Urolithiasis & Upper Urinary Tract Obstruction

Urolithiasis (stone disease) and the obstruction it produces are among the most consistently tested urology themes in NEET PG. This note packs stone chemistry, the imaging hierarchy, the ESWL–URS–PCNL decision matrix, and hydronephrosis grading into an exam-ready format.

Definition & burden

Urolithiasis is the formation of calculi anywhere in the urinary tract (nephrolithiasis = kidney; ureterolithiasis = ureter; cystolithiasis = bladder). Stones cause disease chiefly by obstructing urinary flow, producing renal colic, hydronephrosis, infection and, if neglected, loss of renal function.

Peak incidence is in the 30–50 year age group, with a male:female ratio of ~3:1 (narrowing in recent decades due to obesity in women). Lifetime recurrence is high — roughly 50% within 5–10 years — so metabolic prevention is part of the answer to "definitive management".

High-yield: The single commonest stone overall worldwide and in India is calcium oxalate (~70–80%). The commonest stone in a patient with chronic/recurrent UTI is struvite (triple phosphate). The commonest radiolucent stone is uric acid.

Classification of renal calculi

Stones are classified by composition, which drives radiodensity, urinary pH association and treatment.

Stone type	%	Radiology	Urine pH	Key association	Crystal shape
Calcium oxalate	70–80%	Radio-opaque (densest)	Variable/acidic	Hypercalciuria, hyperoxaluria, idiopathic	Envelope (dihydrate) / dumbbell (monohydrate)
Calcium phosphate	~10%	Radio-opaque	Alkaline (>6.5)	Type 1 (distal) RTA, hyperparathyroidism	Wedge prism
Struvite (Mg-NH₄-PO₄)	10–15%	Radio-opaque (less dense)	Alkaline (>7)	Urease-producing UTI (Proteus, Klebsiella)	Coffin-lid
Uric acid	5–10%	Radiolucent	Acidic (<5.5)	Gout, high purine, myeloproliferative, ileostomy	Rhombic/rosette
Cystine	1–2%	Faintly opaque (ground glass)	Acidic	Cystinuria (autosomal recessive)	Hexagonal (benzene ring)
Xanthine / 2,8-DHA / drug (indinavir)	rare	Radiolucent	—	Xanthinuria, allopurinol overuse, HIV drugs	—

High-yield: Radiodensity order on KUB (most → least opaque): Calcium phosphate ≥ Calcium oxalate > Struvite > Cystine > Uric acid (lucent). Mnemonic for the radiolucent stones: "U-X-I" → Uric acid, Xanthine, Indinavir (also 2,8-dihydroxyadenine). Pure uric acid stones are the classic ones that vanish on plain film but show on CT.

Struvite / staghorn — special status

Struvite stones form only in alkaline urine generated by urease-splitting organisms (urease → urea to ammonia → ↑pH → precipitation of magnesium ammonium phosphate + carbonate apatite). They grow rapidly to fill the pelvicalyceal system as staghorn (coraliform) calculi. Proteus mirabilis is the classic organism (also Klebsiella, Pseudomonas, Ureaplasma); E. coli is NOT urease-producing, a favourite distractor.

High-yield: A staghorn calculus is most often struvite (infection stone); the second commonest cause of a staghorn is cystine. Untreated staghorns → xanthogranulomatous pyelonephritis, urosepsis and renal loss → complete clearance is mandatory (PCNL is treatment of choice).

Etiology & pathophysiology

Stone formation requires urinary supersaturation with the salt, deficiency of inhibitors, and a nidus.

• Supersaturation: low urine volume (dehydration — the universal risk factor), high solute (calcium, oxalate, urate, cystine).
• Inhibitors of crystallisation: citrate (chelates calcium — hence hypocitraturia is a major risk), magnesium, pyrophosphate, Tamm-Horsfall protein, nephrocalcin.
• Nucleation/aggregation: Randall's plaques — subepithelial calcium phosphate deposits at the renal papillae act as anchoring sites for calcium oxalate stones.

