One kidney is not visualized in the fetus
1. Unilateral renal agenesis
2. Ectopic kidney, especially pelvic kidney
3. Multicystic dysplastic kidney that has involuted
4. Severe unilateral renal dysplasia
5. Less commonly, a horseshoe/crossed-fused renal anomaly where the anatomy is misleading.
Ultrasound confirmation of unilateral renal agenesis
Look for the combination of:
• Kidney absent from its renal fossa
• Absent ipsilateral renal artery on color Doppler
• Empty renal fossa replaced by adrenal gland, which may have a characteristic elongated/flattened appearance
• Careful search of the pelvis for an ectopic kidney
• Normal contralateral kidney
• Normal bladder
• Usually normal amniotic fluid, because the other kidney provides adequate urine production
A useful sign is the "lying-down adrenal sign", where the adrenal occupies the renal fossa and becomes elongated because the kidney is absent.
Prognosis
If this is isolated unilateral renal agenesis with:
• Normal contralateral kidney
• Normal contralateral renal artery
• Normal bladder
• Normal amniotic fluid
• No other structural anomalies
the prenatal prognosis is generally good.
However, there is an increased lifetime risk of:
• Hypertension
• Proteinuria
• Reduced renal function
• Hyperfiltration-related renal injury
Also assess:
• Contralateral kidney morphology
• Renal pelvis
• Renal artery
• Bladder
• Amniotic fluid
• Spine
• Cardiac anatomy
• Genital tract where assessable
• Umbilical arteries
Long-term risks
The main issue is that one kidney has to do the work of two, producing glomerular hyperfiltration. Over many years this can predispose to:
• Hypertension
• Albuminuria/proteinuria
• Gradual reduction in GFR
• Chronic kidney disease in a minority
The most important prognostic factors
Very favorable:
• Normal contralateral kidney
• Good compensatory enlargement
• Normal renal artery and blood flow
• Normal bladder
• Normal amniotic fluid
• No contralateral CAKUT
• No proteinuria
• Normal BP and renal function
Higher risk:
• No compensatory hypertrophy of the remaining kidney
• Abnormalities of the remaining kidney/urinary tract
• Proteinuria
• Hypertension
• Reduced GFR
Goals
1. Maintain healthy weight
Obesity increases the hyperfiltration burden.
2. Avoid excessive salt intake
A normal balanced diet is preferable. There is no need for severe protein restriction in an otherwise healthy child.
3. Good hydration, particularly during significant vomiting, diarrhea, fever or strenuous activity.
4. Avoid unnecessary nephrotoxic medications, particularly repeated NSAID exposure such as ibuprofen, unless specifically advised by the treating doctor.
5. Treat UTIs promptly.
6. Monitor BP and urine albumin/protein throughout life.
The baby has one functioning kidney, but that kidney usually enlarges and provides adequate renal function.
Most children with an isolated solitary kidney grow and develop normally.
The important precaution is lifelong surveillance for blood pressure, proteinuria and renal function rather than restriction of normal life.
