This is indeed a difficult and
fairly complex subject, but I will try to explain it as simply as I can.
When one builds up waste products in the body from protein, one of the
wastes is phosphate. This chemical is found in large amounts in most proteins
and, therefore, needs to excreted in the urine in large amounts. When the
filters aren't functioning, it builds up in the body ( usually when kidney
function is down to one-third of normal or less).
As phosphate builds
up, certain mechanisms work to make calcium go down (most is probably excreted
in the urine) to keep a balance in the two. When the blood calcium gets down, a
gland in your neck called the parathyroid is turned on, producing a chemical
parathyroid hormone which, among other things, releases calcium from bones, in
or to bring the blood calcium back up. However, since the phosphate is
elevated, the calcium is rapidly excreted (since one cannot lower the phosphate
and the calcium and phosphate operate in opposite directions, the calcium must
come down). This complex action causes a steady loss of calcium from the bones
(secondary hyperparathyroidism), thereby weakening them (it's kind of like
removing the walls from girders of the building: after a while it is not
functional).
Another problem deals with Vitamin D. In order to replace
the calcium being lost, one needs to absorb the calcium in what one eats. One
needs Vitamin D, which is activated by sunlight and then made more active,
first by the liver and then by the kidney. Comparably, the Vitamin D one eats
has a function of approximately "one", after the liver it is "two" and after
the kidney it is "one hundred". The activation of Vitamin D occurs in the
tubules (which are defective) and, therefore, the cystinotic does not have
active Vitamin D to absorb calcium from the intestines to replace body losses.
Rickets result from the active Vitamin D deficiency.
How does the
physician stop this progressive renal osteodystophy (bone disease caused by
kidney disease)? Usually one can control this process by giving medication
during meals to bind phosphate, so it not absorbed and the process is stopped
early. Frequently this is not enough, however, and calcium and a Vitamin D
product need tobe given so there is adequate calcium absorbed to be put back
into bones and keep them growing. Without treatment, the bones become very
painful, the patient usually refuses to walk, the bones hurt, and the child
becomes very irritable and might even refuse to be held. The bones become
brittle, with bowing of the legs, widening of the wrists and ankles and
multiple small fractures which occur even without trauma. As a consequence,
there is a slowing or absence of growth.
The fine balance of these
medications and the interplay of calcium, phosphorous and Vitamin D on bones,
kidneys and the intestines unfortunately requires frequent blood testing and
changes in medicine and is a difficult task. In my experience, this needs to be
supervised by a physician experienced and skilled in this area, in order to
prevent the consequences of these metabolic problems.
Sheldon
Orloff, MD Sub-Chief of Renal Medicine/Kaiser Hospitals