Mild head trauma not resulting in a concussion may cause damage to the blood-brain barrier.
From the article:
In the study, which was published online Sept. 5 in the Journal of Neurotrauma, scientists scanned the brains of adolescent and adult rugby players with a special type of magnetic resonance imaging. They found damage to the protective barrier that separates the brain from bloodborne pathogens and toxins in roughly half of adolescent rugby players after a full season — even those who did not report a concussion. Professional mixed martial arts fighters showed similar damage after a fight.
Extremely frequent BBB disruption in rugby players:
Ten of 19 adolescent rugby players showed signs of blood-brain barrier disruption by the end of the season. The barrier breakdown appeared on the scans as red blips concentrated throughout the inner regions of the brain. The researchers also scanned eight college rugby players after a match and saw disruptions of the barrier in two of them. Notably, the injuries most study participants experienced were below the current bar for mild head trauma since they did not suffer a concussion.