UROLITHIASIS 0094-0143/97 $0.00 + .20

Urinary stone disease has afflicted hu- mankind since antiquity. A bladder stone was found in an Egyptian skeleton more than 7000 years old.8 Urolithiasis is a common disorder affecting 1% to 5% of the population in indus- trialized countries with a lifetime risk of 20% in white men and 5% to 10% in women.37 The recurrence rate without treatment for calcium oxalate renal stones is 10% at 1 year, 35% at 5 years, and 50% at 10 years.53 In the United States, urolithiasis accounted for 0.9% of hos- pital discharges with a mean stay of 3 days, costing 1.83 billion dollars in 1993.4 An under- standing of the mechanism of stone formation helps to manage patients better, thereby sig- nificantly reducing the morbidity and health care costs associated with urolithiasis. In this article the authors discuss the physical basis of stone formation followed by the patho- physiology of the various types of urinary stones.

MECHANISM OF STONE FORMATION

K.C. Balaji, MD, and Mani Menon, MD

PHYSICAL CONCEPTS

Urolithiasis is a consequence of complex physical processes. The major factors are su- persaturation and crystallization, inhibitors, complexors, promotors, and matrix. The se- quence of events leading to urinary stone formation is as follows: saturation + supersaturation + nucleation

+ crystal growth or aggregation -+ crystal retention + stone formation

Saturation, Supersaturation, and Ksp

Most authorities believe that the most im- portant driving force behind stone formation is the state of saturation of urine. When a salt is added to a solvent it dissolves in the solvent until a particular concentration is reached, beyond which no further dissolution is possible. At this point the solvent is said to be saturated with the salt (Fig. 1). If more salt is added it crystallizes in solution provided the temperature and pH are unchanged. The concentration at which saturation is reached and crystallization begins is called the ther- modynamic solublity product (Ksp). This is a constant, equal to the product of the concen- tration of the pure chemical components of the solute at saturation. For example, at phys- iologic pH, calcium phosphate (Ca,[PO,],OH), a common stone salt, exists as follows20:

Ca,(P04)30H = 5Ca' + 3P04-+OH

Ksp = [Ca+ +]5*[P04-]3*[OH-]

where Ksp is the thermodynamic solubility product or the product of the ionic concentra- tion of Ca++, PO-, and OH-. At this point crystals of calcium phosphate form in pure solution.

The situation for crystallization is different in urine. Urine is a complex solution and contains ions that interact with calcium and

From the Division of Urology, Department of Surgery, University of Massachusetts Medical Center, Worcester, Massachusetts

UROLOGIC CLINICS OF NORTH AMERlCA

VOLUME 24 * NUMBER 1 * FEBRUARY 1997 1

2 BALAJI & MENON

Concentration Product

0

Phenomena

Nucleation will occur

Inhibitors generally not effective

Crystal growth will occur

Crystal aggregation will occur

Inhibitors will impede or prevent crystallization

De novo nucleation is very slow

Heterogenous nucleation may occur

Matrix may be involved

Crystals will not form

Existing stones may dissolve

Formation Product

Solubility Product

Figure 1. Effect of modifiers on solutes in urine at different states of saturation. (from Resnick MI, Pak CYC: Physiochemistry of stone formation. In Urolithi- asis: A Medical and Surgical Reference. Philadelphia, WB Saunders, 1990.)

phosphate. Inhibitors and complexing agents allow higher concentration of calcium phos- phate to be held in solution than in pure solvents. Urine is, thus, metastable with re- spect to calcium phosphate. As the concentra- tion of calcium phosphate is increased, a threshold is reached at which urine can hold no more salt in solution. The concentration at which this occurs is the formation product (Kf) of calcium phosphate in urine.

The state of saturation for a specific solute in urine can be estimated by using a variety of techniques. Pak et a133 have developed the term activity product ratio to estimate calcium oxalate and calcium phosphate saturation. EQUIL 93, a software program, has been used by many prominent researchers in urolithiasis to measure the state of saturation.2 Newer programs will be available through the in- ternet.

