Disk herniations have been classified according to the way in which the herniation occurs. Type I herniation (see figure below) refers to a large tear in the annulus allowing a large quantity of nucleus pulposus to escape, usually in an acute and profound way. The majority of this type of herniation occurs in the chondrodystrophic breeds but can be caused in any breed by extreme physical activity or trauma. Type II herniation (see figure below) refers to small partial tears in the annulus fibrosus which allow nuclear material to escape into the annular area resulting in bulges with only occasional actual "escapes" of nuclear material outside the annulus fibrosus. Type II herniations are the most common type seen in fibroid degenerating disks but can occur in chondroid degenerating disks as well. Typically these herniations are more insidious than the type I herniation.
When injury occurs, a chain of metabolic events is set into motion which, if allowed to go unchecked, can result in irreversible damage to the spinal cord. The spinal cord is extremely sensitive to a lack of oxygen and glucose, its only energy sources, which can only be delivered by a viable vascular supply. Consequently, if vascularity is compromised by disruption or occlusion, the spinal cord starts to die. The degree and reversibility of this damage depend on the length of time the spinal cord goes without proper oxygen and glucose delivery and the degree of actual physical derangement of neurologic tissue that occurs. When the spinal cord dies it liquifies. This liquified state is called malacia. Once that occurs, nothing can reverse the process
Proprioceptive (position sense) nerve fibers are the largest and therefore the most susceptible to pressure. Proprioceptive losses can be seen with only slight amounts of pressure applied to the spinal cord. Clinically proprioceptive deficits are recognized as incoordination (ataxia). If proprioceptive losses are the only ones identified on the neurologic examination, the prognosis is usually good and less aggressive therapy is usually adequate.