(Courtesy of E Dawson)
Tests for stereopsis
Stereopsis is measured in seconds of arc (1° = 60 minutes of arc; 1 minute = 60 seconds). It is useful to remember that normal spatial visual acuity is 1 minute and normal stereoacuity is 60 seconds (which equals 1 minute). The lower the value the better the acuity. Various tests are employed using different test principles. Random dot tests (e.g. TNO, Frisby) provide the most definitive evidence of high grade BSV. Where this is weak and/or based on ARC, contour-based tests (e.g. Titmus) may give more reliable evidence of stereopsis.
TNO
The TNO random dot test consists of seven plates of randomly distributed paired red and green dots which are viewed with complementary red-green spectacles.
•Within each plate the dots of one colour forming the target shape (squares, crosses etc.) are displaced horizontally in relation to their paired dots of the other colour so that they have a different retinal disparity from those outside the target.
•Some control shapes are visible even without red-green spectacles (Fig. 18.19A) while the test targets are only visible to an individual with stereopsis, while wearing red-green spectacles (Fig. 18.19B).
•The first three plates are used to establish the presence of stereoscopic vision and subsequent plates to quantify it.
•Because there are no monocular clues, the TNO test provides a truer positive measurement of stereopsis than the Titmus test, but can give false negative errors when fusion is poor.
•The disparities measured range from 480 to 15 seconds of arc tested at 40 cm. Most children are able to do this (and the Frisby test) from about 4 years of age.
Fig. 18.19 TNOtest. (A) Control shape; (B) control and test targets
Frisby
The Frisby stereotest consists of three transparent plastic plates of varying thickness.
•On the surface of each plate are printed four squares of small randomly-distributed shapes (Fig. 18.20). One of the squares contains a ‘hidden’ circle, in which the random shapes are printed on the reverse of the plate. The patient is required to identify this hidden circle.
•The test does not require special spectacles because the disparity is created by the thickness of the plate and can be varied by increasing or decreasing the working distance, which must be accurately measured.
•The disparities measured range from 600 to 15 seconds of arc. It is important not to allow the subject to tilt the plate or move their head during testing as this will provide monocular clues.
•A simple screening test with a choice of one stereoscopic picture from a plate of two provides an easy preferential looking test for the presence of stereopsis in very young patients.
Fig. 18.20 Frisby test
Lang
The Lang stereotest does not require special spectacles; the targets are seen alternately by each eye through the built-in cylindrical lens elements.
•Displacement of the dots creates disparity and the patient is asked to name or point to a simple shape, such as a star, on the card (Fig. 18.21).
•The Lang test can often be used to assess stereopsis in very young children and babies, who may reach out to touch the pictures.
•The examiner can also observe the child's eye movements from picture to picture on the card. However, the cards must be held exactly parallel to the plane of the face for the effect to be seen and the Frisby screening test may be superior simply for demonstrating stereopsis (e.g. to confirm BSV in infants with suspected squint).
•The degree of disparity is quite gross, ranging 200–1200 seconds of arc at 40 cm.
Fig. 18.21 Lang test
Titmus
The Titmus test consists of a three-dimensional Polaroid vectograph consisting of two plates in the form of a booklet viewed through Polaroid spectacles. On the right is a large fly, and on the left is a series of circles and animals (Fig. 18.22). The test is performed at a distance of
40cm.
1 Fly is a test of gross stereopsis (3000 seconds of arc), and is especially useful for young children.
•The fly should appear to stand out from the page and the child is encouraged to pick up the tip of one of its wings between finger and thumb. In the absence of gross stereopsis the fly will appear as an ordinary flat photograph.
•If the book is inverted, the targets will appear to be behind the plane of the page. If the patient states that the fly's wings are still ‘popping out’, then they are not appreciating true stereoscopic vision.
2Circles comprise a graded series which tests fine depth perception. Each of the nine squares contains four circles.
•One of the circles in each square has a degree of disparity and will appear forward of the plane of reference in the presence of normal stereopsis. The disparities measured range from 800 to 40 seconds of arc.
•If a patient perceives the circle to be shifted to the side, then they are not appreciating stereoscopic vision, but are using monocular clues instead.
3The animals are similar to the circles test but consist of three rows of animals, one of which will appear forward of the plane of reference. The degree of disparity ranges from 400 to 100 seconds of arc.
Fig. 18.22 Titmus test
Frisby–Davis distance stereotest
This consists of a large cube with an open front through which four small objects are visible. Testing is usually performed at 6 metres. The patient has to decide which of four objects within the box is closest to them.
Tests for binocular fusion in infants without manifest squint
Base-out prism
Base-out prism is a quick and easy method for detecting fusion in children. The test is performed by placing a 20 base-out prism in front of one eye (in this case the right – Fig. 18.23). This displaces the retinal image temporally with resultant diplopia. The examiner observes corrective eye movements as follows:
aThere will be a shift of the right eye to the left to resume fixation (right adduction) with a corresponding shift of the left eye to the left (left abduction) in accordance with Hering Law (Fig. 18.23B).
b The left eye will then make a corrective refixational saccade to the right (left re-adduction) (Fig. 18.23C).
cOn removal of the prism both eyes move to the right (Fig. 18.23D).
d The left eye then makes an outward fusional movement (Fig. 18.23E).
