Midterms Review Flashcards
Describe the following conversions:
Degree to circle
Minute to degree
Second to minute
1 degree = 1/360 of a circle
1 minute = 1/60 degree
1 second = 1/60 minute
5 minute = 5/60 degrees
How does letter size correlate to subtending distance?
The larger the letter the bigger the distance.
Describe recognition, detection, resolution, and localization acuity.
I have to be able to see the letter and know what is it
Baby looks to the object until they can’t see it anymore
Resolve= can you see it (tumbling E and Landolt C)
What is the smallest gap between lines that can be resolved
What happens to the angular subtense when you halve the viewing distance? When you double the viewing distance?
Halve the viewing distance, angular subtense doubles
Double the viewing distance, angular subtense halves
What does OD, OS, and OU stand for?
Ocular dextra
Ocular sinister
Ocular uterque
When testing visual acuity when do we stop?
Identify the lowest line that the patient can read at least half the letters correctly.
How do you calculate MAR? How do you calculate logMAR?
Take reciprocal of VA and divide.
VA= 20/40 MAR= 40/20= 2’
Take log for MAR for logMAR
How is the logMAR chart organized (spacing)?
5 letters on every line
Each line is 0.10 logMAR less then previous
Space between each letter is equal to the width of the letter
Space between each line is equal to the height of the letters in the lower line
How is logMAR scored? What is a better score?
Each line= 0.10 logMAR
Each letter= 0.02
Lower score is better
Every line or letter read correctly subtract
When using a mirror how can the viewing distance be calculated?
Viewing distance is the patient’s distance to the mirror plus the mirror distance to the chart.
Does MAR change with viewing distance?
NO
What are the next steps in testing VA if moving the patient closer to the chart does not work? Is finger counting reliable?
Hand Motion (are my hands moving)
Light Projection (where is the light)
Light Perception (is light on or off)
No light perception
Finger counting is very non-standardized and not reliable
Where is near VA tested?
40cm (16in)
Also at the distance where they commonly perform near work.
Describe the Jaeger chart.
Letter range from J1 to J20
Very unstandarized and therefore unreliable
Used by ophthalmologists
Describe the M notation chart for near VA.
M is the denominator of the Snellen fraction
When testing at 40cm if smallest letter that can be read is 0.5M, VA= 0.40/0.5
Describe the Point (N) chart for near VA.
One point = 1/72 of an inch
N5 = 5/72 inches high
Refers to size of block so lowercase letter is half
What factors affect VA?
Blur (blur circle formed on retina when image is focused in front or behind)
Contrast (does not affect VA after a certain point in healthy normal eyes)
Retinal Eccentricity (how far a point on the retina lies from the fovea)
What method is used to reduce the size of the blur circle? What does it change, what stays the same?
Pinhole aperture
Restricts light going through to create smaller blur circle
Improves VA, DOES NOT improve refractive error (does not change were the light focuses)
What is the fovea and how does retinal eccentricity involve it?
Fovea is the most sensitive part of the retina
Location of best vision
Images straight ahead are placed on fovea
Eccentricity is angle that an object away from straight view makes with eye and away from retina
As angle increases, VA decreases (eye turn)
What percentage of the adult USA population is myopic?
42%
By 2050 what population of the world is expected to be myopic?
50%
What percentage of the world’s population over 50 years of age has presbyopia?
100%
What are spherical refractive errors? Examples?
Having the same refractive error in all directions (meridia)
Myopia
Hyperopia
What is the most common type of myopia?
Axial
What is the Far Point of the unaccommodated eye conjugate with?
Retina
How do you calculate far point of a myopic or hyperopic eye? What is far point of emmetropic eye?
Distance from far point to front of eye= 1/ refractive error in diopters
Myopic in front
Hyperopic behind
Emmetropic optical infinity
How can refractive error be corrected?
With lens whose second focal point coincides with the patient’s far point
Lens whose focal length is equal to the distance from the lens to the far point
*Image at optical infinity, object lies at first focal point
What happens when we change the bvd?
We must change the power of the correcting lens
Differentiate the length used to calculate the the power of the eye from the length used to calculate Rx of a lens?
Far point length is used for eye (FP or fc = bvd)
Focal length (from lens to FP) is used for Rx
How does CL and spectacle prescriptions compare for myope and hyperopes?
Myope: spectacle has a more minus prescription
Hyperope: CL has a more plus prescription
What needs to be done to refract a hyperope?
Stop accommodation
Fogging (turn patients into myope by giving them too much plus lens power, overcorrect)
VA will start to drop once fogged
What is total hyperopia and manifest hyperopia? What are they calculated for?
Total hyperopia= cycloplegic drug gets rid of all accommodation
Manifest hyperopia= maximum plus lens that gives best VA
Latent Hyperopia (amount that patient was accommodating without drug)= total- manifest
How can fogging be used to calculate hyperopia? What is the difference referred to?
Fogging can be used to minimize but not remove all accommodation.
Facultative hyperopia= hyperopia under fog - hyperopia before fogging (maximum plus lens)
What is absolute hyperopia? How does this effect the patient’s accommodation?
The amount of hyperopia that a patient has that exceed the maximum amount the patient can accommodate.
These patients don’t usually accommodate because it won’t be enough.
What is astigmatism? What is it corrected by? What is the difference between meridia?
