Introduction to binocular single vision (BSV)

Editor's Notes

• #25: Normal correspondnce is called when both fovea have a common visual direction and the retinal elements nasal to the fovea in one eye corresponds to the retinal elements temporal to the fovea in the other eye. Retinal correspondence is abnormal when the fove of one eye has a common visual direction with an extrafoveal area in the other eye. This is generally seen if the angle of squint is small and the extrafoveal point is close to the fovea.
• #33: Measurement of Angle of Anomaly: The angle of anomaly denotes the degree of shift in visual direction. It is determined by calculating the difference between the objective and subjective angles of deviation. Procedure of estimating the angle of anomaly - For this the use of SMP slides is made. The arms of the synaptophore are set at zero. Both the arms of the instrument are moved by the examiner while alternately flashing the light behind each slide until there is no further fixation movement of the patient’s eye (alternate cover test). The reading of both the arms is noted at this moment and the sum total of the reading of both the arms gives the objective angle of anomaly. The subjective angle of anomaly is the angle at which the visual targets are superimposed. The interpretation of this test is as follows- Angle of Anomaly = Objective Angle – Subjective Angle If Subjective Angle = Objective Angle → NRC If Subjective Angle < Objective Angle → ARC If Angle of Anomaly = Objective Angle →Harmonious ARC (full sensory adaptation) If Angle of Anomaly < Objective Angle →Unharmonious ARC
• #35: Red Filter Test: If one examines the visual field of a patient with heterophoria by placing a red filter in front of the habitually fixating eye while the patient is looking at a small light source, number of different responses can be elicited. The patient may report that two lights are seen, a red one and a white one. In esotropia the images appear in homonymous (uncrossed) diplopia, with the red light to the right of the white one when the red filter is in front of the right eye. In exotropia the images appear in heteronymous (crossed) diplopia, with the red light to the left of the white light when the red filter is in front of the right eye. This represents NRC. The patient may report that only one pinkish light in the position of the white fixation light is seen i.e. the red and white images appear to be superimposed. This is clearly an abnormal response in presence of heterophoria. This is termed Harmonious ARC. The patient may report that two lights in uncrossed or crossed diplopia are seen, depending upon the direction of deviation but the measured distance between the double images proves to be smaller than expected from the magnitude of deviation. This represents unharmonious ARC. Suppression is said to occur when the patient reports only a single light (usually the white light) but occasionally red depending upon the density of the red filter and the degree of the dominance of the fellow eye.
• #36: Bagolini's Striated Glasses Test: For this the patient fixates a small light, after being provided with plano lenses with narrow fine striations across one meridian (micro Maddox cylinders). These glasses do not affect the vision or the accommodation of the patient. The fixation light is seen as an elongated streak. The lenses are usually placed at 45 degree OS and 135 degree OD (cover the patients glasses, if he wears any) and the patient fixates for distance or near. The interpretation of this test is as follows- Crossing of the lines at right angles to each other If cover test reveals no shift and fixation is central, the patient has NRC If cover test reveals a shift, harmonious ARC is present Foveal suppression scotoma (fixation point scotoma) with peripheral fusion, if no shift occurs with cover test, NRC exists, if shift occurs, ARC exists Single line represents suppression
• #37: Worth Four Dot Test: This is a simple test utilizing red-green color dissociation. It is more dissociating than the bagolini glasses and so less physiological. The apparatus for this test consists of a box containing four panes of glass, arranged in diamond formation, which are illuminated internally. The two internal panes are green, the upper one is red and lower one is white. The patient wears red and green goggles (as a convention red in front of right and green in front of left). The test can be performed separately for distance and near vision. The interpretation of this test is as follows- A) The patient sees all the four dots. normal binocular response with no manifest deviation (NRC with no heterotropia) Harmonious ARC with manifest squint. B) The patient sees five dots. uncrossed diplopia with esotropia, red dots appear to the right crossed diplopia with exotropia, red dots appear to the left of the green dots. C) The patient sees three green dots, suppression of right eye. D) The patient sees two red dots, suppression of left eye.
