Many cats with CRF will also suffer from low levels of potassium, an essential mineral which is required for many purposes including normal muscle function. Low blood potassium (hypokalaemia) results from an inadequate intake of potassium in the diet (through not eating enough) and excessive loss in the urine.
In mild cases, hypokalaemia contributes towards loss of appetite, lethargy and general weakness. In more advanced cases, the muscle weakness is severe; in some cases, the cat will be unable to hold its head up (so-called ventroflexion of the neck).
It also seems that low levels of potassium may directly affect kidney function and contribute to CRF. Potassium supplementation of hypokalaemic cats with kidney failure often results in improved renal function. By contrast, feeding a potassium-restricted acidifying diet to normal cats has been shown to actually induce renal damage.
Hypokalaemic polymyopathy is considered to be the commonest cause of generalised muscle weakness in cats and the majority of these cases are related to CRF. Routine assessment of potassium levels in the blood is therefore recommended in cats with CRF, and supplementation made where necessary.
Non-acidifying, low-protein diets help to maintain potassium concentrations, but in more severe cases, further additional potassium salts may be required. Potassium gluconate is preferable to potassium chloride which is unpalatable and may cause gastrointestinal irritation.
Although low-dose oral potassium supplementation has been advocated for all cats with CRF, a recent study showed no benefit in potassium supplementing cats with normal blood potassium levels.