Kidney Stone
PKGhatak, MD
Kidneys filter blood and eliminate harmful waste products from the body. The amount of work the kidneys perform is staggering. In a day the kidneys filter 325 liters of the blood and produce 180 liters of glomerular filtrate. The reabsorption of most of the water, sodium, calcium and phosphate and entire glucose load take place in the renal tubules. In the end, 1 to 1.5 liters of urine is produced in a day and the urine contains 1 to 2 meql of sodium out of 2,400 meql filtered, 100 % of sugar (about 162 gm), 95 % of calcium and 75 to 85 % of phosphates are reabsorbed. The waste products are urea, uric acids, creatinine, and creating, nitrate and sulfates. The amount of organic compounds in the urine of an adult on a regular western diet are as follows: urea 30 gm, creatinine 1.4 gm, ammonia 1 gm, uric acid 0.5 gm, protein less than 200 mg, and creatine 100 mg.
The worldwide incidence of kidney stones is increasing. Turkey reports that 14 % of its population experienced at least one attack of renal colic from real stone. In the USA the incidence is 11 % in males and 7 % in females. In the USA North Carolina has the highest reported kidney stones. The peak incidence of kidney stones is between 40 and 45 years of age.
Mechanism of renal stone formation.
Two things have to happen for stone formation in the kidney. The supersaturation of any of these chemicals must take place - calcium, oxalate, uric acid and urea must take place. The second requirement is the presence of some foreign material or denuded cells in the glomerular filtrate around which the crystallization takes place and that grows into a stone.
Conditions favor stone formation.
Limited water intake, high sugar and high salt intake are the important causes in otherwise healthy individuals. A diet containing an excess amount of calcium. phosphate, oxalate, protein and less citrate. Magnesium, uromodulin and pyrophosphate in the filtrate produce an imbalance promoting the precipitation of crystals. Obesity, excessive physical exertion in hot humid conditions, certain medications and heredity are also risk factors. Distal tubular acidosis, hereditary oxalourea and cystinosis are examples of autosomal recessive inheritance. Mendelian dominant inheritance is due to the mutation of 30 genes but much works remain to be completed to define their importance.
Incidence of kidney stones
|
Type |
Children % |
Adult % |
note |
|
Calcium oxalate |
50-60 |
60 - 80 |
Most common |
|
CaPO4 Apatite |
25 |
15 |
pH 6.8 to 7.4 |
|
hydroxyapatite |
|
2 |
Very hard stone |
|
Struvite |
10 |
4 |
Infection and pH >7.8 |
|
Uric acid |
3 |
10 |
Acid urine |
|
Cystine |
5 |
1 |
|
|
Other |
|
2 |
|
Types of Renal Stones:
Kidney stones are classified as Calcium stones, Struvite stones, Uric acid stones and Cystine stones. Calcium stones are calcium oxalate stones, calcium phosphates Apatite and Struvite stones.
Formation of calcium oxalate stone.
In the basement membrane of the thin loop of Henle, calcium oxalate is deposited. It erodes through the basement membrane and accumulates in the subepithelial space renal papilla. This is known as Randell's plaque. Randell's plaque breaks down the cell layer and falls into the lumen. This provides the site for crystallization and stone formation.
Oxalate is present in leafy vegetables, rhubarb, root vegetables - potato, aram (taro), beets and almonds and cashew. Patients with Crohn's disease and colostomy develop fat malabsorption of fat leaving oxalate free in the small intestine for absorption resulting in high serum oxalate and kidney stones. In genetic disorders- primary oxalurea and cystinosis are characterized by repeated sone formation.
Calcium phosphate stone.
Apatite and Struvite.
Tripple calcium stone is called Apatite. When one or more calcium molecules are replaced by Magnesium and rarely by iron molecules the stone is called Struvite.
Calcium phosphate stones account for 15 % of all renal stones.
Calcium and phosphates are essential elements of the body and are present in every living cell. Metabolic processes generate serum calcium and phosphate in addition to food intake. Vitamin and calcium fortification of food also add to it. The current trend to take calcium and vitamin D supplements favors a further increase of these elements. High salt intake and diabetes are risk factors. If water intake lags behind then calcium reaches saturation points and calcium crystal forms.
Struvite stones.
Calcium phosphate stones in the renal pelvis occasionally become infected by the Proteus, Pseudomonas, Klebsiella group of bacteria. These bacteria generate ammonium from urea. Urea is the final product of amino acid metabolism. In alkaline urine, ammonia combines with urea. Ammonia urates stones grow rapidly in the pelvis of the kidney and the stone takes the shape of branched calyxes and these stones are known as Staghorn calculus. Patients with urinary retention require an indwelling catheter, infection by these bacteria produces calcium deposits around the catheter and multiple bladder stones.
Uric Acid Stone.
Purine and pyrimidine are nitrogen bases for the nucleotides which form nucleic acids. The end product of purine metabolism is uric acid in humans, in lower animals the end product is Allantoin. High serum uric acid leads to gout and gouty arthritis. Uric acid is prone to form crystals in the urine when the pH of urine is acidic. Increased breakdown of the nucleus of cancer cells during the treatment of leukemia and lymphoma are a risk factor for uric acid stones.
Cystine stone.
Cystine stones are large stones and stones recur rapidly. This condition is inherited as an autosomal recessive mode. The patients have high blood amino acid cystine. It is filtered in the urine and forms stones. High sodium in the diet accelerates stone formation.
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