Richard M. Jerram, BVSc, DACVS
Veterinary Specialist Group @ Unitec, Mt. Albert, Auckland, New Zealand
Spinal surgery has become an important part of the repertoire of the small animal surgery, particularly for those in specialist referral practice. There is something very satisfying about seeing that paralysed dog return to normal function after spinal surgery. There are a number of surgical procedures described and there remains considerable debate amongst surgeons as to which surgical procedure to choose for a given condition. While there are no hard and fast rules about what must and must not be done, I believe that some general perioperative guidelines would be very helpful.
Clearly, making accurate surgical decisions in animals with spinal disorders relies on a complete and comprehensive neurologic examination and appropriate preoperative diagnostic testing. Almost certainly, most dogs and cats will require advanced imaging such as myelography, computed tomography, or magnetic resonance imaging to determine whether a surgically treatable lesion is present. The only possible exception to this would be animals with obvious spinal fractures on plain radiography. Most veterinary neurosurgeons typically divide the spine into three clinically relevant areas; the cervical spine, the thoracolumbar spine, and the lumbosacral spine. Spinal surgery is generally performed in these areas for intervertebral disk disease, fracture/luxation, neoplasia, or infection.
As spinal surgery is a complex and involved surgical intervention to the patient, it is critically important that concurrent disease processes (particularly metabolic disorders) are evaluated prior to surgery as these may significantly affect clinical outcomes. A number of considerations can be assessed and prepared for prior to anesthesia and surgery. Preoperative blood tests should be considered including hematology and a biochemical profile, urinalysis would also be indicated particularly in dogs with diskospondylitis or neurologic dysuria. When neoplasia is a differential diagnosis, thoracic radiographs and possibly abdominal ultrasonography are indicated. In addition, echocardiography should be considered in large or giant breed dogs that have a breed predisposition for cardiac disease such as the Doberman pinscher or a Great Dane. Probably one of the most frustrating intraoperative complications during spinal surgery is haemorrhage. In particular breeds of dogs (Doberman pinschers) preoperative hemostatic tests such as buccal mucosa bleeding time (BMBT), activated clotting time (ACP), von Willebrand factor, or a complete coagulation profile are indicated. The use of perioperative prophylactic antibiotics in spinal surgery is contentious, however, I would recommend their use if the dog has concurrent metabolic, dermatological, or dental disease; if a surgical implant is expected to be used; or if the surgical time is likely to exceed 90 minutes. Typically a penicillin-derivative or a cephalosporin would be the ideal choice. The use of appropriate analgesia is strongly recommended particularly the use of opioid drugs such as morphine, fentanyl, or oxymorphone. The use of corticosteroids preoperatively in spinal surgical cases is extremely controversial and the most current information would indicate that the consequences of complications such as gastrointestinal ulceration or perforation far outweigh the overinflated benefits of neuroprotection. The use of nonsteroidal anti-inflammatory medications also would not be expected to have any neuroprotective benefit and probably should be avoided preoperatively.
There are several important anesthetic considerations with regard to spinal surgical patients. Many of these animals will be dehydrated and rehydration prior to anesthetic induction is recommended. In addition, animals with a severe cervical spinal cord injury may have hypoventilation requiring ventilatory assistance prior to and during surgery. Regular blood gas analysis is recommended in these patients. Most spinal surgical patients are maintained on isoflurane in oxygen and recovery from anesthesia should be smooth.
Cytological assessment of the cerebrospinal fluid (CSF) can also be an aid in determining whether surgical decompression is indicated particularly when neoplasia is suspected. For instance, the presence of a spinal cord compressive lesion and large numbers of lymphocytes in the CSF could indicate the presence of lymphoma which may be responsive to chemotherapy without surgical intervention. In many cases the decision to proceed with surgery must be made prior to obtaining the CSF cytology results but postoperative evaluation can still be useful when deciding on follow-up treatment pathways.
Decision-Making in the Cervical Spine
In my view, animals with cervical spinal cord compression are far less likely than those in the thoracolumbar region to have a satisfactory clinical recovery with medical management such as cage confinement and pain relief alone. I believe that this is due to their inability to completely rest the neck and the difficulties associated with using neck braces. The most common surgical procedure performed in the neck is the ventral slot procedure which is typically indicated for cervical type I and type II intervertebral disk disease. Dogs that have had a protracted history of cervical spinal pain (greater than six weeks) or dogs that have had an acute onset of severe cervical spinal pain would appear to benefit from surgical intervention. In the preoperative planning and intraoperatively, decisions may need to be made on whether the cervical spine requires stabilization. This is logically recommended in cases with cervical vertebral fracture/luxation but the decision will be more challenging in animals without perceived instability. Determining whether a cervical spinal lesion is dynamic or static can be challenging and is often a subjective assessment. If myelography is used to obtain the diagnosis then lateral radiographs taken with distraction, dorsoflexion or ventroflexion of the head can be used to demonstrate either an increase or decrease in the size of the compressive lesion. If changes in the degree of compression occur with these additional radiographic views then vertebral instability is strongly suspected and stabilization is probably warranted. In particular, the Doberman pinscher appears to have a genetic predisposition for cervical vertebral instability, therefore, stabilization should probably always be considered in these dogs. In some cases, assessment of gross instability subjectively during the ventral slot procedure may mean that an intraoperative decision to stabilize the spine is necessary.
Most small breed dogs with cervical intervertebral disk disease are unlikely to have significant spinal instability that would require stabilization; however, this may not be the case in larger breeds of dogs. The use of fenestration of the adjacent intervertebral disks during ventral slot surgery is somewhat controversial. It has been shown that fenestration of the ventral aspect of the disk results in vertebral instability, however, I believe that fenestration of discs that have radiographically evident calcification of the nucleus pulposus is indicated.