Temperature and pH always should be specified in a crystallization process. Temper- ature is less of a variable in humans because most reactions take place around 37°C. Varia- tion in pH is clinically important. In the cal- cium phosphate system, as the pH increases more phosphate exists in ionic form, thereby reducing the solubility of calcium phosphate in urine. Clinically, calcium phosphate stones form in alkaline urine. Uric acid stone is an- other good example of effect of pH on uro- lithiasis. The solubility of urate is increased to more than 25 times when pH is changed from 5 to 7. This property is used in treatment of patients with uric acid stones.20

Nucleation

The process by which the earliest crystal nuclei form in pure solutions is called homo-

MECHANISM OF STONE FORMATION 3

geneous nucleation. These nuclei form in a fashion similar to water droplets forming when one breathes out on an extremely cold day. Nuclei form the first crystals that do not dissolve and have a characteristic lattice pattern. In urine, nuclei usually form on ex- isting surfaces, a process called heteroge- neous nucleation. Epithelial cells, urinary casts, red blood cells, and other crystals can act as nucleating foci in urine. The saturation needed for heterogeneous nucleation is much less than for homogenous n~cleation.4~ Once a nucleus is formed, particularly if it is an- chored, crystallization can occur at lower chemical pressures than required for the for- mation of the initial nucleus.

Crystal Formation

Many processes are involved in the forma- tion of crystals that are seen in freshly voided urine. The initial nuclei can grow by the pre- cipitation of additional salt on the lattice framework. This is an inefficient process, if the goal of the process is to form kidney stones. It takes between 5 and 7 minutes for urine to flow from the glomerulus to the col- lecting duct. The earliest site of stone forma- tion in human beings is the papillary duct or the collecting duct tubule, where the diameter is 50 to 200 pm. Finlaysonlo has calculated that it takes between 90 minutes and 1500 years for a crystal nucleus to grow to a diam- eter of 200 pm, this time depending on the state of supersaturation of the urine. Clearly, free-floating nuclei simply pass innocuously into the renal pelvis given these constraints.

Crystal Aggregation

Another useful concept in understanding urolithiasis is crystal aggregation. Once nuclei are formed they bounce apart from each other, float freely, and become kinetically ac- tive. If they remain independent and float freely they are washed away by urine flow. Under certain circumstances, however, these nuclei come in close contact and due to chem- ical or electrical forces can bind to each other, a process called crystal aggregation. Al- though it is impossible for crystal growth alone to give rise to a crystal large enough to occlude the lumen of the collecting duct, aggregates of crystals easily can attain such a size.24 Although crystal growth alone cannot

explain the genesis of urinary calculi, the combination of crystal growth and crystal ag- gregation can. Patients who form stones have larger crystal aggregates in the urine than individuals without stones.

Crystal Retention

Another condition that may lead to calcium oxalate stone formation is crystal retention. In most instances, crystal aggregates are too fragile to occlude a collecting duct long enough to give rise to a stone. It is suggested that crystals should be attached in some way to the epithelial cells to allow them to grow to such a size as to become a kidney stone. If a crystal is retained in the kidney, growth can occur for long periods of time whenever urinary supersaturation or aggregation of new crystals occurs. Many kidney stones have a layered structure suggesting intermit- tent growth most likely during periods of supersaturation. Anatomic abnormalities in the kidney, such as medullary sponge kidney or ureteropelvic junction, or even an increase in crystal epithelial adherence, can lead to crystal retention. Finally, alteration in renal tubular function resulting in increased solute transport into the cells or interstitial spaces can give rise to crystal deposition and re- sulting stone formation.25, 27

Inhibitors

In normal urine, the concentration of cal- cium oxalate is four times more than its solu- bility, and in fact precipitation occurs only when the supersaturation is 7 to 11 times its solubility.6 This is possible because many modifiers of calcium oxalate crystallization are present in the urine. Calcium stone for- mers excrete considerably more calcium and oxalate than normal Many individ- uals who excrete increased calcium and oxa- late do not form stones. Many investigators believe that this is due to inhibitors in the urine. Stone formation may depend on the balance between the saturation and inhibitors in the urine. Robertson et a142 have derived a saturation inhibition ratio that is highly spe- cific in distinguishing stone formers from nor- mal subjects.

