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Hypoxic-Ischemic Encephalopathy(HIE)

By: Dr. Mary Rose Paradis

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Hypoxic ischemic encephalopathy (HIE) describes the result of asphyxiation of a foal during the birth process. While the foal is in the uterus of the mare, he receives all of his oxygen and nutrients from the placental blood flow. If for some reason this blood flow is interrupted, the foal will suffer. For short periods of time, the foal may be able to send the oxygenated blood to his brain and heart preferentially. If it is interrupted for longer periods, the brain will become oxygen starved, which will result in the death of certain cells and the leakage of fluid into other cells in the brain tissue causing the brain to swell.

Because the brain is encased in the hard bones of the skull, it doesn't have much room to swell. This creates pressure and further destruction of nervous tissue. The pressure within the brain may cause some of the blood vessels going to the brain to narrow, further decreasing blood flow and oxygen to the brain.

Causes

  • Severe maternal illness – colic, endotoxemia
  • Placentitis – infection of the placenta
  • Twinning
  • Premature placental separation – "red bag"
  • Labor induction
  • Dystocia – difficulty foaling
  • Excessive hemorrhage from the umbilical cord

    A thorough history may be helpful to your veterinarian in making the diagnosis of HIE. For example if the foal and the placenta were expelled together at the birth or if you saw a "red bag" (premature placental separation) at the time of birth, then you could be fairly certain that the placenta had separated while the foal was in the uterus. If you have a foal that was malpositioned and took some time and force to deliver, then you might suspect that the foal suffered some hypoxia while in the birth canal.

    The clinical signs of HIE may be singular and static, such as a lack of suckle reflex, or they may be progressive to seizure and coma. The progression is probably due to the continued swelling of the brain. The signs maybe full blown at birth or they may occur gradually over the first 72 hours of life. If the signs are progressive they may follow a pattern similar to the following scheme:

  • After birth the foal stands and maybe nurses
  • Within a few hours the foal may start to wander away from his dam
  • The foal loses his ability to suckle
  • The foal may then show signs of depression and becomes recumbent
  • The foal may develop a vacant look in his eyes
  • The foal can begin to have seizures. Seizures may be severe with the foal becoming rigid, throwing his head back (opisthotonus) and paddling his legs. They are generally unaware of their surroundings and will vocalize – sounding like a barking dog. More subtle seizures may be missed. They could present as roving eye movements, tongue movements, grimacing and repetitive blinking.

    These signs may progress rapidly within a few hours or they may take a few days. If the foal recovers from this problem it usually does so in the reverse order of the appearance of the signs, taking sometimes up to a week to be neurologically normal again.

    Disturbed mental function in the equine neonate can occur with other diseases besides HIE. Some of these include:

  • Hypoglycemia. A foal with a blood glucose of less than 40 mg/dl will show similar signs of depression and may progress to seizures.

  • Trauma. Occasionally mares are not as agile around their foals as they should be. Head trauma should be ruled out as a possible cause for mental dysfunction.

  • Developmental abnormalities. Infrequently a foal is born with a congenital neurology problem, such as hydrocephalus.

  • Septic meningoencephalitis. In equine neonatal septicemia, the widespread infection may spread to the brain and the covering of the brain (meninges).

  • Tetanus. A foal with tetanus may be confused with a foal with seizures because they become very rigid.

  • White muscle disease. This is a muscle disease caused by a deficiency in selenium and vitamin E that produces signs of weakness and recumbency.

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