Download Running Head: Amblyoscope

Amblyoscope
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Running Head: Amblyoscope
AMBLYOSCOPE
What is the major amblyoscope?
The major amblyoscope is the basic instrument used for measuring or training
binocular vision, for stimulating vision in an amblyopic eye and for increasing fusion
of the eyes, also known as synoptophore and the troposcope. (Fig. 1) (Amblyoscope
1, 2008)
The optical design of the major amblyoscope:
The major amblyoscope contains the electrical components and controls needed for its
operation. On this base are mounted two tubes, each of them containing a light source
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for the illumination of the slides, a slide carrier, a reflection mirror, and an eyepiece
lens o + 6.50 spheres. The distance from the slide to the mirror and from the mirror to
the eyepiece (the patient's eye) is 15 cm, which equals the focal length of the lens.
Therefore, the eye is focused for distance and no accommodation has to be exerted,
providing the patient is emmetropic or rendered so by wearing his correction. A
millimeter scale enable us to adjust the distance between the arms to the patient's
interpupillary distance, and ahead and chinrest assure his proper and comfortable
positioning.
Each tube moves around a semicircular scale graduate in degrees, centrads, and prism
diopters. The graduation from the zero mark inward represent base-out prisms or
degrees of convergence (+), while those from the zero mark outward represent base-in
prisms or degrees of divergence (-). A scale located on the upper outer side of each
tube and graduated in prisms diopters permits the measurement of vertical deviation
after the tubes have been moved up or down by means of a knob. In rotating the lamp
housings, adjustments for cyclodeviations can be made and a scale graduated in
degrees permits their measurement.
The two tubes may be moved separately or together by means of knobs and levers.
Light switches permit the simultaneous or alternate illumination of the tubes (Fig. 2)
(Hurtt& Rasicovici&Windsor, 1986).
Amblyoscope
Figure 1. The major amblyoscope.(amblyoscope 2,2008).
Figure 2. The optics of the major amblyoscope (Fiona, 2008).
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Diagnostic uses of the major amblyoscope:
1. Measurement of the objective and subjective angle of deviation.
2. Measurement of angle kappa.
3. Measurement of primary and secondary deviation.
4. Measurement of deviation in cardinal directions of gaze.
5. Estimation of status of binocular vision.
a. State of retinal correspondence: normal, abnormal, lack of any.
b. Presence and type of suppression.
c. Presence of fusion and measurement of fusional amplitudes.
d. Presence of stereopsis.
The therapeutic uses of the major amblyoscope:
It is used in the treatment of:
1. Suppression.
2. Abnormal retinal correspondence.
3. Eccentric fixation (only in models that have special attachments with Haidinger
Brushes).
4. Accommodative esotropia (dissociation training).
5. Heterophorias and intermittent heterotropias (improvement of fusional amplitudes)
(Hurtt& Rasicovici&Windsor, 1986).
How we can measure the angle of deviation for near on the major amblyoscope?
Minus 3.00D spheres can be inserted in the lens holders situated in front of the
eyepiece lenses. The patient has to exert 3.00D of accommodation in order to get a
clear image of the slides. In doing so, each eye exerts 3 of convergence for each
diopter of accommodation –in other words, 9∆ of convergence in one eye or 18∆ of
convergence, in both eyes-considering the interpupillary distance as being 60 mm.
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(For a smaller interpupillary distance, the convergence requirement is less; for a
bigger one it is more, providing the Ac/A ratio is normal.) When recording the angle
of deviation, we must keep this in mind and either subtract 18∆ from or add 18 to the
major amblyoscope readings (the first for cases of convergent deviation, the latter for
divergent deviation). In order words, a major amblyoscope recording of 20 bases out
will be recorded as 2∆ ET, while a reading of 20∆ base in will be recorded as 38∆ XT
(Hurtt& Rasicovici&Windsor, 1986).
Describe the measurement of the objective angle on the major amblyoscope?
After making sure that the patient is comfortable seated and correctly positioned in
front of the instrument, we adjust it for the patient's interpupillary distance and insert
first-grade targets (dissimilar targets, for example, the lion and the cage) into the slide
holders. The arms of the instrument are unlocked. The lion is placed in front of the
fixing eye (the right eye) and the light in front of the left eye is turned off. The right
arm is set at zero and the left one in the vicinity of zero on the base- out side for
esodeviations or on the base-in side for exodeviations. After making sure that the
patient is accurately fixing the lion, the light in front of the left eye is turned on and
the light in front of the right eye turned off. The patient is asked to look directly into
the center of the cage. If the left eye moves out to pick up fixation, the left arm is
moved into a more divergent, or less convergent, position. If the eye moves
downward the tube has to be raised; if it moves upward the tube has to be lowered.
The alternate flashing is continued and the tube adjusted until there is no movement in
either eye when it picks up fixation. The reading on the horizontal scale in front of the
left arm, as well as the one of the vertical scale, represents the objective angle of
deviation. For instance, if the left arm is at 15∆ base out and has to be raised 2∆, the
objective angle is recorded as 15∆ ET and 2∆ LHT. The objective angle can be
measured with either eye fixing and in all cardinal direction of gaze (Hurtt&
Rasicovici&Windsor, 1986).
