unit 1 (week 2) Flashcards
With the rule
Steepest corneal meridian is near the verticle
Flattest corneal meridian is near the horizontal
Figure 8 shape
Against the rule
Steepest corneal meridian is near the horizontal
Flattest corneal meridian is near the verticle
Bow tie shape
Regular astigmatism
Refractive error is constant over the refracting surface
When the axis is 90 degrees apart
Irregular astigmatism
Refractive error varies over the refracting surface
When the axis is anywhere between 180 and 90
5 types of regular astigmatism
-Simple myopic astigmatism
-Compound myopic astigmatism
-Simple hypermetropic astigmatism
-Compound hypermetropic astigmatism
-Mixed astigmatism
Mixed astigmatism
One line focus lies in front (negative) and one lies behind (positive) the retina
Simple myopic astig
One line image is formed in front of the retina and one on the retina
Once principal power of the correcting lens is negative and the other is plano
Compound myopic astig
Both line images are formed in front of the retina
Both principal power of the correcting lens are negative
Simple hyperopia astig
One line image is formed behind the retina and one ON the retina
One principal power of the correcting lens is positive and the other plano
Compound hyperopic astig
Both line images are formed behind the retina
Both principal powers of the correcting lens are positive
Subjective refraction
Results depends on the patients ability to discern changes in clarity
Ex. phoropter, trial frame
Objective refraction
Results depends puerly on the examiners judgement to determine the optimum optical correction
Ex, Retinoscopy, autorefractor
Subjective refraction 3 phases
- To correct the spherical element of the refractive error
- The determination of the astigmatic error (if present)
- Balancing and/or modification of the refractive correction to ensure optimal visual performance and patient comfort
Symptoms of uncorrected myopia
Clear near vision, blurred distance vision
Near sighted
Symptoms of uncorrected hyperopia
Blurred near vision, good distance
Far sighted
Does objective refraction determine the initial element of refraction
Yes - doing an auto refraction or using the retinoscope makes it alot easier to get the rx, without doing this it would take much longer to find
If a pxs PD is smaller than phoropter PD
Minus lenses will induce BI prism
Plus lenses will induce BO prism
If a pxs PD is larger than phoropter PD
Minus lenses will induce BO prism
PLus lenses will induce BI prism
Any lens moved away from the eye becomes more positive or negative?
Positive
How does autorefractor work
-Infred light source is projected into the eye
-The reflection passes out to reach a light sensor
-The calculation of refractive error is based on analysis of how the eye influecnes the infared radiation
How will an autorefraction results be most accurate
With cycloplgia or good accomodative control
What errors are most common in autorefractor
Poor fixation
Accommodative fluctuation (proximal accommodeation in young people)
Media difficulties (cataract)
Disadvantages of autorefractor
-May over minus
-Good fixation required: errors with nystagmu / poor attention span
-Errors with media opacities - cataracts, asteroid hyalosis, corneal changes
-Errors with small pupils
-Errors with some case of pseudophakia, high ametropia, amblyopia (lazy eye) or reduced central acuity
Advantages of autorefractor
-Reliable alternative to retinoscopy
-Good accuracy after cycloplegia - better than retinoscopy when pupil is dilated - reflex can be misleading
-Useful if subjective unlikely to be reliable ; learning difficulties / cognitive impairments
-Easy for assisting staff
-Fast and efficient - can use in large scale screening ex/school
