shaved and aseptically prepared skin area extend to the middle third to half of the dorsal-ventral neck region on each side. The cranial limit of this area for cranial cervical and midcervical approaches is the external occipital protuberance. For midcervical and caudal cervical approaches, the caudal limit of the surgical site is approximately at the level of the dorsal spinous process of T2 or T3. The cranial limit of the surgical site for caudal cervical approaches is approximately at the level of C3.



For a cranial cervical approach, incise the skin on the dorsal midline from the external occipital protuberance to the midcervical region (C4 or C5 dorsal spinous process level). Locate the midline fibrous raphe of the superficial cervical musculature (Fig. 40-16). Incise this connective tissue with a No. 11 blade, and carefully continue the midline incision cranially and caudally with Metzenbaum or Mayo scissors, controlling hemorrhage with monopolar and/or bipolar cautery. Reflect and retract the superficial cervical musculature to expose the paired splenius muscles. Divide these muscle bellies as well as those of the underlying biventer cervicis muscles to expose the next layer of paired muscles—the rectus capitis dorsalis muscle cranially and the spinalis cervicis muscle caudally (Fig. 40-17). Identify the nuchal ligament extending caudally from the caudal aspect of C2 at this level. Using a combination of sharp and blunt dissection, elevate the rectus capitis dorsalis muscle off of the C2 dorsal spine. Sharply transect the attachment of the nuchal ligament off the caudal aspect of C2, and divide the muscle bellies of the spinalis cervicis muscles over C3. Using a Freer elevator, bluntly elevate these muscles laterally, exposing the dorsal lamina of C3 (Fig. 40-18). The approach for access to the C1-C2 cervical region is discussed in Chapter 39 (suboccipital craniotomy) (see p. 1446). The initial dorsal approach to the midcervical and caudal cervical regions is similar to that for the cranial cervical approach, with incision of the median raphe of the superficial cervical muscles and retraction of these muscles laterally. To access the dorsal laminae of C4 through C7, divide the nuchal ligament on midline and retract the paired components laterally with Gelpi retractors, exposing the underlying epaxial musculature. Using Mayo scissors, circumferentially cut the tendinous attachments of the spinalis cervicis and multifidus muscles to the dorsal spinous processes, then bluntly elevate these muscles off the dorsal lamina with Freer elevators (small dogs and cats) or Army/Navy osteotomes (large and giant dog breeds). Remove attachments of the multifidus muscles to the articular processes using a combination of periosteal elevators and bipolar cautery (Fig. 40-19, A and B). Remove the dorsal spinous processes of the desired surgery site with rongeurs or bone cutters. Use a high-speed air drill to create the laminectomy defect.






The defect should extend the length of the vertebrae of interest and should extend laterally to the level of the articular processes (Fig. 40-20, A through C). In medium to giant breed dogs, the bone will change in color and character as in the ventral slot procedure (i.e., outer cortical, inner cancellous, inner cortical). In cats and small breed dogs, the dorsal lamina in the cervical region often is very thin, and an obvious inner cancellous bone layer may not be apparent.




Healing of the Spinal Cord


Three basic categories of change occur after acute injury to the spinal cord: (1) direct morphologic distortion of neuronal tissue, as well as severe axonal injury, (2) vascular changes, and (3) biochemical and metabolic changes. Controversy exists regarding the relative importance of these closely related categories in causing functional deficits. Direct morphologic damage to spinal cord tissue (e.g., laceration, compression, stretching) resulting in axonal interruption is believed to be irreversible and untreatable because central axons do not regenerate sufficiently to restore function.


In the case of reversible spinal cord injury, a characteristic series of events occurs during the repair process. Approximately 2 days after injury, a heterogeneous population of small cells (most likely of hematogenous origin), including polymorphonuclear, lymphocyte, macrophage, and plasma cells, invade the traumatized tissue. This is essentially an inflammatory response. Within 7 to 20 days, fibroblasts make their appearance and begin laying down scar tissue. A concurrent glial reaction consisting of astrocyte proliferation and expansion of processes occurs. Depending on the extent of spinal cord injury, moderate to extensive fibrosis, nerve fiber degeneration, and multifocal malacia may be seen. Axons that have been severed, compressed, or stretched start to regrow until they reach the edges of the scar mass. Regeneration of spinal cord axons sufficient to restore function has not been achieved; however, if enough axons remain intact, return to clinically acceptable motor function can be expected.



