Abdominal Ultrasound and Abdominal Radiograph to Diagnose Necrotizing Enterocolitis in Extremely Preterm Infants

Abstract

Necrotizing enterocolitis (NEC) is an important contributor toward
mortality in extremely premature infants and Very Low Birth Weight
(VLBW) infants. The incidence of NEC was 9% in VLBW infants
(birth weight 401 to 1,500 grams) in the Vermont Oxford Network
(VON, 2006 to 2010, n = 188,703).1 The incidence of NEC was 7%
in 1993, increased to 13% in 2008, and decreased to 9% in extremely
preterm infants (22 to 28 weeks gestation) in the Neonatal Research
Network Centers (1993 to 2012).2 The incidence of surgically treated
NEC varies from 28 to 50% in all infants who develop NEC.3 Surgical
NEC occurred in 52% in the VON cohort.1 In this cohort, the odds of
surgery decreased by 5% for each 100 gram increase in birth.
The incidence of surgical NEC has not decreased in the past
decade.4 The mortality from NEC is significantly higher in infants
who need surgery compared to those who did not (35% versus 21%).1
The case fatality rate among patients with NEC is higher in those
surgically treated (23 to 36%) compared to those medically treated (5
to 24%).3 In addition to surgery, NEC mortality rates are influenced
by gestational age, birth weight,1,2,5 assisted ventilation on the day of
diagnosis of NEC, treatment with vasopressors at diagnosis of NEC,
and black race.6,7
Extremely preterm infants who survive NEC are at risk for severe
neurodevelopmental disability and those with surgical NEC have a
significantly higher risk of such delays (38% surgical NEC versus 24%
medical NEC).8 Diagnosis of necrotizing enterocolitis is challenging
and it is usually suspected based on non-specific clinical signs. Bell’s
criteria and Vermont-Oxford Network criteria help in the diagnosis
of NEC.
Bell’s criteria, commonly used for diagnosis, staging, and planning
treatment of NEC, were described in 1978 and modified in 1986.9,10
Bell’s stage I signs are non-specific: temperature instability, lethargy,
decreased perfusion, emesis or regurgitation of food, abdominal distension,
recurrent apnea, and on occasion, increased support with
mechanical ventilation. Abdominal distension and emesis are more
common than bloody stools in very preterm infants compared to term
infants.7 Abdominal radiographic findings are an integral part of Bell’s
criteria. Identification of Bell’s stage I NEC (early NEC) with abdominal
radiograph is challenging, as the features on abdominal radiograph
(normal gas pattern or mild ileus) are non-specific. With progression
of NEC to Bell Stage IIA, the symptoms (grossly bloody stools,
prominent abdominal distension, absent bowel sounds) and features
on abdominal radiographs (one or more dilated loops and focal pneumatosis)
are more specific.
On the other hand, the Vermont Oxford Network criteria for NEC
consist of at least one physical finding (bilious gastric aspirate or
emesis, abdominal distension or occult/gross blood in the stool in
the absence of anal fissure) and at least one feature on abdominal
radiograph (pneumatosis intestinalis, hepatobiliary gas, or pneumoperitoneum).
1 These features correspond to Bell Stage IIA or Stage
IIB and are not features of early NEC. Thus relying solely on abdominal
radiograph for diagnosis of early NEC, as is practiced currently,
has significant drawbacks especially in extremely premature infants.7
Ultrasound has been suggested to improve the percentage of infants
diagnosed with early NEC.11 However, this imaging modality is not
used routinely in the diagnosis or management of NEC.
As the incidence of surgical NEC and mortality from NEC continues
to be high, the literature to demonstrate the shortcomings of
abdominal radiographs and promise of abdominal ultrasound in diagnosis
of NEC is reviewed.