Metabolic drivers to recall

• Hypercalciuria (commonest abnormality): absorptive, resorptive (primary hyperparathyroidism), renal-leak.
• Hyperoxaluria: primary (enzyme defect), enteric (fat malabsorption / ileal resection, Crohn's, jejunoileal bypass → unbound fatty acids bind calcium → free oxalate absorbed), dietary.
• Hyperuricosuria: gout, high purine diet, tumour lysis.
• Hypocitraturia: distal (type 1) RTA, chronic diarrhoea, acidosis.
• Cystinuria: defective tubular reabsorption of Cystine, Ornithine, Lysine, Arginine ("COLA").

High-yield: Distal (type 1) RTA produces calcium phosphate stones (alkaline urine + hypocitraturia + nephrocalcinosis). Type 2 (proximal) RTA does NOT form stones. Primary hyperparathyroidism is the classic systemic cause of recurrent calcium stones.

Clinical features — renal/ureteric colic

The hallmark is acute, severe, colicky loin pain radiating to the groin ("loin-to-groin"), with the patient writhing and restless (contrast with peritonitis where the patient lies still).

• Site of pain by stone position: upper ureter → flank/loin; mid-ureter → iliac fossa (mimics appendicitis on right); lower ureter/VUJ → suprapubic, scrotum/labia, tip of penis, with dysuria, frequency, urgency (mimics cystitis).
• Associated: nausea, vomiting (shared coeliac/renal innervation), gross or microscopic haematuria (~85%).
• Examination: renal angle tenderness; usually a soft abdomen. Fever + colic + obstruction = obstructed infected system = urological emergency.

High-yield: Pain in renal colic correlates with acute distension of the collecting system, not stone size — a tiny VUJ stone can be agonising while a large staghorn may be silent. Stones impact at 3 narrowings: pelvi-ureteric junction (PUJ), pelvic brim (crossing of iliac vessels), and vesico-ureteric junction (VUJ — narrowest).

Diagnosis & investigation of choice

Imaging hierarchy

Modality	Role	Notes
Non-contrast CT KUB (NCCT)	Investigation of choice / gold standard	Detects ALL stones incl. radiolucent uric acid; gives size, site, Hounsfield units (density), hydronephrosis; no contrast/allergy risk
Ultrasound (USG KUB)	First line in pregnancy & children; screening	Operator-dependent; misses mid-ureteric stones; shows hydronephrosis
Plain X-ray KUB	Follow-up of known radio-opaque stone; cheap	Misses radiolucent & small stones; bowel gas obscures
IVU/IVP	Largely historical	Shows anatomy + function but needs iodinated contrast; superseded by CT
MR urography	Pregnancy when USG inconclusive	No radiation; poor at showing the stone itself

Diagnostic flow: Suspected renal colic → NCCT KUB (or USG if pregnant/child) → confirm stone size, site, density, degree of obstruction → urinalysis (RBCs, crystals, pH, nitrites) + culture → serum creatinine, calcium, urate → metabolic/24-h urine work-up if recurrent.

High-yield: NCCT KUB is the single best investigation for acute stone disease. In pregnancy, USG is first line (radiation avoidance). Hounsfield units predict ESWL success: >1000 HU stones (and cystine) fragment poorly and favour endoscopic surgery.

Stone analysis & metabolic evaluation

Retrieved/passed stones should undergo chemical/crystallographic analysis — this is the most direct way to target prevention. A 24-hour urine (volume, calcium, oxalate, citrate, urate, cystine, pH, sodium) is done for recurrent/bilateral stones, children, solitary kidney, or strong family history.

Hydronephrosis & upper-tract obstruction

Obstruction → rise in intraluminal pressure → dilatation of the collecting system (hydronephrosis) ± ureter (hydroureter) → tubular atrophy and, if unrelieved, irreversible parenchymal loss.

SFU (Society for Fetal Urology) grading on USG

Grade	Findings
0	No dilatation
1	Mild dilatation of renal pelvis only
2	Dilated pelvis + a few calyces visible
3	Dilated pelvis + all calyces uniformly dilated, normal parenchyma
4	Grade 3 + thinning of renal parenchyma (cortical loss)

High-yield: Grade 4 = parenchymal thinning = the level at which renal function is threatened. Causes of bilateral hydronephrosis point to a sub-vesical cause: bladder outlet obstruction (BPH), posterior urethral valves (boy), retroperitoneal fibrosis, neurogenic bladder. PUJ obstruction is the commonest congenital cause of unilateral hydronephrosis.

