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11.7.35 In an applicant with a tropia, the examiner will note that during the cover/uncover test the two eyes
move in unison. When one eye adducts the other abducts and vice versa. This maintenance of the
misalignment of the visual axes is the essence of a tropia.
11.7.36 If the deviation is a phoria, by definition the eyes are straight when fusion is allowed. As soon as
one eye is covered, fusion is prevented. There will be no shift in the uncovered eye because it is already looking
at the letters on the chart but the eye behind the cover will drift into its misaligned position. It may take a few
seconds for the misalignment to occur, so the examiner should not hurry the test. When the cover is removed,
the deviated eye will straighten out because fusion is now possible. In applicants with good fusion, the recovery
movement will be rapid. In those with less efficient fusion the recovery will be slower and may require the
patient to blink or make a conscious effort to bring the eyes together.
11.7.37 This drift into the deviated position behind the cover and the recovery movement (fusional
III-11-46 Manual of Civil Aviation Medicine
movement) is the essence of a phoria. Throughout the cover/uncover test in an applicant with a phoria there is
no shift of the uncovered eye. This is the distinction between a phoria and a tropia. The direction of the drift into
the deviation shows if the phoria is eso, exo, hyper or hypo.
11.7.38 The amount of the ocular deviation can be measured using prisms but in most situations it will be
sufficient for the examiner to detect that there is a significant deviation and then refer the applicant to the
appropriate vision care specialist.
Maddox rod
11.7.39 The Maddox rod is a device which prevents fusion by presenting completely different images of a
light source in each eye. It is a ribbed glass which can be fitted into a frame having markers, which show how
the eyes are aligned, and a calibrated rotary prism (Herschel prism) to measure the deviation of the visual axes
in prism dioptres. Looking at a small light source through the device, one eye sees the light and the other eye
sees a straight line which can be horizontal or vertical depending on the orientation of the ribbed glass in the
Maddox rod. When the ribs are horizontal, the perceived line is vertical and vice versa.
11.7.40 With the ribbed glass horizontal (the perceived line will be vertical), the applicant looks at a small
light source 5 to 6 m (16 to 20 ft) distant and adjusts the rotary prism until the line runs through the centre of the
light. The examiner reads the number indicated on the scale of the instrument which indicates the deviation, if
any, whether it is eso or exo, and how much. The ribbed glass is rotated 90 degrees so that it is vertical (the
perceived line will be horizontal), and the applicant again adjusts the rotary prism so that the line runs through
the centre of the light. The scale reading gives the vertical deviation, if any, in prism dioptres.
11.7.41 A simple Maddox rod with no rotary prism can be used and will indicate orthophoria or a deviation.
The amount of the deviation can be measured with loose prisms or a prism bar. If a simple Maddox rod is used,
the examiner must remember that eso deviation will cause displacement of the vertical line to the same side
as the eye looking through the ribbed glass (uncrossed diplopia), and exodeviation will cause displacement of
the line to the opposite side (crossed diplopia). For vertical deviations, the rod is placed in front of the right eye
in which case an upward deviation of the horizontal line indicates a left hyperdeviation, a downward
displacement indicates a right hyperdeviation.
11.7.42 The Maddox rod can be used to test ocular alignment at near by holding the light source at 1/3 m
(1 ft) or a Maddox wing can be used. This is a hand-held instrument with a vertical partition separating the
vision from the two eyes thus preventing fusion. One eye sees red and white arrows and the other eye sees a
graded cross. The applicant looks through the device with both eyes open and reports the positions of the
arrows. The figure at which the white arrow is pointing is a measure of the horizontal deviation. The red arrow
indicates the vertical deviation.
11.7.43 The Maddox rod and wing are ingenious instruments, useful for screening examinations, but they
have shortcomings. First, they are entirely subjective, second they cannot distinguish between phoria and
tropia, third it is possible for the applicant to move the vertical line by exerting voluntary convergence, and
finally they present entirely abnormal viewing conditions for the visual system and may indicate
non-orthophoria when in the real world situation fusion occurs when similar images are presented to each eye.
　
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