Complex sentence: In the brain, this swelling increases intracranial pressure (ICP), which leads to the first observable symptoms of water intoxication: headache, personality changes, changes in behavior, confusion, irritability, and drowsiness.
Split into simpler sentences: In the brain, this swelling increases intracranial pressure (ICP). It is this increase in pressure which leads to the first observable symptoms of water intoxication: headache, personality changes, changes in behavior, confusion, irritability, and drowsiness.