This could be Chronic Renal Failure (CRF) – the leading cause of death in older cats. The kidneys fail to eliminate waste, the waste accumulates and the cat eventually is poisoned by the waste that the kidneys are unable to filter. It is important to distinguish between chronic and acute renal failure.
Chronic kidney failure is when only 30% of kidney capacity might be functioning, so symptoms might not occur until 70% of renal function is lost. It is a progressive and irreversible deterioration. Acute renal failure has a quick onset and might be due to antifreeze or toxins, infectious diseases and it may be possible to have renal function restored. Diet control, drugs and fluid therapy can help.