[Nephrogenic metaplasia of the urinary tract]

Nephrogenic metaplasia or nephrogenic adenoma is a rare, benign disease arising in the urothelium, whose glandular structure resembles that of renal tubules. More than 350 cases have been reported in the literature, mostly in adults and with a male predominance. The mean age of onset is 31 years for women and 44 years for men (range: 3 weeks-83 years). The tumour can occur at any level of the urinary tract, but bladder involvement is predominant (72%) compared to ureteropelvic (19%) or urethral (9%) lesions. The pathogenesis remains unknown, but the hypothesis usually adopted is metaplastic transformation of urothelial cells in response to prior aggression of the urothelium (surgical operations, stones, trauma, etc.). This tumour is an incidental finding in 20% (bladder) to 90% of cases (urethral or ureteropelvic sites). In the other cases, the presenting symptoms and endoscopic appearance are nonspecific. Pathological examination is able to distinguish nephrogenic adenoma from other tumours (transitional cell carcinoma, adenocarcinoma), although these other tumour types may also be associated. The tumour must be treated conservatively, as there is no risk of malignant degeneration. The elimination of predisposing factors is essential in every case, but is not always able to prevent the recurrences observed in 37 to 60% of cases. In the particular context of renal transplant recipients, in whom this disease has been exceptionally reported, prolonged follow-up is necessary due to the absence of any data concerning the long-term course in this clinical context, and the possible carcinogenic risk of immunosuppressant therapy.