Suture Materials and Special Instruments


Neurosurgical instrumentation required for cervical spinal surgery is similar to that discussed for brain surgery (p. 1446). For dorsal laminectomies in the cervical region, deep right-angled Gelpi retractors are helpful in providing exposure. Angled burr guards are also needed to create a shelf under a ventral slot for anchoring polymethylmethacrylate (PMMA) in the PMMA plug technique for caudal cervical spondylomyelopathy (CCSM) (see p. 1486).



Postoperative Care and Assessment


Critical care observation for the first 24 hours postoperatively includes monitoring respiration, administering analgesics, and observing for gastric dilatation-volvulus (particularly in large and giant breed dogs) and seizure activity (particularly if the patient had preoperative myelography). Blood gas analysis is warranted in animals with ventilatory compromise. Postoperative analgesia is provided by low doses of opioids, thus reducing their respiratory depressant effects (see Table 12-3 on p. 141). Fluids should be administered at maintenance rates and the patient turned every 2 hours until sternal. Corticosteroid administration is generally discontinued postoperatively. If neurologic status deteriorates after surgery, corticosteroids may be given until the cause is determined or corrected. Ambulatory patients may be discharged 3 to 5 days after surgery and should be confined for 2 to 4 weeks. They should walk on a harness for 4 to 8 weeks. Nonambulatory patients are treated with frequent hydrotherapy, physiotherapy, an elevated padded cage rack, frequent turning, and bladder expression three to four times a day. They should be kept clean and dry to prevent decubital ulcers. Avoid indwelling urinary catheters because they are a frequent cause of urinary tract infection. A supporting cart helps nurse patients to an ambulatory status. Advantages of a cart include unhindered eating, drinking, micturition, and defecation. Additionally, animals with motor function are encouraged to ambulate, and an erect position facilitates physiotherapy. Nonambulatory patients may be discharged when owners are able to care for them. Neurologic examinations should be performed at 1, 2, 3, 6, 9, and 12 months postoperatively, or until improvement has ceased.





Specific Diseases



Cervical Disk Disease




General Considerations and Clinically Relevant Pathophysiology


Degenerative disk disease is a common problem in dogs but a relatively infrequent clinical disorder in cats. Two types of disk degeneration (type I and type II) typically cause two distinct types of disk disease (Figs. 40-21 and 40-22). In chondroid (type I) degeneration, the normally gelatinous nucleus pulposus loses water-binding capacity, undergoes degradation of the glycosaminoglycan components, and often becomes calcified. The dorsal annulus often weakens, and the abnormal nucleus pulposus contents extrude through the weakened annulus into the vertebral canal. The severity of spinal cord damage caused by type I disk extrusion is believed to be related to the rate of extrusion (force of impact or concussion), the duration of compression, and the amount of disk material extruded. Fibroid (type II) degeneration involves progressive thickening of the dorsal annulus fibrosus, which protrudes dorsally into the vertebral canal over an extended period of time. Type I and type II degeneration can occur concurrently; these terms do not impose limitations on the pathologic behavior of abnormal disks. The phenomenon of “acute-on-chronic” disk ruptures occurs occasionally, in which a dog with chronic signs of probable type II protrusion worsens rapidly owing to sudden extrusion of nucleus pulposus into the vertebral canal (type I extrusion).





Diagnosis



Clinical Presentation



Signalment.

Hansen type I extrusions typically occur in small breed dogs, particularly the chondrodystrophic breeds (e.g., Dachshund, Beagle, Bassett Hound, Shih Tzu, Pekingese, Lhasa Apso). The Dachshund is by far the most commonly affected breed. Hansen type II disk protrusions typically occur in nonchondrodystrophic, larger breed dogs. Either type of disk disease can occur in any breed of dog, however, and both types have been reported in cats. Large dog breeds that seem to be most commonly encountered with type I disk extrusions include mixed breed dogs, German Shepherds, Labrador Retrievers, Doberman Pinschers, and Rottweilers.


Hansen type I extrusions typically occur in dogs 3 years of age or older (peak ages in small dogs of 3 to 6 years) but may occur in dogs younger than this. A wide age range has been reported for type I disk extrusions in cats; they often occur in older patients. Hansen type II protrusions typically occur in dogs 5 years of age and older.




