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that overlaps the fixing eye is suppressed. The remainder of the visual field of the deviating eye is not
suppressed. Thus, the deviating eye always contributes to the overall binocular field of vision in a strabismic
patient in two ways. Neither the area corresponding to the blind spot of the fixing eye nor the peripheral
temporal crescent area in the deviating eye is suppressed. The binocular field is smaller (narrower) in esotropic
patients and larger (wider) in exotropic patients.
8. The retinal midline divides the temporal retina and one side of the brain from the nasal retina and
the other side of the brain. When the image of the fixation target crosses the midline from the nasal side to the
temporal side or vice versa, it operates a “trigger” mechanism (the hemiretinal trigger mechanism) that
determines whether diplopia or suppression occurs. Suppression develops in the visually immature patient in
order to avoid diplopia. The image of the fixation object always falls on the same side of the retina of the
deviating eye and is suppressed. If, however, the deviation is changed from esotropia to exotropia or vice versa,
this is a new situation and diplopia is triggered. It is the change in position of the retinal image from one half of
the retina to the other half that triggers the change from suppression to diplopia and vice versa whenever the
visual fields overlap. Thus the risk to get outside the suppression area and become diplopic is the risk to
change from esotropia to exotropia or vice versa.
9. The monofixation syndrome is characterized by a minor heterotropia with paracentral fixation and
good peripheral fusion. There is suppression of the macula of the deviating eye only. The risk of diplopia is
minimal and depends on the peripheral fusional amplitude, which maintains ocular alignment.
10. Suppression is not equally deep in all patients. To make a patient aware of the images perceived
by the deviated eye, one must reduce the retinal illuminance in the fixating eye until the patient sees double.
This is best done with a series of red filters of increasing density in the form of a ladder (Sbisa bar14). The
patient fixates a small light source, and the filters are placed in front of the fixating eye. Some patients see
double with a light density filter; others require a heavier-density filter before they recognize their diplopia. The
lighter the density of the filter needed to produce diplopia the more superficial is the suppression indicating an
increased risk of diplopia. In individuals with normal fusion, placing graduated neutral-density filters in front of
either eye will, at a certain density level, prevent fusion and induce two lights either together (orthophoria) or
apart from each other (diplopia with heterophoria).
Anomalous retinal correspondence
11. Anomalous retinal correspondence (ARC) is a neural adaptation to eye misalignment in which
non-corresponding retinal points are linked in the visual cortex to provide binocular fusion. When both eyes are
used, ARC works by using a change in the visual direction of the retinal points in the deviating eye so that an
extrafoveal point in that eye corresponds with the fovea in the straight eye. As with suppression, ARC can exist
in either eye in alternating strabismus. In some individuals the fusion mechanism is weak, the ARC may be
unstable, and there is a risk of diplopia. Other persons with ARC have peripheral fusion (including some motor
fusion reserve) and even gross stereopsis. In such cases, diplopia is very unlikely.
14 Sbisa bar: a Bagolini filter bar, manufactured by Sbisa Industriale SpA, Italy.
III-11-62 Manual of Civil Aviation Medicine
SYMPTOMS
Asthenopia
12. Symptoms of asthenopia include redness, dryness, discomfort, a feeling of heaviness in the eyes
and inability to use the eyes for more than a short period of time. In some cases there may be ocular pain or
headaches. The symptoms may indicate decreased accommodation, ametropia or heterophoria, sometimes
with reduced fusional amplitudes and are usually more pronounced while doing close work. Other conditions
such as conjunctivitis and anterior uveitis may cause similar symptoms.
Patients with asthenopia require full ocular examination including refraction, measurement of accommodation
and evaluation of ocular alignment and binocular status.
Diplopia
13. Double vision (diplopia) means that a single object is seen in two different locations.
Diplopia, even if intermittent, it is generally incompatible with safe flying. Single vision in gaze straight ahead,
down and to the sides is required for safety. Some individuals who have diplopia only in the extremes of
up-gaze to the sides may be acceptable for flying duty. Monocular diplopia from any cause is disqualifying.
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Manual of Civil Aviation Medicine 2(69)
