The signs that develop following disc damage are the result of:
- Pressure of the herniated disc material on the spinal cord (compression component).
- Bruising of the spinal cord caused by the impact of the disc as it is herniate (concussion component).
It is not possible to say how much each of these components is contributing to the signs in an individual animal by examination alone. Myelography, CT or MRI scans can help to determine how much the spinal cord is being compressed. However, it can be very difficult to assess how much bruising has occurred (even with the specialised techniques). This concussion can sometimes be seen as spinal cord swelling.
The cables making up the spinal cord are organised into groups depending on their function within the nervous system. The most superficial cables are those running from the leg to the brain. Their main function is to send messages to the brain about the position of the leg and body in space. Because this group of nerves is the most superficial, they are the first to be affected by pressure from a slipped disc. Damage to these nerves results in the animal being wobbly on his legs.
As we move deeper into the spinal cord, the next group of cables are the ones from the brain sending messages to move the legs. Damage to these cables results in weakness of the legs, which can progress to total paralysis.
The deepest cables (in the centre of the spinal cord) are the ones responsible for informing the brain that the bladder is full, and finally the one carrying pain sensation from the limbs from the brain. Loss of function in these cables results in the animal not being able to urinate and being unaware of painful stimulation in the toes.
As an animal recovers from spinal damage, their nerve functions return in the reverse order to that in which they disappeared. Depending on the site of spinal damage (neck, back or lower back), these signs may affect only the back legs or the front ones as well. Rarely, a slipped disc can cause lameness by trapping one of the spinal nerves as it exits the spine.