EOM function was quantified in terms of ROEM by measuring ocular excursions (ductions) for each eye on the Goldmann perimeter using, as in a previous CPEO study [8], the modified uniocular field of fixation technique [9]. This isolates the action of each individual muscle by recording excursions only along the axes corresponding to that muscle’s primary field of action. These are plotted on standard Goldmann charts with pre-printed testing axes. For the right eye the axes are: 0° (lateral rectus, LR); 67° (superior rectus, SR); 141° (inferior oblique, IO); 180° (medial rectus, MR); 216° (superior oblique, SO); and 293° (inferior rectus, IR). The left eye uses a chart which is the mirror image of the right eye chart. Testing was performed uniocularly with chin and head fixation and excursions repeated to ensure measurement consistency within 5°.