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A 1-day-old term baby boy presented with bilious vomiting and was found to have mild renal impairment. No abnormality was detected on antenatal ultrasound scans. He had a normal upper gastrointestinal contrast study, which ruled out malrotation as the cause of his bilious vomiting. The surgical team requested an ultrasound of his renal tract to look for any structural abnormality that may be causing renal impairment. Both kidneys were normal in size and had similar appearances. Look at the longitudinal (figure 1) and transverse (figure 2) ultrasound images of the right kidney and answer the following questions.
There is hydronephrosis. (True or False?)
There are renal stones. (True or False?)
The renal cortex is hypoechogenic (dark). (True or False?)
The apex of the medullary pyramids are hyperechogenic (bright). (True or False?)
This finding is rare. (True or False?)
The cause of these appearances is unknown. (True or False?)
These appearances resolve within a few days. (True or False?)
This appearance is only seen in neonates. (True or False?)
Answers to the questions can be found on page 02
Answers to the Questions on page 01
(1) False, (2) False, (3) False, (4) True, (5) False, (6) True, (7) True, (8) False.
The ultrasound images show hyperechogenicity at the apex of multiple renal medullary pyramids (arrowheads in figure 3 and figure 4), fading up to the middle of the pyramid. The renal cortex is normal in appearance and the kidneys are not hydronephrotic. Renal medullary pyramids are normally hypoechogenic (dark in appearance) compared with the renal cortex at all ages, but this is accentuated in the neonatal period because the immature cortex is thin and hyperechogenic (brighter), making the medullary pyramids appear more hypoechogenic and relatively large. Kidneys of newborns are also often lobulated in outline. Transient hyperechogenicity of the renal pyramids in neonates and infants is common, seen in up to 58% of neonates and mimics medullary nephrocalcinosis and renal stones. One or multiple renal pyramids may be affected, and occasionally hyperechogenic material is also found in the bladder on ultrasound. The appearances usually resolve within the first few days of life, with the physiological increase in glomerular filtration rate. Sometimes it may take longer, particularly in premature neonates on intensive care units. It is important to recognise this as a transient phenomenon in the neonatal period, which does not require further investigation.
There have been several attempts to explain the cause for these appearances; deposition of Tamm-Horsfall protein or urate crystals have been suggested. However, the cause remains uncertain at present. This appearance may also be seen in older infants with hypernatremic dehydration.
Competing interests None declared.
Provenance and peer review Not commissioned; internally peer reviewed.