## Overview of Hypercalciuria (ICD-10: E83.52) Hypercalciuria, defined as excessive calcium excretion in the urine, is a metabolic condition recognized by an elevated 24-hour urinary calcium excretion exceeding 200-300 mg/day (or >4 mg/kg/day in children). While often asymptomatic, its primary clinical significance lies in its strong association with nephrolithiasis, or kidney stone formation. Understanding its pathophysiology, clinical manifestations, diagnostic approach, and management strategies is crucial for preventing recurrent stone formation and mitigating potential long-term complications such as bone demineralization. ### Pathophysiology The mechanisms underlying hypercalciuria are diverse and broadly categorized into three main types, although overlap often exists: * **Absorptive Hypercalciuria:** This is the most common form, characterized by increased intestinal absorption of calcium, leading to elevated serum calcium levels that transiently suppress parathyroid hormone (PTH) secretion. The kidneys, in response to the filtered calcium load, excrete the excess. This can be due to an exaggerated response of intestinal receptors to vitamin D or a primary abnormality in intestinal calcium transport. * **Renal Leak Hypercalciuria:** In this variant, the renal tubules fail to adequately reabsorb filtered calcium, leading to direct renal calcium wasting. This often results in a mild decrease in serum calcium, which stimulates PTH secretion. The elevated PTH then attempts to restore serum calcium by increasing bone resorption and enhancing renal calcium reabsorption (though this is ineffective in the face of the primary tubular defect) and vitamin D activation. * **Resorptive Hypercalciuria:** This type is characterized by increased bone resorption, leading to the release of calcium into the bloodstream and subsequent urinary excretion. This is typically seen in conditions of increased bone turnover, such as primary hyperparathyroidism, thyrotoxicosis, prolonged immobilization, or certain malignancies. In primary hyperparathyroidism, persistently elevated PTH directly enhances bone breakdown and renal calcium reabsorption, but the sheer load of filtered calcium can still lead to hypercalciuria despite the reabsorptive efforts. Idiopathic hypercalciuria, a diagnosis of exclusion, encompasses cases where no identifiable secondary cause is found, representing a significant proportion of patients. Genetic factors also play a role, with various inherited disorders affecting calcium transport proteins or regulatory pathways. ### Clinical Presentation Hypercalciuria is frequently discovered incidentally during evaluation for other conditions, as it often presents without overt symptoms. However, its most significant clinical manifestation is recurrent **nephrolithiasis** (kidney stones), particularly calcium oxalate or calcium phosphate stones. Patients may present with: * **Acute renal colic:** Severe, intermittent flank pain radiating to the groin, often accompanied by nausea, vomiting, and hematuria (gross or microscopic blood in urine). * **Recurrent urinary tract infections (UTIs):** Stones can act as a nidus for bacterial growth. * **Polyuria and polydipsia:** In some cases, chronic hypercalciuria can impair renal concentrating ability, leading to increased urine output and thirst. * **Bone demineralization:** Chronic calcium loss can contribute to osteopenia or osteoporosis, increasing fracture risk, especially in the resorptive and renal leak forms. * **Pediatric considerations:** Children may present with unexplained hematuria, abdominal pain, enuresis, or failure to thrive. ### Diagnostic Criteria Diagnosis relies on a comprehensive evaluation including both blood and 24-hour urine tests: * **24-hour urine collection:** This is the cornerstone of diagnosis, measuring calcium, creatinine, sodium, oxalate, citrate, and uric acid. Hypercalciuria is confirmed by urinary calcium >200-300 mg/24 hours or >4 mg/kg/24 hours. Ratios like calcium/creatinine can also be useful, especially in children. * **Blood tests:** * **Serum calcium:** Typically normal in idiopathic absorptive and renal leak hypercalciuria, but elevated in primary hyperparathyroidism or vitamin D intoxication. * **Serum phosphorus:** Can be low in absorptive hypercalciuria. * **Parathyroid hormone (PTH):** Suppressed in absorptive hypercalciuria, elevated in renal leak hypercalciuria (secondary hyperparathyroidism), and markedly elevated in primary hyperparathyroidism. * **Serum creatinine:** To assess renal function. * **Vitamin D levels:** To rule out vitamin D excess. * **Imaging studies:** Renal ultrasound, KUB X-ray, or computed tomography (CT) scan of the abdomen and pelvis are used to detect kidney stones and assess for hydronephrosis. * **Differential diagnosis:** Distinguishing between the types of hypercalciuria often involves a combination of these tests, sometimes requiring specific calcium loading tests or dietary manipulation. ### Standard of Care Management of hypercalciuria focuses on preventing stone formation, preserving bone health, and addressing underlying causes: * **Dietary and lifestyle modifications:** * **Increased fluid intake:** Crucial for all patients to dilute urine and reduce supersaturation of calcium salts. Aim for >2.5 liters of urine output daily. * **Moderate calcium intake:** Contrary to popular belief, severe calcium restriction is generally not recommended, as it can worsen bone demineralization and increase oxalate absorption, leading to calcium oxalate stones. Aim for adequate dietary calcium (1000-1200 mg/day). * **Low sodium diet:** High sodium intake increases urinary calcium excretion. * **Moderate protein intake:** Excessive animal protein can increase calcium excretion and decrease urinary citrate. * **Avoid excessive oxalate-rich foods:** If hyperoxaluria is present. * **Pharmacological therapy:** * **Thiazide diuretics (e.g., hydrochlorothiazide, chlorthalidone):** These are first-line agents for reducing urinary calcium excretion by increasing renal tubular calcium reabsorption. They are particularly effective in absorptive and renal leak hypercalciuria. * **Potassium citrate:** Used to raise urinary pH and citrate levels, which inhibit stone formation and aggregation, especially for calcium oxalate and uric acid stones. * **Phosphate supplements:** Can be used in specific cases of absorptive hypercalciuria to decrease calcium absorption. * **Bisphosphonates:** May be considered for patients with significant bone loss. * **Treatment of underlying causes:** * **Parathyroidectomy:** For primary hyperparathyroidism. * **Cessation of offending medications:** Discontinuation of loop diuretics, excessive vitamin D, etc. * **Management of granulomatous diseases or malignancies.** * **Monitoring:** Regular follow-up with 24-hour urine collections and imaging studies is essential to assess treatment effectiveness and monitor for recurrence of stones.