Strabismus - Fundamentals of Clinical Ophthalmology.pdf

STRABISMUS
Bifoveal fusion
Normal BSV field
= central fusion
Central suppression of R
Central suppression of R
R. Esotropia with
R. suppression
Smaller BSV field
R. Exotropia with
R. suppression
Larger BSV field
Figure 3.3 Diplopia arises from simultaneous
appreciation of two different images of the one object
due to loss of binocular vision. It arises because the
image (dark grey triangle) in the non-fixing eye falls
on a location outside the macula (black cross) and is
projected to a different point in space. The image
in the non-fixing eye is more faint than that in the
fixing eye
disruption of fusion. In severe head injury with
bilateral superior oblique palsy, the inability to
fuse may be caused by the excyclotropia, usually
of more than 10°.
Suppression
Suppression is a mechanism which eliminates
visual confusion and diplopia by removing an
unwanted image. It may also eliminate fusion
and stereopsis in the mature visual system in the
adult, for example in long-standing unilateral
cataract. The suppression scotoma is roughly
2·5° in diameter 4 and differs in size and shape in
strabismus patients with exotropias compared to
esotropias. 5 The extreme peripheral temporal
field of each eye has no overlap with the other
eye, is monocular, is not suppressed in
amblyopia and contributes to the extent of the
Figure 3.4 Fields of binocular vision and
suppression. Note: (i) In normal binocular vision
there is central fusion. The peripheral temporal field
of each eye has no overlap from the other eye, is
monocular, is not suppressed, and contributes to
the binocular field, even in the presence of strabismus.
(ii) The double arrow-head represents the width of
the binocular field of vision, being smaller in esotropia
than in exotropia
visual field (Figure 3.4). Suppression may also
occur in the adult as a result of an adaptation to
visual confusion.
Suppression develops in the immature visual
cortex as a response to differing inputs from each
eye and is a barrier to the development of fusion.
In the immature visual system its presence can
eradicate the fusion and stereopsis that have
already developed. The whole of the visual field
of the deviating eye is suppressed. The overlap
with the visual field of the fixing eye is
suppressed and the remainder of the visual field
of the deviating eye is not suppressed. 6 The
binocular field is wider in an exotropic patient as
a consequence and smaller in an esotropic
patient (Figure 3.4).
Suppression is an important aspect to
consider in surgery. If there is suppression of the
retina there is no diplopia; however, if surgery
overcorrects the strabismus, diplopia may be
evoked. In the visually immature child,
suppression will develop in response to the new
eye position, but in the visually mature adult,
diplopia may persist and be troublesome.
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