The most common cause of a dog's back legs giving out is a herniated or ruptured intervertebral disc, according to the College of Veterinary Medicine at University of Missouri. A condition known as degenerative myelopathy, a slow, progressive disorder that affects the spine in older dogs, also causes weakness and paralysis.
Paralysis in dogs occurs when any part of the animal's nervous system is damaged enough to cause a disruption in the communication between the brain and the legs. Herniated discs in a dog's spine, especially in breeds that are low to the ground with long backs such as the Dachshund and Basset Hound, can cause weakness and paralysis, notes petMD.
Degenerative myelopathy is a disease of the spinal cord that typically appears in older dogs around seven or eight and may cause complete paralysis within six months to a year, explains University of Missouri. It can occur first in one limb, and then appear in the other, until the dog may be forced to drag its hind legs. Degenerative myelopathy can also cause urinary and fecal incontinence. Breeds particularly prone to this disease include the Welsh Corgi, German Shepherd and Boxer. A veterinarian uses diagnostic tools such as X-rays or MRIs to determine if a dog is showing signs of a slipped disc. Other causes of paralysis include injuries, infections, strokes, tumors and cysts.