What is lower urinary tract obstruction in a fetus, and how is it diagnosed and treated?
LUTO is often the result of posterior urethral valves and affects males much more often than females. In females, posterior urethral valves do not occur, so other etiologies are considered. Ultrasound diagnostic findings include:
- • A markedly distended urinary bladder, typically with a “keyhole” shape
- • Severe oligohydramnios.
- • Hydronephrosis and hydroureter.
- • Hyperechoic renal parenchyma and cysts (evidence of renal dysplasia).
- • Male gender.
- • Decreased fetal thoracic circumference (evidence of pulmonary hypoplasia).
If the fetus is female, then additional evaluation for other causes (e.g., cloacal abnormality) is undertaken. The oligohydramnios leaves little room for fetal movement including breathing motion, which adversely affects lung development. The cephalad displacement of the diaphragm by the large bladder and abdominal organs is a contributing factor. Assessment of the fetal renal function, either by fetal vesicocentesis and urine electrolyte analysis or fetal cord blood measurement of serum beta 2 -microglobulin can be used prior to performing an ultrasound-guided vesicoamniotic shunt, the treatment of choice for LUTO.
On occasion, the fetal urinary bladder is distended, but the amniotic fluid volume is normal or near normal. In these instances, US is used to search for the way in which the bladder is emptying. It is sometimes a pressure-dependent phenomenon with the urethra (or urachus) opening when the pressure in the bladder becomes great enough.