Shock wave lithotripsy is the only non-invasive stone treatment: focused shock waves fragment the stone, and the fragments pass spontaneously. It is best for smaller, lower-density stones in favourable anatomy, and its success is highly dependent on stone composition, density, and body habitus.
Mechanism
Relatively weak shock waves are generated externally and focused on the stone, building to sufficient strength only at the target to fragment it. Comminution occurs by six mechanisms: shear stress, squeezing-splitting, acoustic cavitation, superfocusing, spall fracture, and dynamic fracture.
Shock-Wave Generators
| Generator | Notes |
|---|---|
| Electrohydraulic (spark gap) | Effective; largest focal zone; less pain (energy spread over a large skin area). Drawbacks: shot-to-shot pressure variation, short electrode life |
| Electromagnetic | Produces plane or cylindrical shock waves |
| Piezoelectric | Insufficient power to reliably fragment renal stones |
The unmodified Dornier HM3 remains the gold-standard lithotripter; newer machines are more portable with lower anaesthetic requirements but are less effective. Imaging is by fluoroscopy, ultrasound (no radiation but operator-dependent and poor for ureteric stones), or both.
Contraindications
- Obstruction distal to the stone
- Pregnancy
- Uncorrected coagulopathy / bleeding diathesis
- Untreated UTI
- Arterial aneurysm near the stone (renal or aortic)
- Inability to target the stone (skeletal malformation)
Technique and Optimisation
- A slow rate (~60 shocks/min) fragments more effectively and is more protective of the renal vasculature than 120 shocks/min (at the cost of longer treatment time).
- "Ramping up" the energy (pre-treatment with 100–500 low-energy shocks) is renoprotective.
- General anaesthesia reduces stone motion and improves stone-free rates.
- Antibiotic prophylaxis is not required without a UTI (no invasive procedure is performed).
- Routine pre-SWL stenting is not recommended for ureteric stones (no stone-free benefit), though a stent is advised for renal stones >20 mm to aid passage and avoid steinstrasse.
- Alpha-blockers after SWL improve stone-free rates. Percussion, diuresis, and inversion modestly aid fragment passage.
Factors Predicting Failure
- Skin-to-stone distance >10 cm (morbid obesity)
- Stone density ≥1000 HU
- Resistant compositions (most to least resistant): cystine > calcium phosphate (brushite) > calcium oxalate monohydrate > matrix
- Renal anomalies (horseshoe kidney, calyceal diverticulum) and unfavourable lower-pole anatomy (narrow infundibulopelvic angle, narrow infundibulum, long lower-pole calyx)
If initial SWL fails, offer endoscopic therapy; patients with partial clearance may undergo repeat SWL, whereas those with no fragmentation should proceed to endoscopic intervention.
Complications
- Acute renal injury — shock waves rupture vessels and damage tubules (63–85% show some injury within 24 h on the HM3). Aggravated by more shocks, a shorter inter-shock interval, higher voltage, older-generation devices, and extremes of age; mitigated by a slow rate and ramping.
- Subcapsular / perinephric haematoma — risk factors (mnemonic TD COACH): Thrombocytopenia, Diabetes, Coagulopathy, Obesity, Age, Coronary heart disease, and pre-existing Hypertension (the strongest). Most resolve over weeks.
- Pain and cardiac arrhythmias (resolve on stopping); rare injury to adjacent organs (liver, skeletal muscle).
- Late (chronic) changes: accelerated rise in blood pressure, decreased renal function, increased stone recurrence, and induction of brushite stone disease.
Key Exam Points
- Six comminution mechanisms; three generators (electrohydraulic, electromagnetic, piezoelectric); the Dornier HM3 is the gold standard.
- Contraindications: distal obstruction, pregnancy, coagulopathy, untreated UTI, nearby arterial aneurysm, untargetable stone.
- ~60 shocks/min is more effective and more renoprotective than 120/min.
- Most SWL-resistant stones: cystine, brushite (calcium phosphate), and calcium oxalate monohydrate.
- Failure predictors: skin-to-stone distance >10 cm, density ≥1000 HU, and unfavourable lower-pole anatomy.
- Post-SWL haematoma risk follows TD COACH; pre-existing hypertension is the strongest factor.
- No antibiotic prophylaxis is needed without a UTI, and there is no routine pre-stenting for ureteric stones.