PHILADELPHIA (AP) Critics of the proposed $1 billion settlement of NFL concussion claims want a full U.S. appeals court in Philadelphia to review a recent court decision that upheld the deal.

The challengers believe the lead players' lawyers negotiated away compensation for the key complaint in the original lawsuit, a brain disease known as chronic traumatic encephalopathy, because the science is still being developed.

In their appeal Thursday, they said the lead players' lawyers traded away the CTE issue in exchange for higher awards for less common problems, such as Parkinson's disease and dementia.

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''Rather than ... deleting the CTE claims from the complaint, however, class counsel proceeded to settle those claims for no compensation, while obtaining recovery for other, rarer, conditions that are not exclusively associated with head trauma,'' lawyer John J. Pentz wrote in asking the entire 3rd U.S. Circuit Court to rehear the case.

A three-judge panel unanimously upheld the settlement earlier this month, finding it perhaps imperfect but nonetheless reasonable.

The lead players' lawyers, in response, said further appeals delay payments to ailing players and their families, including that of former Philadelphia Eagle Kevin Turner, who died last month at 46 after battling Lou Gehrig's disease.

Senior U.S. District Judge Anita Brody first approved the settlement last year after twice sending it back to lawyers for revisions. Individual players could receive up to $5 million in the case of severe brain trauma, which could result in NFL payouts over 65 years totaling more than $1 billion, including interest and lawyer fees.

The first concussion lawsuits, filed in 2011, accused the NFL of concealing known links between football and brain injuries and failing to properly address the headaches, dizziness and dementia reported by players and retirees.

The NFL has long denied any intentional misconduct. Last month, a league official testifying before Congress acknowledged for the first time a link between football and CTE.

Also Thursday, in what may be a sign of things to come given the huge legal fees at stake, former Chicago Bears star Gale Sayers and six others sued their original lawyers in the case. As first reported by The Legal Intelligencer, they say the firms have improperly put liens on their future awards.

The settlement includes a potential $112 million in legal fees for the lead players' lawyers.