Astrocytosis (astrogliosis) is an abnormal increase in the number of astrocytes due to the destruction of nearby neurons, typically because of hypoglycemia or oxygen deprivation (hypoxia). It usually takes place in prion infections. It is especially prevalent in fatal familial insomnia, a prion infection affecting the thalamus and circadian rhythms. The disease causes insomnia to the point that death results.

This manifests as astrocytic edema, hypertrophy, proliferation, and increased motility.