Quantitative assessment of influence of aging on optokinetic nystagmus

Nystagmus Associated With the Absence of MYOD Expression Across the Lifespan in Extraocular and Limb Muscles

Purpose

The extraocular muscles (EOMs) undergo significant levels of continuous myonuclear turnover and myofiber remodeling throughout life, in contrast to limb skeletal muscles. Activation of the myogenic pathway in muscle precursor cells is controlled by myogenic transcription factors, such as MYOD. Limb muscles from MyoD-/- mice develop normally but have a regeneration defect, and these mice develop nystagmus. We examined MyoD-/- mice to determine if they have an aging phenotype.

Methods

Eye movements of aging MyoD-/- mice and littermate controls (wild type) were examined using optokinetic nystagmus (OKN). We assessed limb muscle function, changes to myofiber number, mean cross-sectional area, and abundance of the PAX7 and PITX2 populations of myogenic precursor cells.

Results

Aging did not significantly affect limb muscle function despite decreased mean cross-sectional areas at 18+ months. Aging wild type mice had normal OKN responses; all aging MyoD-/- mice had nystagmus. With OKN stimulus present, the MyoD-/- mice at all ages had shorter slow phase durations compared to wild type age matched controls. In the dark, the MyoD-/- mice had a shorter slow phase duration with age. This correlated with significantly decreased fiber numbers and cross-sectional areas. The EOM in MyoD-/- mice had increased numbers of PAX7-positive satellite cells and significantly decreased PITX2-positive myonuclei.

Conclusions

The absence of MYOD expression in aging mice causes a decrease in on-going myofiber remodeling, EOM fiber size, and number, and is associated with the development of spontaneous nystagmus. These results suggest that muscle-specific mutations can result in nystagmus, with increasing aging-related changes in the MyoD-/- EOM.

Age-related changes of blood-oxygen-level-dependent signal dynamics during optokinetic stimulation

The present study aimed to reveal the effects of age on the temporal profile of the positive blood-oxygen-level-dependent response (PBR) during low-velocity optokinetic nystagmus (OKN) in healthy subjects. We were specifically interested in comparing these effects with the effects of age on the PBR elicited by pure visual and motor tasks. Therefore, we conducted 2 additional control experiments: a checkerboard experiment (visual stimulation) and a finger-tapping (motor task) experiment. Whereas the oculomotor performance of the subjects remained unaltered, the temporal profile of the PBR changed significantly with increasing age in visual and oculomotor areas. None of the control experiments revealed significant age-related PBR changes. Thus, this study demonstrates that the PBR changes during OKN occur before any changes in the oculomotor performance can be detected. These effects of age are specific for the OKN task and probably reflect both changes in the neurovascular coupling and changes in the neural processing during OKN.

Age related change of optokinetic nystagmus in healthy subjects: a study from infancy to senescence

BACKGROUND

Optokinetic nystagmus (OKN) gain is asymmetrical between temporal to nasal (TN) and nasal to temporal (NT) stimulation in infancy and decreases at older ages. The age at which OKN gain becomes symmetrical and decreases is debated. The aim was to investigate OKN over the whole lifespan in a large sample of healthy subjects.

METHODS

In a prospective, cross sectional study OKN was tested monocularly using TN and NT small field stimulation. Stimulation velocity was 15 degrees /s and 30 degrees /s for children aged under 1 year (n = 97), and 15 degrees /s, 30 degrees /s, 45 degrees /s, and 60 degrees /s for older subjects (1-9 years, n = 66; 10-89 years, n = 86). Gain was measured using infrared oculography.

RESULTS

Significant OKN gain asymmetry in favour of TN versus NT stimulation was found during the first 5 months of life (p<0.05). Only at 11 months of age was OKN symmetrical in 100% of the subjects. The percentage of children with symmetrical OKN decreased with increasing stimulus velocity. OKN gain increased in the second and third years (p<0.05 for 15 degrees /s), remained stable until 50 years of age, and showed a small but significant decrease afterwards for the tested velocities (between 6% and 18%, p<0.05).

CONCLUSIONS

Infrared oculography is an accurate method to assess OKN, especially in children. Knowledge about change of OKN in healthy subjects could be helpful to interpret OKN in patients with abnormal binocular vision or lesions of the central nervous system.

Aging effects upon pursuit eye movements

Quantitative assessment of aging effects upon pursuit eye movements was done in step-ramp stimulus conditions using 32 normal individuals. Eye movements were recorded with infrared reflection oculography. The target was a spot of 0.5 degree red lazar light. The light spot was blanked for 5 ms while the mirror galvanometer moved to a new position. Eye and target position were sampled at 250 Hz and analysed by a personal computer. In onward stimulation in which 2 degrees, 4 degrees, 6 degrees and 8 degrees position steps were followed by fixed ramp speed (10 degrees/s), and also in backward stimulation in which 2 degrees, 4 degrees, 6 degrees step positions were followed by 9 degrees, 17 degrees and 27 degrees/s, eye acceleration increased depending upon increase of retinal slip velocity in the younger group below 49 years. Among the factors effecting aging effects, the cerebrum might be important because visual recognition and eye acceleration are performed in the parietal lobe.