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c) Optic nerve disorders can cause central, sector or sometimes horizontal hemianopic defects.
11.5.16 The term hemianopia is widely used to describe visual field loss restricted to one half of the visual
field. Strictly speaking the term means total loss of one half of the visual field. Clinically it is frequently the case
that although the vision loss is restricted to one half of the field, the loss is neither total nor does it occupy the
entire half field. In such cases the correct term is hemidysopsia. The term half-field defect covers all types of
defects limited to one half of the visual field, but is rarely used.
11.5.17 Glaucoma is one of the most important causes of field defects. The earliest changes are usually
nerve fibre bundle defects in the form of small, arcuate, paracentral scotomas which enlarge as the disease
progresses. Sometimes nasal defects occur and in the later stages the visual field is reduced to a small central
or temporal island. See Figure 11-14.
11.5.18 The pigmentary retinopathies (retinitis pigmentosa) and other tapeto-retinal degenerations tend to
affect the mid-peripheral portion of the retina first and cause ring scotomas which enlarge and eventually leave
only a small island of central vision.
11.5.19 Lesions involving the centre of the optic chiasm classically cause bitemporal hemianopias, while
those involving the optic tracts and optic radiations produce contralateral homonymous hemianopic defects,
which may be partial or complete. The shape, location and symmetry of these hemianopic defects help in
localizing the causative lesion.
11.5.20 Not all visual defects will disqualify an applicant from flying or from air traffic control duties but any
applicant with a visual field defect requires neuro-ophthalmological evaluation.
11.6 MONOCULARITY
11.6.1 One eye provides about 140 degrees of vision in the horizontal plane. Even allowing free
movements of the head, a monocular pilot can never have as extensive a field of vision at any given moment
as a normal binocular individual. The question of depth perception is also of concern in a monocular individual.
It is important to understand that while a monocular individual has no stereopsis, he does not lack depth
perception. At a distance beyond 10 m (30 ft) stereopsis becomes less important than monocular clues in
judging depth. Monocular individuals cannot perform tasks such as photo-interpretation which requires
stereopsis, and they have difficulty performing visual tasks requiring fine detail discrimination at close range but
they usually have good depth perception at distance which is provided by the following monocular clues:
Part III. Medical Assessment
Chapter 11. Ophthalmology III-11-39
Figure 11-14. Typical glaucomatous scotoma
(right eye)
a) Aerial perspective — distant objects appear bluish with blurring of their contours due to
preferential scattering of the short wavelength light by the atmosphere.
b) Distribution of light and shade including shadows — conveys much information as to shape
and solidity of objects.
c) Overlapping of contours — an object partially concealed by another is interpreted as being
behind it.
d) Geometrical perspective — horizontal planes appear to intersect in the plane of the horizon,
and this produces a foreshortening and alteration in the images of all objects of any significant
size in the visual field.
e) Apparent size — the apparent size of a known object allows designation of a distance of that
object from the observer.
f) Parallax — parallactic displacement of objects relative to each other when the eye is moved is
one of the most important monocular clues in depth perception. When a middle plane is
regarded, objects beyond it appear to move in the same direction as the observer, while
objects in near planes appear to move in the opposite direction.
11.6.2 The Annex 1 requirement for normal visual fields precludes licensing of monocular pilots except
under the flexibility clause (Standard 1.2.4.8).
11.6.3 Before assessing a monocular applicant’s fitness under this flexibility clause, an adaptation period
of at least six months should be allowed following the loss of vision. The assessment should include practical
flight testing in the case of a pilot or practical testing in the air traffic control environment in the case of an air
traffic controller and should be conducted by a suitably qualified person in consultation with the Aviation
Blind
spot
III-11-40 Manual of Civil Aviation Medicine
Medicine Section of the Licensing Authority.
11.6.4 The following points should be considered by a Contracting State prior to granting a licence to a
monocular pilot or air traffic controller:
　
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