Physical Examination Findings


In small breed dogs, type I cervical disk disease usually affects cranial cervical disks (C2-C3 most commonly) and typically causes severe neck pain, usually with inapparent or mild neurologic deficits. In large nonchondrodystrophic dogs, the most common site for type I cervical disk extrusions is at the C6-C7 intervertebral disk space; these dogs also tend to present with acute onset of severe neck pain. In the author’s experience, neck pain tends to be more severe with cranial versus caudal cervical disk extrusions. The patient often adopts a “nose-down” posture with an arched back (Fig. 40-23). When turning, these dogs tend to move the head and the neck as one unit, rather than bending at the neck. Fasciculations of the neck musculature can often be appreciated, especially upon palpation of the neck. Clinical signs of a root signature may be apparent. Type II cervical disk disease may result in clinically appreciable neck pain but rarely to the degree encountered in type I cervical disk disease. Type II cervical disk disease usually causes slowly progressive paresis. This can occur as an isolated process or as a component of caudal cervical spondylomyelopathy. Impaired ambulatory function can range from unilateral thoracic limb lameness to tetraplegia with respiratory compromise.




Diagnostic Imaging


Traditional spinal imaging for pets with suspected disk disease consists of plain radiographs followed by myelography (Fig. 40-24), with both performed under general anesthesia. CT and MRI have been found to be useful imaging modalities in the diagnosis of intervertebral disk disease (Figs. 40-25 and 40-26). In some instances (e.g., lateralized type I extrusions), CT or MRI may be preferred. MRI is rapidly becoming the standard imaging modality for dogs and cats with suspected intervertebral disk disease. In addition to providing superior anatomic detail in disk disease patients compared with myelography and CT, MRI is the best modality for diagnosing other spinal disorders that may have similar clinical presentations as intervertebral disk disease and is associated with fewer side effects than myelography. Normally, the nucleus pulposus has high signal intensity on T2-weighted images (the annulus is hypointense). The nucleus pulposus of a degenerative disk is hypointense, and the distinction between the nucleus and the annulus fibrosus may be lost.





The typical imaging finding (myelography, CT, or MRI) in disk extrusion/protrusion is focal extradural spinal cord compression centered over a disk space. Sometimes, a disk extrusion will lacerate a venous sinus, leading to large extradural accumulations of hemorrhage (in addition to disk material), which can cause extensive cord compression or swelling. Occasionally, a type I disk extrusion will be lateral enough that myelographic results are normal. In such lateralized extrusions, hyperesthesia with or without unilateral limb lameness (ipsilateral to the extrusion) is a more likely clinical sign of dysfunction than overt proprioceptive or voluntary motor deficits. Another important consideration in such a clinical scenario is the presence of syringomyelia, which usually is not evident on myelography or CT imaging (requires MRI). In some cases of high-velocity concussive disk-related injury, severe motor deficits (due to a small amount of rapidly extruding disk material) may occur with minimal evidence of extradural compression on imaging studies; in such cases, evidence of spinal cord swelling is typically noted. This may be difficult to distinguish from a vascular event (see fibrocartilaginous embolic myelopathy [FCE] discussion) if an imaging modality other than MRI is used.



Laboratory Findings


Results of complete blood count (CBC) and serum biochemical profile analyses are typically normal. A stress leukogram may be observed on CBC results as a result of underlying disease, prior administration of corticosteroids, or a combination of these factors. Hepatic enzyme elevations may be evident with dogs that have been treated with glucocorticoids. Urinary tract infection may be present, especially in dogs treated with dexamethasone. Pyuria and hematuria may or may not be present if the patient is still receiving steroids. Even if collected before myelography is performed, cerebrospinal fluid (CSF) is often not evaluated if clear evidence of a disk compression is present on imaging. Results of CSF analysis with type I disk extrusions are variable. Although CSF findings in type I disk extrusions have classically been characterized as normal to mildly inflammatory, moderate to marked pleocytosis with increased protein concentration is a common finding (Windsor et al, 2008).



Differential Diagnosis


A tentative diagnosis of cervical disk disease is based on characteristic signalment, historical complaints, and clinical features. A definitive diagnosis requires spinal imaging, as well as surgical findings of extruded disk material. Other diseases that can cause similar or identical signs of cervical disk extrusion/protrusion include inflammatory/infectious conditions (e.g., corticosteroid-responsive meningitis, diskospondylitis, granulomatous meningoencephalomyelitis), syringomyelia, traumatic fracture/luxation, neoplasia, and congenital abnormalities (e.g., Chiari-like malformation [CLM], atlantoaxial instability, atlanto-occipital overlapping). It is important for the surgeon to realize that, although it is uncommon, some patients with intracranial lesions will exhibit neck pain as the primary or sole clinical feature. If imaging of the cervical region in a patient with obvious neck pain is normal, imaging of the brain should be considered.



