Neonatal hypoxic-ischemic encephalopathy: cooling therapy criteria
Neonatal hypoxic-ischemic encephalopathy (HIE) occurs when reduced oxygen delivery and blood flow injure the developing brain around the time of birth. The resulting illness can range from mild abnormalities that resolve quickly to severe encephalopathy with seizures, cerebral edema, multiorgan dysfunction, and long-term neurodevelopmental disability. Therapeutic hypothermia, commonly called cooling therapy, is the principal evidence-based intervention for carefully selected newborns with moderate or severe HIE.
Cooling must begin early. The treatment window is generally limited to the first six hours after birth, so decisions often occur while clinicians are stabilizing the infant, reviewing delivery records, interpreting blood gases, and arranging transfer to a neonatal intensive care unit. The process requires a coordinated team because temperature control, respiratory support, seizure monitoring, cardiovascular care, and family communication must proceed together.
The subject remains central to perinatal and neonatal medicine, the fields represented by the scientific meetings described on the FAOPS 2020 congress site. Although the Tokyo congress was canceled in 2020, its academic focus reflects the continuing importance of consistent neonatal neurological assessment and evidence-based treatment.
Why early cooling matters
During a hypoxic-ischemic insult, the initial lack of oxygen and glucose can cause energy failure, loss of cellular ion balance, and neuronal injury. A temporary period of apparent recovery may follow, but secondary energy failure can develop over the next several hours. This phase involves excitotoxicity, inflammation, oxidative stress, and mitochondrial dysfunction. Cooling reduces metabolic demand and may limit these processes.
Randomized trials and subsequent follow-up studies have shown that whole-body or selective-head hypothermia can lower the risk of death or significant neurodevelopmental disability in term and near-term infants with moderate-to-severe HIE. Benefits are greatest when treatment starts promptly and is delivered under a standardized protocol. Cooling is not a substitute for resuscitation, ventilation, circulatory support, glucose management, or seizure treatment.
A newborn should not be cooled solely because of a low Apgar score, an abnormal cord gas, or a difficult delivery. Eligibility depends on the combination of biochemical evidence, a credible perinatal event, and the neurological examination. This combined approach helps identify infants likely to benefit while limiting exposure in babies with mild or unrelated neurological abnormalities.
Biochemical and perinatal screening
Most protocols begin with an infant who is at least 35 or 36 weeks’ gestation, depending on local guidance and institutional policy. A common biochemical threshold is an umbilical cord or early neonatal blood gas showing a pH of 7.0 or less or a base deficit of 16 mmol/L or greater. These values suggest substantial metabolic acidosis, particularly when they fit the clinical history.
Some protocols allow an intermediate biochemical range, such as a pH of 7.01–7.15 or a base deficit of 10–15.9 mmol/L. In this situation, additional evidence is usually required: an acute perinatal event, a five-minute Apgar score of five or less, or the need for assisted ventilation for at least ten minutes. Examples of acute events include placental abruption, uterine rupture, cord prolapse, severe cord compression, or prolonged maternal hypotension.
When cord blood is unavailable, a blood gas obtained within the first hour can help establish eligibility. Documentation should include the sample time, source, pH, carbon dioxide, bicarbonate, base deficit, lactate when available, Apgar scores, resuscitation measures, and the timing of spontaneous breathing. A single number should be interpreted alongside the infant’s clinical condition rather than treated as an automatic treatment trigger.
Neurological examination and severity
The neurological examination determines whether the newborn has moderate or severe encephalopathy. Standardized examinations commonly evaluate six categories: level of consciousness, spontaneous activity, posture, tone, primitive reflexes, and autonomic function, including pupils, heart rate, and respiration. Abnormalities in at least three categories are often used to support moderate or severe HIE, although exact criteria vary between protocols.
Moderate encephalopathy may involve lethargy, reduced activity, hypotonia, weak suck, incomplete Moro reflex, or an abnormal autonomic pattern. Severe encephalopathy is characterized by stupor or coma, absent activity, marked flaccidity, absent suck and Moro reflexes, fixed or poorly reactive pupils, irregular breathing, or the need for mechanical ventilation because of neurological depression. Seizures strongly support significant brain injury but are not required for eligibility.
The examination should be repeated because sedation, neuromuscular blockade, hypoglycemia, sepsis, respiratory failure, and medications can obscure neurological findings. A newborn who initially appears mildly affected may deteriorate within the treatment window. Conversely, a baby with transient depression that rapidly resolves may not meet criteria for therapeutic hypothermia. When findings are uncertain, consultation with a neonatologist and review of the complete clinical timeline are essential.
| Assessment domain | Findings that commonly support cooling | Important considerations |
|---|---|---|
| Gestational age | Usually at least 35–36 weeks | Follow the local protocol and trial population |
| Biochemical evidence | pH ≤7.0 or base deficit ≥16 mmol/L | An early neonatal gas may be used if cord gas is unavailable |
| Intermediate gas values | pH 7.01–7.15 or base deficit 10–15.9 mmol/L | Usually requires an acute event plus low Apgar or prolonged ventilation |
| Clinical history | Sentinel hypoxic event, difficult resuscitation, or prolonged low Apgar | Establish timing and duration of the insult |
| Neurological status | Moderate or severe encephalopathy, often across at least three examination categories | Repeat the examination and account for confounders |
| Treatment timing | Initiation as soon as possible, generally within six hours | Do not delay referral while waiting for every test |
| Monitoring | Continuous temperature, cardiorespiratory, oxygenation, and neurological surveillance | Use aEEG or EEG when available, especially for seizures |
Starting and maintaining therapeutic hypothermia
The usual target is a core temperature of approximately 33–34°C, maintained for 72 hours. Whole-body cooling is widely used, while selective head cooling is available in some centers. The infant should be placed on a validated cooling device with continuous core temperature measurement, usually through a rectal or esophageal probe according to local practice. Skin temperature alone is not reliable for confirming the therapeutic range.
