Author Reply Re: Elbaset MA, Ezzat O, Elgamal M, Sharaf MA, Elmeniar AM, Abdelhamid A, et al. Supernormal differential renal function in adults with ureteropelvic junction obstruction: Does it really exist? Indian J Urol 2020;36:205-11.

Dear Sir,

We thank the readers for their queries about our work and believe that our reply will provide more details about this topic.

The old concept of Koff et al. mentioned in the author's comment is countered by new trends in managing patients with UPJO conservatively in case they are asymptomatic and the renal function is stable.[12] In this report, not only did all patients have obstructed kidneys evident by renographic prolonged T½ (23 [16-34] min) but were also symptomatic (29 patients with recurrent flank pain and 2 patients presented with recurrent pyelonephritis).[3] The decision of corrective surgery in this subset of patients could not depend only on T½ value but clinical symptoms as well.

We agree with the theory that renal hydronephrosis is not always a pathological sign but actually a compensating mechanism designed to protect the kidney from high pressures and renal damage. Loss of this adaptive mechanism initially leads to the appearance of patient's symptoms prior to renal function deterioration.[4] Persistence of these symptoms with delayed intervention could initiate renal function deterioration.

We agree with the authors in the necessity of standardized methodology for renogram technique application; we described it thoroughly in the manuscript methodology.[3] The amount of hydration, the fullness of the bladder, and the position of the patient are precautions that should be taken into consideration and are done routinely in our center to increase the accuracy of the renographic results. Evaluation of normalized residual activity, pelvic excretion efficiency, output efficiency, and cortical transit time are analogous to transit time, especially in the presence of standardized renogram protocol.[56]

Although computed tomography (CT) may be used to determine differential function, it cannot replace nuclear renal scan entirely to quantify obstruction.[7] Authors in their comment mentioned a study done by Sarma et al. which was encountered by some limitations. If patients presented with acute on top of chronic pyelonephritis, renal tissue edema would hamper the accurate detection of renal volume. Second, most patients evaluated in chronic obstruction group had poorly functioning renal units. Accordingly, the use of CT-detected renal parenchymal volume as a predictor of differential renal function. (DRF) determination in the snDRF group is questionable. Finally, nephron damage may occur before measurable volume loss and the CT estimate may lag behind a renal scan to indicate decreasing function.[7]

Which type of scintigraphy, 99 mTc-DMSA or 99 mTc-MAG3, should be used in the evaluation of DRF is still a matter of debate. Some authors considered that different isotopes could yield substantially different results in the same patient.[6] On the other hand, it is proven that 99 mTc-DMSA and 99 mTc-MAG3 renograms in the evaluation of DRF are comparable. In a previous study, it was concluded that if a 99 mTc-MAG3 scan has been performed to assess drainage, a 99 mTc-DMSA scan specifically to estimate DRF is unnecessary, provided that the DRF is within normal limits on the 99 mTc-MAG3 scan and there is no proven scarring on previous imaging.[8] Based on that, a 99 mTc-MAG3 renogram was done commonly at our center for adults in the evaluation of UPJO taking advantage of obstruction evaluation. Hence, we could not expose our patients to further radiation exposure by doing 99 mTc-DMSA. We could suppose that further prospective studies might open the horizons to fill those gaps in this topic of the literature.