Fig. 18.23 Base-out prismtest
Most children with good BSV should be able to overcome a 20 prism from the age of 6 months; if not weaker prisms (16 or 12 Δ) may be tried but the response is harder to observe.
Binocular convergence
Simple convergence to an interesting target can be demonstrated from 3 to 4 months. Both eyes should follow the approaching target symmetrically ‘to the nose’. Over-convergence in the infant may indicate an incipient esotropia. Divergence may reflect a tendency to divergence or simply lack of interest in the target.
Tests for sensory anomalies
Worth four-dot
This is a dissociation test which can be used with both distance and near fixation and differentiates between BSV, ARC and suppression. Results can only be interpreted if the presence or absence of a manifest squint is known at time of testing.
1Procedure
aThe patient wears a green lens in front of the right eye, which filters out all colours except green, and a red lens in front of the left eye which will filter out all colours except red (Fig. 18.24A).
b The patient then views a box with four lights: one red, two green and one white.
2Results (Fig. 18.24B)
•If BSV is present all four lights are seen.
•If all four lights are seen in the presence of a manifest deviation, harmonious ARC (see ‘synoptophore’ below) is present.
•If two red lights are seen, right suppression is present.
•If three green lights are seen, left suppression is present.
•If two red and three green lights are seen, diplopia is present.
•If the green and red lights alternate, alternating suppression is present.
Fig. 18.24 Worth four-dot test. (A) Red-green glasses; (B) possible results
Bagolini striated glasses
This is a test for detecting BSV, ARC or suppression. Each lens has fine striations which convert a point source of light into a line, as with the Maddox rod (see below).
1Procedure
aThe two lenses are placed at 45° and 135° in front of each eye and the patient fixates a small light source (Fig. 18.25A).
b Each eye perceives an oblique line of light, perpendicular to that perceived by the fellow eye (Fig. 18.25B).
cDissimilar images are thus presented to each eye under binocular viewing conditions.
2 Results (Fig. 18.25C) cannot be interpreted correctly unless it is known whether or not strabismus is present:
•If the two streaks intersect at their centres in the form of an oblique cross (an ‘X’), the patient has BSV if the eyes are straight, or harmonious ARC in the presence of manifest strabismus.
•If the two lines are seen but they do not form a cross, diplopia is present.
•If only one streak is seen, there is no simultaneous perception and suppression is present.
•In theory, if a small gap is seen in one of the streaks, a central suppression scotoma (as found in microtropia) is present. In practice this is often difficult to demonstrate and the patient describes a cross. The scotoma can be confirmed with the 4 prism test (see below).
Fig. 18.25 Bagolini test (A) Striated glasses; (B) appearance of a point of light through Bagolini lenses; (C) possible results
4prism test
This test differentiates bifoveal fixation (normal BSV) from a central suppression scotoma (CSS) in microtropia and employs the principle described in the 20 test (Hering law and convergence) to overcome diplopia.
1In bifoveal fixation the response is as follows:
aThe prism is placed base-out in front of the right eye with deviation of the image temporally and movement of both eyes to the left (Fig. 18.26A).
bThe left eye converges to fuse the images (Fig. 18.26B).
2In left microtropia with CSS the response is as follows:
a |
The patient fixates a distance target with both eyes open and a 4 prism is placed base-out in front of the left eye with |
|
suspected CSS. |
bThe image is moved temporally in the left eye but falls within the CSS and no movement of either eye is observed (Fig. 18.27A).
cThe prism is then moved to the right eye which adducts to maintain fixation; the left eye similarly moves to the left (Hering), but the second image falls within the CSS and no refixation movement is seen (Fig. 18.27B).
Fig. 18.26 4 prismtest in bifoveal fixation. (A) Shift of both eyes away fromthe prismbase; (B) fusional refixation movement of the left eye
kanski 7th
Fig. 18.27 4 prismtest in left microtropia with a central suppression scotoma. (A) No movement of either eye; (B) both eyes move to the left but there is absence of re-fixation
Synoptophore
The synoptophore compensates for the angle of squint and allows stimuli to be presented to both eyes simultaneously (Fig. 18.28A). It can thus be used to investigate the potential for binocular function in the presence of a manifest squint and is of particular value in testing young children (from age 3 years), who generally find it enjoyable. It can also detect suppression and ARC.
•The instrument consists of two cylindrical tubes with a mirrored right-angled bend and a +6.50 D lens in each eyepiece (Fig. 18.28B, top). This optically sets the testing distance as equivalent to about 6 metres.
•Pictures are inserted in a slide carrier situated at the outer end of each tube. The two tubes are supported on columns which enable the pictures to be moved in relation to each other, and any adjustments are indicated on a scale.
