Muscle Cramp - Kidney Failure

Muscle cramp is mainly caused by low level of calcium (hypocalcaemia) which is due to excessive accumulation of phosphates (hyperphosphatemia) in the blood that can not be discharged out of the body because of impairment of renal functions and declined glomerular filtration rate (GFR).

In the early stage of renal failure, the calcium and phosphorus in the blood can remain in normal range and there is usually no clinical manifestations. Only in the middle-late stage (GFR<2ml/min) can there be hyperphosphatemia and hypocalcaemia.

The reasons of hypocalcaemia include:

1. Strict restriction on food cause inadequate supply of vitamin D;

2. Due to the lack of 1-hydroxylase in chronic kidney failure patients, the active vitamin D3 can not be produced and this will affect the absorption of calcium;

3. The decreased glomerular filtration rate will cause decreased excretion of phosphorus. The retention of phosphate iron in the blood will increase the serum phosphate and even mild hyperphosphatemia can cause hypocalcaemia so as to maintain their product constant.

In case of acidosis, there is larger proportion of free calcium though the total serum calcium is low. This reduces the neuromuscular irritability and patients can have no muscle cramp even though the blood calcium is low.

In some cases the supplement of calcium can not control muscle cramps and this may not only related to serum calcium but also related to the lesions in the brain neural system.