Urinary Tract Calculi II: Pathophysiology and Clinical Manifestations

Kidney stones form when urine becomes supersaturated with stone-forming substances like calcium, oxalate, and uric acid that exceed the urine's ability to dissolve them. Low urine volume from inadequate fluid intake increases mineral concentration, promoting crystal formation. Changes in urine pH also influence stone formation, as solubility of these substances varies with pH levels, facilitating nucleation and crystal growth into larger stones.

Urine pH directly influences which types of stones form. High pH promotes calcium phosphate stone formation, while low pH favors uric acid stones. The solubility of stone-forming substances varies with pH; some substances are more likely to crystallize under acidic conditions, whereas others form stones in alkaline conditions. This pH-dependent solubility is a key factor in determining stone composition and formation risk.

Pain location varies by stone position. Renal pelvis stones cause deep, aching costovertebral pain with hematuria. Ureteral stones produce severe, colicky pain radiating to the thigh and genital area with strong urge to void. Bladder stones cause irritation and may obstruct the bladder neck, leading to urinary retention. Renal colic presents as acute pain with nausea and vomiting, reflecting the stone's position and degree of obstruction.

Certain substances inhibit stone formation by reducing available stone-forming minerals. Citrate, for example, binds with calcium, decreasing the amount available to form calcium oxalate stones. These inhibitor substances work by lowering the concentration of free stone-forming constituents in urine, thereby reducing supersaturation and crystal aggregation. Understanding these inhibitors is important for developing prevention strategies.

When stones reach the lower urinary tract, patients experience increased urinary urgency and frequency, dysuria (painful urination), and passage of blood-stained urine. Hematuria may be visible or detected microscopically due to stone irritation of the urinary tract lining. These symptoms differ from upper tract manifestations and signal that the stone has progressed distally, requiring prompt assessment and potential intervention.

Low urine volume, often resulting from inadequate fluid intake, increases the concentration of stone-forming minerals in the urine. This elevated concentration raises the risk of crystals forming and aggregating into stones. Decreased urine volume directly reduces the dilution capacity of urine, making supersaturation more likely and accelerating the nucleation and growth of crystalline structures into larger calculi.

Severe kidney stone obstruction can trigger nausea and vomiting, reflecting the body's response to intense pain and urinary tract irritation. If infection develops, patients may experience fever and chills, and urine may become cloudy or foul-smelling. Large stones causing complete obstruction may lead to urinary retention and incomplete bladder emptying. These systemic manifestations indicate the need for urgent medical management.