Relieving obstruction emergently: an obstructed, infected kidney (fever, pyonephrosis, sepsis, rising creatinine, or solitary obstructed kidney) needs urgent decompression — percutaneous nephrostomy (PCN) or retrograde JJ/DJ ureteric stent — before any definitive stone treatment. Definitive stone removal is deferred until sepsis resolves.

Management

Acute renal colic

1. Analgesia first → NSAIDs (diclofenac/ketorolac) are first-line (reduce ureteric oedema and peristalsis); opioids if NSAID contraindicated. Paracetamol is a safe adjunct/alternative.
2. Antiemetics + IV fluids for vomiting (avoid aggressive over-hydration, which worsens colic).
3. Medical Expulsive Therapy (MET): alpha-blocker (tamsulosin) for distal ureteric stones to aid passage.
4. Treat infection, decompress if obstructed + infected.

High-yield: NSAIDs > opioids for renal colic pain. Stone passage probability depends on size: <5 mm → ~70–90% pass spontaneously; 5–7 mm intermediate; >7–10 mm unlikely → intervention. MET (tamsulosin) most benefits distal ureteric stones 5–10 mm.

Indications for active intervention

Stone unlikely to pass / persistent pain / obstruction with infection / deteriorating renal function / solitary kidney / bilateral obstruction / large or staghorn stones / patient preference or occupation (e.g., pilots).

Choosing the procedure — the core exam matrix

Procedure	Best indication	Avoid / limits
ESWL (extracorporeal shock-wave lithotripsy)	Renal/upper-ureteric stones <2 cm, radio-opaque, low HU	Stone >2 cm, cystine/calcium-oxalate-monohydrate/>1000 HU, pregnancy, bleeding diathesis, untreated UTI, distal obstruction, aortic/renal aneurysm, obesity
Ureteroscopy (URS) ± laser (semi-rigid/flexible/RIRS)	Mid & distal ureteric stones; ESWL failures; pregnancy (safe); bleeding diathesis	Needs anaesthesia; ureteric injury/stricture
PCNL (percutaneous nephrolithotomy)	Large renal stones >2 cm, staghorn, lower-pole >1 cm, ESWL-resistant	Bleeding risk, infection; more invasive
Laparoscopic / open / robotic	Very large/complex, concurrent anatomical correction (e.g., PUJ obstruction)	Rarely needed now

High-yield (NEET PG favourite):

• Renal stone >2 cm or staghorn → PCNL (treatment of choice).
• Renal/upper-ureteric stone <2 cm → ESWL.
• Lower-ureteric stone → ureteroscopy + laser.
• Stone in a pregnant woman needing intervention → ureteroscopy (ESWL & PCNL contraindicated); temporise with JJ stent / PCN if needed.
• Mid-ureteric stones are the hardest for ESWL (overlying pelvic bones/vessels) — favour URS.

Decision flow: Confirmed stone → size & site & HU → <5 mm & no obstruction → conservative + MET** → **persistent/larger → match to ESWL vs URS vs PCNL** → **staghorn/>2 cm → PCNL → post-procedure stone analysis + metabolic prevention.

Composition-specific & preventive management

• Universal: high fluid intake to keep urine output >2.5 L/day, reduce sodium, moderate animal protein, maintain normal dietary calcium (low-calcium diets paradoxically increase oxalate absorption and stones).
• Calcium stones / hypercalciuria: thiazide diuretics (reduce urinary calcium); potassium citrate for hypocitraturia.
• Uric acid stones: urinary alkalinisation with potassium citrate / sodium bicarbonate to pH 6.5–7 ± allopurinol — uric acid stones can be dissolved medically (the classic "no surgery needed" stone).
• Struvite: complete surgical clearance (PCNL) + culture-directed antibiotics; urease inhibitor acetohydroxamic acid as adjunct.
• Cystine: high fluids + alkalinisation + tiopronin or D-penicillamine (thiol binders).

