What might be Doodle's handicap in "The Scarlet Ibis"?What was wrong with Doodle?

Athough the reader is never told exactly what is wrong with Doodle, the sickly little brother in James Hurst's short story "The Scarlet Ibis," he undoubtedly was suffering from a weak heart, physical disabilities and possibly even mental retardation. William Armstrong is unable to walk until he is forcibly taught by Brother, though he can crawl backwards, reminiscent of a doodlebug--thus the origin of his nickname. His parents originally think that he would not survive and, later, that he may not be "all there," but Brother later determines that, in fact, "he's all there!" This may or may not have been true, since Doodle's speech and learning skills seem to have been somewhat arrested as well.