SAN DIEGO, CA USA (UroToday.com) - In this plenary lecture, Dr. Dean Assimos discussed the role of imaging in helping to appropriately treat kidney stone disease.
He reviewed a number of imaging characteristics that have been studied and shown to be predictors of treatment success. In particular, he reviewed the prognostic value of stone burden, stone attenuation, skin-to-stone distance (SSD) and collecting system anatomy.
Dr. Assimos opened his discussion by stating that “size does matter,” alluding to the fact that stone burden has been proven to be the greatest predictor of results for shockwave lithotripsy (SWL), ureteroscopy (URS), and percutaneous nephrolithotomy (PCNL). For SWL, studies have shown that the stone-free groups have lower stone volumes compared to those with residual stones. Specifically, one study recommend if stone volume is >500mm3, then the patient should not undergo SWL because of poorer outcomes. In addition, a couple of nomograms for SWL have been developed and both include stone burden as a part of the equation. For URS, studies have looked at several measures of stone burden including volume, surface area, and stone diameter. Of these measures, stone volume has been demonstrated to be the best predictor of ureteroscopic treatment success. Similar results have been found with PCNL, where stone area >2500mm2 has been shown to be associated with lower success rates. CROES has also developed a nephrolithometry scoring system to predict success with PCNL, and this includes stone burden as a parameter.
Next, Dr. Assimos reviewed the evidence for stone attenuation as a predictor. Several studies have shown that when stone attenuation, on CT imaging, is greater than >1000 Hounsfield units (HU), stone-free rates with SWL for renal stone is only 54.5%. Stone attenuation also has an impact on success for the treatment of ureteral stones, but the effect is less prominent. Stone attenuation is also very useful as it can help to identify uric acid stones, which typically have very low attenuation and can be treated with alkalization therapy. Similarly, stone attenuation can help to identify matrix stone, which demonstrates minimal attenuation and no enhancement. Dr. Assimos also reviewed the methods for measuring stone attenuation, including measurement of the maximal and average HU. The current evidence supports measuring the average HU as the best predictor for treatment outcome.
Dr. Assimos then moved on to review the role of SSD. Several studies have demonstrated SSD to be a predictor of treatment outcome with SWL for renal stones. Studies have shown SSD greater than 9 cm or 10 cm, depending on which study you look at, is predictive of poorer treatment outcomes. This association of SSD and treatment success has been demonstrated in adult patients for renal stones, but not for ureteral stones or in the pediatric population.
Collecting system anatomy, specifically the presence of hydronephrosis, was then highlighted by Dr. Assimos as a predictor of poor treatment outcome in both SWL and PCNL. In particular, studies have shown a longer time to clearance of stones and a need for a greater number of treatments in patients with ureteral stones and hydronephrosis treated with SWL. For patients with staghorn stones, severe hydronephrosis has been shown to correlate with lower stone-free rates with PCNL.
Dr. Assimos then moved on to discuss the role of imaging in predicting success with medical expulsive therapy (MET). He reviewed the findings of a recent study that demonstrated longitudinal stone diameter on coronal CT as the only predictor of stone passage for patients on MET. Specifically, this study showed the threshold for stone passage was 4.5 mm for proximal calculi and 3.99 mm for distal calculi. The best way to measure longitudinal diameter is using the coronal CT images in the bone windows with magnification.
Lastly, Dr. Assimos underscored the importance of imaging characteristics in evaluating the likelihood of success with endopyelotomy. The presence of significant hydronephrosis, a crossing vessel, and poor renal function of the affected kidney are all predictive of poor outcomes.
The take-home message, as Dr. Assimos concluded, is that the information garnered from imaging studies helps direct management of patients harboring renal and ureteral stones. This information also helps predict results and can be useful for counseling patients.
Presented by Dean G. Assimos, MD at the American Urological Association (AUA) Annual Meeting - May 4 - 8, 2013 - San Diego Convention Center - San Diego, California USA
Chair, Department of Urology, UAB School of Medicine, Birmingham, AL USA
Reported for UroToday.com by Michael Ordon, MD; UC Irvine Medical Center, Orange, CA USA