By David A. Zackson, M.D.
Assistant Clinical Professor of Medicine Division of Nephrology
New York Hospital-Cornell Medical Center
Increased urinary dilution (from forced hydration) and strong urinary alkalinization (from oral alkalinizing agents) are two of the most effective methods for the treatment and prevention of cystine kidney stones (calculi). Neither of these modalities reduces the total urinary excretion of cystine; that appears to be relatively stable for each cystinuric individual. Rather, increasing urinary dilution reduces the concentration of urinary cystine (I.e., the amount of cystine contained in each milliliter of urine while the total output of cystine remains unchanged). By contrast, urinary alkalinization increases the solubility of urinary cystine (I.e., the amount of cystine that can be dissolved in each mL of urine without it precipitating out of solution to form cystine crystals, sand, gravel and stones). These therapeutic effects on cystine concentration and solubility are complementary. Not only can they decrease cystine stone formation, but, if applied with sufficient vigor over sufficient time, can even dissolve large cystine calculi--a process accelerated by lithotripsy which cracks cystine calculi and increases their (therapy-exposed) surface area.
The benefits of forced hydration and urinary alkalinization are obtained with wide safety margins. Forced hydration has no serious adverse effects while with urinary alkalinization serious risks are minimal. In treating Cystinuria, a wide safety margin is particularly appreciated because the only other established method of treating Cystinuria is with penicillamine and Thiola, drugs which carry high risks of serious allergy and toxicity. (For simplicity, in this article, the generic drug penicillamine--brand names Cupramine and Depen--will be referred to by their generic appellation. Thiola, the only brand available for the generic variously known as alpha-mercaptoproprionylglycine, tiopronin and MPG, will be referred to by its brand name.) The action of penicillamine and Thiola is to decrease the total urinary excretion of cystine. Since the stone-inhibiting benefits of forced hydration-alkalinization are complementary to the cystine reductions from penicillamine and Thiola, a strongly applied regimen of the former may entirely replace the more toxic agents or at least permit reduction in their dosage. This is of particular importance for patients with proven intolerance of penicillamine or Thiola, or who are in fear of their adverse effects, E.g., patients who already have reduced kidney function, or women who desire pregnancy. In fact, cystinuric women under treatment with penicillamine or Thiola who become pregnant can frequently have these agents discontinued and go through pregnancy treated only by forced hydration and urinary alkalinization--albeit under an extreme burden voiding frequency.
INTENSITIES OF CYSTINURIA
The frequency and severity of kidney-stone formation vary widely in cystinurics. Some patients experience very frequent sand and gravel and form at least two to three cystine stones per year, sometimes huge in size (staghorn calculi). Others form only one or two small calculi over decades. By combining data of 24-hour-urinary cystine excretions, the urine-solubility characteristics of cystine, and clinical experience correlating these parameters with stone formation rates, the following broad classification of Cystinuria intensity can be suggested:
(1) Generally insignificant (I.e., cystine stones rarely form): up to 150 mg/day;
(2) Mild: 150--400 mg/day;
(3) Moderate: 400--800 mg/day;
(4) Moderately severe: 800--1,200 mg/day;
(5) Severe: 1,200--2,000 mg/day;
(6) Very severe: over 2,000 mg/day.
In general, the greater the 24-hour-urine excretion of cystine, the greater the cystine stone problems. Variations from this rule, however, include the following situations:
- Obstruction in the urinary tract can induce intense cystine stone formation behind the obstruction even though the total urinary excretion of cystine may be only modest; obstruction has this stone-inducing effect for all types of kidney stones, not just cystine.
- (2) Without current explanation, some patients with fairly unimpressive urinary cystine concentrations (E.g.,only 200 mg/liter), who in theory should have ceased forming cystine calculi, nevertheless avidly continue to do so. In contrast, some subjects with urinary cystine concentrations as high as 800 mg/liter only rarely experience stone events.
- (3) When numerous retained cystine calculi are present--with an extensive cumulative surface area--percolating high volumes of dilute, strongly alkalinized urine over these stone surfaces dissolves cystine which is then added to the total urinary cystine excretion. Such increments in cystine excretion, however, reflect successful therapy and do not increase the stone formation rate. (However, in such patients it may be impossible to calculate the true baseline urinary cystine excretion until they are rendered stone free, or the regimen of hydration and alkalinization is temporarily discontinued.)
