A 2-day-old boy is evaluated in the newborn nursery. The patient was born at an estimated 40 weeks gestation to a 26-year-old primigravida who received no prenatal care. He has been taking 30 mL (1 oz) of a cow's milk–based formula every 3 hours since birth. The mother says the boy seems uncomfortable. She is worried about dehydration because he has been spitting up formula after his last few feeds and has only had one damp diaper and a single stool since birth. Birth weight was 3,200 g (7 lb 1 oz), and weight today is 3,260 g (7 lb 3 oz). Temperature is 36.7 C (98.1 F), pulse is 150/min, and respirations are 66/min. Pulse oximetry is 88%on room air. Lung examination shows diminished breath sounds bilaterally. Cardiac examination is normal. The abdomen is distended. The urethral meatus appears intact. A small sacral dimple with intact skin overlies the coccyx. His skin is warm and pink. Chest radiograph reveals diminished lung volumes bilaterally with a normal heart size. The patient is placed on supplemental oxygen. Which of the following is the best next step in the evaluation of this patient's condition?
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This newborn boy has abdominal distension, poor urine output, and respiratory distress. In addition, rather than the normal expected weight loss in the first few days of life, this patient has weight gain (likely due to retained urine). In conjunction with a normal cardiac examination, these findings are suspicious for posterior urethral valves (PUV), the most common cause of urinary tract obstruction in newborn boys.
Patients with PUV have a persistent congenital posterior urethral membrane that obstructs the flow of urine, leading to reduced urine output and a distended bladder. Some patients may have a weak urinary stream, grunt, or strain as they attempt to pass urine. In utero, impaired fetal urinary excretion causes oligohydramnios and subsequent lung hypoplasia, which can lead to diminished lung volumes and postnatal respiratory distress (eg, hypoxia, tachypnea, decreased breath sounds), as in this patient.
The first step in evaluation is a renal and bladder ultrasound. Findings consistent with PUV include a dilated bladder with bilateral hydroureters and hydronephrosis. Voiding cystourethrogram is performed if ultrasound is abnormal; evidence of posterior urethral dilation supports the diagnosis. Management involves bladder drainage and electrolyte correction followed by cystoscopy to confirm the diagnosis and ablate the PUV.
(Choice A) Echocardiogram diagnoses congenital heart disease, which can present with neonatal respiratory distress but often also causes a heart murmur and/or cardiomegaly on chest x-ray. In addition, abdominal distension is atypical.
(Choice B) Lumbosacral MRI evaluates for spina bifida, which can cause urinary retention due to neurogenic bladder dysfunction. Cutaneous manifestations may include a deep sacral dimple above the gluteal cleft with a hair tuft or abnormal pigmentation. This patient's sacral dimple with an intact base is a normal finding that does not require workup.
(Choice C) Pyloric ultrasound detects pyloric stenosis, a condition that most commonly presents in boys age 3-5 weeks with postprandial projectile vomiting. This patient's age makes pyloric stenosis highly unlikely. In addition, respiratory distress is not associated with pyloric stenosis.
(Choice E) Upper gastrointestinal series evaluates for malrotation, which should be suspected in any neonate with abdominal distension and bilious emesis. This patient has regurgitation (ie, spit up), which is more consistent with physiologic gastroesophageal reflux.
Educational objective:
Posterior urethral valves present in newborn boys with bladder distension, decreased urine output, and respiratory distress (due to oligohydramnios and subsequent lung hypoplasia). Initial evaluation includes renal and bladder ultrasound and voiding cystourethrogram.