Strabismus - Fundamentals of Clinical Ophthalmology.pdf

1 Concepts in strabismus
Normal binocular vision
Normal binocular single vision is the ability of
the brain and visual cortex to fuse and integrate
the image from each eye into a single perception.
It implies bifoveal fusion and a high degree of
stereopsis (40 seconds of arc). Normal binocular
vision develops after birth from early infancy and
is completed with fusion and stereopsis by the age
of 8–10 years. Its maturation is associated with a
maturation of visual functions in both sensory
and motor systems.
Strabismus
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Strabismus is a misalignment of the eyes, such
that the visual axes of each eye are not
simultaneously directed at the object of regard.
The misalignment may be present in a
particular direction of gaze or in all directions
of gaze.
Strabismus implies an impairment of binocular
vision.
The close interrelationship between sensory
and motor system development means that
strabismus may arise from a disturbance of either
sensory or motor development, particularly in
the younger patient where the visual functions
are more vulnerable.
The near reflex
The concept of the near reflex is important in
the maintenance of binocular vision at near and
in the effect of refractive error in the development
of strabismus. It is also important in the
understanding of strabismus and mechanisms of
accommodative strabismus. The eye has the
capacity to vary its focus from distance to near
and this ability (accommodation) is linked in a
synkinesis to the power of convergence of the
eyes and pupillary constriction in what is known
as the “near reflex”. The near reflex occurs on
visual fixation at near, as in the act of reading.
There is evidence that newborn infants can
accommodate shortly after birth; as early as 1
month infants are able to intermittently fixate
on near targets as determined by retinoscopy. 1
Because of smaller pupils and poorer central
vision, newborns have a greater depth of focus
than adults and accommodation is not as exact as
that seen in later development.
During accommodation the lens of the eye
becomes a stronger convex lens in near focus as a
consequence of the effect of the sphincter action
of the ciliary muscle, relaxation of the supporting
zonule and the elasticity of the lens capsule. With
greater effort for near focus, there is a greater
convergence as binocular reflexes maintain each
eye focused on the object of interest.
The importance of the maturity of the near
reflex is seen clinically in the Bruckner test which
is used for screening amblyopia in strabismus.
It is said that in the child, particularly under
the age of 4 months, the lack of precision of
accommodation prejudices the reliability of the
Bruckner test. 2 In the Bruckner test, the eye
fixating a direct ophthalmoscope light gives a
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