Evaluating compensatory hypertrophy: a growth curve specific for solitary functioning kidneys

While compensatory hypertrophy is expected in solitary kidneys, the definition of appropriate hypertrophy remains unclear. The normal renal growth rate in children age 1 to 18 years with 2 kidneys has been defined as 0.28 × age (years) + 6.1. Solitary kidneys appear to grow faster and larger and, thus, require a separate growth curve.

The records of all patients 18 years old or younger with solitary functioning kidneys were reviewed from 2001 to 2011. Exclusion criteria were greater than SFU (Society for Fetal Urology) grade 2 hydronephrosis, posterior urethral valves, vesicoureteral reflux or any ipsilateral obstruction. Ordinary least squares regression modeled the renal length as a function of age by using only the initial sonogram per subject. The distribution of mean kidney length by age was plotted and compared to published normal values.

A total of 91 subjects were included in the study. Patients were evenly split by laterality and gender. Multicystic dysplastic kidney comprised 34% and solitary kidneys 66% of subjects. Of these subjects 55% underwent their first sonogram at younger than 1 year old. There were 61 subjects with multiple sonograms but the initial 91 were included in the study. Age was a better predictor of renal length for subjects age 1 year or older (r(2) = 0.7312) than for those younger than 1 year old (r(2) = 0.6138). For children age 1 to 18 years we used the equation, length = 0.38 × age + 7.2. Solitary kidney values were approximately 2 standard deviations greater than normal values.

The equation 0.4 × age (years) + 7 can be used to accurately estimate expected renal length in children (age 1 to 18 years) with solitary kidneys and can be used as a quick reference to evaluate for renal compensatory hypertrophy.