Urinary inhibitors may be organic or inor- ganic. Howard et allsa in 1967 showed that urine from normal patients prevented the set-

4 BALAJI & MENON

ting of Portland cement, whereas urine from stone formers did not. They originally postu- lated that the inhibitor substance in the urine was a peptide but subsequently stated that the substance was pho~phocitrate.~~

Urinary inhibitors have been identified for the calcium phosphate and calcium oxalate systems but not for the urate system. Magne- sium, citrate, pyrophosphate, and nephrocal- cin make up most of the inhibitors in the urine for the calcium phosphate crystal sys- tem.I9 Calcium oxalate crystal formation is inhibited by citrate, pyrophosphate, glyco- saminoglycans, RNA fragments, and nephro- calcin with much of inhibition by large molecular weight compounds.12, 22, 32 These in- hibitors inhibit crystal formation at very low concentration. For instance, pyrophosphate at concentration of M can inhibit growth of calcium phosphate It is suggested that these potent inhibitors are adsorbed to the active crystal growth sites where they block further crystal growth and aggregation. RNA fragments increase nucleation but de- crease growth and aggregation. Glycosami- noglycans, like chondroitin sulphate, de- crease crystal aggregation but are less effective against crystal

Nephrocalcin and Tamm-Horsfall protein are urinary glycoproteins that are potent in- hibitors of calcium oxalate monohydrate crys- tal aggregati~n.~~ Nephrocalcin, which is syn- thesized in the proximal tubule and the thick ascending loop of Henle, is a potent inhibi- tor of calcium oxalate monohydrate crystal growth in simple solutions. Urinary nephro- calcin from calcium oxalate stone formers de- creases calcium oxalate monohydrate crystal aggregation 10-fold less than the urinary nephrocalcin from normal subjects. Urinary nephrocalcin from calcium oxalate stone for- mers lacks the y-carboxyglutamic acid, which is normally present in two to three residues per molecule. Another peptide, lithostathine, is a protein that colocalizes with nephrocalcin in the kidney but is immunologically differ- ent.54 Tamm-Horsfall mucoprotein in the urine inhibits crystal aggregation but not growth. It is produced in the thick ascending loop of Henle and distal tubule and is the most potent inhibitor of aggregation identi- fied so far. At equimolar concentrations Tamm-Horsfall protein is 10 times more po- tent than nephr~calcin.'~ Although no quanti- tative difference appears in the urinary excre- tion of Tamm-Horsfall protein between stone formers and normal subjects, Tamm-Horsfall

protein exists in a self-aggregated form in stone formers, reducing its effectiveness as an aggregation inhibitor.

Tamm-Horsfall protein and nephrocalcin are potent inhibitors of calcium oxalate crys- tallization in simple solutions. When inhibi- tion was studied in urine, urinary prothrom- bin fragment 1 (Fl) was identified as the most potent inhibitor.49 These studies are prelimi- nary and the definite role of F1 in crystal inhibition is under investigation.

Uropontin is an aspartic acid-rich protein that shares the N-terminal amino acid se- quences with human osteopontin. It is an im- portant inhibitor of calcium oxalate crystal oxalate growth.44 Osteopontin is produced by mouse kidney cortical cells in culture and is present in the distal renal tubules of stone- forming rats.55 Uropontin and osteopontin are major components of calcium oxalate mono- hydrate stone matrix. Under certain condi- tions these substances may promote crystal adherence to renal epithelial cells. This is thought to be facilitated by the functional Arg-Gly-Asp cell binding sequence.38

a,-Antitrypsin is a serum protein that pro- duces the alpha peak on serum protein elec- trophoresis. a,-Antitrypsin or a related pro- tein has been identified in renal This protein plays an important role in inflamma- tion but is not known to bind to calcium. Its presence in these stones suggests that the crystal may have been in contact with red blood cells during their growth. Blood cells are known to cause crystal adherence.

Complexing Agents

Substances that form soluble complexes with the lattice ions for specific crystals, such as calcium oxalate, are called complexing agents. These agents decrease free ionic activ- ity and thus reduce the level of saturation of the stone-forming substance. Citrate is a po- tent complexor of calcium and reduces the ionic calcium in the urine with consequent reduction in the supersaturation of calcium salt. Citrate exerts its maximum effect at a pH of 6.5. magnesium, a divalent cation, com- plexes oxalate in the calcium oxalate In the calcium phosphate system citrate and magnesium act as complexors and inhibitors. In the calcium oxalate system citrate acts as a complexor and inhibitor, whereas magnesium is a complexing agent only.

