What's Wrong with Arnie?

After the bathtub incident, Arnie is afraid of any kind of body of water. He gets very upset and starts to hurt himself when he tries to wake his mother and she never wakes up. The diagnosis for Arnie Grape that fits most appropriately is Autistic Disorder

Does Arnie Grape have intellectual disability?

Gilbert Grape lives in a small town with his younger sister, his obese mother, and his brother Arnie, who has an intellectual disability. As a young adult, Gilbert becomes the patriarch of the family, and therefore must provide for his dysfunctional family and act as the primary caretaker for his brother.

How does Arnie die?

His affair with Betty ends when she leaves town in search of a new life following her husband's death; he drowned in the family's wading pool after suffering a heart attack.