Type I IVDD is defined by which event?

• A. Chronic bulging of the annulus fibrosus
• B. Insidious bulging of annulus fibrosus
• C. Acute rupture of annulus fibrosus and extrusion of nucleus pulposus into spinal canal
• D. Fibrocartilaginous embolism

Answer: (C)

Type I IVDD is defined by an acute rupture of the annulus fibrosus with extrusion of nucleus pulposus into the spinal canal, causing rapid spinal cord compression. This event is typical in chondrodystrophic dogs, where the nucleus pulposus degenerates and calcifies, so a sudden extrusion can occur with little warning. Because the extrusion happens abruptly, signs appear suddenly and can be severe.

In contrast, disc problems described as chronic bulging of the annulus fibrosus (Type II) involve a gradual, degenerative process leading to slower, progressive signs rather than an acute crisis. Fibrocartilaginous embolism is a different mechanism altogether—vascular in origin and causing spinal cord ischemia rather than disc extrusion.