Medical Management


Treatment of acute and chronic disk disease is a subject of considerable debate, but a number of guidelines have been established. Positives and negatives are associated with surgical and nonsurgical management of disk disease patients, and clients need to be informed of the benefits and risks associated with either approach.


Patients with suspected type I cervical disk extrusions often are first treated successfully nonsurgically if they exhibit mild to no neurologic deficits (i.e., mainly neck pain) and have not had repeated episodes of pain. Medical management traditionally consists of strict cage confinement for 3 to 4 weeks, with or without anti-inflammatory medication. The cage or crate should be of such a size that the patient can change positions but cannot walk around or jump. Activity should be restricted to short walks to urinate/defecate, at which times the owner can assess the progress of the patient. If the patient fails to improve or worsens at any time during the confinement period, surgical options should be pursued.


Administering anti-inflammatory drugs to a patient who is exhibiting signs of an extruded disk without concurrently confining that patient is contraindicated. Anti-inflammatory drugs alleviate the patient’s pain, and most dogs will subsequently become more active. Increased activity is thought to cause more pressure to be placed on the abnormal disk by adjacent vertebrae; subsequently, more disk material is extruded into the vertebral canal, and clinical signs acutely worsen. It is also contraindicated to concurrently administer steroidal and nonsteroidal anti-inflammatory drugs (NSAIDs) to disk disease patients because this combination increases the chances of severe gastrointestinal complications. It is important to realize that subclinical gastroduodenal ulceration is likely to be present in dogs with type I disk extrusions, even without administration of potentially ulcerogenic drugs (e.g., NSAIDs, glucocorticoids); therefore, the use of such drugs should be minimized (see also p. 490).


Traditional recommendations for medical management of dogs with type I disk extrusions have recently been called into question, especially those dealing with cage confinement and glucocorticoid administration. In a retrospective report evaluating medical management of dogs with presumptive type I cervical disk extrusions, no association was found between duration of cage confinement and success of medical therapy (Levine et al, 2007). Additionally, no beneficial effect of glucocorticoid administration on the success of medical management was reported. Use of NSAIDs was positively associated with successful outcome. These findings suggest that an extended cage confinement recommendation often is not adhered to by dog owners, and that it may be an excessive recommendation in the first place. The average confinement period was approximately 2 weeks in this study, which may be a more realistic recommendation. In addition to anti-inflammatory medications, oral opiates may be considered for dogs experiencing discomfort (e.g., tramadol, 2 to 4 mg/kg every 8 to 12 hours) and/or pregabalin (2 mg/kg PO every 12 hours).


Many dogs will respond favorably to medical management. In the author’s clinical experience, 50% to 70% of ambulatory patients with presumptive type I cervical disk extrusions (cervical or thoracolumbar) will have an initial positive response to medical therapy, although many of these dogs tend to have recurrence of clinical signs. In the aforementioned retrospective study, sustained success was achieved with medical management in approximately half of patients, whereas initial success with subsequent disease relapse was experienced by about 30% of dogs. Medical failure was considered a failure from the onset of its institution in 18.1% of cases. The mean follow-up time from onset of disease until collection of data was approximately 3 years.


Type II disk disease that is not associated with cervical spondylomyelopathy is typically managed medically with restricted activity and anti-inflammatory drugs. When surgical intervention is indicated for a disk disease patient, spinal imaging and CSF analysis are typically performed on the anesthetized patient immediately before surgery. Disk fenestration is considered to be an optional ancillary surgical procedure. Fenestration involves removing a segment of annulus fibrosus so that future extrusions will likely occur through that opening, rather than into the vertebral canal. Fenestration is performed in the ventral aspect of the disk in the cervical region and in the lateral aspect of the disk in the thoracolumbar region. Disk fenestration is considered to be a prophylactic measure. The efficacy of fenestration remains to be definitively proven. At the present time, fenestration remains a potentially effective but unproven method of decreasing recurrence of disease for type I extrusions and its use is a matter of clinician preference. Patients with type II disk disease are often controlled adequately for long periods of time with medical therapy.

Sep 11, 2016 | Posted by in SMALL ANIMAL | Comments Off on Surgery of the Cervical Spine
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