MECHANISM OF STONE FORMATION 5

Promotors

Pure promotors of urolithiasis are rare. Cer- tain substances, however, can act as promo- tors at a certain stage of crystal formation and inhibitors at a different stage. For instance, glycosaminoglycans promote crystal nucle- ation but inhibit crystal aggregation and growth.26 Tamm-Horsfall protein, depending on its state of aggregation, may act as a pro- motor or an inhibitor of crystal formation.

Role of Matrix

The presence of noncrystalline organic ma- trix in urinary calculus first was described by Anton Von Heyde in 1684.' The matrix of most urinary calculi is 3% by weight. Rarely, a calculus that is most mostly matrix can be seen, almost invariably in the presence of re- nal infection.8 Cystine stones have 10% ma- trix.28

Chemical analysis of matrix revealed 65% hexosamine and 10% bound water. The ma- trix contains substances similar to uromucoid found in the urine, except that the matrix lacks the 3.5% sialic acid found in the urinary uromucoid. It is suggested that the urinary enzyme sialidase may play a role in cleaving sialic acid from the matrix.8

Boyce et all described an immunologically unique substance from the urinary stones called substance A, distinct from other uro- mucoids in the urine. One third of substance A is carbohydrate and two thirds are protein. This substance was found in all calciferous stones, in kidneys of patients with stone dis- ease, and in the urine of patients who had recent inflammation secondary to infection, infarction, or cancer. Other investigators tried to immunologically identify substance A but instead found three or four other antigens unique to the stone.8 In a more contemporary study, experimental nephrolithiasis in rats was associated with a reduction in the uri- nary excretion of low molecular weight pro- tein and its selective incorporation in the kid- ney stones.*l

Whether matrix is a cause or consequence of urinary stone is undecided. Finlaysonlo suggested that the matrix is nothing more than a coprecipitate with the crystals that form stone. Finlaysonlo later, however, con- cluded that the matrix and stone interaction may be more complex than simple adsorp- tion. The concentric lamellated structure of

organic matrix is similar to renal calculi. Scan- ning microscopic studies of broken calcium oxalate stones have shown fibrous material bridging the stone crystals. These findings suggest that the matrix may be a ground sub- stance for stone formation. Matrix probably originates from the proximal tubule.8 These investigators noted intranephronic calculi in renal tubules in patients with idiopathic cal- cium urolithiasis, which were laminated in structure. The lamination alternated between the matrix and the stone analogous to larger stones. No such microliths, however, were found in patients with struvite, cystine, or uric acid stones.

More recently, Du Toit et a19 suggested that a factor in urolithiasis is an alteration in the excretion of the enzymes urokinase and siali- dase. According to their theory, decreased urokinase and increased sialidase in urine leads to the formation of mineralizable stone matrix. Proteus mirabilis and Escherichia coli decrease urokinase and increase sialidase ac- tivity. Up to one third of patients with cal- cium stones have a history of urinary tract infection, usually associated with E. coli.18 E. coli, a non-urease-producing organism, may cause urolithiasis by producing matrix sub- stances that in turn increase crystal adherence to the epithelium. The sequence of events in urolithiasis and its modifiers are shown in Table 1.

Summary of Physical Aspects of Urolithiasis

A prerequisite for urinary stone formation is urinary crystal formation. For this, urine must be supersaturated with the offending

Table 1. PHYSICAL EVENTS AND MODIFIERS IN UROLlTHlASlS

Physical Events Modifiers ~~

Saturation

Supersaturation Citrate and magnesium Nucleation Glycosaminoglycans Crystal growth Pyrophosphate, magnesium,

citrate, and uropontin Crystal aggregation Tamm-Horsfall protein and

nephrocalcin Crystal retention Anatomic renal abnormalities;

alteration in tubular cell transport (e.g., oxalate or calcium transport); increased crystal epithelial adherence

Concentration of solute, pH, and temperature

6 BALAJI & MENON

salt. This occurs when excretion of the chemi- cals that constitute the crystals increases. The crystallization potential of urine is related not only to the concentration of the compounds in question but also to the presence or ab- sence of other compounds, such as com- plexors, inhibitors, or promotors of the crystal in question (see Fig. 1).