•The synoptophore can measure horizontal, vertical and torsional misalignments simultaneously and is valuable in determining surgical approach by assessing the different contributions in the cardinal positions of gaze.
Fig. 18.28 (A) Synoptophore; (B) optical principles and grading of binocular vision
Grades of binocular vision
Binocular vision is graded on the synoptophore as follows (Fig. 18.28B, bottom):
1First grade (simultaneous perception, SP) is tested by introducing two dissimilar but not mutually antagonistic pictures, such as a bird and a cage.
•The subject is then asked to put the bird into the cage by moving the arm of the synoptophore.
•If the two pictures cannot be seen simultaneously, then suppression is present.
•Some retinal ‘rivalry’ will occur although one picture is smaller than the other, so that while the small one is seen foveally, the larger one is seen parafoveally (and is thus placed in front of the deviating eye).
•Larger macular and paramacular slides are used if foveal slides cannot be superimposed.
2Second grade (fusion). If simultaneous perception slides can be superimposed then the test proceeds to the second grade which is the ability of the two eyes to produce a composite picture (sensory fusion) from two similar pictures, each of which is incomplete in one small different detail.
•The classic example is two rabbits, one lacking a tail and the other lacking a bunch of flowers. If fusion is present, one rabbit complete with tail and flowers will be seen.
•The range of fusion (motor fusion) is then tested by moving the arms of the synoptophore so that the eyes have to converge and diverge in order to maintain fusion.
•The presence of simple fusion without any range is of little value in everyday life.
3Third grade (stereopsis) is the ability to obtain an impression of depth by the superimposition of two pictures of the same object which have been taken from slightly different angles. The classic example is the bucket which is appreciated in three dimensions.
Detection of abnormal retinal correspondence
ARC can be detected on the synoptophore as follows:
aThe subjective angle of deviation is that at which the SP slides are superimposed. The examiner determines the objective angle of the deviation by presenting each fovea alternately with a target by extinguishing one or other light and moving the slide in front of the deviating eye until no movement of the eyes is seen.
bIf the subjective and objective angles coincide then retinal correspondence is normal.
cIf the objective and subjective angles are different, ARC is present. The difference in degrees between the subjective and objective angles is the angle of anomaly. ARC is said to be harmonious when the objective angle equals the angle of anomaly and inharmonious when it exceeds the angle of anomaly. It is only in harmonious ARC that binocular responses can be demonstrated; the inharmonious form may represent a lesser adaptation or an artefact of testing.
Measurement of deviation
956 / 1137
Hirschberg test
The Hirschberg test gives a rough objective estimate of the angle of a manifest strabismus and is especially useful in young or uncooperative patients or when fixation in the deviating eye is poor.
•A pen-torch is shone into the eyes from arm’s length and the patient asked to fixate the light. The corneal reflection of the light will be (more or less) centred in the pupil of the fixating eye, but will be decentred in a squinting eye, in the direction opposite to that of the deviation.
•The distance of the corneal light reflection from the centre of the pupil is noted; each mm of deviation is approximately equal to 7° (one degree ≈ 2 prism dioptres).
•For example, if the reflex is situated at the temporal border of the pupil (assuming a pupillary diameter of 4 mm), the angle is about 15° (Fig. 18.29A); if it is at the limbus, the angle is about 45° (Fig. 18.29B and C). This test is also useful in detecting pseudostrabismus, which may be caused by the following conditions:
1Epicanthic folds may simulate an esotropia (Fig. 18.30A).
2Abnormal interpupillary distance; if short may simulate an esotropia and if wide an exotropia (Fig. 18.30B).
3Angle kappa is the angle between the visual and anatomical (pupillary) axes (see Fig. 18.1).
•Normally, the fovea is situated temporal to the anatomical centre of the posterior pole. The eyes are therefore slightly abducted to achieve bifoveal fixation and a light shone onto the cornea will therefore cause a reflex just nasal to the centre of the cornea in both eyes (Fig. 18.31A). This is termed a positive angle kappa.
•A large positive angle kappa may simulate an exotropia (Fig. 18.31B).
•A negative angle kappa occurs when the fovea is situated nasal to the posterior pole (high myopia and ectopic fovea). In this situation, the corneal reflex is situated temporally to the centre of the cornea and it may simulate an esotropia (Fig. 18.31C).
Fig. 18.29 Hirschberg test. (A) The right corneal reflex is near the temporal border of the pupil indicating an angle of about 15°; (B) the left corneal reflex is at the limbus indicating an angle of about 45°; (C) right corneal reflex is at the limbus in a divergent squint
(Courtesy of J Yanguela – fig. A)
Fig. 18.30 Pseudostrabismus. (A) Prominent epicanthic folds simulating esotropia; (B) wide interpupillary distance simulating exotropia
Fig. 18.31 Angle kappa (A) Normal; (B) negative simulates an exotropia; (C) positive simulates an esotropia
