| Abstract |
Purpose: The oculomotor system provides a spectacular window into the brain and
the central nervous system. As long as different types of eye movements are
controlled by different neural networks in the brain, investigation of the oculomotor
behaviour constitutes a very useful diagnostic tool for a variety of neurological
disorders. Recent researches have revealed the major part of the neuroanatomical
and neurophysiological substrate of eye movements. Therefore it became possible to
attribute disordered properties of eye movements to dysfunction of specific neuronal
populations or structures in the brain. The aim of this study was to assess fixation and
smooth pursuit movements in patients with Parkinson’s disease and compare them
with healthy age-matched subjects.
Methodology: 16 patients with Parkinson’s disease (8 mildly affected, 8 with
advanced PD), aged 54-82 yrs, were examined. 12 out of 16 were receiving
medication. 9 healthy volunteers, aged 51-72 yrs, with no history of neurological or
visual disorders were studied as a control group. A group of 10 young healthy subjects
(26-34 yrs) was also examined. Eye positions signals were recorded with a headmounted,
video-based eye tracker (Eyelink II, SR Research, Canada) at a sampling
rate of 500Hz and an angular resolution of <0.01º. Fixational eye movements were
recorded using a circular target (profiled spot) of 0.229º diameter at 100cm distance.
Two contrast levels were evaluated (100% and 10%). Smooth pursuit movements were
recorded using a circular (1.948º diameter) sinusoidal moving target at 50cm
distance, in both directions (horizontal and vertical). Three different target
frequencies were used (0.2, 0.5, 1.0Hz). Stimuli were displayed on a SONY, GDM 520
monitor by means of a VSG 2/5 card (CRS, UK). All subjects had their heads restrained
with a chin rest. Recordings were binocular and the duration of each trial was 15sec.
The system was calibrated prior to each separate recording session. Data were
retrospectively analysed using a Matlab program created specifically for this
purpose. Fixation stability was assessed by Bivariate Contour Ellipse Areas (BCEAs)
(68% of fixation time). Fourier transformation was used to analyse smooth pursuit
movements. Pursuit gain (mean eye amplitude over target amplitude), frequency
gain (eye frequency over target frequency) and saccades (velocity and
acceleration criteria: υ > 50º/sec, a > 2000º/sec2) during smooth pursuit, were
determined. The results were analysed by repeated measures ANOVA and post hoc
tests (Fisher’s Protected LSD).
Results: Significant reduced fixation ability was found in the old group when
compared with young participants: an increase in age implies larger ellipse areas.
Contrast was also found to have strong effect on the size of BCEAs, with fixation to
low contrast target generating larger ellipses. No influence of Parkinson’s disease on
fixation stability was found. The difference between mildly and severely affected
patients was of no statistical significance too. As for the smooth pursuit movements,
many statistical differences were revealed. Results showed that smooth pursuit gain
reduced significantly with increasing age and target frequency. For the latter,
statistical differences were found between 0.2-1.0Hz and 0.5-1.0Hz (no significant
difference was found between 0.2-0.5Hz). Target direction had a significant influence
on pursuit gain as well. Parkinson’s patients were strongly impaired in the pursuit gain
when compared with controls but no difference was found among the mild and the
advanced group. Finally, there was a significant interaction between group and
target frequency to smooth pursuit gain. Regarding frequency gain, strong effects of
age (as age increases, frequency gain decreases), target frequency (decreased
frequency gain with increasing stimulus frequency) and direction, were found. PD
patients did not differ from age-matched control subjects in their frequency gain. No
difference was found between the two groups of PD patients as well. Finally, as for
the saccades during smooth pursuit, the effects of age, target frequency and
Parkinson’s disease, were found significant. Older subjects make more saccades than
the young ones whilst PD patients make less when compared with the control group.
No statistical difference was found between mildly and severely affected patients. All
subjects execute more saccades as target frequency increases. Also, the interaction
between direction and group was found significant. Healthy subjects perform more
saccades at the vertical pursuit whereas PD patients perform less.
Conclusions: A possible explanation for the decreased fixation ability of the old
subjects, are the saccadic intrusions which occur as the age increases. It is
noticeable though, that no difference between PD patients and control group was
found (saccadic intrusions are strongly present in many neurological disorders, as PD).
Accurate estimation of their number would be therefore helpful in a future study. The
increasing area of fixation as the target contrast decreases is attributed to the
diminished ability of the visual system to detect low contrast stimuli. Both age and
Parkinson’s disease affect the smooth pursuit movements. The restriction of the range
of ocular motility (primarily in the vertical plane) in old subjects as well as in PD
patients is attributed to changes in the orbital tissues. As for the saccades, older
subjects have to make more saccades in order to keep the line of sight on the
moving target. On the contrary, PD patients are unable to produce the right number
of saccades needed, probably due to their bradykinesia. Dysfunction of the basal
ganglia, which play an important role to saccade execution, constitutes another
possible reason for this disability. The fact that no difference was found between
mildly and severely affected patients is very important, since eye movement disorders
seem to be present by the early stages of the disease.
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Μελέτη της οφθαλμοκινητικότητας σε ασθενείς με Parkinson