Most urine specimens, particularly if stored, contain crystals; however, most indi- viduals do not form kidney stones. As a group, urinary stone formers excrete larger crystals and crystal aggregates than healthy individuals. Urine from patients with recur- rent calcium oxalate stones tends to have higher calcium and oxalate saturations and lower inhibitors than urine from patients without stones. A mathematically derived saturation-inhibition index discriminates be- tween stone formers and controls with greater than 90% accuracy. Free crystals formed within the renal tubule are unlikely to have the ability to grow large enough to occlude a collecting duct and form a stone if the urine is flowing freely. Crystal retention or aggregation is required for crystals to be transformed to stones. Anatomic abnormali- ties, such as medullary sponge kidney or in- creased crystal adherence to tubule epithe- lium, can cause crystal retention. Bacterial infection may promote calcium oxalate stone formation by increasing mineralizable matrix, which in turn promotes crystal adherence. Finally, altered renal epithelial transport may result in intracellular or interstitial crystalliza- tion. Crystals retained in the kidney can be- come the nucleus for stone formation.

MECHANISM OF FORMATION OF VARIOUS TYPES OF STONES

The various types of urinary stones and their incidence in United States are listed in Table 2. The pattern of urolithiasis shows sig- nificant geographic variation. For instance, close to 90% of urinary stones in India are pure calcium oxalate, whereas only 14% of urinary stones are pure calcium oxalate in Is- rael.*

Calcium Oxalate Stones

Calcium oxalate in pure form or in combi- nation with calcium phosphate is the most common component of urinary stones. The

Table 2. TYPES OF UROLlTHlASlS IN THE UNITED STATES

Forms of Urolithiasis Incidence (W

Pure calcium oxalate stones 33 Mixed calcium oxalate and phosphate 34 Struvite 15 Uric acid 8 Pure calcium phosphate stones 6

Artifacts and others 1

Data from Prien EL Sr, Gershoff SF: Magnesium oxide-pyridox- ine therapy for recurrent calcium oxalate calculi. J Urol 112:509, 1974.

Cystine 3

causes of calcium oxalate stones are as fol- lows:

Idiopathic hypercalciuria Hypercalciuric conditions Low urinary citrate H yperoxalaluria Hyperuricosuria

Idiopathic Hypercalciuria Hypercalciuria in the presence of normal

serum calcium is termed idiopathic hypercalci- uria. Between 30% to 60% of all patients with calcium oxalate kidney stones have increased urinary calcium excretion in the absence of elevated serum calcium. In 1974, Pak et a135 suggested that idiopathic hypercalciuria is of heterogenous origin:

Absorptive hypercalciuria with a primary abnormality of increased calcium absorp- tion

Renal hypercalciuria characterized by pri- mary renal leak of calcium

Resorptive hypercalciuria secondary to in- creased bone demineralization.

Coe and associates, however, eschew these subdivisions and believe that hypercalciuric nephrolithiasis is caused by multiple distur- bances in renal tubular function, a distur- bance in phosphate transport, and accelerated 1,25-dihydroxyvitamin D synthesis resulting in intestinal calcium reabsorption.28

The primary defect in absorptive hypercal- ciuria, inherited as an autosomal dominant trait, is increased passive mucosal absorption of calcium and oxalate in the jejunum. Some investigators restrict this term to patients in whom the intestinal hyperabsorption of cal- cium is primary, whereas others use it to include patients in whom the increase in cal- cium absorption is secondary to alterations

MECHANISM OF STONE FORMATION 7

in 1,25-dihydroxyvitamin D, production and action. In many patients, calcium absorption is mediated through a vitamin D-inde- pendent mechanism, but 1,25-dihydroxyvita- min D levels are elevated in up to 50% of patients with absorptive hypercalciuria. The increased uptake of calcium results in a slight, transient elevation of serum calcium, which then suppresses parathyroid hormone (PTH) secretion. This reduces 1,25-dihydroxyvita- min production and a new steady state is achieved with high-normal serum calcium levels and low-normal PTH and vitamin D levels. The filtered load of calcium is elevated, but the PTH-dependent renal tubular reab- sorption of calcium is inhibited, causing hy- percalciuria (Fig. 2).

