Strabismus - Fundamentals of Clinical Ophthalmology.pdf

STRABISMUS
Sensory evaluation – binocular vision,
fusion, and stereopsis
Tests of binocular vision are important not
only to exclude strabismus, but also to assess the
extent of visual impairment.
Stereopsis
In children, the Titmus Fly and the Lang tests
are useful determinants of stereopsis. High grade
stereopsis implies good visual acuity in both eyes
and some degree of fusion. Poor stereopsis may
occur with poor monocular vision in each eye or
diminished acuity in either or both eyes. 7 The
Titmus Fly test is widely used and uses
dissociation polaroid glasses. There is a larger
representation of a house fly and smaller
representations of animals and circles. The
Lang test involves a random dot stereogram
incorporated into a card and has the advantage
that glasses need not be used. Other stereopsis
tests include the TNO random dot stereopsis test.
Testing of monofixation syndromes
The testing of a stable strabismus angle with
peripheral fusion and a monofixation syndrome
(MFS) is important and is the result obtained in
optimally treated congenital esotropia. The
diagnosis of MFS requires evidence of absence
of central binocular vision or bifoveal fixation
and the presence of peripheral binocular vision
and fusion.
The diagnosis of absence of bifoveal fixation
requires the documentation of a macular
scotoma.
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Firstly, useful and more commonly used tests
include Bagolini striate glasses (see following
section). In the presence of a microsquint, if
there is peripheral fusion the patient will see a
cross. If the patient has sufficient cognitive
maturity, they may be able to describe the
scotoma.
Secondly, the AO vectograph project-O chart
slide has value in documentating the
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facultative scotoma of MFS. Patients with
MFS delete letters that image in the eye with
the fixation scotoma.
Thirdly the testing of stereoacuity provides
useful evidence of gross stereopsis indicating
peripheral fusion and about 200–300 seconds
of arc of stereopsis. Fine stereopsis of more
than 40 seconds of arc implies the patient has
bifoveal fixation. Another useful motor test for
monofixation is the 4 D base out prism test
(Figure 6.8). During binocular viewing,
quickly introduce a 4 D base out prism. Both
eyes will move away from the base of the
prism – if binocular vision is normal, the eye
not covered by the prism will make a corrective
convergent movement. This response occurs
no matter which eye the prism is placed over.
By contrast in MFS, when the base out prism
is introduced in front of the fixing eye, there is
the movement of both eyes away from the base
of the prism, but the eye with MFS does not
take up free fixation. If the prism is placed in
front of the non-fixing eye, there may be no
movement. Occasionally MFS causes a switch
fixation each time the prism is moved. This test
is probably the least reliable to diagnose the
presence of macular scotoma. 8
Fusion
Worth 4-dot test Fusion tests include
the Worth 4-dot test involving four illuminated
dots arranged in a diamond with the red above
and the white below and two horizontal green
dots between and lateral to these (Figure 6.9).
The four dots are viewed through red and
green glasses that are complementary to the
illuminated colours. When fusion is present,
four dots are seen, with the white dot changing
colour due to retinal rivalry. If diplopia is
present, five dots will be seen. If suppression is
present, the child will see two or three. The test
has been criticised because the artificial need to
wear green and red filters breaks down phorias
and because the test underestimates in some
people.
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