Renal hypoplasia refers to a congenitally small kidney where there is essentially normal residual parenchyma but smaller calyces, lobules and papillae. This is in contrast to renal atrophy where renal development was initially normal but the kidney has become smaller secondary to various other pathologies. In an adult patient the distinction of the former from the latter however can be difficult.
Renal hypoplasia can present as unilateral or bilateral. In cases of unilateral renal hypoplasia, it is common to have compensatory hypertrophy of the contralateral kidney.
Due to the essentially normal renal parenchyma, these kidneys can still produce normal urine, however in a reduced volume. This low volume of production causes urine stasis and predisposes to urolithiasis and urinary tract infections.
Renal hypoplasia can be divided into two broad groups:
complete (global) renal hypoplasia
segmental renal hypoplasia: Ask-Upmark kidney