Hypoxic-Ischemic Encephalopathy
The
neurological condition of the newborn during the early neonatal (immediate
after birth) period is important in determining the cause of the brain injury
and in predicting its severity over the long term. The term hypoxic-ischemic
encephalopathy (HIE) is a clinical diagnosis of impaired neurological
function that is usually made by a neonatologist or pediatric neurologist
after delivery. A frequent cause of HIE is fetal asphyxia. Many clinicians
use the criteria in Sarnat HB, Sarnat MS. Neonatal encephalopathy
following fetal distress. Arch Neurol 1976; 33:696-705 to grade the
severity of a newborn's neurologic condition after birth.
What is the significance of Seizures?
Seizures
often develop in infants who sustained severe birth asphyxia during the first
24 hours of life. Since seizures can exacerbate a pre-existing brain injury,
it is imperative that appropriate and timely treatment be implemented.
However, researchers do not know for sure whether seizures are the manifestation
of a severe brain injury, or whether they cause additional injury. In any
event, when evidence of early-onset neonatal seizures is combined with
evidence of other neurologic abnormalities resulting in a diagnosis of HIE, a
compelling argument can be made that the newborn sustained an acute
hypoxic-ischemic brain in-sult.
The Importance of Blood Gas Studies
An
analysis of the newborn's acid-base status, together with the results of
subsequent blood gas studies, is important in creating a link between birth
asphyxia and brain damage. The analysis of umbilical cord blood provides
information about how the fetus responded to decreased or inadequate oxygen
supply. The term "perinatal asphyxia" is frequently used by
pediatricians and neonatologists to describe a brain injury caused by hypoxic
and ischemic complications leading to blood gas abnormalities at the time of
birth.
It is interesting to note
that even though the American College of Obstetricians and Gynecologists
suggests that the blood's pH should be less than 7.0, if it is going to be
used as a factor establishing a link between birth asphyxia and neurologic
injury, this narrow view is not widely supported by the literature. Many
other studies confirm that birth asphyxia can cause brain damage even when
umbilical cord arterial pH is greater than 7.0. nevertheless, it appears that
infants with severe intrapartum asphyxia, quantified by an umbilical artery
pH of less than 7.00, are more likely to develop permanent brain injuries.
The Importance of Neuroimaging Studies
Technological
advances in CT scanning and magnetic resonance imaging (MRI) have opened new
frontiers for establishing a link between asphyxia and hypoxic-ischemic brain
injury. While some physicians continue to question whether cerebral palsy can
be caused by birth asphyxia, many pediatric neuroradiologists acknowledge
that brain damage can result from acute obstetrical emergencies causing total
or near total oxygen deprivation, or as a result of partial, intermittent
episodes of oxygen deprivation over longer periods of time. Imaging studies
reveal different patterns of brain damage in infants suffering acute total
asphyxia, as compared to prolonged partial asphyxia. Sometimes, the films can
distinguish between hypoxia or ischemia that was acute and produced brain
damage in a short period of time, (10 or 15 minutes), and a prolonged hypoxia
or ischemia that was present for a longer period of time, (1 hour or longer),
before it produced irreversible brain damage.
Each one of the imaging techniques
has strengths and weaknesses. It is also important to know whether a
particular imaging study was performed at the optimal time to pinpoint when
the asphyxial brain damage may have occurred. It is common for clinicians to
order cranial ultrasounds, blood gas tests, and EEG's to assess an
asphyxiated newborn. Cerebral edema (swelling) often occurs after an
asphyxial episode, and seizures can exacerbate the edema and prolong the
period during which brain cells are vulnerable.
Remember!
The
quality and timing of newborn care may affect the severity of the brain
damage that was caused by birth asphyxia. The question that parents will have
to consider in some cases is whether the newborn received timely and
appropriate treatment to reduce the potential adverse effects of systemic and
neurologic complications present immediately after birth, or developing
shortly thereafter. Some of the goals thought to improve the neurologic
outcome of a newborn include:
- Maintaining normal blood
glucose,
- Maintaining normal blood
pressure,
- Preventing or controlling
seizures, and
- Preventing or minimizing
cerebral edema
A
question that often requires the assistance of a qualified expert in
pediatric neurology and/or neonatology to answer is whether the brain cells
that were compromised during the initial asphyxial episode were going to die
regardless of how carefully and rapidly the newborn's complications were
corrected. In other words, was the sequence of events that caused asphyxial
brain damage continue to produce additional damage during the first few days
of life, or did the failure to stabilize the infant and correct the blood gas
abnormalities cause additional brain injury?
