Dr. Mihir Trilok Kothari, K08617
Introduction:
High grade stereopsis is the function of bifoveal fusion. Presence of fine stereopsis [stereoacuity< 40″ (seconds of arc) on TNO test] essentially confirms an efficient and ‘normal’ binocular input.1 Measurement of Stereoacuityis found useful in
- Monitoring the control of ocular alignment in intermittent strabismus1,2 viz. the intermittent exotropia,3-5 the intermittent esotropia1 and accommodative esotropia.6
- Evaluating the functional outcome of strabismus surgery.7
- Comparing the effects of early intervention in infantile strabismus syndromes viz. congenital esotropia.8
- Screening of amblyogenic factors in children viz. anisometropia, significant ametropia and strabismus.9
In a strabismus clinic, measurement of distance stereopsis is particularly useful in the management of intermittent exotropia and esotropia of divergence paralysis type. This is mainly because reduction in the distance stereopsis is a sensitive sign of early deterioration of these strabismic disorders and generally makes a case for the surgical intervention.10
Various tests have been used for the measurement of stereoacuity for the near.11Titmus/Wirt Fly test, Randot test and TNO test are among the more popular ones. For the distance stereopsis, the Mentor BVAT SG12 and Frisby Davis distance stereo test (FD2)13 are fairly accurate and versatile. However, the distance stereoacuity tests are not commonly available in the clinics and they are relatively expensive.
In the past, we had demonstrated that TNO test, that uses red-green random dot stereopairs (anaglyphics), can be combined with an inverted binocular telescope to measure the global, crossed disparity induced, central stereopsis.14
The present study was done to find 1) whether the presence of distance stereopsis on CP 690 chart could be predictive of presence of stereopsis on Inverted telescope-TNO system, 2) to assess the effect of increasing anisomyopia on distance stereopsis on CP690 chart and 3) to find the magnitude of anisometropiathat lead to loss of stereopsis on both CP690 and Inverted telescope-TNO system.
Methods:
This prospective cohort study was carried out in a standalone Pediatric Ophthalmology and Strabismus tertiary teaching eye centre in the Western India. The orthophoric subjects having the best corrected vision of 6/6 in each eye within the age group of 10-45 years, were included in the study. A pediatric optometrist performed complete ophthalmic examination. Ocular motility evaluation was reconfirmed by a fellowship trained strabismologist. Once the subject was diagnosed as ‘normal’, an oral consent was obtained explaining the nature of the test and the need of the study.
The eligible subjects were tested first on a standard distance stereopsis chart using polaroid lenses and CP690 projector chart (NidekInc, Japan) calibrated for 6 meters distance. This was followed by a repetition of the test with increasing aniso myopia (induced by introducing + lenses in the steps of 0.5DS) in front of the left eye.The responses were recorded as per the following stereopsis values.
On the TNO test (TNO institute of Vision, Netherlands). Care was taken to hold the TNO chart at 40cm (measured with a tailor’s tape) in a brightly lit room by the same investigator in the same room conditions each time. The subjects were allowed enough time to recognize the 3 dimensional image in the test. Once the subject could appreciate the stereopsis, a binocular telescope (Galilean design, 26mm, 2X, Camma Inc. China) was held in an inverted fashion in front of the red-green goggles by the subject. The inter pupillary distance (IPD) was adjusted by the subject, the test was run again and the response was recorded in the same fashion. Care was taken to maintain the test distance at 40 cm and the subjects were given enough time and explanations to complete the test.This was followed by a repetition of the test with increasing aniso myopia (induced by introducing + lenses in the steps of 0.5DS) in front of the left eye. The stereopsis was not graded on the TNO test. Only first 3 plates were utilized.
Results:
10 adults aged 30.6years ±10(10-42) of which 4 were males were included. All the patients had full stereopsis (1”) on distance CP690 chart. With fogging, the stereopsis was abolished completely at + 3DS in front of the left eye. Lower levels of blur +0.5DS to +2.5DS was apparent as a blur but was unable to degrade the stereopsis on CP690 chart.
All the patients who had stereopsis on CP690 chart also had stereopsis on Inverted Telescope-TNO Test system. On that chart also, the subjects lost stereopsis at +3.0DS giving 100% correlation with CP690 chart. Lesser amount of induced anisometropia was not associated with the loss of stereopsis.
Discussion:
The TNO test is a popular test used to assess the near stereoacuity. In this test red-green anaglyphics are presented in a random dot stereopair. When viewed with the Red-Green filter (goggles) it induces the stereo effect that measures the global, central, crossed disparity threshold (central stereoacuity). Inversion of the chart (upside down) can test the stereoacuity for the uncrossed disparity. The advantages of TNO test over Titmus/Wirt fly test are two folds. 1) There are no monocular clues and 2) Red-Green colour optimizes the stereo contrast for the measurement (akin to the measurement of the Cone function with Photopic conditions- the optimisation of Cone system)
By definition, stereoacuity is the smallest depth difference we can see, that is, a depth discrimination threshold. Thus stereoacuity can be thought of as the “resolving capacity” of stereopsis, much as visual acuity in the resolution limit of the spatial vision. In other words it is the measure of the horizontal binocular disparity that results from the images in each eye being formed on the closely spaced non-corresponding retinal points within Panum’sfusional area. This disparity can be measured16 using the formula n=2ax/d2. Where n is the angular stereo disparity in radians (to get the stereoacuity in arc seconds mulitply with 2,06,256), 2a is the inter pupillary distance (IPD), d is the fixation distance and x is depth interval. The fixation distance in the inverted telescope distance can be calculated using the formula u = 1/v independently for each lens of the telescope (eye piece and the objective lens). The image of the first lens acting as the object for the second. Once the fixation distance is calculated, stereoacuity can be derived by measuring the IPD. The disparity value can also be calculated for any distance by multiplying the original value on TNO test chart by a factor 40/d where d is the viewing distance in cm.17
There are a few limitations of the TNO test. 1) It dissociates eyes, 2) it induces crowding phenomenon that reduces the visual acuity in the amblyopic eye more severely and there by reducing the stereopsis, 3) TNO test induces colour rivalry between the two eyes and 4) the test produces chromatic aberration. These are also some of the reasons why the stereoacuity norms on TNO test are different (lower) than that with Titmus/Wirt Fly test.
Similarly each of the distance Stereoacuity tests have their limitations and merits. We do believe that an inverted telescope system can be used with any other near stereoacuity chart. However, the test would need to be standardized, normative values need to be calculated and the repeatability needs to be assessed. So long as one is consistently using the same stereoacuity chart, the interpretation would remain the same.
The effect of pictorial monocular clues was eliminated by the use of random dots and the use of telescopic system. However, the effect of convergence and accommodation though less likely to be clinically significant, may affect the measurements.
In the past the investigators have reported a graded reduction on near stereopsis and binocular vision (fusion) per diopter of induced anisometropia.[18]Anisomyopia was found to be more degrading than the anisohyperopia and +3DS blur (causing anisomyopia of 3DS) was associated with complete loss of stereopsis.[19]
In the present study we found presence of stereopsis on CP690 was predictive of presence of stereopsis o inverted Telescope-TNO chart. However, there was no graded reduction of stereopsis on distance stereopsis using CP690 chart. 3D of induedanisomyopia was universally associated with loss of stereopsis.
In conclusion, novel inverted telescope-TNO system works well to assess presence or absence of distance stereopsis. 3D of anisomyopia can be potentially very damaging to the distance stereopsis. Lower levels of anisomyopia is compatible with intact stereopsis.
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