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Nephrolithiasis refers to the formation of stones within the urinary tract, most commonly within the kidney, from which they may pass into the ureter causing an acute obstructive episode, classically termed renal colic.

• Calcium oxalate stones are the most common type overall, associated with hypercalciuria, hyperoxaluria, and low urine volume.

• Uric acid stones, notably radiolucent on plain X-ray unlike calcium containing stones, are associated with a persistently low urine pH, often seen in the context of gout, metabolic syndrome, or high purine intake.

• Struvitestones, associated with chronic urinary tract infection by urease producing organisms such as Proteus species, can grow rapidly to form large staghorn calculi filling the renal pelvis and calyces.

• Cystinestones are rare and reflect an underlying inherited disorder of amino acid transport.

• Renal colic classically presents as sudden onset, severe, colicky flank pain, often radiating anteriorly toward the groin as the stone migrates distally down the ureter, reflecting the shared visceral innervation along this path.

• The pain is characteristically severe enough that patients are unable to find a comfortable position and are often restless, in contrast to the stillness typically seen with peritonitic pain, a useful bedside distinguishing feature.

• Nausea and vomiting frequently accompany the pain. Hematuria, either visible or microscopic, is common, though its absence does not exclude a stone.

• Lower ureteric stones, as they approach the bladder, can cause urinary frequency and urgency, sometimes mimicking a urinary tract infection.

Fever in the context of renal colic is an important red flag, suggesting an obstructed, infected system, which represents a urological emergency given the risk of rapid progression to urosepsis in an obstructed kidney (see Sepsis entry).

• Non-contrast CT of the kidneys, ureters, and bladder (CT KUB) is the investigation of choice, offering high sensitivity and specificity for detecting stones of essentially any composition (with the exception that certain drug induced stones, notably from indinavir, may not be well visualized), and providing information on stone size, location, and the degree of any resulting hydronephrosis.

• Ultrasound is a reasonable first-line alternative, particularly to avoid radiation exposure in pregnancy or in children, though it is less sensitive than CT, particularly for smaller ureteric stones, and is better at detecting hydronephrosis than the stone itself in many cases.

• Urinalysis is performed in every case, both to assess for hematuria and, critically, to look for evidence of infection, since the combination of an obstructing stone and infection constitutes a urological emergency.

• Renal function should be checked, particularly relevant if the stone is bilateral, affecting a single functioning kidney, or if there is any suggestion of pre-existing chronic kidney disease.

• Metabolic workup, including stone analysis of any passed or retrieved stone, serum calcium, uric acid, and, in recurrent stone formers, a 24 hour urine collection assessing calcium, oxalate, citrate, and uric acid excretion, is pursued in patients with recurrent stones, to identify a modifiable underlying metabolic driver.

Differentials include other causes of acute flank pain, including pyelonephritis (see Urinary Tract Infection entry), musculoskeletal pain, and, importantly, ruptured or leaking abdominal aortic aneurysm in an older patient, which can present with a similar pattern of flank pain and should be considered, particularly where the pain is atypical or the patient is hemodynamically unstable.

Analgesia is a priority and should be given promptly, since renal colic is among the most severe pain syndromes encountered in clinical practice. NSAIDs (diclofenac, either oral, rectal, or intramuscular depending on the clinical setting) are considered first line, offering effective analgesia and, through their effect on ureteric smooth muscle and glomerular filtration, some benefit in reducing ureteric spasm, in addition to pure pain relief. Opioids are used as an alternative or adjunct where NSAIDs are contraindicated or insufficient. Antiemetics are given as needed for associated nausea and vomiting.

Most stones smaller than approximately 5 to 6 millimeters will pass spontaneously and can be managed conservatively with analgesia, hydration, and outpatient follow up, alongside straining of urine to catch and later analyze the passed stone where feasible.

• Medical expulsive therapy with an alpha blocker such as tamsulosin is commonly used as an adjunct for stones in this size range, or slightly larger, to relax ureteric smooth muscle and facilitate passage, particularly for more distally located stones.

Larger stones, or those failing to pass with conservative management within a reasonable timeframe, require active intervention.

• Extracorporeal shock wave lithotripsy fragments the stone using externally applied shock waves, allowing the resulting fragments to pass more easily.

• Ureteroscopy, with direct visualization and either laser fragmentation or basket retrieval, is used for stones not amenable to or failing lithotripsy, particularly for ureteric stones.

• Percutaneous nephrolithotomy is reserved for larger renal stones, including staghorn calculi, given the need for a more substantial approach to clear the stone burden.

• An obstructed, infected kidney is a urological emergency and requires urgent decompression, typically with a ureteric stent or percutaneous nephrostomy, alongside broad spectrum antibiotics, following the same urgency principles as any obstructed, infected system (analogous to the approach for cholangitis, see that entry), since definitive stone treatment should be deferred until the infection is controlled and the system decompressed.

Prevention of recurrence is guided by the metabolic workup and stone type where available: increased fluid intake to maintain a high urine output is a universal recommendation regardless of stone type, alongside dietary modification (reduced sodium and animal protein intake, adequate but not excessive dietary calcium) and, where indicated by the metabolic workup, specific pharmacological measures such as thiazide diuretics for hypercalciuria, or allopurinol and urinary alkalinization for uric acid stones.

Referral: urology for any stone requiring active intervention, recurrent stone disease warranting metabolic workup, or any evidence of an obstructed, infected system requiring emergency decompression.