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
What can tomey MR-6000 measure
Topography
Refractin
Keratometry
Tonometry
Pachymetry
Dry eye analysis tool
When doing auto refraction it is important to double check the measruments to see if..
Has a standard deviation of less than 0.25 and within 10 degrees
WHo is good to use retinoscope on
Pxs who are unavailable to cooperate in a subjective refraction - Babies and pxs with mobility issues
-Young children
-Development delay
-Low vision
-Non english speaking pxs
-Malingeres (pretending/faking)
What parts of the eye can a retinoscope view
lens
iris
cornea
Static retinoscopy
Accomodation is controlled or suppressed
Dynamic retinoscopy
Accomodation is allowed to occur
how does retinoscope work
- a streak of light is shone into the pxs eye and the practitioner observes the movement of light refelcted from patients retina (reflex)
If the reflection is moving with the retinoscope..
It means the px is hyperopic so correct with plus lens
If the reflection is moving against the retinoscope
It means the px is myopia so correct with minus lenses
How do you know you reached the endpoint of retinoscope
Occurs when the pupil fills or glows with light
Working distance
Impossible for the practitioner to work at infinity
The usual working distance is an arms length (50-66cm)
Preparing the retinoscopy
Adjust the trial frames
-match the distance pd
-level
-panto and vertex should be at sensible values
Add lens to the trial frames
-spheres should be places in the back cells
Do you need to be in light or dark room for retinoscopy
Dark room - so pupil dilates which makes the reflex more visible
When working on the visual axis w retinsocope
Work within 5 degrees of the visual axis
Use your right eye to test the patients right eye and use your left eye to test that pxs left eye
Fogging
Fog the fixating eye to ensure accommodation is relaxed
-If overdone can induce accommodation
-SHould be less than 2D
-Check for againt movement along all meridians in the fixating eye
Basic method for retinoscopy
- Initial lens selection
- Identification of the principal meridians
- Neutralisation of the refractive error along the principal meridians
Working distance of 66cm would need what lens
+1.50Ds
A working distance of 50cm will need what lens
+2.00DS
