Dissociated horizontal deviations (DHD): terminology and causes - a review

Purpose: To classify dissociated horizontal deviations (DHD) based on the e tiology of the vergence that causes the dissociation of the squint angle. Methods: Dissociated strabismus can reliably be diagnosed by a change of th e squint angle caused by an alternation in the fixation from one eye to the other, when all the other conditions (head posture, direction of gaze, fix ation distance, accommodation) remain unchanged. The decisive diagnostic to ol is the reversed fixation test: During monocular fixation with one eye, t he squint angle of the other eye is neutralized using a synoptometer or a p rism. Then, the fixation is changed to the other eye, which keeps its posit ion behind the prism. If the change in fixation causes a change in the posi tion of the previously fixating eye, the deviation is dissociated. If the p osition of the previously fixating eye does not change, the deviation is no t dissociated. Results: Dissociated strabismus can be detected and differentiated from bot h comitant and incomitant non dissociated strabismus by the reversed fixati on test. The dark red glass test is less sensitive arid less specific for d issociated strabismus than the reversed fixation test. If a difference in a ccommodative convergence is excluded, possible causes of DHD are nystag-mus -attenuating convergence, substitution of version eye movement by convergen ce, and a horizontal side effect of the vertical eye muscles in asymmetric DVD. Conclusion: Dissociated strabismus is an effect of vergence which is brough t about by the change in the fixation or dominance from one eye to the othe r eye or in the interocular ratio of afferent input (luminance). Dissociate d strabismus can be classified based on its direction or due to its etiolog y.