Neuropathy

• Neuropathy, a condition associated with diabetes, is nerve damaged caused by hyperglycemia, which is high blood glucose levels. When levels are high, some cats suffer from low potassium levels due to frequent urination also contributing to the weakening of the back legs. Exercise assists the cat's body in reducing the levels. Also, checking glucose levels at home is helpful in beginning to define the problem; however, only a vet should make the final diagnosis.
  
  

Constipation

• Check for signs around the litter box.litter image by Jerome Dancette from Fotolia.com
  
  A common condition in cats with chronic renal failure, constipation causes cats to have issues walking on unsteady and inefficient legs. Other signs include loss of appetite, defecating or urinating right outside the litter box, vomiting and/or dry stools. If the condition goes untreated, a dangerous build up of toxins may result in the cat's system.
  
  

Saddle Thrombus

• Blood clots are dangerous for cats.blood image by Sergey Galushko from Fotolia.com
  
  Also known as a blood clot in the legs, saddle thrombus, another common condition amongst felines with chronic renal failure, particularly affects the rear legs. The blood flow to the legs is halted and the legs become very cold and often completely lame. If the animal is still able to walk, it will have a limp. The condition can be life threatening, so the cat must be brought to the vet immediately.
  
  

Hypertrophic Cardiomyopathy

• Heart issues cause leg problems.big heart little heart image by cat from Fotolia.com
  
  A cat with hypertrophic cardiomyopathy will experience severe heart troubles because the heart muscle thickens and cannot function properly. In addition to labored and rapid breathing, weight and appetite loss and lethargy, weakness or paralysis of the hind legs develop. The sudden onset of weak legs is a serious medical emergency and must be treated immediately.
  
  

Other Potential Issues

• A number of conditions and illnesses cause leg weakness.Virus image by Denis Makarov from Fotolia.com
  
  According to Dr. Mike Richards, a veterinarian on vetinfo.com, rear leg weakness is a symptom of a wide range of diseases, further indicating that a trip to the vet is always the best idea. Other conditions that can potentially cause leg weakness are cancer, toxoplasmosis (parasite infections), feline leukemia, feline immunodeficiency virus (FIV) and feline infectious peritonitis (a viral infection).