Chapter 40 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 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). 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. 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. 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. 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). 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 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). 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.
Surgery of the Cervical Spine
General Principles and Techniques
General Considerations
Preoperative Management
Anesthetic Considerations
Surgical Anatomy
Standard Surgical Approaches to the Cervical Spine
Ventral Approach to the Cervical Spine
Dorsal Approach to the Cervical Spine
Suture Materials and Special Instruments
Postoperative Care and Assessment
Cervical Disk Disease
General Considerations and Clinically Relevant Pathophysiology
Diagnosis
Diagnostic Imaging
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Surgery of the Cervical Spine
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