For reasons that are unclear, increased sodium intake can significantly elevate urinary cystine excretion and exacerbate cystine stone formation, in some subjects more than others. Restriction of methionine-containing foods (milk, meat, eggs, etc.) results, in most patients, in a small decrease in total urinary cystine excretion. Usually, the cystine decrement is neither clinically significant nor worth the (considerable) efforts involved; in rapidly growing children the requisite protein restrictions can even lead to nutritional deficiencies. However, because some patients lower their cystine outputs more dramatically than others, advanced protein restriction--with close monitoring of its hoped-for cystine reductions--is worth a brief trial in patients who prove refractory to other measures.
CYSTINE SOLUBILITY IN URINE
It has been observed that if the urinary concentration of cystine is approximately 300 mg/liter, or less, new calculi generally do not arise, nor already formed stones enlarge. If the urinary concentration of cystine is much below 300 mg/liter, calculi may even slowly dissolve (although this usually requires concomitant strong urinary alkalinization.) If the urinary concentration of cystine is above 300 mg/liter, cystine calculi may form, or enlarge--the higher the cystine concentration, the more rapid the process. A urinary concentration of 300 mg/liter may be regarded as the "clinical saturation point" for cystine. The clinical message from this data is that, if at all possible, the urinary concentration of cystine should be kept at, or below, 300 mg/liter. For the rare patients who continue to form cystine calculi at unexpectedly low cystine concentrations (E.g., even as low as 200 mg/liter), therapy should be directed at strong urinary alkalinization and keeping urinary cystine concentrations at even lower levels.
URINARY DILUTION AND FORCED HYDRATION
Clearly, it is not the total cystine excretion which determines whether or not cystine calculi will form but the urinary cystine concentration. E.g., even if the cystine output of a patient were (a huge) 2,000 mg per day, if the concentration of cystine in the urine could be diluted down to 200 mg/liter--I.e., by forcing enough water ingestion to form 10 liters of urine per day--cystine calculi would be unlikely to form. In contrast, if the urinary cystine output of a patient were only 400 mg/day, but only 1 liter of urine per day were formed, the resultant urinary cystine concentration (I.e., 400 mg per liter, or twice that in the previous example) might be sufficient to initiate stone formation.
Since to stop cystine calculi in most patients it is sufficient to dilute urinary cystine concentration down to 300 mg/liter, for a patient with a daily cystine excretion of 600 mg this would mean forcing a 24-hour urine volume of 2 liters (2,000 ml, or about 2 quarts). For a patient with a daily cystine excretion of 1,200 mg, achieving the protective cystine concentration would necessitate a 24-hour urine volume of 4 liters, etc. In such a subject, to achieve the even safer (lower) cystine concentration of 200 mg/liter, a daily urine volume of 6 liters would be required--a formidable undertaking!
For cystinurics who require such huge daily urine volumes, it is advisable to consume at least 2 large glasses of water every 2 hours during the day, with 2 more glasses upon retiring to force voiding at least twice during the night, coupled with ingestion of another glass of water at each nocturnal awakening. It may take several months of patient effort until such an onerous regimen is adapted to--with an ability to readily fall back to sleep being a significant asset in this regard. As a rule of thumb, the urine should be kept "very light in color," approaching that of tap water, making it a habit to observe this aspect with each voiding--and increasing the amount and/or rate of water consumption no matter what it might already be if the urine color remains too dark. (Maintain awareness that coloring dyes in medications, or capsules, may also darken urine.)
INSENSIBLE WATER LOSSES
Protection against cystine stone formation comes not from how much liquid is drunk, but from how much urine is passed. Under different circumstances, different amounts of liquid will be required to achieve protective volumes. Most ingested water is excreted from the body as urine. However, significant additional losses of water from the body occur via sweating, evaporation from skin surfaces, excretion as liquid stool, and as moist air exhaled during respiration. These are called "insensible losses," and can readily mount to 1,500 ml/day--or much more if environmental temperatures are high, or fever, heavy sweating, diarrhea, or deep rapid respirations are present (as during illness.) Thus a cystinuric who requires a daily urine volume of 4,000 mL might readily have to drink 5,500 ml of water per day to reach this goal and perhaps double this amount if insensible water losses are very pronounced (e.g., as in vigorously playing tennis, with heavy associated sweating, during very warm weather.)
