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Abstract: PO2017

When Less Is More: Phosphate Homeostasis Insights from a Microvillus Inclusion Disease Patient

Session Information

Category: Pediatric Nephrology

  • 1700 Pediatric Nephrology

Authors

  • Robinson, Cal, The Hospital for Sick Children, Toronto, Ontario, Canada
  • Yanchis, Dianna, The Hospital for Sick Children, Toronto, Ontario, Canada
  • Garofalo, Elizabeth Marie, The Hospital for Sick Children, Toronto, Ontario, Canada
  • Lemaire, Mathieu, The Hospital for Sick Children, Toronto, Ontario, Canada
Introduction

Microvillus inclusion disease (MVID) is a rare, severe congenital secretory diarrhea caused by recessive MYO5B or STX3 mutations. Multiple cases of MVID with partial proximal tubule (PT) defects are reported (mostly hyperphosphaturia). While MYO5B is expressed in PT cells, MVID patients have normal PT brush border on kidney biopsy, and the PT defect resolves after intestinal transplant. Therefore, it is unlikely that the MYO5B genotype is causally related to the proximal tubulopathy.

Case Description

Like all patients diagnosed with MYO5B-MVID, our patient required cycled total parenteral nutrition (TPN). She was referred to nephrology at age 2 for persistent hypophosphatemia despite escalating TPN phosphate (PO4) content, and nephrocalcinosis. Urinary PO4 wasting was confirmed given the low (<65%) tubular reabsorption of phosphate (TRP). FGF-23 and PTH were elevated. A 24 hr balance study (on/off TPN) revealed that TRP was lowest and FeNa highest (~2%) while on TPN (these values were improved after 6h without TPN). It also confirmed that the negative PO4 balance was only due to renal losses. We hypothesized that high TPN electrolyte concentrations caused an obligate phosphaturic response. Gradual reductions of TPN sodium (Na+) (by 13%), then TPN PO4 (by 70%) over 4 mo led to normalization of serum PO4 (Figure), TRP (83-91%) and FeNa (~0.3%).

Discussion

We propose that excessive TPN Na+ and PO4 promoted a strong phosphaturic response: the combination of several physiologic factors likely explains this unusual phenomenon. Of interest, the intermittently negative TRP suggest that tubular phosphate secretion must have contributed to the massive phosphaturia. A counterintuitive reduction in TPN Na+ and PO4 reduced renal PO4 wasting without impacting serum Na+. We surmise that other MVID cases of PO4 wasting were also probably due to unusually high TPN electrolyte concentrations. Detailed balance studies are invaluable tools to assess complex fluid/electrolyte disorders.