In renal hypercalciuria, the underlying ab- normality is a primary renal wasting of cal- cium. Urinary losses of calcium reduce serum calcium levels and cause a secondary eleva- tion in PTH. This results in 1,25-dihydroxy- vitamin D production and an increase in in- testinal calcium absorption. Serum calcium normalizes in the presence of elevated serum PTH and vitamin D. Thus, intestinal calcium absorption is increased in absorptive and re- nal hypercalciuria: the key difference between the two disorders is that parathyroid function is suppressed in absorptive hypercalciuria but stimulated in renal hypercalciuria. The cause of renal calcium leak remains unidenti-

t Serum Pi + t 1 :25 OHD

I

fied. Some patients have elevated serum os- teocalcin levels.27 In others, there is an ante- cedent history of urinary tract infection, but the cause-effect relationship is not clear. Many have low urinary citrate excretion. In some patients, a sodium load induces all the biochemical changes of renal hypercalciuria, whereas in others urinary prostaglandin se- cretion is increased.

A third condition that results in hypercalci- uria is increased bone resorption, usually caused by subtle hyperparathyroidism. These patients have parathyroid adenomas but do not have impressive hypercalcemia.

Idiopathic hypercalciuria occurs in about 10% of normal individuals and in about one half of patients with calcium oxalate renal calculi. A syndrome of renal phosphate leak with elevated vitamin D levels and hypercal- ciuria has been detected in members of a Bedouin tribe. Animal models of renal and absorptive hypercalciuria exist. These obser- vations suggest that idiopathic hypercalciuria may be inherited as an autosomal dominant trait, but do not exclude the possibility of poly- genic control of calcium e~creti0n.l~ In an ex- haustive study by Pak,32 a substantial propor- tion of patients with hypercalciuria also had hyperuricosuria, hyperoxaluria, hypocitratu- ria, or increased urinary sodium excretion. This coexistence of multiple abnormalities suggests that environmental or nutritional

Non-vitamin D factors

I t Jejunal calcium absorption

I +Serum calcium (high normal) I

t PTH I

t Filtered calcium

Urinary calcium excretion

Figure 2. Mechanism of absorptive hypercalciuria. Pi = serum phosphorus; OHD = organic heart disease; PTH = parathyroid hormone. (From Menon M, Krishnan CS: Evaluation and medical management of patient with calcium stone disease. Urol Clin North Am 10:595-615, 1983.)

8 BALAJI & MENON

factors rather than an underlying genetic de- fect may be the cause of hypercalciuria.

Primary Hyperparathyroidism

Primary hyperparathyroidism is caused by a PTH-secreting adenoma of the parathyroid glands. PTH increases serum calcium by the following mechanisms:

PTH stimulates osteoclasts to demineral- ize bone by breaking down the bone crystal apatite. The dissolution of apatite results in the release of calcium and phosphate into the bloodstream. PTH causes calcium reabsorption by the kidneys and decreases renal absorption of phosphates. PTH stimulates production of 1,25-dihy- droxyvitamin D, by the kidneys, which in turn increases intestinal reabsorption of calcium. PTH does not seem to have an direct effect on intestinal calcium ab- ~orption.~

Hypercalcemia causes hypercalciuria, which predisposes to urinary calcium stone formation. Pak et a135 have suggested that increase in urinary calcium crystallization was a result of decreased inhibitors or in- creased promotors. Decreased urinary citrate has been shown in hyperparathyroid patients with renal calculi.'

Hypercalcemia of Nonparathyroid Origin

In an outpatient setting hyperparathyroid- ism is the most common cause of hypercalce- mia, whereas malignancy is the most com- mon cause in an inpatient setting.39 Other common causes of hypercalcemia-causing urinary stone disease include granulomatous diseases, hyperthyroidism, glucocorticoid-in- duced hypercalcemia, pheochromocytoma, immobilization, and thiazide diuretics. These can be distinguished from hyperparathyroid- ism by serum parathormone levels. Serum PTH is elevated in primary hyperparathy- roidism, whereas it is generally lower in other hypercalcemias.

