A patient reports that her left arm feels as though it has become enormous and grotesquely swollen, though there is no actual physical change and she does not deny that the arm belongs to her. What is this?

This is body image disturbance: a distorted internal sense of the size of a body part, here a felt enlargement of the arm, occurring without any actual physical change and without denial of ownership, exactly as described. It is not a nihilistic delusion, since body image disturbance differs from a delusion about the body (such as a nihilistic delusion that a body part does not exist) in that body image disturbance is often, though not always, rooted in a demonstrable disturbance of sensory processing rather than a purely psychiatric belief system, and here there is no denial of existence at all, only a distortion of size. It is not depersonalisation, which involves a broader sense of detachment from the self rather than a specific distortion of body part size. And it is not hemispatial neglect, which involves reduced awareness of one side of space rather than a distorted felt size of a body part.