The difference between two different meridia of the same eye.
Cylindrical lens
Cyl lens is in direction of zero power (sph that is corrected first) and 90 degrees away is the maximum power axis.
How do you transpose between minus cyl and plus cyl?
- New sph is the sum of the original sph and cyl
- Change sign on cyl
- Change axis to 90 away
What does the direction of the reflex movement reveal about the far point?
Neutral= at retinoscope
With= behind eye or behind retinoscope (doesn’t matter which it is behind)
Against= between retinoscope and eye
What parts os the Rx found through retinoscopy are affected by working distance?
Sphere
What happens as accommodation occurs?
Refractive power changes
Contraction of ciliary muscle
Central aperture diameter decreases
Tension in zonules decreases
Lens bulges forward
Central radius of curvature decreases
Lens sinks due to gravity
Where are near and distant objects focused during accommodation?
Near focused at the retina
Distant focused in front of retina, lens has too much power
Describe stimulus to accommodation and how to calculate it.
Stimulus to accommodation= required power - unaccommodated power
Power needed to focus an object at a specific distance on the retina minus uncorrected refractive power of eye
What is the difference in stimulus to accommodation between an axial myope (or hyperope) and a refractive myope ( or hyperope)?
They are exactly the same
What is Tonic accommodation?
Caused by underlying tones of the ciliary smooth muscle
Never fully relaxes
0.50D
What is convergent accommodation?
Stimulated when the eyes converge towards one another
1 meter angle (6) of convergence with stimulate 0.40D of accommodation to give convergent accommodation to convergence ratio of 0.40D/6.
Describe blur driven accommodation.
Accommodation to clear retinal blur
Image is accommodated to be focused on the retina
Describe proximal accommodation.
Caused by awareness of nearness of an object
Even if image is at infinity our brains will recognize a near object is near and we start accommodating.
How does accommodative response compare to accommodative stimulus?
Stimulus is what we should do and response is what we actually do.
For low dioptric stimuli <1D, response is greater then stimulus (lead accommodation)
For high dioptric stimuli >1D, response if less then stimuli (lag accommodation)
Only at one point does response=stimulus
What is keratometry used to measure?
Measure the power (and astigmatism) of the anterior corneal surface, the most refractive element of the eye.
Specifically the annulus around the apex
Find site of ocular astigmatism
Fitting CL
For refraction
See if ametropia is axial or refractive
Corneal health
What is the difference between a corneal and a noncorneal astigmatism as shows on the keratometry reading?
Corneal astigmatism shows a difference between the meridia that is subtracted to find the cyl.
Noncorneal astigmatism shows a spherical reading for keratonomy but a cyl for refraction
Describe wtr and atr astigmatism.
WTR- vertical meridian 90 has the highest dioptric power, correct by a minus cyl x180
ATR- horizontal meridian 180 has the highest dioptric power, corrected by a minus cyl axis 90
How does wtr and atr astigmatism change with age?
Below 45 y/o, wtr astigmatism is more common
Above 45 y/o, atr astigmatism is more common
How to calculate ocular astigmatism?
Add corneal astigmatism and non corneal astigmatism
What is Javal’s Rule?
OA= 1.25 CA + 0.50 atr
How does keratometry compare to topography?
Keratometry only measures a small areas around the cornea
Topography measures the entirety of the corneal shape.
Where is the near power measured on the bifocal lens?
On the surface it was added onto either the front or the back.
How to caluclate the distance power and near add on a bifocal lens? Does add require both meridians?
Distance power = distance bvd
Near add= near fvp - distance fvp
Add is spherical so only one meridian must be measured
Compare and contrast the effect of prisms and lenses on light rays.
Prisms bend light, don’t change the vergence (parallel beams come out parallel)
Lenses change the vergence of the incident beam (parallel beams do not emerge parallel)
Describe the direction that light is defeated after going through a prism.
Light is deviated towards the BASE.
Image is displaced towards the APEX.
Describe how to calculate a prism diopter.
One prism diopter is a displacement of 1cm at a viewing distance of 1m.
Displacement (cm)= prism power x viewing distance (m)
What is the difference between ground (worked) prisms and prism by decentration?
Ground- same magnitude of prism all over the lens
Prism by decentration- the amount of prism varies across the lens (when patient looks away from center of lens)
What is the distance between center (DBC)?
Millimeter distance between the optical center of each lens.
Often mistaken for PD
How do prisms affect the position of an image in the Lensometer? What is the description of each displacement? What is the reason for this displacement?
The image is not in the center of the reticle.
Displaced down = base down
Displaced up= base up
Temporal= base out
Nasally= base in
Lensometer magnifies and inverts the image
Describe the orientation of plus (convex) vs minus (concave).
Plus- base of triangle meets to form optical center
Minus- point of triangles meet to form optical center
What do regular reflected corneal mires appear as? What kind of mirror do they act as?
Clear and Regular (round)
Convex mirrors
In patients with corneal astigmatism, how do the reflected mires appear?
Oval rather than round
In patients with both blurred and irregular reflected mires, what do this indicate?
Warping of the cornea due to CL overwear
How do reflected mires appear in dry eye patients?
Appear clear immediately after a patient blinks, but quickly go out of focus until the patient reblinks (spreading a new layer of tears)