Low Urinary Citrate

Citrate complexes urinary calcium and re- duces its ionic concentrati~n.~~ It inhibits spontaneous and heterogenous nucleation of calcium oxalate crystal. Citrate restores the inhibitory reactivity of Tamm-Horsfall pro-

tein.16 Although mole-for-mole its inhibitory activity is less than other potent inhibitors, it is present at higher concentrations than other inhibitors in urine; therefore, it plays an im- portant physiologic role as an inhibitor. The most important cause of hypocitraturia is metabolic acidosis, which causes increased proximal tubular reabsorption of citrate. Hy- pocitraturia is seen in 15% to 63% of patients with urolithiasis.

Hyperoxaluria

Eighty percent of urinary oxalate is endoge- nous in origin and 10% is dietary in origin. There are several causes of hyperoxaluria: Primary hyperoxaluria, type 1, is caused by deficiency of the enzyme a1anine:glyoxylate aminotransferase in the liver. Primary hyper- oxaluria, type 2, or L-glyceric aciduria is a much rarer variant caused by deficiencies of hepatic enzymes D-glycerate dehydrogenase and glyoxylate reductase, which cause in- crease in urinary oxalate and glycerate excre- tion. Enteric hyperoxaluria occurs in patients with short bowel syndrome or malabsorption. Some patients with idiopathic recurrent cal- cium oxalate urolithiasis have mild hyperoxa- luria or increased transport of oxalate by red blood cells. Hyperoxaluria causes oxalate crystal formation, which combines with uri- nary calcium to form calcium oxalate stones.

Hyperuricosuria

Excessive dietary intake of purine is the most common cause of hyperuric~suria.~ In the absence of gout, serum uric acid is nor- mal. This suggests that such patients may have an abnormality in the renal handling of urate. Uric acid promotes calcium oxalate crystallization by facilitating the formation of nuclei. Well-oriented calcium oxalate crystals are deposited on uric acid crystals when crys- tals of uric acid are added to supersaturated calcium oxalate solution. Sodium hydrogen urate and uric acid crystals can initiate cal- cium oxalate crystal formation in seeded solu- tion; however, sodium hydrogen urate crys- tals are not seen in fresh urine or kidney ~ tones .~

Robertson40 has suggested that sodium urate may produce calcium oxalate stone dis- ease by nullifying the effectiveness of natu- rally occurring inhibitors of calcium oxalate crystal growth. Monosodium urate can ad- sorb to glycosaminoglycans, such as heparin,

MECHANISM OF STONE FORMATION 9

and other naturally occurring urinary macro- molecules, such as glycopeptide, reducing the inhibitory activity of these macromolecules against crystal growth of calcium oxalate.12, 34

Struvite Stones

Struvite stones are composed of magne- sium, ammonium, and phosphate mixed with carbonate. These stones are formed in urine with a pH of greater than 7.2 and ammonia in the urine, produced by urease-producing bacteria. Urease hydrolyzes urea into ammo- nia and carbon dioxide. The pK of ammonia is 9, hence the ammonia formed combines with hydrogen ion from water to form ammo- nium. The carbon dioxide formed by hydroly- sis of urea is hydrated to carbonic acid (pK 4.5), which dissociates to bicarbonate ion and a proton. The reaction is illustrated as fol- lows:

NHZCONH, + H2O - 2NH3 + CO, 2NH3 + H,O ++ 2NH4+ + OH-

CO, + H,O - H + HC03- - H+ + C032-

Thus, hydrolysis of urea produces an acid (carbonic acid) and a base (ammonia). Neu- tralization of the base, however, is incom- plete.

P. rnirabiIis is the most common organism associated with struvite ~alculi.4~ The other mechanism by which bacterial infection can induce stone formation is by increased crystal adherence. Parsons et a136 showed that ammo- nia damages the glycosaminoglycan layer lining the bladder mucosa, which in turn increases bacterial adherence. Damage to the glycosaminoglycan layer also causes in- creased adherence of struvite crystals in the bladder.l3 Its possible that bacterial infection in the renal pelvis causes glycosaminoglycan layer damage, thereby facilitating bacterial adherence, tissue inflammation, production of organic matrix, and crystal matrix interac- tion.

