Fixation is the primary driver of dissociated vertical divergence
Dissociated vertical divergence (DVD) remains one of the most controversial ocular motor disturbances. It is characterized by the slow ascent of either eye followed, after a variable period of time, by a slow descent back to its neutral position. Although it is generally associated with infantile esotropia, it can accompany other forms of binocular misalignment that develop in early infancy.
Alfred Bielschowsky, M.D., defined the essential role of luminance disparity in DVD in 1931. The importance of fixation was demonstrated in 1944 by Adolph Posner, M.D. "It remains unknown yet today, however, whether a binocular luminance disparity can trigger DVD in the absence of a pre-existing binocular fixational stimulus," says Michael C. Brodsky, M.D., an ophthalmologist at Mayo Clinic's campus in Rochester, Minnesota.
"This issue is crucial to understanding the pathophysiology of DVD," says Dr. Brodsky. "To address it, our research team performed video-oculographic eye movement recordings in subjects with DVD while independently controlling for luminance and fixational disparity in the two eyes." Study results were published in Investigative Ophthalmology & Visual Science in 2015.
6 subjects, 6 controls, 4 conditions tested
Dr. Brodsky's team measured vertical eye position in six subjects with DVD (ages 11-47 years, five females, one male) and six controls (ages 16-40 years, five females, one male) using video-oculography under conditions of change in fixation and luminance.
Diagnosis of DVD was based on these clinical criteria: a clinical history of infantile strabismus and the finding of a hyperdeviation of each eye when covered on alternate cover testing in the primary position.
Exclusion criteria included an inability to perform the eye movement recording protocol (age younger than 8 years), a refractive error greater than 3 diopters (unless corrected with contact lenses) or a history of vertical muscle surgery.
Simultaneous horizontal, torsional and vertical eye movements were recorded using the SensoMotoric infrared video-oculographic system, a measurement system for acquisition of eye movements based on noninvasive video image processing technology using head-mounted infrared video cameras.
The testing protocol for all subjects involved four specific conditions, performed in consecutive order, in succession, and at less than one-minute intervals:
- Alternate occlusion without fixation (monocular darkening)
- Alternate increased luminance without fixation (monocular flashlight)
- Alternate occlusion with fixation in darkness (crossbar)
- Alternate occlusion with fixation (room light)
Control subjects showed no vertical divergence under any testing conditions, but in subjects with DVD:
- In monocular darkening, when fixation was precluded with a translucent filter and bright light was shined into one eye to produce a marked binocular luminance disparity, some subjects had a small induced vertical divergence causing the illuminated eye to be lower than the nonilluminated eye.
- In monocular flashlight, when fixation was precluded with a translucent filter while alternate occlusion produced a mild binocular luminance disparity, a smaller vertical divergence of the eyes (not statistically significant) occurred.
- In crossbar, when alternate occlusion produced reversal of monocular fixation in the dark with essentially no change in peripheral luminance disparity, there was a significant vertical divergence movement causing the covered eye to be relatively higher than the uncovered eye.
- In room light, the amplitude of the crossbar vertical divergence was similar to that measured under conditions of alternate occlusion in a lighted room, where there also was a significant average relative upward movement of the covered eye.
"Our results confirm that DVD is driven primarily by fixation, and only to a very minor degree by isolated binocular luminance disparity," says Dr. Brodsky. "They suggest that peripheral binocular luminance disparity modulates DVD at the cortical level primarily when superimposed upon fixation. DVD develops when fixation (or fixational effort) evokes facultative cortical suppression of the peripheral retina, which suggests that this secondary effect provides the cortical output signal that ultimately triggers DVD."
Dr. Brodsky, whose research in dorsal light reflex was published in Archives of Ophthalmology in 1999, concludes: "These findings may explain how primitive visual reflexes can be driven through the human visual cortex and provide a novel neurological template for the re-emergence of subcortical visual reflexes in the setting of higher cortical dysfunction."
For more information
Ghadban R, et al. Relative roles of luminance and fixation in inducing dissociated vertical divergence. Investigative Ophthalmology & Visual Science. 2015;56:1082.
Brodsky MC. Dissociated vertical divergence: A righting reflex gone wrong. Archives of Ophthalmology. 1999;117:1216.