• #38: Hering Bielschowsky After-Image Test: This is a highly dissociating orthoptic test in which battery- powered camera flash is used to produce a vertical after image in one eye and a horizontal after image in the other eye. The center of flash is covered with a black mark (serves as a point of fixation and protects the fovea). Once an afterimage is created in each eye, the position of the images in relation to each other no longer depends on whether the eyes are open, closed, straight or crossed. The interpretation of this test depends on the fixation behaviour. Procedure - Each eye fixates on the center black mark of a glowing filament, first presented horizontally to the eye with a better visual acuity and then vertically to the poorer eye for 20 sec in a darkened room while the fellow eye is occluded. The patient indicates the relative position of the two gaps in the center of each afterimage. The gaps correspond to the visual direction of each fovea if central fixation is present. Interpretation of results - Cross response: A symmetrical cross with the central gaps superimposed indicates a normal bifoveal correspondence (if eccentric fixation is excluded). This is irrespective of any deviation between the two eyes, i.e. any eso or exo-deviation with NRC still gives a symmetrical cross response. Asymmetrical crossing: In case of ARC the horizontal and vertical lines have their center separated, the amount of separation dependent on the angle of anomaly. A patient with right esotropia sees the vertical afterimage displaced to the left and a case of right exotropia sees the vertical afterimage displaced to the right. Single line with a gap: A single line with a gap indicates suppression in the fellow eye.
• #39: Foveo-Foveal Test of Cuppers: Cuppers test for retinal correspondence determines whether the two foveas have common or different visual directions. It permits quantitative analysis of the angle of anomaly when eccentric fixation is present. Procedure - The patient fixates with the normal eye on the central light of a Maddox scale via a plano mirror, which for the convenience of the examiner is turned in such a manner that the amblyopic eye looks straight ahead. The visuoscope asterisk is projected by the examiner onto the fovea of the amblyopic eye. The figure of the Maddox scale on which the patient sees the asterisk indicated the angle of anomaly. Modification - To determine which parts of the peripheral retina in the deviating eye have acquired a common visual direction with the fovea of the fixating eye, the patient is asked to guide the Visuoscope until he sees the asterisk superimposed on the central light of the Maddox cross.The examiner views the fundus when this task is completed and notes the position of the asterisk, which indicates the location of retinal elements having a common visual direction with the fovea of the sound eye.
• #42: Stereograms: Stereogram with three concentric circles and a check dot for each eye is to be seen with both eyes together. Stereograms with three eccentric circles are to be seen with each eye separately. If the patient reports seeing concentric circles, it means stereopsis is present. If they are seen eccentrically one may ask whether the inner circles are closer to the right or left of the outer circle. It determines whether the disparate elements are suppressed in the right or the left eye. Vectographs: Consists of Polaroid material on which the two targets are imprinted so that each target is polarized at 90 degrees with respect to the other. Patient is provided with Polaroid spectacles so that each target is seen separately with the two eyes. Titmus stereo test – A gross stereoscopic pattern representing a housefly is provided to orient the patient and check for gross stereopsis (threshold 3000 sec of arc). Can be used in young children. Disadvantage of this test is that it can test only near stereopsis. Polaroid test – Two common types: A) Contain three rows of animals, one animal in each row imaged disparately (threshold 100, 200 and 400 sec. of arc respectively). The child is asked which one of the animals stands out. The animal figures contain a misleading clue. In each row one of the animals correspondingly imaged in two eyes is printed heavily black. A child without stereopsis will name this animal as the one that standsout. B) Contains nine sets of four circles arranged in the form of a diamond. In this sequence the upper, lower, left or right are disparately imaged at random with thresholds ranging from 800 to 40 sec of arc. The child is now asked to push down the circle that stands out, beginning with the first set. A child with limited stereopsis will make mistakes or find no circle to push down. E- Random Dot Test - This test consists of two cards, one with a ‘E’ stereo figure and the other stereoblank. A model of the figure is shown to the child before the test. The child is provided with Polaroid glasses and seated at 50 cms from the cards is required to point out the card which contains the "E". Random Dot Stereogram of Julesz - Random dot stereogram, when viewed monocularly,convey no visual information and is seen as scattered random dots. When viewed binocularly, a square pattern appears in vivid depth above or below the level of the page. This test exposes the child to visual demands that are more difficult than those that occur under more casual conditions of seeing.