Intervertebral Disc Disease
What are intervertebral discs?
Intervertebral discs are important cushion-like structures that are found between the vertebrae in the spine (except the first 2 vertebrae). They provide biomechanical support and allow the spine to flex, extend and rotate.
The structure of an intervertebral disc is similar to that of a jam doughnut. There is an outer structure called the annulus fibrosus (dough of the doughnut) that encircles an inner structure called the nucleus pulposus (jam filled centre of the doughnut). The outer layer is very strong and fibrous, while the inner centre is jelly-like and is very well hydrated. This relationship between the annulus fibrosus and the well hydrated nucleus pulposus is what allows the disc to function normally.
What is intervertebral disc disease
Interveretbral disc disease is a degenerative condition of the intervertebral discs that can ultimately lead to herniation and spinal cord compression.
The type of degeneration that occurs can, but not always, be breed specific. In chondrodystrophic breeds (breeds that have disproportionately short and curved limbs eg dachshund and Shih Tzu), the degeneration is called chondroid metaplasia. The nucleus loses the ability to hold water and becomes more like cartilage, or even mineralises. This results in the disc not being able to function normally like a cushion and can result in disc extrusion and spinal cord compression. That is, the jam squishes out through the dough and squashes the spinal cord. This is called Hanson type I disc disease.
The discs of nonchondrodystrophic breeds usually undergo fibrous metaplasia. This results in bulging of the annulus (protrusion) and spinal cord compression. That is, the dough becomes larger and squashes the spinal cord. This is called Hansen type II disc disease.
Sometimes, a small amount of nucleus explodes out of the disc like a shot gun pellet out of a gun and directly damages the spinal cord. This is called a shot-gun disc extrusion, low-volume, high-velocity disc extrusion or Hansen type III disc disease.
Signs of intervertebral disc disease
Regardless of the type of disc disease, IVDD usually results in variable degrees of pain and neurological dysfunction. These signs may develop gradually or very suddenly.
Signs that your pet is in pain may be overt, (eg yelping, crying) or may be more subtle, (eg panting, hunched back, reluctance to turn the neck, reluctance to jump or climb stairs).
In addition, the ability of the brain to communicate with the body is affected and results in neurological dysfunction. If a garden hose is connected to a tap and the tap is turned on, water exits the end of the hose. The tap can be thought of as the brain, the hose the spinal cord and the water, the messages. If the garden hose is kinked or squashed, water flow reduces or fails to exit the hose completely. If a spinal cord is squashed, message transfer is slowed or stops completely resulting in difficulty walking (eg a wobbly gait) or complete paralysis (an inability to move the limbs).
The ability to feel pain may also be affected. In severe cases, a patient may lose the ability to feel pain in their limbs completely.
The location of the spinal cord injury will affect the signs that the patient shows. Neck injuries result in all four limbs being affected; if the back is affected, then only the hind limbs will be affected. Sometimes, one leg may be more affected than another. The ability to urinate may also be affected.
How is intervertebral disc disease diagnosed?
Accurate and timely diagnosis is required to direct appropriate treatment. Initially, neurological examination is required to suggest that a patient has IVDD. However, there are many different diseases that may show similar signs to IVDD. Therefore, imaging and sometimes, cerebrospinal fluid testing are required.
Plain radiographs (x-rays) may show that there is evidence of IVDD (mineralised discs, narrow disc spaces), but are generally not accurate in confirming the diagnosis or the exact location of the disc extrusion/protrusion. Rather, advanced imaging is required.
Magnetic resonance imaging (MRI) is a very specialised imaging modality that uses a computer and a strong magnet to produce images of the body. It is able to confirm the location and severity of the disc extrusion/protrusion accurately.
Computed tomography (CT) uses x-rays and a computer to produce images of the body. This test may require the addition of contrast (a special dye) to help highlight the spinal cord.
Cerebrospinal fluid testing may also be required. This is obtained using a spinal tap. The test can be likened to a “blood-test” for the brain and spinal cord.
How is intervertebral disc disease treated?
Intervertebral disc disease is treated conservatively or surgically.
Conservative treatment is generally considered for those patients who have spinal pain and/or only very mild neurological signs. This may consist of strict rest, pain relief, anti-inflammatories, muscle relaxants and physiotherapy. If the patient is having difficulty urinating, then appropriate bladder care that may include manual expression or catheterisation may be required. The patient must be monitored very closely for evidence of worsening of signs or failure to improve.
Surgical treatment is considered for those patients who have spinal pain and more significant neurological signs, or when conservative treatment fails. The aim of surgery is to relieve the pressure caused by the disc on the spinal cord. This is called, spinal decompression. In the neck, this is achieved by accessing the spinal canal from the underside of the neck and performing a ventral slot. A slot of disc and bone is removed and the extruded disc material is removed. In the back, decompression is achieved by removing a section of bone, generally from the top side of the spinal canal, and removing the extruded disc. This is called a hemilaminectomy.
What is the prognosis for intervertebral disc disease?
The prognosis depends on the severity and duration of the compression, as well as the treatment method.
Patients treated conservatively may recover if their clinical signs are only mild. The aim is for the disc to heal and for the spinal cord inflammation to resolve. Unfortunately, some conservatively patients can become worse or have a repeat episode of extrusion of the same disc.
Patients treated surgically generally have a very high likelihood of recovery, as long as they retain sensation in their paws and have not been compressed for too long. In general, a patient remains hospitalised after surgery, receiving 24/7 care until they are able to walk and toilet by themselves. When they are discharged, strict rest and rehabilitation exercises may be required for approximately 8 weeks after surgery.