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).

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.

Definitions

Cervical disk disease refers to degeneration of the nucleus pulposus (Hansen type I or type I degeneration) or annulus fibrosus (Hansen type II or type II degeneration) components of cervical disks and the clinical consequences of such degeneration. Type I degeneration is also called chondroid degeneration, whereas type II degeneration is called fibroid degeneration. Radiculopathy means pathology of the nerve roots and is often manifested as pain due to compression by disk material; when a radiculopathy leads to lameness or holding up a thoracic limb because of suspected irritation/pain, this is called a root signature.

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

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.