Comparison of growth in primary Fanconi syndrome and proximal renal tubular acidosis

To compare the difference between primary proximal renal tubular acidosis (PRTA) and Fanconi syndrome (FS), and to find out possible risk factors for growth retardation, we studied the long-term growth, clinical, laboratory, and radiological findings associated with the treatment of six children with primary FS and 15 children with PRTA. The ages of the children with FS were much older than those with PRTA at initial diagnosis (7.03±3.82 vs. 1.63±1.56 years). The height standard deviation score (SDS) at the start of treatment was significantly lower in FS than in PRTA. Catch-up growth was noted in PRTA at the end of follow-up (initial height SDS −2.13±1.10 vs. last height SDS −1.33±1.43, P=0.023 by paired t-test), whereas apparent linear growth impairment was found in FS in terms of overall growth velocity index (82.70±8.37%) and height SDS (initial −3.25±0.95 vs. last −3.15±0.31, P=0.791). There was also a higher rate of rickets occurrence in FS (3/6 vs. 0/15 in PRTA). Hypophosphatemia during the follow-up period was more frequent for FS than PRTA (69.2±26.1% vs. 7.0±25.8%, P<0.001), whereas metabolic acidosis (blood HCO₃<20 mmol/l) was less efficiently corrected in PRTA (49.1±20.5% vs. 25.2±21.6% in FS, P=0.028). Moreover, the height ΔSDS correlated well with the mean serum P level during the treatment period in these patients (R=0.528, P=0.014 for all children; R=0.917, P=0.01 for FS patients). Our data suggest that metabolic acidosis may not be the sole factor causing growth impairment in FS. Correction of metabolic acidosis may indeed improve growth in PRTA but not in FS. This study indicates that factors other than metabolic acidosis, such as phosphate depletion and delayed diagnosis/treatment, should be considered to be important causes of growth retardation in FS.