HOW IS THE KIDNEY AFFECTED IN CYSTINOSIS?

In order to understand how kidney function is impaired in cystinosis, one needs to understand how it works normally. There are two kidneys, one on each side in the back. Each kidney is made up of one million nephrons, the working unit of the kidney.
Each nephron has two parts:
     1. a glomerulus or filter.
     2. a tubule, a long collecting duct for the urine.

Blood comes into the kidney to be filtered. Blood consists of :
     1. a water part with various dissolved chemicals, such as salt, sodium bicarbonate, potassium, sugar, calcium, etc. and
     2. protein particles and white and red blood cells, too big to dissolve in the water. As the blood comes through the kidney, the water part of blood and the dissolved chemicals are filtered and start down the long tube.

Along the tube are cells that have hooks that recognize if you need to take back (reabsorb) the water, salt and bicarbonate, etc. If you need more water, it is reabsorbed, and similarly for the other chemicals. If you drink alot, you urinate a lot and if you don't drink as much, you don't urinate as much. In essence, 99% of everything that starts down the tube is reabsorbed, otherwise an adult would have to urinate and, therefore drink 50 gallons per day.

In cystinosis, these hooks don't work normally. For some reason, the cystine crystals deposit in the cells lining the tubules and alter their function. This is a long slow process, but by interfering even a little bit, with the normal mechanisms of control, the balance of chemicals in the body is disturbed tremendously. Fortunately, the kidneys are mainly reserve and medications can control most of the chemical disturbance until 90% of the kidney function is lost.

What happens then? Another function of the kidney is to filter waste products. When you eat protein, some is used to rebuild new cells, ect., and some is not (since your body is not 100% efficient). The excess protein and waste products are usually handled by the kidneys, resulting in the characteristic yellow color and ammonia smell of urine. After progressive renal deterioration, the damaged tubules collapse, causing a backup that destroys the filters (glomeruli), resulting in a loss of the ability to rid the body of normal waste products. The substances appear in the blood as blood urea nitrogen (BUN), or creatinine, and with build up in the body are toxic (poisonous). The long slow buildup is tolerated until approximately 10% of the kidney function remains. At this point, further intervention is necessary to prevent uremic (literally urine in the blood) poisoning. This therapy is called dialysis and cleans the blood of these waste products and usually corrects most of the metabolic disturbances of the body.

The kidney actually also has other functions since it controls water excretion, acid and base excretion, high blood pressure, bone minerals, etc., which need to be discussed at a future time.

At this time, experimental therapy is aimed at using substances to remove the cystine from the cells and, therefore prevent or slow down this relentless and progressive decline of kidney function in cystinosis.


Sheldon Orloff, MD Sub-Chief of Renal Medicine/Kaiser Hospitals

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