1Amblyoscope
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 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).
Figure 1.The major amblyoscope.(amblyoscope 2,2008).
Figure 2.The optics of the major amblyoscope(Fiona, 2008).
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. (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 20bases 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 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 (EDCBA012345) 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 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).
Figure 4. Slides of the fusion (Work class).
Figure 5. Slides of the Stereoacuity(Work class).
Advantages of the major amblyoscope:
- simplest to carry out
- enabling vertical and torsional elements of the deviation to be accurately assessed
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)
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:
(Amblyoscope 2, 2008)refers from:
(Amblyoscope 3,2008)refers from: