Nephrocalcinosis is defined by the excessive accumulation of calcium deposits within the functional tissue of the kidneys, known as the parenchyma. This deposition occurs primarily in the renal tubules and the surrounding interstitial spaces, often localized within the inner part of the kidney called the medulla. It is distinct from nephrolithiasis, or kidney stones, which are mobile, discrete stones that form within the collecting system. Although separate entities, the underlying metabolic factors causing nephrocalcinosis often increase the risk of forming kidney stones, and the two conditions frequently occur together. The presence of calcium salts within the kidney tissue indicates a systemic metabolic disturbance requiring specific investigation and management.
How Calcium Deposits Form in the Kidney
Calcium deposit formation is a direct consequence of a sustained imbalance in the concentration of calcium and other crystal-forming substances in the kidney’s filtering system. This process begins when the concentration of calcium phosphate or calcium oxalate in the tubular fluid exceeds its limit of solubility, leading to the formation of microscopic crystals. These crystals, typically forming in the distal regions of the nephron, adhere to the internal walls of the tubules. They eventually accumulate in the inner medullary interstitium, where they aggregate to form larger deposits.
Two main metabolic disturbances drive this process, often working in combination to promote crystal precipitation. The first is hypercalcemia, an abnormally high level of calcium in the blood, which increases the amount of calcium filtered by the kidney. The second disturbance is hypercalciuria, which is an excessive amount of calcium in the urine, even if blood calcium levels are normal.
Various systemic conditions can precipitate these imbalances, including primary hyperparathyroidism, which elevates blood and urinary calcium levels, and distal renal tubular acidosis (RTA). Certain medications, such as loop diuretics or excessive intake of Vitamin D or calcium supplements, can also lead to the sustained hypercalciuria necessary for the condition to develop. The environment within the kidney tubules, especially an overly alkaline urine pH, further reduces the solubility of calcium phosphate, promoting crystal formation and tissue deposition.
Identifying the Condition
Nephrocalcinosis often develops slowly and may not cause distinct symptoms in its early stages, frequently being discovered incidentally during imaging for an unrelated issue. The symptoms that do emerge are typically a reflection of the underlying metabolic disorder or a complication such as an associated kidney stone.
When the condition progresses, patients may experience flank pain, which can become severe if a kidney stone forms and attempts to pass through the urinary tract. Another common sign is blood in the urine, known as hematuria, which can be visible or microscopic. Impaired kidney function resulting from the tissue calcification can also lead to polyuria, or the need to urinate excessively, and frequent urinary tract infections (UTIs). Increased thirst is also common.
Diagnostic Procedures
Confirming the presence of nephrocalcinosis and determining its underlying cause requires a combination of imaging and laboratory tests. Imaging studies are used to visualize the calcium deposits within the kidney tissue.
Renal ultrasound is often the preferred initial screening tool due to its low cost, lack of radiation exposure, and ability to detect even mild to moderate calcification. The deposits appear as characteristic areas of increased brightness, or echogenicity, typically concentrated in the renal pyramids.
While computed tomography (CT) scans are the most sensitive method for detecting calcification, ultrasound is frequently the preferred choice for pediatric or pregnant patients. A plain abdominal X-ray (KUB) can also show the deposits, but it is less sensitive than ultrasound or CT.
Laboratory analysis is essential to identify the specific metabolic defect driving the calcium precipitation. Blood tests measure serum levels of calcium, phosphate, and parathyroid hormone (PTH) to rule out conditions like hyperparathyroidism or Vitamin D toxicity. The most informative test is the 24-hour urine collection, which measures the daily excretion of key stone-forming and inhibiting factors, including calcium, oxalate, citrate, and total urine volume. The results of this metabolic profile directly guide the selection of appropriate treatment.
Management and Treatment Options
The primary goal of managing nephrocalcinosis is to stabilize the condition by correcting the underlying metabolic disorder, as the existing tissue deposits are largely irreversible. Treatment is highly individualized based on the results of the 24-hour urine collection and serum tests.
A high fluid intake is recommended, aiming for a daily urine output of at least two liters. This high volume dilutes the concentration of calcium and other solutes, thereby reducing the supersaturation that drives crystal formation.
Dietary modifications are also foundational, particularly restricting sodium intake to no more than 2,300 milligrams per day. A low-sodium diet is crucial because excessive sodium intake increases calcium excretion in the urine. Conversely, calcium intake should not be excessively restricted but maintained at recommended levels, typically 1,000 to 1,200 milligrams daily, as low dietary calcium can paradoxically increase the absorption of oxalate, another crystal-forming substance.
Pharmacological intervention often includes thiazide diuretics for patients with hypercalciuria. These medications, such as hydrochlorothiazide, work directly on the distal convoluted tubule of the kidney to increase the reabsorption of calcium back into the bloodstream, thereby lowering the amount of calcium excreted in the urine. The hypocalciuric effect of thiazides is maximized when combined with a low-sodium diet.
Another common medication is potassium citrate, particularly for those with low urinary citrate levels, a condition called hypocitraturia. Citrate is a potent natural inhibitor of calcium crystallization; potassium citrate supplements work by binding to calcium ions in the urine, reducing their activity and simultaneously increasing the urinary pH. Ongoing monitoring of kidney function and urine chemistries is necessary to ensure the treatment is effective and to make adjustments as the underlying metabolic condition changes.