How is the subjective angle of deviation measured on the major amblyoscope?
If the patient claims superimposition (the lion is in the cage) at his objective angle,
this angle is also his subjective one. If this is not the case, the arms are moved back to
zero and the patient is instructed to fixate steadily on the lion while he is moving the
left arm until the lion is in the cage. This is his subjective angle. At this point, the
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orthoptist should, by means of rapid alternate flashing, check whether or not the eyes
move when the patient is asked to fixate on each picture in turn. This is done mainly
to make sure that an actual change in the angle between the visual axes has not
occurred, as happens frequently through relaxing or increasing the accommodative
effort or in cases of a variable angle deviation. In the majority of cases the
determination of the subjective angle is not as simple as described previously. The
patient may never succeed in putting the lion in the cage, and it may suddenly be seen
on the other side of the cage ( in a crossed or homonymous position in convergent
deviations and in an un a crossed or homonymous position in divergent deviations), or
there may be too much suppression. In such cases the crossing point is considered to
be the subjective angle (Hurtt& Rasicovici&Windsor, 1986).
How is the angle of anomaly determined on the major amblyoscope?
The difference between the objective angle and the subjective angle represents the
angle of anomaly. For instance, if a patent with convergent strabismus has an
objective angle of deviation of 20∆ bas out and a subjective angle of 6∆ base out
(BO), his angle of anomaly is 14∆ (Hurtt& Rasicovici&Windsor, 1986).
How is the angle kappa determined on the major amblyoscope?
A special slide is placed in front of the eye under observation. It consists of a row
of numbers and letters (E D C B A 0 1 2 3 4 5) at 1º intervals. The patient is asked to
look at the zero. If the corneal reflex is on the nasal side of the pupil the angle is
positive; if it is on the temporal side it is negative. The patient is asked to look in turn
at either one letter or one number until the reflex is centered. The degree of deviation
corresponding to the letter or number is then recorded. For instance, if the right eye is
the one to be tested and the corneal reflex is centered when the patient looks at the
letter C, the patient has a 3ºnegative angle kappa in the right eye (Hurtt&
Rasicovici&Windsor, 1986).
How assessment of the retinal correspondence?
Simultaneous perception:
Simultaneous perception slides are dissimilar. Subjectively, the patient moves the
tube before the non-fixating eye and attempts to overlap the images. Simultaneous
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perception is absent if the patient is unable to see the two images at the same time or
if one image repeatedly disappears or the images jump. It is easier to superimpose
with larger slides, and if Simultaneous perception is not achieved with smaller slides,
it is useful to reassess with peripheral slides before assuming lack of SP (Fig. 3)
(Fiona, 2004)
Fusion:
Sensory fusion is present if, at the subjective angle, the patient is able to
superimpose both fusion slide images and is able to see both control images (one from
each slide). The motor fusion range is assessed by converging and diverging the tubes
whilst asking the patient to state when fusion breaks. The patient will appreciate
diplopia or will suppress one of the controls. If the patient has difficulty viewing the
targets or loses one of the controls, larger slides may be used. Foveal fusion is more
difficult to maintain than peripheral (Fig. 4) (Fiona, 2004)
Stereoacuity:
Both tubes are locked at the fusion angle. Gross stereopsis slides are inserted and
the patient asked whether or not there is a stereoscopic effect. Detailed stereopsis is
assessed using Braddick slides (Fig. 5) (Fiona, 2004)
Figure 4. Slides of the Simultaneous perception (Work class).
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Figure 4. Slides of the fusion (Work class).
Figure 5. Slides of the Stereoacuity (Work class).
Advantages of the major amblyoscope:
1. simplest to carry out
2. enabling vertical and torsional elements of the deviation to be accurately
assessed
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Disadvantages
Horizontal inaccuracies may be occurring due to convergence accompanying the
(unnecessary) accommodative effort and patients often exert. Thus the synoptophore
readings may show a larger convergent or smaller divergent angle than is, in fact, the
cases). (Amblyoscope 3, 2008)
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Reference
Stein, H.A., Slatt, B.J. & Stein, R.M. (1994). The Ophthalmic Assistant (6 ed) St.
Louis: Mosby-Year Book Inc.
Rowe F, (2004). Clinical Orthoptics (second edition). USA: Blackwell publishes.
Hurtt & Rasicovici &Windsor, (1986). Comprehensive review of orthoptics and
ocular motility.
(Amblyoscope 1, 2008) refers from:
http://medical-dictionary.thefreedictionary.com/amblyoscope
(Amblyoscope 2, 2008) refers from:
http://www.takagi-j.com/seihin_e/katarogu_e/mt364_e.html
(Amblyoscope 3,2008) refers from:
http://www.palopticlub.com/vb/showthread.php?s=38328663c10a2441d403485fa60a
136e&t=2325&page=1