POTENTIAL ADVERSE EFFECTS FROM FORCED HYDRATION
Adverse effects from forced hydration are primarily of a nuisance type, but may be very perplexing. Large urine volumes force frequent, highly inconveniencing voidings. Indeed, certain professions may be rendered almost impossible by this aspect, e.g., mailmen, policemen, sterile-gowned surgeons, etc. Commuting via public transportation without adequate bathroom facilities can become nightmarish. Nocturnal voiding forces frequent awakening and some patients cannot fall back to sleep, resulting at times in near ruinous daytime fatigue, with job loss or falling asleep at the wheel. The small bladder capacities of men with prostatism, and women in their last trimester of pregnancy, exact particularly cruel penalties. However, serious medical risks from forced hydration are vanishingly rare. In general, one need not fear drinking "too much water" since normal kidneys can excrete up to 20--22 liters per day of an ingested water load. Rarely, a patient ingesting a large water load who is also on thiazide diuretics, high dosages of nonsteroidal anti inflammatory drugs (e.g., Naprosyn, Advil, Aleve, etc.), or a very-low sodium diet, may retain water and experience a drop in serum sodium with resultant muscle cramps and headache. Patients with advanced renal disease should also observe special precautions. On balance, however, the advantages of forced hydration in preventing cystine stones far outweigh any downside risks especially when the alternatives are a concentrated stone-forming urine, or potentially toxic drugs such as penicillamine or Thiola.
SIGNIFICANCE OF URINARY PH
Urinary pH signifies the degree of acidity or alkalinity of the urine: the lower the number, the more acidic the urine; the higher the number, the more alkaline the urine. Because of limits on the degree of urinary acidity or alkalinity which can be generated by human kidneys, urine pH ranges only from 4.5 to 8.0. As approximate definitions, urine pHs of 4.5--5.5 are regarded as highly acid; 5.5--6.5 as mildly acid to neutral; 6.5--7.0 as neutral to mildly alkaline; and 7.0--8.0 as strongly alkaline. All pH measurements are based upon a logarithmic scale (i.e., a mathematical system of 10 being multiplied upon itself) as in an earthquake-measuring scale. In a logarithmic system, small-appearing differences signify huge mathematical increments: E.g., with a pH increment of only 2 units, a urine pH of 7.0 is one-hundred times more alkaline than a pH of 5.0, and with a further pH increment of only 0.3 units, a urine pH of 7.5 is twice again as alkaline as a urine pH of 7.2.
EFFECTS OF PH ON CYSTINE SOLUBILITY
At urine pHs below 6.5, the solubility of cystine is only 300 mg/liter. At pH 6.5, cystine solubility starts a very slow rise--barely perceptible until pH reaches 7.0 when it has risen to 400 mg/liter. By pH 7.5, however, cystine solubility has climbed to 600 mg/liter and starts to elevate even more rapidly. By pH 8.0 it has climbed to over 1,000 mg/liter, with some graphed results suggesting even more dramatic elevations. (Note: at urine pH 9.0, the solubility of cystine reaches 1,400 mg/liter which would be incredibly useful for dissolving cystine stones. Although urine pH cannot rise above 8.0 via any oral alkalinizing regimen, certain chemicals at pH 9.0--10.0 (solutions of THAM and sodium hydroxide) can be infused directly into the collecting system of the kidneys--where cystine stones collect--via percutaneously introduced catheters. At medical centers specializing in these techniques, such "super alkalinization" can be safely maintained for several days at a time, rapidly dissolving cystine stone fragments.) Cystine calculi dissolve only very slowly when exposed to dilute-alkaline urine because dissolution only occurs from stone surfaces, which are limited. Even though lithotripsy commonly is not successful in breaking up cystine stones so that they pass from the urinary tract, lithotripsy treatments can introduce myriads of cracks into a large cystine stone. This vastly increases the total surface area of the stone over which dilute-alkaline urine can percolate, grossly accelerating the rate of stone dissolution.
