Dissociated Deviation in the Infantile Strabismus Syndrome

I thank Martín Gallegos-Duarte for his letter and his interest in the subject of dissociated vertical divergence (DVD). He shares his observations and scientific contributions about the origin of dissociated strabismus, which are interesting. I do think that the Gordian knot of the origin of DVD is cut with a neuroanatomical sword. His conclusions, based on his research, lead him to believe that any solution of this difficult problem must at least include the cerebral cortex.

Indications that cortical visual pathways are involved in DVD are: 1) the ability to produce alternating hypertropia in DVD by way of attention and intentional fixation changes; 2) the balanced interaction between both eyes due to interocular suppression; and 3) the light intensity–response dependency in DVD, which is similar to the same dependency in the visual cortex. We used these considerations to propose a hypothesis on the neural mechanism for DVD (corticotectal lateralization). We postulated that DVD originates from dominance of nasal retinal ganglion cells, cortical suppression and insufficiently developed neuronal coupling between the superior colliculi. As a result the visual cortex in one hemisphere is more active and this asymmetrical activation is transferred to the brainstem.^1–3

So, I do agree with Dr. Gallegos-Duarte that unbalanced cerebral cortical activation initiates DVD and that every hypothesis on the origin of DVD must include the cerebral cortex. I also firmly agree with his phrase “dissociated deviation,” which includes both a vertical (DVD) and a horizontal (DHD) manifestation only. It doesn’t include dissociated torsional deviations. He also writes that the cortex itself does not elicit torsional elevation movements. These arguments corroborate our hypothesis of corticotectal lateralization. In the mechanism presumed by us, it is assumed that excitation of the corticotectal pathway gives rise to a downward eye movement while inhibition leads to elevation of the eye. In DVD a downward stimulus will remain dissociated at the level of the superior colliculus due to a failure in the linkage between the colliculi. The same neural mechanism explains dissociated horizontal divergence (DHD). For horizontal vergence Alvarez showed that latency for divergence depends on the initial vergence angle, while convergence is independent of this angle.⁴ Therefore, similar to vertical eye movements, we assume that convergence is realized by corticotectal excitation and divergence by inhibition of this pathway. So, inhibition of one corticotectal pathway may lead both to elevation of one eye (DVD) and or an exodeviation (DHD). In primates both the cortex and the superior colliculus are two-dimensional. So, corticotectal dissociation only includes dissociation in two dimensions: DVD and DHD. The torsional eye movements in DVD are not dissociated; there is no torsional vergence. The torsional movement is the same torsion we encountered in disparity-induced vertical vergence.⁵ Most likely a purely vertical signal, at the level of the cortex and superior colliculus, becomes a cyclovertical signal after the interstitial nucleus of Cajal.

Therefore, latent nystagmus, DVD and DHD are all explained by a similar pathophysiology: due to a lack of binocular input, the developing human cerebral cortex fails to orchestrate and change the monocular organization of the brainstem oculomotor structures.

This presumed hierarchical control system in which abnormal cortical development leads to fluctuating corticotectal output to subcortical oculomotor control systems, however, is not malleable with Brodsky’s hypothesis concerning the righting reflex of fish.⁶ A theory about the origin of DVD must include the cerebral cortex, but it should explain also why the elevating eye in DVD extorts. Besides the many irrefutable reasons why DVD is not a righting reflex (it is a body reflex instead of an eye reflex; the eye movements in fish are opposite to Bielschowsky’s phenomenon in DVD; anatomical structures responsible for the righting reflex in fish are absent in the primate brain), the eye movements of DVD and disparity-induced vertical vergence are impossible to explain in the realm of fish neuroanatomy.