Are cysteine stones amenable to shockwave lithotripsy?

Efficacy of Shockwave Lithotripsy for Cystine Stones

Cystine stones are generally resistant to shockwave lithotripsy (SWL) and alternative treatment modalities such as ureteroscopy or percutaneous nephrolithotomy should be considered as first-line therapy for these stones. 1

Characteristics of Cystine Stones and SWL Response

• Cystine stones have structural characteristics that contribute to decreased SWL fragility, making them among the hardest stones to fragment with shockwave therapy 1
• In vitro studies demonstrate that cystine stones require significantly more shock waves for complete comminution (5937 ± 6190 per gram) compared to other stone types such as uric acid (400 ± 333 per gram) or calcium oxalate monohydrate (965 ± 900 per gram) 2
• Cystine stones may be barely opaque on standard imaging or fluoroscopy, potentially compromising shock-wave focusing and treatment efficacy 1

Predictors of SWL Success for Cystine Stones

• Stone morphology may predict SWL outcomes - cystine stones with rough-appearing external surfaces on plain film imaging are more amenable to fragmentation with shock-wave energy than those with smooth contours 1
• CT attenuation coefficients correlate with SWL success - smooth-type cystine stones have significantly higher attenuation values and are more resistant to shock-wave fragmentation 1
• Stone size is a critical factor - small cystine stones (<10mm) may respond to SWL, while larger stones show poor stone-free rates 1

Treatment Recommendations

• SWL monotherapy should not be used for patients with staghorn or partial staghorn cystine stones due to poor stone-free rates 1
• For small cystine stones (<10mm), SWL may be considered but with lower expectations for success compared to other stone compositions 1, 3
• Technology currently utilized for intracorporeal lithotripsy during ureteroscopy (URS), including holmium laser, ultrasonic, and pneumatic devices, can readily fragment cystine stones and should be considered as primary treatment options 1
• Percutaneous nephrolithotomy (PNL) has emerged as the treatment of choice for larger cystine stones based on superior outcomes and acceptably low morbidity 1, 4

Combined Approach Considerations

• For larger or complex cystine stones, a combined approach may be necessary 1
• When combination therapy is used, initiating treatment with PNL followed by SWL shows clear advantages over starting with SWL, requiring fewer shock wave treatments per renal unit (1.5 vs 4.3) 4
• The "sandwich therapy" approach (percutaneous debulking, followed by SWL of residual stones, and finally percutaneous nephroscopy) may be considered for complex cases 1

Special Considerations

• Patients with cystinuria typically have their first stone event early in life, are prone to recurrent stones, and consequently are subject to repetitive removal procedures 1
• These patients are at risk for developing renal insufficiency over time, making minimally invasive and effective stone removal particularly important 1
• Prophylactic medical therapy and close follow-up are essential to limit recurrence in these patients 1

Conclusion

For cystine stones, the treatment algorithm should prioritize:

1. Small stones (<10mm): Consider URS as first-line, with SWL as a potential alternative in select cases
2. Larger stones (≥10mm): PNL as first-line therapy
3. Staghorn or complex stones: PNL-based approach or combination therapy