Paralysis in a cat's back legs can be caused by a slipped disk, a spinal problem or cardiomyopathy. The best way to determine the actual cause, according to Pet MD, is to examine the symptoms that lead to paralysis in the back legs.
A cat that shows signs of pain in the neck, spine or legs could be experiencing a spinal problem which leads to back leg paralysis, also known as paraplegia. A visit to the vet, where blood tests and urine samples can be taken, will help to determine if a spinal issue is causing the paralysis. An x-ray can show if a slipped-disc is pressing against the spinal cord and causing paralysis in the hind legs. The cat could have a bacterial, viral or toxin-related infection.
A more serious cause of back leg paralysis is cardiomyopathy, a form of heart disease in cats, according to Veterinary Medical Clinic. Cardiomyopathy occurs when a cat's heart is unable to pump the proper amount of blood to meet the demands of its body. According to Manhattan Cats, one of the complications of heart disease in a cat is aortic thromboembolism, a blood clot which forms in the heart and travels through the bloodstream. If a cat experiences aortic thromboembolism, it will not be able to move its hind legs.