The continuous cervical dorsal laminectomy procedure is indicated for animals with multiple sites of spinal cord compression particularly when this compression is dorsally or laterally located. This procedure is technically demanding and appears to have a higher morbidity for postoperative inability to ambulate, therefore, careful pre-surgical planning is imperative. Patients with ventral spinal cord compression at multiple sites can be more challenging because performing the ventral slot procedure at multiple spaces is likely to result in instability. These animals should be stabilized using polymethylmethacrylate plugs or other spinal stabilization methods such as locking plate technology.
Decision-Making in the Thoracolumbar Spine
The most common reason for performing surgical treatment in the thoracolumbar region of the spine is type I intervertebral disk disease. In most cases, animals that are able to ambulate unassisted even if they have mild-moderate ataxia/paresis are probably candidates for non-surgical management with strict cage confinement and analgesic therapy. In my opinion, any dog with the inability to ambulate without assistance should be considered a candidate for spinal cord decompression. Spinal surgery should also be considered in dogs that have had several (> 3) episodes of spinal pain in a relatively short period (six months). The duration of the absence of deep pain sensation as a prognostic indicator for thoracolumbar disk extrusion has been the subject of a number of clinical reports. Overall recovery rates for dogs with loss of deep pain sensation range from 25% to 76%. It has generally been accepted that the loss of deep pain perception for longer than 48 hours is associated with a grave prognosis (less than 5% success rate). In our hospital, decompressive or exploratory spinal surgery is generally offered for animals that have lost deep pain sensation. Owners are advised that neurological recovery may not occur and that structural loss of spinal cord integrity based on findings of a durotomy may indicate a grave prognosis.
The most commonly performed surgical procedure is the hemilaminectomy which allows for adequate access to the lateral and ventral aspects of the vertebral canal to allow removal of extruded or protruded intervertebral disk material and possibly some decompression of the spinal cord. The hemilaminectomy procedure has been described using a smaller or mini-approach to eliminate possible instability of the spine, however, I more typically perform this procedure if I am uncertain of the lateralization of the lesion from the preoperative diagnostic testing. In these situations, a mini-hemilaminectomy will allow determination of the presence of this material without removal of the adjacent articular facet. If disk material is evident then the laminectomy site can be extended and if disk material is not present then a surgical approach can be made to the contralateral side. Dorsal laminectomy is rarely indicated for patients with thoracolumbar intervertebral disk disease but may be appropriate when spinal neoplasia is suspected. Stabilization of the thoracolumbar region of the spine is indicated if a fracture/luxation is evident. I would also recommend subjective assessment of the spinal stability in cases that have had suspected traumatic intervertebral disk herniation/damage but do not have strong radiographic evidence of spinal displacement. The affected intervertebral disk may be significantly damaged in these dogs and without stabilization postoperative luxation of the spine is possible. Fenestration of the intervertebral disks in the thoracolumbar region has been a controversial topic for many years. It has been shown that fenestration does reduce the possibility of recurrence of intervertebral disk extrusion postoperatively, however, complete fenestration of all of the thoracolumbar intervertebral disks requires a more extensive surgical approach and prolonged surgical time. Many surgeons recommend fenestration of the affected intervertebral disk and those immediately adjacent during a routine hemilaminectomy procedure. I believe that this is indicated when obvious radiographic calcification of the nucleus pulposus is evident but it is more difficult to recommend when adjacent intervertebral disks appear to be clinically normal.
Decision-Making in the Lumbosacral Spine
Surgical treatment of spinal disorders of the lumbosacral spine has proved somewhat controversial over the years; however, more recently there appears to be a move to providing stabilization techniques as well as decompression. This has probably been advanced by the use of more advanced diagnostic imaging techniques such as computed tomography and magnetic resonance imaging. Dogs with pain in the lumbosacral region as their primary or sole clinical sign should probably all have a period of medical management before proceeding with surgical intervention. This medical management should consist of exercise restriction and analgesic medication. If the clinical signs do not improve or deteriorate and if the advanced imaging studies show evidence of significant intervertebral disk protrusion then surgical decompression using a dorsal laminectomy technique is indicated. The use of computed tomography has provided a more objective assessment of lumbosacral instability as determined by three-dimensional reconstructions of the vertebral foramina and the articular facets. Stabilization has been described using a variety of techniques including Steinman pins and wire, pins and polymethylmethacrylate, trans-facet screws, and locking plates. Distraction of the lumbosacral intervertebral disk space is recommended prior to placement of the preferred stabilization implants. Removal of the articular facets (facetectomy) has also been described to provide superior decompression of the spinal nerve roots, however, this is likely to result in increased instability, therefore, stabilization is recommended when facetectomy is performed. Surgical opening/widening of the vertebral foramen (foraminotomy) has also been recommended, however, this may have been superseded by the distraction/stabilization techniques that have been more recently described. In my opinion, almost all large breed dogs with significant lumbosacral intervertebral disk disease benefit from additional distraction/stabilization. I would typically recommend the use of the trans-facet screw techniques or vertebral body locking plates.
Some of the recommendations made in this discussion and in other surgical descriptions are unproven and open to debate. There is a definite need for a superior evidence-based approach to decision-making in spinal surgery, but this approach will require collective agreement amongst multiple veterinary institutions and specialist veterinary hospitals in coordinating detailed case controlled studies. Unfortunately, current recommendations are often anecdotally and experience-based. Despite this, however, the surgical success rates following spinal surgery are good-excellent which is extremely satisfying for the veterinary neurosurgeon.