You may wonder why we are talking about bone disease. After all, renal disease is about kidneys, not bones, right? Actually, they are related. When the kidneys fail, it affects the bones very much. Chronic Kidney Disease – Mineral and Bone Disorder (CKD-MBD) occurs when the kidneys fail to maintain the proper levels of calcium and phosphorus in the blood, leading to abnormal bone hormone levels. CKD-MBD affects almost all patients receiving dialysis.
Bone changes that occur with CKD-MBD take a long time to appear, so this disease has been called a “silent crippler”. If left untreated, the bones gradually become thin and weak, increasing the chances of breaking a bone. The bones and joints can also become painful.
How does this happen? When the kidneys fail, two things happen. First, they stop filtering out excess phosphorus in the blood, causing phosphorus levels to rise and calcium levels to fall. This causes four small glands in the neck called the parathyroid glands to release a hormone called parathyroid hormone (PTH). PTH draws calcium from the bones into the blood. Over time, this weakens the bones and leads to calcification of arteries and other body tissue. The second thing that happens is that the kidneys stop making active vitamin D hormone (calcitriol) that is received from sunlight and food. The drop in calcitriol causes the body to not be able to absorb calcium from food. Calcitriol and PTH are supposed to work together to keep calcium balanced and the bones healthy. When calcium levels fall because of lack of calcitriol, PTH levels rise and cause bones to lose calcium, so bones weaken. Thus bones are losing calcium since it comes out of the bone, and since it isn’t being replaced through dietary sources, the bones become more and more brittle.
In order to diagnose CKD-MBD, labs are drawn monthly for calcium and phosphorus. PTH levels are drawn every three months. Vitamin D levels are drawn as needed. A bone biopsy may also be performed to see if the bone cells are building normal bone.
CKD-MBD is treated with dietary changes, medications, and dialysis treatments.
Reducing dietary intake of phosphorus is one of the most important steps in preventing bone disease. Almost all foods contain phosphorus, but it is particularly high in processed foods, fast foods, milk, cheese, dried beans, peas, nuts, chocolate, and drinks like cocoa, dark sodas, and beer. These foods need to be limited in order to find success.
Medications called phosphorus binders – such as calcium carbonate (Tums), calcium acetate (PhosLo), sevelamer hydrochloride (Renagel/Renvela), or lanthanum carbonate (Fosrenol) – are prescribed and taken with meals and snacks to bind phosphorus and eliminate it in the bowel rather than letting it get absorbed into the bloodstream.
Increasing dialysis treatment time or flow rate can help control phosphorus too. Be sure not to miss any treatments!
If the kidneys are not making enough calcitriol, a synthetic calcitriol medication (such as Calcitriol, Rocaltrol, Hectorol, or Zemplar) may be prescribed.
A calcium supplement may also be recommended.The drug cinacalcet hydrochloride (Sensipar), lowers PTH levels by imitating calcium’s effects on the parathyroid gland. It causes PTH and calcium levels to drop. Often phosphorus levels will also come down a little with Sensipar use.
If PTH levels cannot be controlled, some or all of the parathyroid glands may need to be removed surgically. This should be viewed as a last resort since it may not solve your bone issues. Removal of the parathyroid gland can also result in a different kind of bone disease, and can make calcium levels drop very low. Low calcium can cause EKG changes, muscle spasms, bone loss, and very low levels can cause seizures. This can also be managed with medication.
In conclusion, the loss of kidney function affects many other parts of the body, including the bones. In this case, the saying “an ounce of prevention is worth a pound of cure” (Benjamin Franklin) totally applies. By being consistent in avoiding phosphorus foods, receiving adequate dialysis, and taking prescribed medications, you can help your bones stay stronger.