Viewing the benefits of urinary alkalinization benefits in reverse, at urine pH 8.0, marked solubilization of cystine is present; at pH 7.5, cystine solubilization is rising rapidly; at pH 7.2, although the solubility increments from alkalinization are measurable, they are of only minimal clinical significance; at pH 6.8--7.0, urinary alkalinization is largely a waste of time for cystinurics. Physicians who are not familiar with the ultra-steep slope of cystine's solubility curve at pH 7.5 may not counsel cystinuric patients correctly about pH matters. One reason for this is that for the much more common (and familiar-to-physicians) condition of uric acid kidney stones, a urine pH of 6.0--6.5 is all that is required for protective solubilization, and a sustained urine pH of 8.0 is deemed undesirable.
pH: To monitor urine pH in cystinurics, a fine grade of urine pH-testing strip paper should be used that can distinguish readings in the 7.0--8.0 range. Nitrazine pH-testing paper has the advantage of being widely available. Its drawbacks, however, include its considerable expense (frequently over $30.00 per roll), and a top pH reading of only 7.5. An inexpensive but high quality urine pH-testing paper, which has proven sufficiently accurate clinically, is available from Micro Essential Laboratories in New York. This pH paper starts its readings at 6.0 and progresses, by increments of 0.2--0.4 pH units, to a top reading of pH 8.0. (Write for: pH paper; range 6.0--8.0; Catalog# 345; Micro Essentials Laboratories, Inc.; 4224 Avenue H; Brooklyn, New York 11210; $12.00 for two rolls. Management requests no telephone orders, please.)
HOW INGESTED ALKALI ALKALINIZES URINE
Alkalinizing agents ("alkali" ) work by neutralizing body-fluid acids, with the excess alkali spilling out through the kidneys to alkalinize the urine. The acids in body fluids arise both from dietary sources (i.e., ingesting acidifying foods, e.g., red meats, or acidic liquids) and from the body's internal metabolism (i.e., cellular activities, e.g., muscle cells during exercise). Alkali can be ingested as alkalinizing foods (e.g., vegetables), ingested as drug store-origin chemicals (e.g., sodium bicarbonate, or Alka-Seltza), produced internally by the kidneys, or "borrowed" from the alkali-rich skeleton.
Both acids and alkali are measured in units termed milliequivalents (mEq). One mEq of any alkali neutralizes one mEq of any acid, and vice versa, whether in a test tube or the body. An average adult produces internally about 70 mEq of acids per day. Since this acid is generally more than is needed to neutralize the body's usual alkali load, the "excess acid" spills into the urine, acidifying it down to a pH 4.5--5.5 Therefore, to strongly alkalinize such urine (raise its pH to 8.0) this individual would have to introduce into his/her body about 70 mEq of alkali, either ingested as alkalinizing food or drug store-origin sodium bicarbonate. (Renal and bone originating alkali are not calculated in this simplified example.) Although such an alkali load would neutralize the body's acid load, it would not yet alkalinize the urine. To do this, the subject would have to ingest slightly more alkali (i.e., in "excess" of what was required to neutralize the acid load) which would then "spill" into the urine, alkalinizing it. The more excess alkali taken, the more rapid the urinary alkalinization, and the higher the resultant urine pH, until the maximum of 8.0 was reached.
The amount of alkali required for this "alkali-acid titration” is not fixed. Rather, it will vary with rates of cellular metabolism, diet (e.g., vegetarian vs. meat-eating), loss of alkaline fluids from the body (e.g., as in alkali-rich diarrhea), or the ingestion of drug store-origin alkali over the preceding several days. Therefore, for a cystinuric to attain the desired degree of urine alkalinity, a "guesstimated" amount of alkali is taken and urine pH measured after about 30--60 minutes (to allow for absorption, metabolic processing and urinary excretion of the "excess" alkali). If the chosen dosage of alkali does not sufficiently elevate urine pH, a larger amount is chosen for the next dose. Frequently, over time, less and less alkali is required to produce the desired degree of urinary alkalinization as alkali stores within the body (probably in the skeleton) progressively build up.
Acetazolamide (Diamox) alkalinizes the urine by a different mechanism: Acetazolamide blocks the kidneys' reabsorption of bicarbonate causing bicarbonate to spill into the urine, strongly alkalinizing it. Acetazolamide also blocks the kidneys' reabsorption of sodium, water and potassium causing large losses of these substances--I.e., acetazolamide is a strong diuretic. Acetazolamide is particularly useful for nocturnal alkalinization of urine in patients who cannot, or do not want to, awaken to maintain forced hydration, and in patients who cannot tolerate high sodium loads E.g., patients with hypertension, edema, or heart problems.
