Endourology and Urinary Stone Disease
Core concepts
Stone questions test composition, imaging, and the choice of intervention based on size, location and anatomy.
Composition: calcium oxalate (most common), calcium phosphate, uric acid (radiolucent, dissolves with alkalinisation), struvite (infection, staghorn), cystine. Trap: assuming all radiolucent stones are uric acid - cystine stones are also poorly radiopaque.
Imaging: non-contrast CT KUB is the gold standard. Ultrasound is first-line in pregnancy and children. Know that a stone in the distal ureter with hydronephrosis plus fever is an emergency requiring decompression, not definitive stone treatment.
Intervention thresholds: observation for small distal stones (<5-6 mm) with good follow-up; SWL for proximal stones <1-2 cm with favourable anatomy; ureteroscopy for distal and mid-ureteric stones and for stones where SWL has failed; PCNL for stones >2 cm or lower pole stones where SWL is unfavourable. Trap: offering SWL for a 2.5 cm stone or for a lower pole stone with a steep infundibulopelvic angle.
Metabolic work-up: indicated for recurrent, bilateral, paediatric, or unusual stones. Know 24-hour urine parameters and the dietary advice that follows each.
Complications: steinstrasse after SWL, ureteric avulsion, sepsis after PCNL, and the fact that a ureteric stent does not prevent all obstruction.
Worked rule of thumb
A 1.8 cm lower pole stone with a narrow infundibulum: SWL clearance is poor, so ureteroscopy or PCNL is preferred. The trap answer is "SWL first because it is non-invasive" - invasiveness is not the deciding variable; clearance probability is.
Exam traps
- Forgetting that infected obstructed systems need decompression first.
- Confusing uric acid and cystine radiolucency.
- Recommending SWL for large or unfavourably located stones.
- Missing the need for metabolic evaluation in a young recurrent stone former.