Download Normal visual pathway routing in dissociated vertical deviation.

Investigative Ophthalmology & Visual Science, Vol. 29, No. 7, July 1988
Copyright © Association for Research in Vision and Ophthalmology
Normal Visual Pathway Routing
in Dissociated Vertical Deviation
Christine Boylan,* Richard Anthony Clement,* and Anne Howrief
Flash visually evoked cortical potentials have been recorded in three groups of age- and sex-matched
subjects; one comprised of subjects with dissociated vertical deviation, one comprised of subjects with
oculocutaneous albinism and one group of controls. The latency of the major positive (P2) component
did not show statistically significant contralateral lateralization on monocular stimulation in either the
dissociated vertical deviation group or the control group. Contralateral lateralization was found in the
albino group at a statistically significant level (P < 0.01). It is concluded that subjects with dissociated
vertical deviation do not possess the typical albino optic pathway misrouting that has been reported.
Invest Ophthalmol Vis Sci 29:1165-1167,1988
The visual system of albinos is characterized by a
misrouting of optic nerve fibers. Somefibersfrom the
temporal retina are misrouted at the chiasma and
cross over to terminate in the contralateral rather
than the ipsilateral hemisphere. This has been shown
anatomically1 and electrophysiologically when the
visually evoked cortical potential (VECP) shows contralateral lateralization on monocular stimulation.2"8
Recent VECP studies using half-field pattern reversal stimulation in subjects with dissociated vertical
deviation (DVD) have concluded that these individuals possess the albino-type misrouting,910 although
there is no anatomical confirmation of such a defect.
Albino-type misrouting can be demonstrated with
full-field stimulation using a simple flash stimulus. In
albinos, monocular responses show contralateral lateralization of the major positive component at a statistically significant level; the latency is shorter over
the hemisphere contralateral to the eye stimulated.7
To investigate the possibility of misrouting in subjects with DVD a study was undertaken with the
method already used successfully in albinos.7'8
months. One had an exotropia first noticed at 18
months of age. The optokinetic responses had been
tested in four of the subjects. Three showed a definite
abnormal uniocular response when the drum was rotated in a nasotemporal direction, and one subject
had a reduced response.
Flash VECPS were recorded from Ol (left hemisphere) and O2 (right hemisphere) referred to C3 and
C4 respectively (10/20 system11). Flash stimulation
was provided by a Grass PS22 photostimulator
(Grass, Quincy, MA) at setting 2 (1363 nits) with a
flash rate of 1.8 per second. The initial 500 msec of
the responses to 50flasheswere averaged by a Nicolet
Pathfinder II (Nicolet, Madison, WI), bandpass
0.5-30 Hz. Binocular and monocular responses were
recorded; care was taken to ensure complete occlusion of the nonstimulated eye. For comparison, two
other groups of age- and sex-matched subjects were
examined under identical conditions. One group
consisted of five control subjects with normal visual
acuities (6/6 or better) and no known ophthalmologic
or neurologic defects, and one group consisted of five
oculocutaneous albinos. Informed consent to undertake examination was obtained in all cases from the
subjects and their parents prior to the study.
Materials and Methods
Five subjects with DVD ranging in age from 6-14
(mean 9.8) years were examined. Four had esotropia,
three with onset from birth and one with onset at 4
Results
The DVD group showed the typical flash VECP
waveform. The major component was a positive
component at around 110 msec (P2) preceded by Nl
and PI components. The responses recorded from
one subject are shown in Figure 1. In common with
the previous study7 the P1-N2 and N2-P2 peak to
peak amplitudes and the PI, N2 and P2 latencies
were measured over both hemispheres on monocular
and binocular stimulation in each subject of the three
From the *Department of Vision Sciences, University of Aston,
and the tOrthoptic Department, Birmingham and Midland Eye
Hospital, Birmingham, United Kingdom.
Submitted for publication: November 2, 1987; accepted December 17, 1987.
Reprint requests: Dr. C. Boylan, Department of Vision Sciences,
University of Aston, Aston Triangle, Birmingham, B4 7ET, England.
