At a news conference Friday, Dr. Margaret Mermin said King's speech is limited to a few words, the Washington Post reported.
"She gets frustrated at times, as anyone would who can't speak," Mermin said.
Mermin said King, 78, suffered a minor heart attack which caused a major stroke. She had recently suffered two minor strokes.
She has continued her husband's work for equality and civil rights, founding the King Center following his 1968 assassination. She was hospitalized in April with atrial fibrillation and, following doctors' advice to avoid stress, she cut back on public appearances over the summer.
Mermin said King had been unwilling to take blood thinners for the atrial fibrillation.
Martin Luther King III said in a statement "expressions of love and concern" have been pouring in for his mother and the family.