Extending the duration of hypothermia does not further improve white matter protection after ischemia in term-equivalent fetal sheep

A major challenge in modern neonatal care is to further improve outcomes after therapeutic hypothermia for hypoxic ischemic encephalopathy. In this study we tested whether extending the duration of cooling might reduce white matter damage. Term-equivalent fetal sheep (0.85 gestation) received either sham ischemia followed by normothermia (n = 8) or 30 minutes of bilateral carotid artery occlusion followed by three days of normothermia (n = 8), three days of hypothermia (n = 8) or five days of hypothermia (n = 8) started three hours after ischemia. Histology was assessed 7 days after ischemia. Ischemia was associated with loss of myelin basic protein (MBP) and Olig-2 positive oligodendrocytes and increased Iba-1-positive microglia compared to sham controls (p < 0.05). Three days and five days of hypothermia were associated with a similar, partial improvement in MBP and numbers of oligodendrocytes compared to ischemia-normothermia (p < 0.05). Both hypothermia groups had reduced microglial activation compared to ischemia-normothermia (p < 0.05). In the ischemia-five-day hypothermia group, but not ischemia-three-day, numbers of microglia remained higher than in sham controls (p < 0.05). In conclusion, delayed cerebral hypothermia partially protected white matter after global cerebral ischemia in fetal sheep. Extending cooling from 3 to 5 days did not further improve outcomes, and may be associated with greater numbers of residual microglia.