Intervertebral Disc Disease and Herniated Discs in Veterinary Medicine
>> Friday, November 30, 2012
The spinal cord is protected by a series of vertebra and small pads that sit between each vertebra, called intervertebral discs. There are 7 cervical vertebra with discs sitting between each except between the first vertebra, sitting adjacent to the skull, (called the Atlas) and second cervical vertebra (the Axis). Then there are 13 thoracic vertebra and 7 lumbar vertebra ( with a disc between each vertebral body) and 3 sacral vertebra (there are no discs between the sacral vertebra). Finally the coccygeal vertebra make up the tail, the number of these vertebra varies depending on the length of the tail.
Image 1
The intervertebral disc serve many important functions in maintaining both the stability and the mobility of the spine. The discs sit between the vertebral bodies, “beneath” or ventral to the spinal cord in veterinary patients (image 1). The disc acts as a joint and as a cushion between two vertebra. The disc is built like a jelly doughnut with a center gel-like middle called the nucleus pulposus and an outer fibrous coating called the annulus fibrosis. The center of the doughnut is placed slightly off center, sitting closer to the spinal cord and the annulus is thinner in that area (Image 2).
Image 2
The discs can degenerate. Dogs are more prone to disc degeneration and herniation than cats. With animals that have congenital (inherited) disc degeneration (common in young dachshunds and spaniels, to name a few) the nucleus of the disc becomes dehydrated. Often these discs appear mineralized or calcified on radiographs.
Older animals experience a fibrous degeneration of the discs that is less evident radiographically. The fibrous degeneration begins on the periphery of the nucleus and moves more centrally. The annulus can degenerate as well. As the disc becomes less ‘healthy’ and more and more fibrous it can no longer accommodate the movements of the spine. The tissues thin and eventually the thinnest part of the annulus gives way allowing the nucleus to move upward, or dorsally, towards the spinal cord.
Image 3
Discs can herniate very abruptly (called a Hansen type 1 herniation) or more slowly and progressively (Hansen Type 2). Discs may also herniate as a result of trauma (like being hit by a car) or as a result of other vertebral deformities leading to abnormal spinal alignment that contributes to the movement of the disc. The thoracic spine from the 2nd to the 10th thoracic vertebra is much less likely to experience a disc herniation due to a ligament, the Intercapital ligament, that runs along the floor of the spinal canal, preventing disc movement into the spinal canal. Therefore, herniated discs are most commonly cervical, thoracolumbar or lumbar in location. The location of the disc herniation will influence how each patient is affected. Patients with a herniated cervical disc may have the function of both their thoracic (front legs) and their pelvic (back legs) affected. Patients that have thoracolumbar disc herniation or a lumbar disc may only have their pelvic limbs, tail and bowel and bladder control potentially damaged. Regardless of the location of the herniated disc, almost all patients will experience pain. The discomfort often helps us in localizing the site of the herniation.
The amount of damage done to the spine is defined by the extent and location of the disc herniation. The larger the volume and force of the herniation, the more damage is done. Type 1 discs cause very acute and forceful trauma that compromises the spinal cord through compression, bruising and alterations in blood flow and oxygenation of the nervous tissue. These patients not only exhibit pain, but decreased or absent motor function. Type 2 discs can be equally as traumatic, but tend to occur more slowly.
Patients that have had a herniated disc may exhibit pain (most common sign), poor coordination (ataxia), inability to place the feet appropriately (knuckling or walking on the tops of their paws), weakness, inability to walk, abnormal posture, and loss of sensation (loss of deep pain).
Any patient that is suspected of a herniated disc (spinal pain, difficulty or an inability to walk) should be seen by a veterinarian immediately. Prompt management either through initiating appropriate medications or surgery can be pivotal in the successful recovery of the patient.
Each patient should have a thorough physical and neurological examination. This allows the veterinarian to localize the lesion or identify the approximate area of the spine that has been injured. These examinations also provide important information regarding the extent of injury and aid in identifying any other health issues that may be contributing to the current problems.
Based on these examinations the doctor is going to recommend diagnostics that are tailored to further assess the patients health and examination the area of the spine that was injured. Initially blood work, thoracic radiographs and spinal radiographs may be discussed. If the patients signs are severe and surgery is to be considered even more detailed evaluation of the of the spine and spinal cord will be needed since we will want to know the exact location of the injury and define the injury accurately, prior to surgery. MRI and myelography (Images 4 and 5 ) are two techniques that allow the localization of a spinal cord lesion. Cerebrospinal fluid analysis may be performed as well to help rule out any other diseases of the nervous system.
MRI of a herniated disc at C4-5
Image 4
Myelogram C3-4 herniated disc
Image 5
The damage done to the cord is not necessarily irreparable. Many herniated discs, just as in human medicine can be medically managed. Careful management by a veterinarian, rest, anti-inflammatory medicine (often steroids) and physical therapy all play a role in the successful recovery of these patients.
More severely affected patients either due to loss of motor function, loss of sensation to the affected limbs, or persistent pain will require surgical intervention. Again, these patients are more severely affected due to the dynamics of disc herniation, the extent of disc compression (the amount of material in the canal) and the location of the disc herniation.
Post surgical recovery can be as short as 2-4 weeks or as long as 4-8 months. Typically, recovery times are directly proportional to the degree of damage prior to surgery. Therefore, a patient with no voluntary movement to the affected limbs and loss of sensation will have a longer recovery time than a patient that has surgery due to chronic discomfort.
Early in our understanding of spinal cord trauma subsequent to herniated discs, veterinarians counseled owners that a patient that had lost deep pain response to the affected limbs would have a 10% or less chance of recovery. Currently, we appreciate that many of these patients will regain their ability to walk, but the recovery will be long and the owner must have a very strong commitment to providing the nursing care and physical therapy necessary for a successful recovery.
Clients, rightly so, are always concerned with the prognosis for their pet. At our hospital our experience is that patients that have a recent onset of signs such as spinal pain, with present or decreased motor function will often be successfully treated medically. It must be recognized that medical treatment is done initially in the hospital and under a clinicians supervision. Should the patient not respond, surgery must be considered.
Patients that have lost motor function, but maintain deep pain are recommended surgery. Success with surgical decompression ranges from 85-90%. Patients that have lost deep pain sensation to the affected limbs have suffered substantial injury and the exact extent is very difficult to appreciate even with the benefit of MRI. These patients are given a 50% chance of significant improvement with a long recovery time expected.
The most common surgeries performed for herniated disc are a hemi or dorsal lamenectomy, usually performed on the thoracic and lumbar spine and ventral slot performed on the cervical spine. The choices of surgical approach depend on the location of the herniated disc within the spinal canal as well as individual preferences of the surgeon.
Care for a patient post-surgery again is dependent on the extent of spinal injury. Patients that are ambulatory need to be restricted in their activity and slowly brought back to ‘normal’ daily activity over a 4-6 week period. Patients that are not ambulatory require confinement as well, passive range of motion exercises, assistance outside to urinate and defecate, possible assistance in urinating and careful observation.
Herniated intervertebral discs can be a frightening event for a pet owner. Prompt intervention, application of appropriate care and diligent home care will typically lead to the pets recovery. Any animal that has experiences a herniated disc, despite the completeness of the care provided, may be left with some deficits, a weak leg or some coordination issues, for example. It is key that the owner receive a thorough over view of their pets condition and an appreciation of the clinicians expectations for their pet so that all expectations are realistic and the owner is prepared for the recovery process ahead.
