Dissociated horizontal deviation.

Despite the emphasis placed on the vertical aspect of the deviation by its currently accepted clinical label, dissociated vertical deviation is well known to include movements in both the torsional and horizontal planes. When the horizontal component is very prominent, dissociated horizontal deviation is suggested as an appropriate label. We report the clinical characteristics and surgical treatment of six patients with a dissociated horizontal deviation that was marked enough to be the reason for seeking medical attention. All patients underwent lateral rectus recession unilaterally or bilaterally, alone or in combination with superior rectus recession. In addition, we report the use of the darkening wedge test to demonstrate the Bielschowsky phenomenon in the horizontal plane. This phenomenon, a dissociated exotropia spontaneously moving toward and crossing the midline to become an esotropia under cover when a progressively darkening filter is placed over the fixing eye, has not to our knowledge been previously reported.