NPC, W4D, Stereo Flashcards
Define near point of convergence (NPC).
Measuring the maximum vergence response
Amplitude of vergence
How do we measure NPC?
Advance a target towards the patient until either
They report diplopia (subj)
You see one eye turn out (obj)
What is the NPC measured when testing?
Distance from target to the patient when diplopia/eye turn out
What is the expected break value for NPC?
5cm/ 20MA / 120prism (assume PD=60) from outer canthus (center of rotation of the eye)
What is the expected recovery for NPC?
7cm from outer canthus
How is NPC recorded?
4cm/6cm + diplopia
8cm/11cm; OS turns out
TTN
What kind of effect does age have on NPC?
NPC DOES NOT decline significantly with age (unlike accommodation)
What does the Worth 4 Dot Test test?
Binocular vision
How well our eyes work together
What does a recording of Grade 1 for W4D indicate?
Simultaneous perception
See out of both eyes at the same time (binocular)
Without this we have no binocular vision at all
What does Grade 2 indicate for W4D?
Flat fusion
Pt has grade 1 and can fuse image
No double vision
What does grade 3 indicated for W4D?
Stereopsis
Grade 1 and 2 and can see in depth
Only can be indicated with a Stereopsis test, if patient has grade 3 you do not need to test for grade 1 and 2
What color filters are placed on the patient for W4D? What are the images for each eye?
Red filter OD, green filter OS
OD sees 2 lights (red and white one as red)
OS sees 3 lights (2 green and white one as green)
Patient may see 2-5 lights in total
What does it mean if the patient sees 4 lights?
Patient has at least flat fusion
Binocular and fusing the white dot (appears as red, green or flickering depending on eye dominance and retinal rivalry)
What does it mean if the patient sees 3 lights?
Patient is suppressing their right eye
Images from left eye
What does it mean if the patient only sees 2 lights?
Patient is suppressing left eye
Right eye images
What does a response of 5 lights mean?
Patient has simultaneous perception (grade 1)
Seeing out of both eyes, NOT fusing the white dot
How can an exo or eso deviation be determined from W4D?
Ask patient where the red dots are relative to the green
Red dots on left= exo
Red dots on right= eso
What is the diagnosis if the red dots are higher then the left?
Left hyperopia
What distances is W4D performed at? What about the handheld device?
6m (20ft), 3m (10ft), 40cm (16in)
Small handheld unit- 3m and 40cm
Define stereopsis.
Binocular perception of depth based on retinal disparity (correspondence)
What must be true regarding the correspondence of reinstall points during stereopsis?
Images must be imaged on non-corresponding retinal points with the disparity not exceeding Panum’s fusional area
What is Panum’s fusional area?
Point on retina away from corresponding point (not necessarily the fovea) where the image will still be single
How much we can under or over converge and still see a single image
Define corresponding retinal points.
Same visual direction
How large is PFA typically when corresponding point is the fovea?
6 to 10 minutes of arc on the retina (does not apply to periphery)
Image will be single within this range
What are monocular depth cues?
Shadow, perspective, overlap, texture, size constancy provide a lot of depth information
How do patients without Stereopsis (one-eyed patients) still see in 3D?
Monocular depth cues
Stereopsis only provides a high level of depth perception
Define stereo-threshold.
The smallest separation between the two objects when the observer can reliably state that one object is in front of the other
Describe the three needle test.
Middle needle is moved
Observer must determine if center needle is in front, behind, or next to the outside of the two needles
What is the Howard-Dolman test?
Same as three needle but only has 2 needle
Observer must determine which is closer to them
What is the frisby test?
Patient must determine which square includes a circle that appears depressed
Circle is painted on the back of the plastic sheet, everything else is on front
Thinner sheets needed for lower stereo-thresholds (better)
Define stereoacuity.
The difference between 2 vergence demands (2 different objects, how much total vergence they require in degrees)
Unit is seconds of arc
What is the average stereoacuity in patients with normal binocular vision?
20 seconds of arc with standard deviation of 10
96% of the normal adult population should have stereoacuity of at least 40 seconds of arc
Why do we have to test for stereopsis?
Assessment of binocular function (highest level)
Screening test in young children (amblyopia, anisometropia, heterotropia)
Job demands
What condition may affect stereopsis in a patient?
Amblyopia, strabismus, unequal refractive error
Do we have to perform W4D if patient has good stereopsis?
No
Is Stereopsis more important for near or distance vision?
Near
What do clinical Stereopsis tests use?
Polarization to present disparate images to each eye
Produces impression of depth
What are some examples of clinical stereopsis tests?
Stereo fly test
Wirt Circles (circles overlap to create 3D effect, imaged at non corresponding point within PFA, circles become more closely overlapped as the test continues (less 3D effect), images are closer to fovea, stereoacuity decreases)
What does turning the wirt circle book upside or upside down polarized sunglasses do to the image?
Fused images are now seen behind the book (going into book)
Uncrossed instead of crossed
What is the lowest recording value for Randot? How can we test for higher levels of stereopsis?
20 seconds of arc (average level of stereoacuity for patient with normal binocular vision)
Changing the working distance to 80cm (halves the angles)
What can be observed regarding the wirt circles when glasses are removed?
They have monocular cues
Do RDS’s have any monocular cues?
No
Describe the magic eye test.
To see charts in 3D requires accommodation in the plane of the picture (for clarity) with relative divergence that the eye are converged to a point behind the plane of the picture (NRV)
Can also be PRV, the image will be behind the plane of the page rather then in front of it
What happens to PFA as we move away from the fovea?
Increases
Always measured from the fovea (think of eccentricity of retina)