AVAILABLE URINARY ALKALINIZING AGENTS
Urinary alkalinizing agents include:
(1) those that are sodium-based;
(2) those that are potassium-based;
(3) the drug acetazolamide.
Examples of sodium-based alkalinizing agents, and their common dosage forms, include:
(1) sodium bicarbonate tablets, (generic only, 0.324 gm or 3.7 mEq; 0.648 gm or 7.5 mEq);
(2) BICITRA oral solution (sodium citrate and citric acid; each mL is equivalent to 1 mEq of bicarbonate).
Examples of potassium-based alkalinizing agent include:
(1) POLYCITRA-K CRYSTALS (potassium citrate and citric acid, 30 mEq per packet for solution in water);
(2) K-Lyte tablets (potassium citrate-bicarbonate, 25 mEq or 50 mEq per tablet for solution in water);
(3) UROCIT-K (slow-release potassium citrate in a wax-matrix tablet, 5 mEq and 10 mEq per tablet). Diamox (acetazolamide) is available in extended-release caplets of 125, 250 and 500 mg.
REGULATING THE ALKALINIZATION REGIMEN
An average cystinuric patient, under average conditions of diet and exercise, requires about 15--20 mEq of an alkalinizing agent to elevate urine pH to 7.5, perhaps 25--30 mEq to reach 8.0. If this amount of alkalinizing agent does not elevate the urine pH to the requisite range (I.e., probably signifying that the body had a larger-than-expected acid load to be neutralized), a larger dose should be administered at the next chosen dosing interval and/or the interval between dosages should be shortened. After an alkalinizing agent elevates urine pH, that elevation will remain for a highly variable duration (E.g., 15 minutes to 6 hours), depending, again, upon the rate of the body's acid loading from diet and internal metabolic acid production.
For some examples, if the alkalinization target were an 8.0 urine pH for merely a 2 hour period, then a single dose of about 15--20 mEq of the chosen alkalinizing agent would likely be sufficient. If the alkalinization target were a urine pH of 8.0 on an around-the-clock basis, then 25--30 mEq three or four times per day would likely be required.The exemplified dosages are on the light side. If a target urine pH elevation is not achieved, be prepared to increase the dose of alkalinizing agent (perhaps markedly), or shorten dosing intervals, or both. Conversely, if a target urine pH is readily achieved, test if the amount of alkalinizing agent utilized was really necessary by lessening its dose or lengthening dosing intervals, etc. Lastly, recall that the longer, and more frequently, a subject takes alkalinizing agents, the easier alkalinizing the urine becomes (as internal alkaline stores build up).
HOW LONG AND HOW FREQUENT THERAPY?
Although approximate guidelines can be suggested, clinical results are the final judge. If over a 6 to 12 month period a given hydration-alkalinization regimen prevents cystine stone formation, or actually dissolves stones, it is likely sufficient and, if the regimen were burdensome, can perhaps (gingerly) be lightened. If stones are not prevented, or dissolved, the hydration-alkalinization regimen must be intensified, or, if the regimen were already maximized, complementary therapy should probably be added (E.g., penicillamine or Thiola), probably on a permanent basis. E.g., for a patient with mild-moderate-intensity Cystinuria (E.g., cystine excretion of about 400 mg/day), with no cystine calculi, where the goals of therapy are primarily prophylactic, maintaining the hydration-alkalinization regimen for merely 2 to 4 hours per day, or perhaps even every other day, is probably sufficient. (This may be analogized to a prophylactic "Roto-Rooter" treatment.) However, if numerous or large cystine calculi are present, and the goal of hydration-alkalinization is stone dissolution, the regimen should optimally be maintained around-the-clock for months at a time, perhaps even permanently. This would be especially true, E.g., for a woman planning pregnancy, or a stone-forming cystinuric planning a trip abroad to undeveloped regions, both being instances where it would be desirable to enter the endeavor with a stone-free urinary tract.
