What is "strephosymbolia"

Strephosymbolia was first coined in 1925 by Samuel Orton to make a distinction between itself and " word-blindness," but no reasons were given for what these distinctions arise from, or what exactly strephosymbolia means other than "a descriptive name for the whole group of children who show unusual difficulty in learning to read." This definition was subsequently changed in 1937 to specify that, as a condition, it creates difficulty in seeing the ends of written words in unfamiliar languages, especially those in which the letters look similar to each other such as "b" and "d", "m" and "n", and "p" and "q."