Passive cooling may begin during stabilization or transport if directed by an experienced neonatal team, but uncontrolled temperature reduction can lead to excessive hypothermia. Active warming should be avoided unless specifically required, and the infant should be transferred promptly to a unit capable of controlled treatment. Temperature, blood pressure, oxygen saturation, glucose, blood gases, coagulation, urine output, and laboratory markers of organ function need regular review.
Rewarming generally occurs slowly after 72 hours, often at approximately 0.25–0.5°C per hour. Rapid rewarming may produce hypotension, electrolyte shifts, pulmonary instability, or increased seizure activity. The post-cooling period is a vulnerable time, so neurological observation and seizure surveillance should continue during and after rewarming.
Monitoring seizures and associated injury
Electroencephalography is an important part of care because electrographic seizures may occur without visible movements, especially in sedated or severely encephalopathic infants. Amplitude-integrated EEG can support bedside monitoring, while conventional multichannel EEG provides more complete seizure detection and background assessment. A normal early tracing does not necessarily exclude HIE or guarantee a normal outcome.
Common complications include respiratory failure, hypotension, pulmonary hypertension, arrhythmias, coagulopathy, thrombocytopenia, renal injury, abnormal glucose, and electrolyte disturbances. Cooling can alter drug metabolism and may increase bleeding risk, so treatment plans should account for the infant’s temperature and organ function. Infection must also be considered because sepsis can mimic or worsen neonatal encephalopathy.
Brain MRI, typically performed after rewarming when the infant is clinically stable, helps define the pattern and extent of injury. Diffusion-weighted imaging can identify early injury, while later sequences may clarify deep gray matter, watershed, white matter, or posterior fossa involvement. MRI findings support prognosis but should be combined with serial examinations, EEG results, developmental follow-up, and the family’s clinical context.
When cooling may not be appropriate
Therapeutic hypothermia is not suitable for every newborn with suspected perinatal compromise. Relative or absolute exclusions vary, but may include very early gestational age, extremely low birth weight, major congenital anomalies, conditions with a poor prognosis unrelated to HIE, uncontrolled bleeding, severe coagulopathy, or an established decision for comfort-focused care. A rapidly fatal illness may make invasive treatment inappropriate even when biochemical criteria are present.
Mild HIE is a particularly difficult category. Some infants with mild abnormalities recover without major disability, while others show later language, motor, behavioral, or learning difficulties. Evidence for routine cooling in mild HIE remains less definitive than for moderate or severe disease, and practice varies. Enrollment in an ethically approved clinical study or careful neurological observation may be considered where appropriate, rather than applying severe-HIE protocols automatically.
Cooling should also be reconsidered when the infant presents outside the usual six-hour window. Selected centers may evaluate late-presenting infants, but the evidence is less established and treatment decisions require specialist judgment. Families should receive clear information about the potential benefits, uncertainties, complications, and the need for long-term developmental surveillance.
Practical recommendations for clinical teams
- Establish a birth-room pathway that triggers rapid assessment of gestational age, blood gas results, Apgar scores, resuscitation duration, and sentinel events.
- Begin controlled temperature management while arranging urgent consultation or transport, avoiding accidental overcooling.
- Perform and document a structured neurological examination before six hours of age, then repeat it when sedation or clinical instability may have affected the findings.
- Use continuous temperature and cardiorespiratory monitoring with early aEEG or conventional EEG when available, especially if abnormal movements or unexplained deterioration occur.
- Explain eligibility, treatment duration, possible complications, and follow-up needs to parents while maintaining respectful shared decision-making.
Follow-up after the cooling period
Discharge planning should begin well before rewarming. Infants treated for HIE may require assessment of feeding, tone, hearing, vision, sleep, and early motor development. Some appear clinically stable in the neonatal period but later develop cerebral palsy, epilepsy, language delay, executive-function difficulties, or behavioral concerns. Early referral to developmental services can reduce delays in receiving support.
Prognosis should never be based on a single variable. Apgar scores, acid-base status, examination findings, seizure burden, EEG background, MRI pattern, and the clinical course each provide useful information, but their predictive value changes with timing and treatment. Discussions with families should acknowledge uncertainty and use understandable language rather than presenting early tests as definitive.
A consistent cooling pathway improves the speed and safety of care, but protocols must remain adaptable to gestational age, comorbid illness, transport conditions, available monitoring, and local expertise. Reviewing cases through neonatal audit, multidisciplinary conferences, and updated clinical guidance helps teams identify delays, refine referral systems, and support better outcomes.
Perinatal clinicians can use these criteria as a framework for urgent assessment, while applying current national guidance and specialist consultation to every individual newborn. Prompt recognition, controlled hypothermia, careful neurological monitoring, and structured developmental follow-up form the essential continuum of care for infants affected by hypoxic-ischemic brain injury.
