Kidney disease is fairly common in older cats and seems to be mainly a disease of old age although cats can get kidney damage from untreated bladder infections, dehydration, medications, anaesthesia complications, eating toxic plants like lilies or licking antifreeze. For some reason, antifreeze is attractive to cats so it's vital that they never have access to it. Lilies, especially easter lilies, are deadly for cats.
If a cat is over 12 years old, kidney disease carries a statistic of around one in three, so it is a good idea to monitor an older cat's health regularly. Signs that a cat may have kidney disease are weight loss, drinking more water than usual and dandruff in the coat. They may also vomit often or have a poor appetite. Some vets recommend yearly blood tests to monitor kidney function.
Some cats go downhill very fast, others can be maintained on a renal diet for a long time and others will need supportive treatments regularly, in the form of subcutaneous fluids and antinausea medications. Fluids are given because a cat with kidney damage no longer concentrates urine adequately and thus loses more water than they are drinking. Cats, unlike humans, can't be told to drink more water, so we have to give them extra, to keep the toxin load in the body lower and maintain good hydration.
My cat Earnest was 12 years old when he was diagnosed with kidney disease. What I leaned later was that he had one kidney that was no longer functioning and another that had reduced function. This may have been caused by emergency surgery he had to have when he was 9 when he had a blockage from bladder stones. Although his kidneys tested normal at the time of his surgery, it is possible that he suffered kidney damage at the time of the surgery.
When he was first diagnosed, he was severely dehydrated and was hospitalised and put on a drip. He did not improve much from hospitalisation and when he was discharged, I was told to give him fluids, blood-pressure medication and a renal diet. I was very upset and I fully expected him to die within a few months. Giving fluids was hard to do at first and he didn't like it much. I learned to warm up the fluids and eventually I was able to switch to smaller needles that hurt less. I joined some online support groups, some of which were helpful and supportive but eventually I realised that they were increasingly filled with ignorant, bullying people who believed that they knew more than the vets did. While some vets aren't up to date at treating kidney disease, many are and my vet is great. I got quite annoyed at all the egos, ignorance and vet-bashing and ultimately quit using these groups.
It took a while to stabilise Earnest and there were many ups and downs in the first year and he stopped eating the renal diet so I fed him whatever he wanted to eat. He ultimately lived another four years, despite his kidney disease. I put this down to giving him fluids daily. In the end, his kidneys were still coping but he had reached the end of his life and he stopped eating. It was expensive and time-consuming to care for him but I didn't mind and I was glad that I got to spend the extra time with him. Not all cats cope well with being treated in this fashion but he was an easy-going cat so the daily treatment didn't impact too negatively on his quality of life. I'm not sure that I would do that for all of my cats, though. I believe it's important to consider the cat's needs before your own. When I was part of online communities, I saw many people pushing all kinds of medical treatments on to their cats to keep them alive at all costs and I decided that he didn't deserve excessive invasive medical intervention, only treatment that kept him comfortable and I would only treat him for as long as he seemed to be enjoying his life. He was very special to me and we had a very close bond as I had him from when he was a tiny 4-week old feral kitten.