Fast-phase instabilities in normally sighted relatives of congenital nystagmus patients--autosomal dominant and x-chromosome recessive modes of inheritance

The Molecular Genetics of Congenital Idiopathic Nystagmus

Congenital Idiopathic Nystagmus (CIN) is genetically heterogeneous. Autosomal dominant, autosomal recessive and X-linked patterns of inheritance have been reported. Linkage analysis has suggested the existence of at least three distinct loci for both autosomal dominant and x-linked forms, although as yet no disease genes have been identified. The pathophysiological mechanisms underlying nystagmus are poorly understood and it is likely that insights may arise from finding and characterizing disease genes. If linkage experiments are used to find "nystagmus genes," their power will depend heavily on accurate phenotyping to avoid misdiagnosis due to masquerading conditions and phenotypic variations within pedigrees.

Saccadic instabilities in albinism without nystagmus

Albinism effects a surprising manipulation of the visual pathway in which some of the normally uncrossed axons of the temporal retina instead cross at the chiasm. An expected consequence of this misrouting is that subjects with albinism will have difficulty in specifying the targets of saccades. Usually albinos have nystagmus so the stability of their saccadic eye movements is not readily accessible, but some albinos do not have nystagmus. In these subjects it was found that they had frequent saccadic intrusions, the sizes of which were correlated with velocities of steady drifts in fixations (r = 0.802, P < 0.05). An explanation for the correlation between the amplitudes of the intrusions and the velocities of the drifts is that it is due to a common failure in the development of a saccadic system which is responsible for converting a given retinal displacement into a matching eye movement, with the extent of the failure reflecting the severity of the misrouting.

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

We 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.

Congenital nystagmus in a (46,XX/45,X) mosaic woman from a family with X-linked congenital nystagmus

X-linked congenital nystagmus is a rare disorder in which affected males manifest binocular uniplanar nystagmus with associated head oscillation. In the families previously reported, affected females have been described. We report on a multigeneration family with X-linked congenital nystagmus with an affected woman. She was a (46,XX/45,X) mosaic. Magnetic resonance images of the brain of affected individuals were normal.