POTENTIAL ADVERSE EFFECTS FROM URINARY ALKALINIZATION
Serious adverse effects from alkalinizing agents are very uncommon provided some minimal precautions are observed. Minor adverse effects are common but can be minimized by appropriate monitoring and regimen modifications.
POTENTIAL ADVERSE EFFECTS FROM SODIUM PREPARATIONS
Sodium bicarbonate frequently gives rise to increased gas formation in the stomach which may lead to excessive belching and chest discomfort. It can also induce lower intestinal bloating and diarrhea. Because of its buffered characteristics, Bicitra is far less problematic than sodium bicarbonate in terms of gastrointestinal tolerance. (Indeed, it is also far less irritating to the gastrointestinal tract than any of the potassium-based agents.) In patients with heart problems or advanced kidney disease, however, sodium-containing agents can induce sodium retention resulting in edema or lung congestion. Sodium loading can also exacerbate hypertension and induce excessive thirst. And all alkalinizing agents can induce over accumulation of bicarbonate in the body (I.e., metabolic alkalosis) with confusion, cramps, weakness, and symptoms of low blood calcium (muscle spasms)--especially in the face of any dehydration and renal disease. Close physician's guidance is necessary if any of these conditions are present, suspected, or develop.
At sustained high urine pH, starting at about 7.3, and becoming more pronounced at 8.0, calcium phosphate salts (apatite) in the urine are much less soluble. Apatite stones can form in the urinary tract and "ring calcifications," I.e., calcium encrustations can form around foreign bodies including catheters, stents and cystine kidney stones. These complications occur more readily in patients who already have low urine citrate values (urinary citrate helps keeps calcium in solution and helps prevent calcium stones or encrustations from forming or enlarging).
Although apatite stone formation, and ring calcifications, are a theoretical risk from any intense, sustained alkaluria, in our experience we have noted it only in association with sodium-based alkalinizing agents. Although we don't fully understand the basis for this advantage of potassium over sodium-based agents, it may be because potassium citrate elevates protective urinary citrate more than does sodium bicarbonate. Also, sodium loading sweeps calcium out through the kidneys elevating urinary calcium--much more in some subjects than others--while potassium-based alkalinizing agents do not possess this disadvantage. Elevated urinary calcium (hypercalciuria) increases the potential for calcium stones formation, and ring calcifications at high urine pHs. (The action of sodium citrate in this regard is not known; as a sodium-based agent it likely also induces hypercalciuria; but, as a citrate salt, it may elevate protective urinary citrate as much as the potassium citrate preparations.)
To monitor whether intense alkalinization therapy is inducing the complication of calcium kidney stones, or calcium encrustations in the urinary tract, it is wise to obtain X-rays of the abdomen every 1--2 months when starting the regimen, then, if no problem has arisen, less frequently; radiation-free sonographic evaluations, although more expensive, may be substituted. If calcification-type complications do not develop within the first 6 months of intense urinary alkalinization, it is unlikely that they will do so in the future provided the regimen is not significantly altered. If calcium stones, or calcium encrustations, are induced by urinary alkalinization, it is best to switch to a potassium-based alkalinizing agent, if tolerated. If this cannot be effected, one might consider drastically lowering dietary phosphate intake, probably coupled with the use of oral aluminum-based phosphate binders, I.e., a Schorr regimen. The Schorr regimen decreases apatite stone formation in the urinary tract, perhaps by grossly lowering urinary phosphate concentrations, or perhaps by the apatite-inhibiting effects of aluminum being excreted in the urine. Lastly, since sodium loading can increase urinary cystine excretion, in patients with refractory, problematic cystine stones, sodium loading should be avoided unless it has been demonstrated not to significantly elevate urinary cystine.
POTENTIAL ADVERSE EFFECTS FROM POTASSIUM PREPARATIONS
Potassium-based alkalinizing agents also can cause stomach and intestinal gas formation but far less frequently than sodium-based agents. However, potassium agents frequently irritate the gastrointestinal lining causing gastritis, "heartburn," and diarrhea, especially in patients with an underlying G.I. sensitivity. These bothersome GI side effects can be mitigated somewhat by taking the preparations only at mealtimes, or by taking lower dosages at more frequent intervals.