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INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE / July 1988
groups. Two-way analysis of variance was carried out
using the hemisphere as one treatment and the stimulation condition (binocular, right or left eye) as the
Binocular
Vol.
29
DVD group
116
114
112
110
108
106
104
102
100
RE
BIN
H
Right hemisphere
0
Left hemisphere
\S
Right hemisphere
©
Left hemisphere
H
Right hemisphere
LE
02-C4
Control group
116
114
112
110
108
106
104
102
100
01 -C3
RE
BIN
LE
Albmo group
100 msec
116-|
114 112110108106104102100-
>P
12] Left hemisphere
r
•
RE
BIN
LE
Fig. 2. Graphic illustration of the results of two-way analysis of
variance of the P2 latency data in the DVD, control and albino
groups. There is no significant interaction effect in the control or
DVD groups. There is a significant interaction effect in the albinos
(P < 0.01). P2 has a shorter latency over the hemisphere contralateral to the eye stimulated. RE = right eye, LE = left eye, BIN
= binocular.
01 -C3
Left eye
02-C4
01 -C3
second. The albino group showed a significant interaction effect for the latency of the P2 component, the
latency being shorter over the hemisphere contralateral to the eye stimulated (F2,8 = 16.385, P < 0.01).
There was no significant interaction effect for any of
the measured variables in either the control group or
the DVD group. A comparison of the interaction effects of the P2 latencies in the three groups is shown
in Figure 2.
Discussion
Fig. 1. TheflashVECPs recorded from the two.hemispheres of a
subject with DVD, on binocular and monocular stimulation. The
responses illustrate the noise-free nature of the recordings obtained
and show the typical flash VECP waveform of a major positive
component (P2) preceded by Nl and PI components.
The DVD group examined did not show the VECP
lateralization that is characteristic of albino-type
misrouting as confirmed in this and a previous
study.7 The lateralization on monocular stimulation
in the DVD group did not differ from that found
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VISUAL PATHWAY IN DVD / Boy Ian er ol.
No. 7
within an age- and sex-matched control group. The
responses recorded from the DVD group had the
usual flash VECP waveform, latencies and amplitudes and were not delayed as reported for the pattern
reversal response.910 Without further anatomical evidence to the contrary, we conclude that misrouting
does not occur in subjects with DVD.
This finding of normal optic projections in DVD is
in contrast to previous results910 and may be because
of the following reasons. On methodological grounds,
the most direct test of albino-type misrouting involves monocular full-field stimulation rather than
half-field stimulation.4"6 Because of the gross asymmetry in the cortical projections, a clear lateralization
of the VECP is observed when stimulation is swapped
from one eye to the other. The use of full-field stimulation avoids the need to interpret the lateralization
that occurs with half-field stimulation in terms of the
"paradoxical" lateralization that occurs in normal
subjects.12 On technical grounds, the pattern reversal
VECP is not the appropriate technique for revealing
misrouting; the nystagmus present with albinism has
been shown to result in very poor responses.4"6 Finally, previous studies of DVD subjects910 described
lateralization of the amplitude of the PI00 component but did not give any statistical analysis of the
results. In many of the albinos studied here and previously,7 individual traces did show amplitude lateralization but, due to the inherent variability of the
VECP amplitude, the lateralization was not statistically significant.
Although the neural misrouting found in albinos is
an exciting discovery, one must proceed with caution
when hypothesizing the existence of this misrouting
in other conditions. It was shown early on that
misrouting is not associated with congenital squint.13
At present, it appears that the type of misrouting
found in albinos is specifically linked to a lack of
melanin in the retinal pigment epithelium.14"16
Key words: dissociated vertical deviation, visually evoked
cortical potential, oculocutaneous albinism, misrouting
Acknowledgments
The authors would like to thank Paul Furlong and Vivica
Van der Vliet for their technical assistance and the orthop-
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tists and consultant ophthalmologists at the Birmingham
and Midland Eye Hospital for arranging for us to see the
subjects.
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