Renal stone formation and passage during space flight can potentially pose a severe risk to crew member health and safety and could affect mission outcome. Although renal stones are routinely and successfully treated on Earth, the occurrence of these during space flight can prove to be problematic.
Causes and countermeasures
Causes
Several factors contribute to the formation of renal stones in space. Dietary changes, bone metabolism, dehydration, increased salt intake as well as decreased urine volumes and increased urine saturation are all possible causes of renal stone formation. It has been noted that spaceflight-induced changes in urine biochemistry are conducive for stone formation.
Other possible causes include: increased urinary pH history of or predisposition to stone formation inflight food stability preservation of food using salt quantity of onboard water supply limited nutritional sources increased diet of animal protein
The most common renal stone is calcium oxalate and is usually caused by treatable metabolic disorders of hypercalcuria (increased calcium levels in the urine). These stones cause pain with passage and blockage and have been known to recur.
Uric acid stones have similar characteristics as the calcium oxalate stones, but occur much more rarely (approximately 5% of all renal stones). Since they are translucent, they cannot be seen by radiographs.
Struvite stones are generated by infections of urease-containing microorganisms that are capable of hydrolyzing the urea in the urine to carbon dioxide and ammonia. Struvite stones can form when urinary pH rises above 7.2, can fill the renal collection system, and can erode into the renal tissue.
Cystine stones, caused by hereditary cystinuria, begin forming in childhood and can grow large enough to fill the renal collection system.
Calcium phosphate stones, or otherwise called brushite, are caused by high urine pH and a supersaturation of calcium phosphate salt in the urine.
Countermeasures
It is more cost effective to prevent stone formation during a mission than is it to treat it. Increased fluid intake will increase urine volume and dilute the stone-forming salts to below the upper risk levels. Avoiding foods that are high in fat and high in oxalate (nuts, pepper, chocolate, rhubarb, spinach, dark green vegetables, fruits) can help reduce hyperoxaluria (excessive urinary excretion of oxalate). Reducing the amount of meats and other purine-containing foods suppresses hyperuricosuria (increased amounts of uric acid in the urine).
Oral alkali (such as potassium citrate) raises the urine's pH and helps suppress calcium oxalate crystal formation. This also works by binding the calcium to form calcium citrate (a crystal growth and aggregation inhibitor). Studies have also shown an additive effect of potassium citrate. The ingestion of potassium citrate has been associated with increased bone density as well as the prevention of bone loss by providing an alkali load averting the bone resorbing effect of sodium chloride excess. Potassium citrate has also been shown to reduce bone loss in postmenopausal women and also improves calcium balance in patients with distal renal tubular acidosis
On the ground
Bed rest studies are used as ground-based analogs to space flight environments. During these studies, the rate of bone loss and subsequent urine composition are similar to those observed in space. A recent ground-based study tested the effectiveness of potassium-magnesium citrate (similar to the currently used potassium citrate) as a countermeasure for renal stones.
Computer based simulations
The Integrated Medical Model (IMM) group at Glenn Research Center in Ohio has been analyzing and optimizing data gathered on renal stone formation since late 2008.
In space
Since the risk of renal stone formation could, as has, result in the loss of a crew member to a mission, regular testing is conducted. To date, there has been only one inflight instance of renal stone formation (described in detail in Valentin Lebedev's book, Diary of a Cosmonaut: Two hundred eleven days in space).
Skylab
The Skylab missions were the first missions that extended past several days in duration. Testing during these missions showed that calcium excretion increased early in flight and almost exceeded the upper threshold for normal excretion.