Attorneys Brian McMonagle and Monique Pressley assist Bill Cosby at court in Elkins Park, Pa. (Matt Rourke/Associated Press)

For decades, comedian Bill Cosby was celebrated as a keen and vigorous observer of daily life. Now, as he faces a criminal charge of sexual assault and myriad allegations that he preyed upon and drugged women, a vastly altered Cosby has emerged: impaired, dependent and, his lawyers argue, blind.

At his December arraignment in suburban Philadelphia, the embattled entertainer carried a thin wooden cane and stumbled over a curb, as two attorneys guided him by each arm into the courtroom.

“He’s a 78-year-old blind man who they’ve chosen to charge,” Cosby attorney Monique Pressley said later. “That’s not a defense to a charge, that’s just a fact.”

How prominent a role will the entertainer’s health play in his multiple legal cases? How was Cosby able to conduct his “Far From Finished” comedy tour last year, where he often sat in a chair but, at his Baltimore appearance, was without a cane and performed workout exercises on the stage? When did his eye condition first arise?

Cosby’s attorneys would not comment for this story, but the comedian’s impaired vision was mentioned in a 2004 sexual­assault lawsuit brought against him by Andrea Constand, a former Temple University official and the alleged victim in the Pennsylvania criminal case. In a decade-old deposition, Cosby testified that his weakening eyesight “is ongoing. It’s getting worse” and refers to his then-ophthalmologist, who has a subspecialty in glaucoma.

Between the deposition and the 2014 storm of sexual-assault allegations, Cosby maintained a rigorous schedule of appearances and comedy concerts, collecting multiple honorary degrees and the 2009 Mark Twain Prize for American Humor, where he did not require assistance appearing on the Kennedy Center stage. He was developing a weekly sitcom with NBC and taped a comedy special for Netflix, both projects ultimately shelved.

In 2010 at Al Sharpton’s annual convention, Cosby acquaintance and news commentator Roland Martin recalls the comedian “was assisted by someone to get to the stage” and initially didn’t recognize Martin. The previous year at the Essence Music Festival, Martin says, Cosby “was observant and did not require the assistance.”

A 2015 defamation lawsuit that Cosby filed against model Beverly Johnson, one of his sexual-assault accusers, notes that “Mr. Cosby has suffered from a degenerative eye condition for years, which has steadily worsened,” and that “for over a year” the entertainer has been “legally and functionally blind.”

Cosby’s right eye appears cloudy and gray in his police mug shot. News articles have variously reported that the comedian has glaucoma, cataracts, macular degeneration and keratoconus, a progressive cornea disease.

“His right eye looks like he might have had a cornea transplant,” said ophthalmologist Christopher Rapuano, chief of cornea services at Wills Eye Hospital in Philadelphia, who has never examined the comedian. From his police photo, Cosby could have a cataract or glaucoma, Rapuano says. “The left eye looks good and clear.”

During a two-day February hearing in Norristown, Pa., part of the aggravated indecent assault criminal case, Cosby never referred to documents or wore glasses, occasionally applying drops to his eyes.

Cosby’s seemingly impaired vision is cited frequently as a possible mitigating factor in a defense motion in the criminal case: “Mr. Cosby has lost his eyesight, hampering his ability to identify the physical appearance of witnesses, to view documents, photographs and video, and thus is limited and in many instances incapable of working with attorneys in preparation of his own defense as well as hindered in his ability to confront evidence offered against him by the District Attorney’s Office.”

Will such arguments succeed?

“It is a creative defense, likely without precedent, that I expect will be unsuccessful,” says Jacob Frenkel, a veteran Maryland criminal defense attorney and former federal prosecutor. “A defendant becoming incompetent or compromised after the offense does not affect culpability for the commission of a crime.”

Under the theory that poor eyesight impairs Cosby’s defense, Frenkel says, “no blind person could ever be prosecuted.”

More essential to his legal defense is Cosby’s health in 2004. “It’s the sight he had at the time of the alleged offense,” says Maryland criminal and civil lawyer Bruce L. Marcus.

In addition to his “loss of eyesight,” the defense motion states that Cosby’s “age and the substantial passage of time have impaired his memories of the relevant events and witnesses.”

“Sort of floating around is the issue of competency,” Marcus says, “to take the position that he’s incompetent to stand trial.”

In the decade-old Constand lawsuit deposition, Cosby said he gave his onetime protege Benadryl to relieve stress and help her sleep. Constand and her attorneys dispute it was Benadryl and say it was a stronger drug that, according to a court filing, made her feel “dizzy and weak” and “only barely conscious.”

At that time of the alleged 2004 assault, Cosby testified it was easy to know what he gave Constand, but that his eyesight had since deteriorated.

“A number of months have passed. When Andrea took the pills, I could see and identify the boxes clearly,” Cosby said. But when he gave the deposition, he testified that “the degeneration of the vision” now required him to use an illuminated magnifying glass to distinguish the antihistamine.

Cosby continued to lead an active life. “Many of our patients have limited vision and visual function but live very productive and fairly normal lives,” says Rapuano, the ophthalmologist.

Cosby was criminally charged days before Pennsylvania’s 12-year statute of limitations expired. That long prosecutorial delay, rather than Cosby’s vision, “ now impairs his ability to defend himself,” says Joe King, a veteran Virginia criminal defense lawyer.

“A good defense lawyer from day one pulls every trick out of his bag,” says Frenkel. “The defense strategy is to do everything possible to ensure it never goes to trial.”