[Dissociated horizontal deviations (DHD): nomenclature and etiology. An orientating attempt in terminological...

To classify dissociated horizontal deviations (DHD) based on the etiology of the vergence that causes the dissociation of the squint angle.

Dissociated strabismus can reliably be diagnosed by a change of the 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, fixation distance, accommodation) remain unchanged. The decisive diagnostic tool is the reversed fixation test: During monocular fixation with one eye, the squint angle of the other eye is neutralized using a synoptometer or a prism. Then, the fixation is changed to the other eye, which keeps its position behind the prism. If the change in fixation causes a change in the position of the previously fixating eye, the deviation is dissociated. If the position of the previously fixating eye does not change, the deviation is not dissociated.

Dissociated strabismus can be detected and differentiated from both comitant and incomitant non dissociated strabismus by the reversed fixation test. The dark red glass test is less sensitive and less specific for dissociated strabismus than the reversed fixation test, if a difference in accommodative convergence is excluded, possible causes of DHD are nystagmus-attenuating convergence, substitution of version eye movement by convergence, and a horizontal side effect of the vertical eye muscles in asymmetric DVD.

Dissociated strabismus is an effect of vergence which is brought about by the change in the fixation or dominance from one eye to the other eye or in the interocular ratio of afferent input (luminance). Dissociated strabismus can be classified based on its direction or due to its etiology.