In the documents, the lawyers claim that the 27-year-old former football player "had a horrendous existence" as a result of a severe chronic traumatic encephalopathy (CTE), which is a degenerative brain disease linked to head trauma.
The complaint, which was obtained by PEOPLE, lays out Hernandez's estate's claim that the former New England Patriot sustained multiple hits to the head while wearing Riddell helmets. Hernandez reportedly suffered from Stage 3 CTE from the injuries, which was a rarity in football players at his age.
"Aaron experienced a chaotic and horrendous existence in many respects, due to his undiagnosed brain injury," the estate alleges in its complaint.
CTE causes "aggression, explosive behavior, loss of concentration, mood swings, depression, apathy, and cognitive impairment," the lawsuit reads in another part of the complaint.
Boston University's CTE Center shared a statement in which they say Hernandez "had early brain atrophy and large perforations in the septum pellucidum, a central membrane."
"Aaron Hernandez's advanced stage 3 of CTE is normally found in the median age of a 67-year-old man," Hernandez family attorney Jose Baez told PEOPLE in September.
After Hernandez committed suicide in his cell in prison, he was acquitted of the double murder charges in the deaths of two men outside a Boston nightclub in 2012.
"While we still maintain that he was innocent [of the murders], the CTE can explain a lot of his behavior," Baez said. "The impulsiveness can be a symptom of CTE. We think that the CTE explains a lot of things that Aaron did, including his supposed suicide."