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diplopia. The paucity of visual clues when flying at night or at high altitude and the physiological stresses of
hypoxia, vibration and high G-loading can interfere with normal fusion mechanisms so that ocular misalignment
is more likely than in the normal terrestrial environment.
11.7.6 Before discussing examination methods, some explanation of the terms used to describe ocular
muscle balance is appropriate. The innervation and coordination of the twelve extraocular muscles so as to
keep the object of regard accurately imaged on the two foveae at all times is complex, and it is not surprising
that in many individuals the ocular alignment is less than perfect.
Definitions
11.7.7 Normal binocular vision is vision in which images from each eye are blended into a single,
unified perception so that there is no diplopia. For this to happen, the eyes must be accurately aligned with each
other. The mechanism for maintaining this alignment involves a motor component in which the extraocular
muscle innervation is precisely adjusted so that both eyes are pointing at the object of regard. It also involves
a sensory component in which the data from each eye are integrated in the cerebral visual cortex. This motor
component together with the sensory component constitute the mechanism called fusion.
11.7.8 Stereopsis (stereoscopic vision) is a special type of binocularity in which small differences in the
retinal images from each eye are used to assign “depth” or position of objects in space. Good stereopsis is
evidence of binocular vision and indicates normal binocular function. However, stereopsis is not essential for
binocular vision, and some individuals with minimal ocular misalignment and/or minimal amblyopia have
binocular vision and use peripheral fusion to maintain ocular alignment without having good stereopsis.
11.7.9 Orthophoria means perfect alignment of the eyes with no tendency for deviation of the visual axes
even when fusion is prevented by covering one eye or by any of the various tests to be described.
8 Panum’s fusional area: the region of binocular single vision. It is the area on the retina of one eye over which
a point-sized image can range and still provide a stereoscopic image with a specific point of stimulus on the
retina of the other eye. Outside Panum's fusional area, physiological diplopia occurs. After Peter Ludvig Panum,
Danish physiologist (1820-1885).
III-11-42 Manual of Civil Aviation Medicine
11.7.10 Tropia is a manifest deviation of the visual axes. The amount may be large or small but the eyes
are misaligned even when there is no mechanical obstruction to fusion. In a tropia, fusion is not happening so
that covering one eye or interposition of any of the test instruments to prevent fusion does not make any
difference to the deviation.
11.7.11 Phoria is a latent deviation which means that there is a tendency for the eyes to become
misaligned but this tendency is held in check by the normal fusion mechanisms. When fusion is permitted, the
eyes are straight. When fusion is prevented, such as by covering one eye or by interposition of a Maddox rod9,
the visual axes become misaligned. As soon as the obstruction to fusion is removed, the deviating eye will align
itself correctly.
11.7.12 The distinction between phorias and tropias is important. Persons with long-standing non-paralytic
tropias rarely have diplopia. On the other hand, persons with a phoria may “break down” and become tropic if
the fusion mechanisms are impaired by such things as fatigue, stress, high G-loading or sedative drugs,
including alcohol. Such a person will be asymptomatic while phoric but may have diplopia when the deviation
becomes a tropia and therefore be at risk of developing double vision during the course of a prolonged or
difficult flight.
11.7.13 Ocular misalignments may be classified according to the direction of the deviation. Collectively
these are called heterophorias or heterotropias:
Inward
deviation
esophoria
esotropia
or
Outward
deviation
exophoria
exotropia
or
Upward
deviation
hyperphoria
hypertropia
or
Downward
deviation
hypophoria
hypotropia
or
Rotational
deviation
cyclophoria
cyclotropia
or
11.7.14 Most phorias are well controlled by fusion and cause no symptoms. In some individuals the
compensation is less satisfactory, and they may have symptoms such as headaches, eye discomfort and
fatigue together with tearing and redness of the eyes and eyelids, all of which are generally worse during
periods of fatigue, stress or general debility from any cause.
11.7.15 It should be noted that the ocular misalignment may not be present at all distances. For example,
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