Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that your girl Pasht is now unable to use her rear legs properly, even though she seems to be able to move both her rear legs and her tail.
Is she able to eliminate urine and stool normally?
She may not be eliminating because she is unable to get into position, or hold herself in position, to go, or if she cannot feel her back legs then she may also not be able to feel a full bladder or have the ability to urinate on her own.
Your cat may have one of a few things occurring.
A physical examination by her veterinarian and diagnostics such as radiographs of her heart and spine, an ultrasound of her heart, as well as cultures of her urine or blood and a complete blood count may help narrow things down.
If she does go outdoors unsupervised trauma to her spine is a possibility but it should leave some external signs, which you don't mention.
Another possibility is diskospondylitis, a bacterial or fungal infection of the intervertebral disks and surrounding vertebrae. It is usually caused by an infection somewhere else that has been spread to the vertebrae by the blood supply. It is a painful condition. Treatment is usually antibiotics or antifungals for at least 8 to 12 weeks, sometimes as long as a year. Some of these cats require surgery to remove as much of the infection as possible and possibly stabilize the spine. Frequent re-evaluations by her veterinarian are necessary. Prognosis depends upon the amount of damage done and the organism causing the infection.
Intervertebral Disc Disease happens when the spongy disc or disc material between the vertebrae slips up and compresses the spinal cord causing pain and weakness or paralysis. It can be treated with anti-inflammatories and rest or surgery depending upon how much function is lost and how much pain the patient is in. Prognosis is much better if the patient's sense of deep pain remains intact and if surgery is done quickly in the cases that require it. It does not usually cause a fever.
Fibrocartilaginous Emboli is a piece of a degenerated intervertebral disc that has broken off and lodged in a small artery or vein near the spinal cord blocking the blood supply to her spinal cord and degeneration of the cord. It is often one sided, only painful the first few minutes to hours, and doesn't cause a fever. Signs don't progress after 24 hours unless another emboli is thrown. Prognosis depends upon how much damage is done initially. Recovery is slow and gradual, the most improvement happens between day 21 and 42, and may not happen at all if her sense of pain perception is lost.
Another possibility is a spinal cord tumor which is compressing the nerves to her rear leg causing weakness. These cats will progressively worsen without treatment. Steroids may help for a while, as they reduce inflammation and swelling, and will shrink some types of tumors for a short time, but eventually symptoms will likely return. Surgery, if the tumor is resectable may be curative.
The most likely cause of her condition is an Aortic Thromboembolism (saddle thrombus). This is a blood clot that formed in the heart, usually secondary to a primary heart disease called cardiomyopathy, or hyperthyroidism causing secondary heart disease, and has lodged in her aorta usually cutting off the circulation to her rear legs. This condition is very painful. It has a higher incidence in males but females can be affected too. Temperature of the affected limbs is sub-normal. Her pads on her rear feet may feel cooler and look darker in color compared to her front foot pads due to poor circulation. Treatment is anti-clotting medication like heparin or low dose aspirin as well as pain medication such as Buprenorphine and treatment of the primary heart disease or hyperthyrpidism that predisposed her to developing a clot. Prognosis is very poor because of the damage done by the clot as well as the underlying heart disease. Recovery may take weeks.
Your girl needs hands on veterinary care as soon as possible. Please let me know if you have any further questions.