rules for evaluating diplopia

Table 16-3. Rules for evaluating diplopia
1. Head tilt: When the weak extraocular muscle is unable to move the eye, the head moves the
eye. Therefore the head tilts and turns in the direction of action of the weak muscle (see Figure
16.9).
2. The image from the nonfixing eye is the false image and is displaced in the direction opposite
the deviation; thus when the patient fixes with the nonparetic eye, the false image is displaced in the direction of action of the paretic muscle (see Figure 16.5).
3. The false image is the most peripheral image and is displaced in the direction of action of the
weak muscle, except when the patient fixes with the paretic eye. When the lateral rectus is
paralyzed, the eyes are esotropic (crossed), but the images are uncrossed (see Figure 16.5A).
The diplopia is worse at a distance and on looking to the side of the weak muscle. When the
medial rectus is paralyzed, the eyes are exotropic (wall-eyed), but the images are crossed (see
Figure 16.5B). The diplopia is worse at near and on looking to the opposite side.
4. The images are most widely separated when an attempt is made to look in the direction of the paretic muscle.
5. Secondary deviation (the angle of ocular misalignment when the paretic eye is fixating) is
always greater than primary deviation (when the good eye is fixating) (see Figure 16.6). Patients who fixate with the paretic eye may appear to have intracranial disease.
6. Comitance: With a comitant strabismus, the angle of ocular misalignment is relatively constant in all directions of gaze. With a noncomitant (paralytic) strabismus, the angle of misalignment varies with the direction of gaze.