Hansen type 1 and Hansen type 2 disc degeneration in dogs are two different conditions that share a common factor: they both impact the disc in a dog's spine. The term "degeneration" is indicative of a condition that degenerates over time. Although both conditions impact the discs of a dog's spine, albeit in a different way, it is possible in some cases for both Type I and type II degeneration to occur at the same time. The “acute-on-chronic” episodes may occur when a dog with chronic signs associated with type II protrusion worsens rapidly leading to sudden extrusion and onset of sudden pain.
A Lesson in Anatomy
Dogs, just like humans come equipped with a spinal cord which is protected by several vertebrae. Dogs have seven cervical vertebrae, thirteen thoracic vertebrae, seven lumbar vertebrae, three sacral vertebrae, and several coccygeal (tail) vertebrae.
In between each vertebrae are several intervertebral discs which are meant to provide stability and flexibility to the dog's vertebral column. These discs provide cushioning and act as shock absorbers.
For sake of comparison, each disc can be compared to a jelly donut, with an outer shell (annulus fibrosus) representing the dough and a a middle portion (nucleus pulposus) representing the jelly.
When there is some sort of degeneration affecting these parts, the ability to absorb shock is diminished, leading to what's known as Intervertebral Disc Disease (IVDD). There are two main types of disc degeneration in dogs: Hansen type 1 and Hansen type 2 disc degeneration.
Hansen Type 1 Disc Degeneration in Dogs
Also known as chondroid degeneration, Hansen type 1 disc degeneration can occur in any dog breed of any size, although it mostly affects chondrodystrophoid dogs.
Chondrodystrophoid is simply a medical term used to depict dogs that have been selectively bred to have short legs and an overall stout appearance, as seen a lot in dachshunds and other breeds such as bulldogs, corgis, Pekingese, Shih Tzu, and basset hounds. This condition often affects young, middle-aged dogs between the ages of three and six.
The discs in predisposed dogs generally start deteriorating in the first two years of life with herniations occurring any time thereafter.
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What happens is that the disc overtime undergoes mineralization changes and the inner gelatinous portion starts dehydrating losing its cushioning abilities. Any movement at this point may trigger herniation.
In this condition there is a sudden, massive extrusion of of the inner, gelatinous part of the disc which squeezes out through the annulus fibrosis, impinging on the spinal cord causing bruising. This type of herniation is sudden, causing the abrupt onset of pain, weakness, and possible partial or total paralysis (loss of functionality) of the limbs.
Depending on which disc herniates, dogs may display different signs. For instance, if the discs of the neck are affected, neck pain is the most common symptom. Partial paralysis is less likely in this case due to the fact that there is more space within the spinal canal and therefore compression is less severe.
If the discs of the thoracic and lumbar regions of the spine are affected, then the front legs may shows signs of partial paralysis and a wobbly drunk-like gait may be observed. If the discs of the lumbar back region are affected, then affected dogs may show lameness of the back legs, back pain and even urinary incontinence.
Hansen Type II Disc Degeneration in Dogs
Also known as fibroid degeneration, Hansen type II disc degeneration mostly affects large dog breeds and non-chondrodystrophoid breeds.
This condition is often encountered in German shepherds, Dobermans, Labrador retrievers and Rottweilers, although it can affect any breed of dog. Most affected dogs do well when they are young and active, but start suffering between the ages of six and eight.
In this condition there is slow, chronic protrusion of material into the spinal cord. A protrusion of the annulus fibrosis, (the outer shell of the donut) bulges into the vertebral canal. Also known as a disc bulge, this protrusion takes place when the disc forms an outpouch that can potentially press against the nerves causing spinal cord compression.
A Hansen type II type of disc degeneration has a slower, more gradual onset compared to type 1. It is often accompanied by arthritic changes. Affected dogs show signs of progressive pain, although it may present as well acutely. Signs include yelping, a reluctance to jump, an arched back, low head carriage, holding the neck stiffly, looking with the corner of the eyes, restlessness and panting.
Did you know? Disc herniation affects dogs in a different way than humans and this is grossly due to some differences in anatomy. In humans, the spinal cord ends and splits into nerve roots at a significantly much higher level compared to a dog. A herniated disc in a human's lower back may therefore lead to severe pain and possibly sciatica in one leg, while a dog suffering from a herniated disc in the same location would be unable to walk.