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a person may be orthophoric at distance and esophoric or esotropic at near. Another individual may be
exotropic or exophoric at distance and orthophoric at near.
11.7.16 There is no absolute correlation between the amount of ocular deviation and symptoms. Some
persons with large phorias are entirely asymptomatic while others with a much smaller deviation have
9 Maddox rod: a set of parallel cylindrical glass rods used in testing for heterophoria. The rods, placed before
the eye, distort the image of a point source of light into a long streak perpendicular to the axis of the rods,
interfere with fusion, and break up binocular vision (see also under Examination techniques). After Ernest
Edmund Maddox, English ophthalmologist (1860-1933).
Part III. Medical Assessment
Chapter 11. Ophthalmology III-11-43
significant problems. In some individuals the ocular misalignment worsens over time so that a small phoria
becomes larger, then progresses to an intermittent tropia and finally a constant tropia. This is particularly likely
in exo-deviations (outward deviation of the visual axes).
11.7.17 Ocular deviations are measured using prisms which are designated by the deviation they produce
in the light passing through them. This deviation can be measured in degrees but the unit most often used
clinically is the prism dioptre (Δ). One prism dioptre is an angle whose tangent is 1/100. A prism having a power
of 1 Δ produces an apparent shift of 1 cm of a object located 1 m distant from the prism. A 5 Δ prism produces
an apparent displacement of 5 cm of an object 1 m from the prism.
11.7.18 As a general rule, symptoms may be expected when the deviations exceed the following:
esophoria 10 prism dioptres
exophoria 5 prism dioptres
hyper or hypophoria 2 prism dioptres
cyclophoria 1 prism dioptre
11.7.19 Applicants whose ocular deviations exceed these values should be referred for evaluation by an
appropriate vision care specialist.
Strabismus
11.7.20 Manifest or latent misalignment of the visual axes is called strabismus and may be classified into:
Paralytic — due to injury or disease affecting the extraocular muscles or their nerve supply;
Non-paralytic — due, probably, to some poorly understood disorder of the fusional mechanisms or
to the central nervous system centres controlling eye movements.
11.7.21 Paralytic strabismus of recent onset is always associated with diplopia and is not acceptable in
flight crew or air traffic controllers.
11.7.22 Non-paralytic strabismus may be congenital or acquired. In the acquired types when fusional
ability is exceeded there may be symptoms which have been mentioned above. In congenital or early onset
strabismus the central nervous system is presented with the problem of resolving intolerable diplopia. Three
adaptations are possible:
a) suppression of the central vision in one or other eye depending on gaze direction. This avoids
diplopia while maintaining good visual acuity in each eye. It occurs in alternating strabismus;
b) continued suppression of the central vision in one eye only. This avoids diplopia but leads to
failure of development of the visual potential in the deviating eye. This probably occurs in the
central nervous system rather than in the eye itself and is called amblyopia ex anopsia. A
similar loss of development of visual potential may occur when there is a large difference in the
refractive error between the two eyes. This is amblyopia ex anisometropia; and
c) a readjustment in the directional values of the various parts of the retina. This is called
anomalous retinal correspondence and avoids diplopia but generally with some sacrifice of
III-11-44 Manual of Civil Aviation Medicine
visual acuity.
Examination techniques
11.7.23 The following examination techniques enable the examiner to detect some of the ocular
misalignments which have been described above and to decide on referral to the appropriate vision care
specialist whenever the screening standards are not met or if significant pathology is suspected.
11.7.24 Abnormal head posture is sometimes an indication of an extraocular muscle weakness. Head turn
to one side is seen in homolateral sixth nerve weakness and head tilt to one side in contralateral fourth nerve
weakness. These abnormal positions are adopted to get rid of diplopia. Examining ocular excursions may
disclose impaired muscle function, but additional testing is often necessary to evaluate ocular misalignments.
The most useful screening tests are cover tests and Maddox rod or Maddox wing.
Cover testing
11.7.25 This is the most useful screening examination to determine ocular alignment. No special
equipment is required. It allows the examiner to distinguish between phorias and tropias, to estimate the
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