Without a doubt, the most common neurosurgical condition presented to veterinarians is IV disc herniation.Herniated discs occur between C2 – C7 and T10 – L5, with C2 – C3 and T12 – T13 being the most common. Chondro-dystophic breeds (Dachshunds, Cocker Spaniels, Lhasa Apsos, etc.) represent the majority of these patients. However, IV disc herniation can occur in almost any breed—even in cats. Diseases such as fibrocartilaginous emboli (FCE), meningitis, and neoplasia can mimic the symptoms of disc herniation and must be ruled out during the diagnostic work-up.
In years past, patients exhibiting pain and/or mild loss of motor function were commonly managed conservatively with cage confinement, steroids, muscle relaxers and pain medications. Even if these patients improved initially, many would have some recurrence of symptoms or worsening of neurological function at a future time. Unfortunately, if the time interval between episodes is more than a few months, an acute disc herniation with a very favorable surgical outcome can become a chronic herniation with a more guarded outcome.
An acutely herniated nucleus is soft and even putty like. Over time it becomes fibrous or even osseous and potentially adherent to the dura of the spinal cord and the floor of adjacent vertebra. Attempts to remove a chronic disc herniation is more challenging, has a higher potential to worsen neurological symptoms and can result in permanent deficits. Therefore, over the past 10 years, the general philosophy as to when to pursue advanced spinal imaging and potential neurosurgical intervention has dramatically changed!
Diagnostic spinal imaging techniques used to diagnose disc herniation include myelogram, CT scan or MRI. Plain film radiography is useful to rule out other diseases such as bony neoplasia, fractures, luxations and discospondylitis. However, plain radiographs cannot be relied upon to diagnose an intervertebral disc herniation or to plan surgical intervention.
MRI is a very useful tool to diagnose disc disease; however it is more expensive than other comparable diagnostic imaging techniques such as CT scan or myelography. In addition, MRI typically requires transport to specialized imaging centers. Some imaging centers do not employ surgeons; therefore the patient must be transported to another facility for surgery necessitating another anesthetic event. Therefore, myelography and CT scan are still the most widely accepted, practical, and cost effective imaging technique used in veterinary medicine.
MRI is generally reserved for cases where CT/myelogram is inconclusive or for suspected intramedullary spinal cord tumors, lumbosacral disease, brachial plexus neoplasia or primary brain abnormalities.
In chondrodystrophic breeds, axial images by computed tomography (CT) will identify the herniated nucleus in the majority of cases, without need for positive contrast material. This makes the study safer to the patient and less costly to the client. In non-chondrodystrophic breeds an injection of radiographic contrast media into the subarachnoid space (myelogram) followed by a CT may be necessary.
CT imaging offers diagnostic accuracy comparable to an MRI but at a fraction of the cost. The surgeons at the DVSC are convinced of the diagnostic benefits of CT for management of neurosurgical diseases, and we are preparing to add a second helical CT to our practices.
The most common surgical procedure used to manage IV disc herniation is either a ventral cervical slot for C2-T1 disc herniation or hemilaminectomy in the thoracolumbar spine. Occasionally a dorsal laminectomy or a hemilaminectomy is performed for a lateralized “foraminal” disc rupture in the cervical region. The prognosis for regaining normal or near-normal motor function after surgery for an acute cervical or T-L disc herniation with preoperative deep pain sensation is well above 90%.
Patients suffering from chronic disc herniation can generally be returned to an ambulatory status, but often some degree of incoordination and occasional weakness may be present for the remainder of the pet’s life.
Patients with preoperative negative sensory status have the most variable response to surgery. In these patients the duration of time from loss of sensation (which is often unknown) until surgical decompression is inversely proportional to the clinical outcome. Basically, the longer the negative sensory status has been present the less chance surgery will result in a favorable outcome. Returning negative sensory patients to an ambulatory status ranges from 0-50%, based on the results of several retrospective studies.
Some surgeons feel the prognosis drops below 25% if the negative sensory status has been present for more than 12 hours, and rapidly approaches 0% after 24 hours of loss of sensation. Surgery is still the best option for the negative sensory pet; however a client should be counseled on the severity of the disease.