Ocular motor abnormalities in achiasmatic mutant Belgian sheepdogs: Unyoked eye movements in a mammal

AbstractWe studied the eye movements of several members of a family of Belgian sheepdogs that includes achiasmatic mutants. Our aim was to identify the types of nystagmus and other ocular motor abnormalities exhibited by the mutants. We also recorded from several unaffected heterozygous carriers of the genetic mutation and from a normal Irish Setter. Mutant dogs exhibited nystagmus waveforms that were occasionally similar to those of humans with congenital nystagmus (CN). Foveating and braking saccades and foveation periods were seen in some waveforms. More common were pendular oscillations of both eyes that were essentially independent in amplitude and phase. At some times there was a pendular nystagmus with a 180 deg phase shift between the movements of the eyes. Similar to the nystagmus, saccades were often unyoked or uniocular. The eye movements of unaffected relatives did not reveal any saccadic instabilities. However, small saccadic intrusions could have been masked by quantization artifacts. Individual dogs from this family provide an animal model of the ocular motor consequences of the disturbed visual input caused by the absence of an optic chiasm and a novel model of CN. Despite any other ocular motor abnormalities present, the CN may be studied in isolation just as in humans it is studied when strabismus and other types of nystagmus are present. Further studies of ocular motor development and function in achiasmatic dogs have the potential to reveal both the organization of the control systems for each extraocular muscle and the role of yoking of the agonist muscles of the two eyes

The congenital and see-saw nystagmus in the prototypical achiasma of canines: comparison to the human achiasmatic prototype

AbstractWe applied new methods for canine eye-movement recording to the study of achiasmatic mutant Belgian Sheepdogs, documenting their nystagmus waveforms and comparing them to humans with either congenital nystagmus (CN) alone or in conjunction with achiasma. A sling apparatus with head restraints and infrared reflection with either earth- or head-mounted sensors were used. Data were digitized for later evaluation. The horizontal nystagmus (1–6 Hz) was similar to that of human CN. Uniocular and disconjugate nystagmus and saccades were recorded. See-saw nystagmus (SSN), not normally seen with human CN, was present in all mutants (0.5–6 Hz) and in the one human achiasmat studied thus far. This pedigree is an animal model of CN and the SSN caused by achiasma or uniocular decussation. Given the finding of SSN in all mutant dogs and in a human, achiasma may be sufficient for the development of congenital SSN and, in human infants, SSN should alert the clinician to the possibility of either achiasma or uniocular decussation. Finally, the interplay of conjugacy and disconjugacy suggests independent ocular motor control of each eye with variable yoking in the dog