Transient renal medullary hyperechogenicity in ultrasound studies of neonates: is it a normal phenomenon and what are the causes?

A prospective ultrasound study of the renal morphology of 51 neonates with no clinical signs of renal impairment showed transient medullary hyperechogenicity of varying intensity in 37%. These findings were made in the first few postnatal days, were not accompanied by any other echographic changes, and disappeared again within the first week of life. They were encountered twice as often in full-term as in premature babies. In parallel with this phenomenon, the concentration of Tamm-Horsfall protein, calcium, and uric acid, and the osmolality were determined in the daily urine of 44 of these neonates. The analyses did not provide any definite clues as to the etiology of the ultrasound findings. It is therefore suggested that the term "Tamm-Horsfall nephropathy or proteinuria" should no longer be used for such sonographic phenomena in the neonatal kidney, but rather, they should be regarded as normal. Furthermore, the term "tubular stasis nephropathy" should be reserved for cases with clinically demonstrable impairment of renal function.