Lee Chung Horn
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  CALCIUM DISORDERS

 

RENAL BONE DISEASE

Most people with kidney failure have some degree of renal bone disease. This is because one of the functions of the kidneys is to help make the bones strong and healthy. For strong bones, the kidneys must be able to maintain a healthy balance of various substances such as: calcium, phosphate and vitamin D in the body. Kidney failure results in abnormal levels of these substances, and so leads to renal bone disease.

Development of renal bone disease

Blood tests will reveal abnormal levels of calcium, phosphate and vitamin D very early in patients with kidney failure. The calcium and vitamin D levels will be too low, while the phosphate level will be too high. Normally the calcium and vitamin D levels are not noticed by the patient until after the start of dialysis, but high phosphate levels causes the patient to have itchy skin. Treatment should be started at an early stage to prevent weakening of the bones. Without treatment, renal bone disease can lead to pain in the bones, especially in the back, hips, legs and knees. The weakened bones also become increasingly prone to fracture. Fortunately, early recognition and treatment of renal bone disease means that bone pain and fractures are now uncommon in kidney patients.

Causes of renal bone disease

Most kidney patients will have a combination of the following causes of renal bone disease:

1. Low vitamin D levels in blood - Vitamin D helps the body absorb calcium from foods to strengthen bones. It is found in some foods such as margarine and butter and mostly made by the skin (a process which only occurs if stimulated by sunlight). Unfortunately, vitamin D found in food and made by the skin cannot be used directly by the body and requires the kidneys to transform vitamin D into the active substance.

2. High phosphate levels in the blood - Foods that contain phosphate include dairy products, nuts and meat. Phosphate is also stored in the body in the bones and also present in the blood. Normally kidneys help to keep the right amount of phosphate, but in kidney failure, phosphate builds up in the blood. Normal phosphate levels in the blood are 0.8 to 1.4mmol/l. In kidney patients phosphate levels are high-rising to more than 2.0mmol/l. Unfortunately phosphates are not easily removed by dialysis and it is, therefore, important to watch your diet to make sure the phosphate levels do not rise too high. High levels can cause severe itching which is uncomfortable.

3. Low calcium levels in the blood - Calcium is stored in our bones and is obtained from foods, especially dairy products, eggs and green vegetables. We also have calcium in our blood. The kidneys normally help to keep calcium in the bones by producing the active form of vitamin D and controlling phosphate levels. Phosphate and calcium levels are controlled together in the blood and tend to act like a see-saw - if one rises then the other falls. A high phosphate level in renal failure tends to lead to a low calcium level. Therefore in kidney failure the calcium drains out of the bones and is lost from the body. This leads to a fall in the level of calcium in the blood. Normal calcium blood levels are between 2.2 and 2.6 mmol/l. In kidney patients the level of calcium in the blood may fall below 2.0 mmol/l.

4. Parathyroid hormone and kidney failure - Parathyroid Hormone (PTH) is a substance produced by four tiny glands called the parathyroid glands, which are situated in the front of the neck. The PTH helps to control blood levels of calcium.In kidney failure, the patient's blood calcium can become low. If this happens the PTH can boost it by causing calcium to drain from the bones. Raised PTH levels are therefore common in kidney failure but aggravate renal bone disease.

How is renal bone disease monitored?

The best guide to progress and severity of renal bone disease is the amount of PTH in the blood, which is obtained by a simple blood test. Changes in blood PTH can tell us about what will happen to the bones in the future; the lower the PTH, the better.

As renal bone disease can be present before dialysis is needed, doctors will often measure a patient's blood PTH before dialysis is required. Once a patient is having dialysis, it is usually measured every six months. If the patient has a high blood PTH, treatment is commenced to prevent the problem worsening. High PTH levels can usually be lowered with medication, but occasionally removal of the overactive parathyroid glands is required.

How is renal bone disease treated?

Treatment may be needed for each of the four main causes of renal bone disease.

1. Treatment of low vitamin D levels - Vitamin D treatment is also needed to prevent renal bone disease. The most common form of treatment is with alfacalcidol, which provides the active vitamin D that is lacking and therefore increases blood calcium levels. Peritoneal dialysis patients receive vitamin D in the form of a tablet, and haemodialysis patients receive it either as tablet, or as an injection during dialysis.

2. Treatment of high phosphate levels - Dialysis removes some phosphate from the blood but most renal patients need further treatment to control phosphate levels Phosphate binders such as calcium carbonate need to be taken just before food to be effective. The target blood phosphate level is less than 1.8mmol/l for a patient on dialysis.

3. Treatment of low calcium levels - Patients on dialysis obtain some extra calcium from the dialysis fluid whether on PD or haemodialysis. This happens because there is more calcium in the dialysis fluid than in the blood. Calcium passes from the stronger solution (the dialysis fluid) into the patient's blood (the weaker solution) by a process called diffusion. Sometimes extra calcium from the fluid is not enough. Therefore they are given extra calcium in form of a drug called calcium carbonate. Although calcium carbonate is used to reduce blood phosphate levels, it can also raise blood calcium levels. Blood calcium levels are also raised by treatment with vitamin D.

4. Parathyroidectomy - In most patients, correcting the blood levels of calcium, phosphate and vitamin D is enough to control renal bone disease, and to cause PTH levels to fall. In some patients, the blood PTH levels continue to rise. When this happens the blood calcium tends to rise to above normal (it is usually low in kidney failure), and the blood phosphate is very high. This combination of an extremely high PTH, a high calcium and a very high phosphate level cannot be treated by dialysis and tablets alone. It is then necessary to carry out an operation to remove the parathyroid glands. This operation is called a parathyroidectomy.

If the operation is not performed, the blood vessels can become "furred up" with calcium, the eyes can become red and itchy and the skin can also become affected due to the calcium deposits. A parathyroidectomy is a very effective operation. It returns blood calcium levels to normal and can prevent these complications worsening. After the operation, frequent checks of the blood calcium level are needed as they can fall to a very low level after the operation. It is often necessary for patients to take high doses of calcium carbonate/ or vitamin D after a parathyroidectomy. These can usually be stopped at a later date.
 


 

Copyright of Lee Chung Horn Diabetes & Endocrinology 2009