Uric Acid Stones

The principal cause of uric acid crystalliza- tion is the supersaturation of urine with re- spect to undissociated uric acid." There is no known inhibitor of uric acid crystallization. Prolonged periods of acidity in the urine pre- dispose to uric acid stones. The most common

finding associated with uric acid stones is not hyperuricosuria (as defined conventionally) but increased urinary acidity. The mean uri- nary pH in patients with uric acid stones is 5.5, and the pH of the first morning urine sample is below 5.7 in 90% of patients.15 These values are lower than in normal subjects or in patients with calcium oxalate urolithiasis.

Patients with gout or uric acid stones ex- crete relatively less ammonium and more ti- tratable acid than do normal subjects. The exact cause, however, of decreased ammonia in the urine of these patients is unclear. These patients also suffer from increased produc- tion of uric acid and impaired renal excretion of uric acid.43, 56 The increased uric acid pro- duction may be due to increased ingestion of purines. Although the exact cause of uric acid overproduction is unclear, it is not due to an enzyme defect in uric acid synthesis. A combination of persistent acidic urine, uri- nary uric acid supersaturation, and low urine volume creates an ideal environment for uric acid crystallization.

Pure Calcium Phosphate Stones

Although calcium phosphate is commonly found in association with oxalate in stones, pure calcium phosphate stones are rare. Cal- cium phosphate stones occur only when the chemical pressure for crystallization is high, and thus they are usually seen in very active stone disease.51 Pure calcium phosphate stones almost always are associated with re- nal tubular acidification defects. When the kidneys lose some of their ability to lower urinary pH, the resulting higher pH increases the divalent and trivalent forms of phosphate, which causes calcium phosphate supersatura- tion.

Cystine Stones

Cystine stones occur only in patients with cystinuria. Cystinuria is an autosomal reces- sive disorder of transmembrane cystine trans- port manifested in the intestine and in the kidneys.50 Cystine is the least soluble of natu- rally occurring amino acids and its solubility is increased from 300 mg/L at a pH of 5, to 400 mg/L at a pH of 7, and greater than 1000 mg/L at a pH of 9. Cystine is poorly soluble at physiologic urinary pH, hence cystine stones are formed in patients with cystinuria.

10 BALAJI & MENON

Miscellaneous Stones References

Rarer stones include dihyroxyadenine stones, xanthine stones, triamterene stones, silica calculi, matrix calculi, and ammonium uric acid calculi. Dihyroxyadenine and xan- thine stones are secondary to inborn errors of metabolism due to deficiency of the enzymes adenine phophoribosyltransferase and xan- thine oxidase, respectively. In addition xan- thine stones can also form in patients with Lesch-Nyhan syndrome and in patients treated with large doses of allopurinol. Pa- tients treated with the potassium-sparing di- uretic triamterene excrete about 70% of the orally administered dose in their urine. They can form pure triamterene stones or stones mixed with calcium oxalate or uric acid. Silica stones are extremely rare in humans and are due to excessive ingestion of silica-containing antacids. Matrix calculi are found in patients with urease-producing infection. The ammo- nium uric acid calculi form in the presence of urealytic infection in addition to increased urinary uric acid, low urinary phosphate, and decreased fluid intake in children of devel- oping countries.

CONCLUSION

Urolithiasis causes significant morbidity to patients in addition to adding substantially to health care costs. Remarkable advances have been made in understanding urolithiasis dur- ing the last 200 years, yet clinicians are far from curing patients of their urinary stone disease. The basis of urolithiasis is crystal formation, retention, and growth. When the urine is supersaturated with a stone-forming substance, it forms crystals in the presence of other favorable factors, namely pH, promo- tors, complexors, and lack of inhibitors. Once formed, these crystals can aggregate or grow to form larger particles and eventually a stone. Urine is a complex solution and suc- cessful stone formation depends on a multi- tude of factors. This explains the varying sus- ceptibility of patients to urolithiasis.

An analogy can be drawn between the uri- nary stone formation and intravascular clot- ting. Virchow's triad describes the three fac- tors involved in intravascular clotting: (1) alteration in the streamlined flow of blood, (2) endothelial damage, and (3) hypercoagulable state of blood. Similarly, alteration in urinary flow, epithelial damage to the lining of the urinary tract, and a decreased saturation inhi- bition ratio of urinary solutes can lead to urolithiasis.

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