Doctors first noticed something was wrong when the TV star, who played fashion-loving teen Lisa Turtle on the popular U.S. sitcom, began alternating between moments of pure joy and constant chatter to times of prolonged silence, coupled with a vacant stare - within a matter of minutes.
The 38 year old's mother, Tricia, tells People magazine, "It's frustrating... They can't explain it... I care deeply about my daughter and I want her to resume her life."
The actress, who has been struggling with the erratic behaviour for years, was recently prescribed medication to help deal with the mental illness, but Tricia insists the condition was brought on by a series of undisclosed events, which left the actress traumatised.
She refuses to divulge further details, saying, "I don't want to go there," but Tricia adds her daughter was devastated when her career stalled after the show's final episode in 1993 and her marriage to husband Miguel Coleman fell apart in 2001.
However, she does have hope Lark will pull through: "She's trying so hard. She says she's delayed, but she's going to get there."