What is the Spinal Column?
The spinal column is a series of back bones interposed by fibrous discs. Each back bone or vertebra has a solid lower section (the body) and a hollow bony tube at the top. The spinal cord is housed in the protective environment of the hollow tube. There are also bony projections that act as anchor points for the muscles.
What is a disc? A disc in this sense means a soft structure between the bones of the spine (vertebrae). It is designed to permit flexibility of the spine in three dimensions whilst ensuring that the flexibility is controlled, the disc is also a shock absorber. Each disc is exactly the same in terms of its structure. In some senses you can think of the disc in a young dog as being like a jam doughnut. There is a blob of jelly in the centre of a dense fibrous capsule. The flexibility and shock absorbing capacity of the disc is largely dependent upon the liquid nature of the gel in the centre. The image to the right shows a cut away of the back bone to reveal the disc and spinal cord. How does disc disease occur? There are breeds such as Dachshunds, Shih Tzu, Lhasa Apso and so on that are called chondrodystrophoid breeds. This means that they have been genetically pre programmed to have shortened limb bones relative to their body (Queen Anne Legs). The same gene that produces this distinctive appearance also causes early dehydration of the gel in the centre of the disc, such that by the time one of these dogs is 12 months old, they have discs that are similar to those of an eight year old German Shepherd (for example). As the centre of the disc is no longer gel like, the shock absorbing nature of the disc is reduced. Now the stress and strain of normal movement is taken by the fibrous capsule. This results in progressive tearing and injury to the fibres and allows the hardened gel to move up towards the spinal cord. How does this result in paralysis? In some dogs, the final barrier between the hardened gel and the spinal cord is breached and the disc material is propelled into the spinal canal and impacts the spinal cord. If this happens very quickly (as is often the case), the small mass of material moving very quickly towards the cord possesses a great amount of energy and this energy is absorbed by the cord producing bruising and injury to the spinal cord. It is this bruising and damage to the spinal cord that causes paralysis. The spinal cord is made up of two types of tissue: White Matter and Gray Matter. In most patients with disc extrusion it is the injury to the White Matter that causes varying degrees of paralysis. The white matter is on the outside of the cord and the grey matter is in the middle. The white matter is like telephone wires allowing communication between the brain and the muscles and joints of the limbs. When white matter is injured (as it always is in this type of disc disease) the result is a break down of communication of varying severity between the brain and the limbs. To continue with the telephone analogy you can think of this as talking to someone on the telephone with interference on the line. If the interference results in you not hearing certain words in a sentence, then it will be difficult to understand what the other person is telling you and it is equally difficult for them to understand you. If the wires are completely damaged then the line will “go dead” and no communication will be possible. The degree of functional loss in your dog’s legs will directly relate to the degree of damage to the white matter (or wires) and the subsequent difficulty in two way communication between brain and legs. Some dogs can walk but appear drunk, others can stand but not walk and some dogs cannot stand at all. In some cases there is impoverished communication between the brain and the bladder. This can result in incontinence. This sign is usually seen in cases that are unable to stand and are thus more severely affected.