Intervertebral Disc Disease
The spinal cord resides in a canal surrounded by a column of bone consisting of the vertebrae and is responsible for transmitting neurological impulses and information between the brain and the periphery. The intervertebral discs (IVD) are located between the ends of adjacent vertebrae which form the spinal column. The discs consist of an outer fibrous ring called the annulus fibrosus and an inner portion called the nucleus pulposus. The IVD functions as a “shock absorber” and stabilizer of the spine. Intervertebral disc disease is a degenerative process of the IVD causing loss of normal disc architecture and function, which can result in disc bulging or herniation into the spinal canal causing trauma to and compression of the spinal cord. Occasionally, an otherwise healthy disc may herniate as a result of trauma. The severity and type of clinical signs (neurological deficits) may depend on the rate of disc extrusion (rapid vs. gradual), the concussive force upon the spinal cord, the volume of herniated disc material, the degree of spinal cord compression, and the location of the affected IVD (cervical vs. thoracolumbar). Chondrodystrophic breeds such as the Dachshund are at an increased risk for IVDD, but other commonly affected breeds include the cocker spaniel, beagle, Shih tzu, Pekingese, Lhaso apso, Welsh corgi, and poodle. Cats may also be affected but much less frequently than dogs.
Clinical signs associated with IVDD vary with the location of the disc herniation. With cervical discs, the most common clinical sign is pain and affected patients may guard their neck, hold their head in a lowered position, have a reluctance to move their head and neck, and may walk with a stiff forelimb gait. Muscle spasms may be visible in the neck and shoulder region and occasionally, the patient may hold up one of the forelimbs---this is known as a radiculopathy or “root signature”. Although paresis (weakness) or paralysis (inability to move) in all four limbs may occur with cervical disc herniation, it is not common. With thoracolumbar IVDD, the patient may have varying degrees of back pain characterized by arching of the back, sensitivity on touch or palpation (hyperpathia), and abdominal guarding. The hind limbs typically have some degree of neurologic deficits ranging from ataxia and incoordination to paresis or paralysis. When the IVDD lesion involves a disc space caudal (behind) the level of the 4th lumbar vertebra, a “root sign” may be present in one of the pelvic limbs. A thorough neurological examination should be performed to localize the lesion (determine what region of the spine is affected) as well as to rule out potential other causes for the neurological deficits including conditions such as spinal fracture/luxation, fibrocartilaginous emboli (a “stroke-like” condition), meningitis, discospondylitis (infection in a disc space), tumors, and other conditions which can mimic IVDD. Typically, confirmation of IVDD is accomplished through imaging of the spine. Historically radiography of the spine and myelography (contrast injection around the spinal cord) were commonly used to diagnose IVDD, however, CT and MRI are now the preferred imaging modalities due to their non-invasive nature and diagnostic accuracy. The choice between these types of imaging depends on individual patient factors such as age, breed, previous history of clinical signs and examination findings. Your surgeon will discuss the pros and cons of each modality with you, as it relates to your pet.
Treatment of IVDD is often based upon the grade or severity of the neurological dysfunction. Conservative, or non-surgical management, is usually considered in mildly affected (pain only, mild ataxia) patients, whereas, surgery is generally considered in more severely affected (non-ambulatory, lack of voluntary movement of the limbs) patients. Conservative management typically consists of strict cage confinement of the patient for 4-8 weeks while avoiding stairs and any interaction or play with other pets. Medications used to treat IVDD may include a non-steroidal anti-inflammatory drugs, an analgesic such as gabapentin, and occasionally muscle relaxants. Surgical treatment, referred to as surgical decompression, involves removal of the disc material from the spinal canal which is causing compression of the spinal cord, and the surgical approach and procedure depend upon the location of the IVDD lesion. With cervical IVDD, the most common surgical procedure performed is called a ventral slot decompression, in which the affected disc space is approached through an incision made on the underside of the neck and a “key hole” opening is made in the vertebral bodies on either side of the affected disc space through which the herniated disc material is removed. Occasionally, when herniated disc material is lateralized in the canal and may not be accessible via the more common ventral slot approach, a lateral approach (on the side, rather than underneath) to the cervical spine is made and a procedure called a hemilaminectomy is performed. Thoracolumbar disc disease is most often treated via a hemilaminectomy in which a small “window” of bone is removed from adjacent vertebrae centered over the disc. Generally speaking, non-ambulatory patients undergoing surgery recover more quickly and more completely (fewer residual neurological deficits) than those patients treated conservatively; however, recovery is dependent upon the presenting neurological status. Patients with intact sensation (the ability to feel when the toes are pinched) typically have a favorable prognosis with recovery rates between 70-100%. Although patients with complete sensory loss may have a less favorable prognosis for return of neurological function and were previously thought to have a poor prognosis, there is emerging evidence to suggest that recovery rates can be as high as 40-60% (58% of dogs with no deep pain at the time of surgery recovered the ability to walk within the 3-month follow-up period in a recent study).
Complications associated with IVDD include urinary tract infections, especially those patients in which normal control over urination has been impaired, fecal incontinence, myelomalacia (progressive spinal cord damage), recurrence at the same or another IVD space, and persistent neurologic deficits. Unless the patient remains paralyzed, most disorders of urination will improve as the overall neurological function improves. A less common but nonetheless severe complication seen after spinal cord injury is a condition called myelomalacia in which the spinal cord tissue continues to degenerate, resulting in a progressive loss of neurologic function, often termed ascending-descending myelomalacia. This problem is typically occurs in approximately 10% of dogs who have lost feeling to their hind legs and occurs within the first few days after spinal cord injury, and may be severe and/or fatal. Prophylactic fenestration is a additional procedure which can be performed at the initial decompressive surgery, which reduces the risk of future disc herniations in that region of the spine. This procedure involves making a small window in the side of the intervertebral disc through which material from the center of the disc is removed and allows remaining material to herniate to the side instead of into the spinal canal. In the lower back, fenestration is typically performed at the intervertebral disc spaces between the 11th thoracic and 3rd lumbar vertebrae. Although this does not entirely eliminate the risk of future herniations, it does decrease the likelihood of a second disc herniation. Recurrence of IVDD at either the same or a different location is possible and occurs in less than 10% of all dogs who have had a prophylactic fenestration and up to 20% of dogs without fenestration. Recovery time after surgery is typically measured in weeks with most patients that have a favorable prognosis resuming normal function within 6-8 weeks. During the postoperative period, typical recommendations for care include strict confinement, administration of pain medications and anti-inflammatories, physical therapy and bladder management, if indicated.