How are seizures diagnosed in hypoxic-ischemic encephalopathy (HIE)?

Updated: Jul 18, 2018
  • Author: Santina A Zanelli, MD; Chief Editor: Dharmendra J Nimavat, MD, FAAP  more...
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HIE is often reported to be the most frequent cause of neonatal seizures. They usually occur 12-24 hours after birth and are difficult to control with anticonvulsants. Large, unilateral infarcts occur with neonatal seizures in as many as 80% of patients. Seizures are often partial (focal) and contralateral to the cortical lesion. About two thirds of newborns with cerebral venous infarcts have seizures. Those with multiple or diffuse lesions and cerebral venous infarcts often have multifocal or migratory seizures. Seizures are observed during physical examination and may confirm the diagnosis. Observation often reveals clonic rhythmic contractions. When holding the limb affected by clonic seizures, the examiner's hand shakes or feels limb movement. Limb flexion or extension does not suppress the clonic activity, as it does in jitteriness and clonus. Newborn infants cannot have generalized seizures due to immaturity of the neuronal pathways connecting the 2 halves of the brain.

Tonic, unilateral, or focal seizures consistently have an EEG signature. In the seizures, unilateral arm and leg posturing is often accompanied by ipsilateral trunk flexion. Generalized tonic posturing (eg, extension of the upper and lower extremities or extension of the legs and flexion of the arms) is related to an EEG seizure in 15% of affected neonates.

Tonic seizures can be seen in neonates with local anesthetic intoxication. Although generalized tonic posturing is infrequently associated with electrical seizures, it is not a benign sign. Of neonates with tonic posturing and an abnormal EEG background, 13% have normal development.

Mizrahi and Kellaway suggested the name brainstem release phenomena because tonic posturing and some subtle seizure-like motor automatisms are probably the result of primitive brainstem and spinal motor patterns liberated because the lack of inhibition from damaged forebrain structures. [35]  However, this tonic posturing is not a seizure and, thus, treatment with antiepileptics does not have benefit unless the infant is having other semiology consistent with seizures.

Subtle seizures may be a part of the HIE picture. Subtle manifestations of neonatal seizures are confirmed on EEG and include apnea; tonic eye deviation; sustained eye opening; slow, rhythmic, tongue thrusting; and boxing, bicycling, and swimming movements. Most still accept that some subtle seizures may be correlated with EEG results. However, publications since the late 1980s have shown that seizures are not as frequent as previously thought and that they are unusual in patients close to term. Several other patterns of subtle neonatal seizures are described without EEG confirmation. The lack of an EEG signature does not exclude CNS pathology because neonates with HIE often have motor automatisms without EEG seizures. Management is controversial, but treatment is not usually beneficial unless more overt seizure activity is noted. [36]

Seizures may be difficult to clinically diagnose in the premature neonate. Subtle seizures associated with ictal EEG changes are not rare in premature infants. The subtle patterns of neonatal seizures in the premature infant include sustained eye opening, oral-buccal-lingual movements (smacking, drooling, chewing), pedaling movements, grimacing, and autonomic manifestations.

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