The only potentially serious adverse effect from potassium-based alkalinizing agents is potassium retention (I.e., hyperkalemia.) When marked, or arising suddenly, potassium retention can be fatal. Potassium retention from potassium alkalinizing agents is a risk in the following situations:
(1) Whenever kidney function is very depressed, or when urinary tract obstruction is present;
(2) Rarely, some older diabetics may unexpectedly experience difficulty with excreting potassium;
(3) When a potassium-taking patient is concurrently being treated with drugs that inhibit renal excretion of potassium. These include certain diuretics (E.g.,Dyazide, Aldactone, Midamor), and "ACE inhibitors" (E.g., Vasotec, Capoten, etc.) In this regard, the ACE-inhibitor, captopril (Capoten) is to be particularly feared because it is sometimes used, and in very high dosage, to treat Cystinuria as a substitute for penicillamine and Thiola (in subjects allergic to the latter.) Captopril can be a very effective drug to lower urinary cystine, at least for a few months, but any cystinuric subject taking daily dosages of more than 50 mEq potassium, and more than 50 mg of captopril, should have it proven that a potentially dangerous degree of potassium retention has not occurred. Because peak potassium retention may not occur until 1-2 hours after the alkalinizing agent has been taken, the serum potassium should be checked then, rather than a fasting state preceding medication administration.
(4) Rarely, when a patient taking very high dosages of potassium is also taking very high dosages of a beta blocker (E.g., Inderal, Lopressor, etc.) since beta blockers may retard the rate at which potassium enters systemic cells.
POTENTIAL ADVERSE EFFECTS FROM ACETAZOLAMIDE
Although acetazolamide sounds like an ideal agent, it has major drawbacks in that in chronic use, at high dosage, it causes too much accumulation of acid in the body (metabolic acidosis), markedly lowers (stone-protective) urinary citrate and elevates urinary calcium (from skeletal leaching.) This combination, in the face of urine at pH 8.0, grossly favors the formation of calcium encrustations and calcium stones in the urinary tract. For these reasons, acetazolamide is frequently used only intermittently, during phases of sodium intolerance. In selected individuals, however, it can be excellently tolerated and may prevent a cystinuric patient from requiring penicillamine or Thiola. In our experience, it also greatly aids nocturnal alkalinization and can be a vital force in cystine stone dissolution (when losses of potassium and bicarbonate are judiciously replaced.)
CONCLUSIONS AND SUMMARY
For the treatment and prevention of cystine calculi, a regimen of urinary dilution and urinary alkalinization provides the best combination of efficacy and safety. Since the components of this regimen are complementary in their actions (decreasing urinary cystine concentration and increasing urinary cystine solubility), they should always be used together. Penicillamine and Thiola lower urinary cystine excretion but they possess a much greater risk of toxicity. Since forced hydration and urinary alkalinization are also complementary in purpose to penicillamine and Thiola, they should always be added to a regimen of the latter. Forced hydration and urinary alkalinization may reduce the requirements of penicillamine and Thiola and, in milder cases, even substitute for them. Significant urinary cystine solubility begins at urine pH 7.5 and peaks at 8.0. Therefore, sufficient alkalinizing agent should be given to always reach this target pH range. Intensity of treatment with alkalinizing agents and hydration, should be determined empirically: intermittent and/or low-intensity regimens are likely sufficient for mild cystinurics whereas high intensity, around-the-clock, long-term regimens will likely be required for severe cystinurics. Penicillamine or Thiola should be initiated only after an optimal regimen of forced hydration and urinary alkalinization has proved insufficient. The intensity of Cystinuria can be defined on the basis of 24-hour-urine cystine excretions, with some exceptions. Potassium-based alkalinizing agents are probably superior to sodium-based ones because they do not elevate urinary cystine or urinary calcium, nor at high urine pH as strongly favor the formation of calcium stones and calcium encrustations in the urinary tract. Acetazolamide is a very useful drug for urinary alkalinization, especially for nocturnal use, sodium retainers and rapid stone dissolution. However, because of its unique adverse effects profile it should be reserved for special situations. Focused and vigorous application of forced hydration and urinary alkalinization are a major modality for the treatment and prevention of cystine calculi.
David A. Zackson, M.D.
Medical Treatment & Prevention of Kidney Stones
450 East 69th Street
New York, N.Y., 10021
(212) 744-4400 (Office)
(212) 535-6932 (Fax)