Eye movements in canine hemichiasma: does human hemichiasma exist?

Abstract

Purpose: We wished to compare the eye movements seen in two Belgian sheepdogs whose crossing retinal fibers from one eye were interrupted at the optic chiasm (hemichiasma) with those seen in dogs lacking any crossing fibers (achiasma). In the latter condition, congenital nystagmus (CN), see-saw nystagmus (SSN), and strabismus result; also, unyoked or uniocular eye movements (saccades and nystagmus) are possible. Methods: Eye movements during fixation were measured using infrared reflection. Data were digitized either at 250 Hz with 8-bit resolution or 200 or 400 Hz with 16-bit resolution. Results: One dog behaved normally, indicating good stereopsis, and had no nystagmus. However, unyoked and uniocular saccades were recorded and the number of fibers from the good eye were close to normal. The other dog mimicked the behavior of dogs with achiasma, including CN and SSN and there was a reduced number of fibers from the good eye and an increased number of fibers to the ipsilateral lateral geniculate. Conclusions: Although bilateral redirection of retinal fibers that would normally cross may be strongly associated with the CN and SSN seen in achiasmatic canines and humans, unilateral redirection is not. The preservation of one ‘binocular’ representation of the central visual field seems to be sufficient to calibrate both horizontal and vertical ocular motor subsystems, thereby preventing the development of CN and SSN; hemichiasma may not result in strabismus. Dogs with either achiasma or hemichiasma make uniocular saccades. The discovery of canine achiasma led to the identification of its human counterpart and canine hemichiasma now raises the possibility of a similar syndrome in humans. Human hemichiasma may mimic achiasma, with CN and SSN, or, unlike achiasma, it might not be associated with CN, SSN, or strabismus, preserving stereopsis. Thus, it might easily go undetected in the human population. However, human hemichiasma may be differentiable from achiasma by imaging the chiasm, or possibly, using VEP.