High-yield: Uric acid and cystine stones can be dissolved by urinary alkalinisation; calcium, struvite, oxalate cannot. Over-alkalinisation, however, risks calcium phosphate precipitation.

Complications

• Acute: obstructive AKI, pyonephrosis/urosepsis (life-threatening), perinephric abscess.
• Chronic: recurrent UTI, hydronephrosis → obstructive uropathy and CKD, hypertension.
• Long-standing staghorn: xanthogranulomatous pyelonephritis (enlarged non-functioning kidney, "putty kidney"); rarely squamous cell carcinoma of the renal pelvis (chronic irritation).
• Procedure-related: ESWL — steinstrasse ("stone street" — column of fragments obstructing ureter), perinephric haematoma; PCNL — haemorrhage, pleural injury; URS — ureteric perforation/stricture.

High-yield: Steinstrasse is a column of stone fragments obstructing the ureter after ESWL — classic post-ESWL complication. SCC of renal pelvis is the malignancy linked to chronic stones/infection.

Key differentials

Renal colic must be separated from other acute flank/abdominal pain:

Differential	Distinguishing clue
Acute appendicitis (right)	Migratory pain, peritoneal signs, patient lies still
Biliary colic / cholecystitis	RUQ, post-fatty meal, Murphy sign
Ruptured/leaking AAA (elderly)	Pulsatile mass, hypotension — must exclude before labelling colic
Ectopic pregnancy / ovarian torsion	Female, +βhCG, adnexal findings
Pyelonephritis	Fever, costovertebral tenderness, pyuria, no colicky writhing
Musculoskeletal / herpes zoster	Dermatomal, no haematuria

High-yield: In an elderly man with sudden "renal colic" and haemodynamic instability, rule out leaking abdominal aortic aneurysm before attributing to a stone — a classic clinical trap.

Recently asked / exam angle

• Commonest stone = calcium oxalate; commonest radiolucent = uric acid; staghorn = struvite; organism = Proteus (urease +).
• Investigation of choice = NCCT KUB; USG in pregnancy; Hounsfield units predict ESWL response.
• Treatment matching: >2 cm/staghorn → PCNL; <2 cm renal → ESWL; lower ureter → URS; pregnancy → URS/stent.
• NSAIDs are first-line analgesia, not opioids.
• Cystine crystal = hexagonal; struvite = coffin-lid; calcium oxalate = envelope.
• COLA mnemonic for cystinuria amino acids; distal RTA → calcium phosphate stones.
• Steinstrasse and contraindications to ESWL (pregnancy, bleeding diathesis, UTI, large/hard stone, aneurysm) are recurrent one-liners.
• SFU Grade 4 hydronephrosis = parenchymal thinning; obstructed infected kidney → urgent PCN/stent.

Rapid revision

1. Calcium oxalate = commonest stone overall; uric acid = commonest radiolucent.
2. Staghorn calculus = struvite (Mg-NH₄-PO₄), from urease-positive Proteus; needs PCNL for full clearance.
3. NCCT KUB is the gold-standard investigation; detects even radiolucent stones.
4. USG is first-line imaging in pregnancy and children.
5. NSAIDs (diclofenac/ketorolac) are first-line for renal colic; tamsulosin (MET) helps distal stones pass.
6. Stones <5 mm usually pass spontaneously; >7–10 mm usually need intervention.
7. Treatment matrix: <2 cm renal → ESWL; >2 cm/staghorn → PCNL; lower-ureteric → ureteroscopy + laser.
8. Pregnant patient needing intervention → ureteroscopy (ESWL and PCNL contraindicated).
9. ESWL contraindicated in pregnancy, bleeding diathesis, untreated UTI, distal obstruction, hard/high-HU (>1000) or cystine stones; steinstrasse is its classic complication.
10. Uric acid and cystine stones dissolve with urinary alkalinisation (potassium citrate); calcium and struvite do not.
11. Distal (type 1) RTA → alkaline urine, hypocitraturia → calcium phosphate stones & nephrocalcinosis.
12. Obstructed + infected kidney = emergency → urgent decompression with PCN or JJ stent before definitive stone treatment; SFU Grade 4 hydronephrosis shows parenchymal thinning.