Late-onset oligomeganephronia in a young adult born extremely preterm with lifestyle-related hyperfiltration stress

Oligomeganephronia (OMN) is a rare congenital renal hypoplasia characterized by markedly reduced nephron number with compensatory glomerular hypertrophy. Although typically diagnosed in childhood, adult-onset OMN is uncommon and often under-recognized. A 29-year-old man born at 28 weeks of gestation as one of triplets, with a birth weight of 740 g, was referred for evaluation of persistent proteinuria. Proteinuria had been intermittently detected for 10 years but remained uninvestigated. Three months before admission, the urinary protein-to-creatinine ratio increased to 1.20 g/gCr with serum creatinine 1.54 mg/dL. He had engaged in regular resistance exercise and consumed protein supplements. On admission, urinary β2-microglobulin was mild elevated, and a discrepancy between creatinine- and cystatin C-based estimated glomerular filtration rate (eGFR) was observed, whereas inulin clearance was preserved. Kidney biopsy revealed markedly enlarged glomeruli, reduced glomerular density, and perihilar variant focal segmental glomerulosclerosis, consistent with OMN with secondary hyperfiltration injury. Treatment with losartan resulted in a rapid reduction in proteinuria and stable renal function over 9 months. This case illustrates that latent OMN in individuals born extremely preterm may remain subclinical until additional hyperfiltration stressors precipitate overt renal injury. Early recognition and renin-angiotensin system inhibition can ameliorate proteinuria and stabilize renal function.