DALLAS – Luci Baines Johnson, the younger daughter of former President Lyndon Johnson, is being treated at the Mayo Clinic for what doctors suspect is a rare autoimmune disorder that affects the nervous system, her physician said Saturday.

Johnson, 62, who was flown to the clinic in Rochester, Minn., on Friday, remained in the neuro-intensive care unit, said Dr. Dudley Youman, her doctor in Austin and a family friend.

Initially, Johnson was admitted to a hospital in Austin on Wednesday after experiencing “extreme weakness” in her arms and legs, Youman said.

Youman said she had a mild viral illness preceding this, a sore throat and hoarseness that may have been a warning sign. He said doctors suspect Guillain-Barre syndrome, which affects the nerves that control movement and can cause paralysis.