objective and subjective refraction: human eyes Flashcards
what is objective refraction ?
. retinoscopy
. still need basic co-operation
. understand what is happening to light entering the eye
. communicate effectively
what is subjective refraction ?
. assessing refractive error by asking Px for their subjective responses
. it can only be done if we have reliable responses from the patient
. understand what is happening to light entering the eye
. communicate effectively
which method of refraction do we use ?
. sometimes , we use only subjective methods and other times only objective
. we use objective refraction as a basis for subjective refraction
what else can we use retinoscopy for ?
. indication of pupil sizes
. pupil response ( wether accommodation is fluctuating)
. idea of clarity of optical media , are there any lens opacity
. signs of corneal conditions
what are the steps of refraction procedure ?
- measure the inter-pupillary distance (PD)
- adjusting trial frame
- measure vision/visual acuity
- retinoscopy
how to measure the inter-pupillary distance (PD) ?
- “look into my left /open eye” ( practitioner closes their RE )
- align zero of ruler scale with tangent to R temporal limbus
- “look into my right/open eye “ ( practitioner closes LE )
- read scale from tangent to L nasal limbus
why it important to measure the inter-pupillary distance (PD) ?
- make sure frame is fitting accurately
- make sure that patient is looking through the centre of any lenses that you are using for refraction
what is phoropter ?
- way to do refraction instead of using a trial frame and trial lenses
what is advantage of phoropter ?
- placed in front a patient’s face - you don’t need set of trial lenses
how to adjust trial frame ?
- with the frame off the PX’s face , set PD
- then put on Px
- adjust bridge height and tighten
- adjust side length and angle
- make fine adjustments when the frame is on
- ensure Px comfort
what are the steps of measuring vision/visual acuity ?
. vision or visual acuity is measured monocularly
. keep room light on
. use an occluder in front of the left eye when measuring the right eye and vice versa
. ask patient to read the the smallest line of letters you can see
. ask if there anything else from the line below that you can read
. record your findings ( Snellen or logMAR)
why do we measure vision/visual acuity first ?
. it gives us a clue about the magnitude of refractive error
. gives you some insight before moving on to retinoscopy
. you have an idea of how well/badly the patient can see without correction
. this help you to know what to expect - whether the patient is emmetropic , hyperopic or myopic , astigmatic and whether a high power prescription is likely to be needed
. you also have some indication of the kind of target the patient will be able to look at while you carry out retinoscopy
how is retinoscopy carried out in humans ?
. ensure that the patient can view the target with the eye not being tested
. start with RE
. light is shone into one eye while the patient views the target and we ensure their visual axis is clear
. occluding lens is removed
. use your right eye to conduct retinoscopy on the patient’s right eye , and keep away from the visual axis of the left eye
. there may be some accommodation despite your best efforts
. the pupil size will vary between and within patients
. the reflex will also vary
how to control / discourage accommodation during retinoscopy ?
- room light off
- use a 6m target, appropriate size for level of vision
- further control for accommodation by using green target on the duochrome
why do we ask patient’s to use green target on duochrome to control accommodation ?
- green wavelengths are shorter than red wavelength
- when looking at green , the target becomes more blurred with accommodation
- when looking at red , longer wavelength , the target becomes clearer with accommodation
- we don’t want patient to accommodate so we asm them to look at circles on green background
what is another way to control/discourage accommodation during retinoscopy ?
- fogging
- before starting ret on RE, check whether there is a with movement in LE ( hypermetropic ). if so , change this to ‘against’ or ‘reversal’ with a positive lens to discourage Px from accommodating while you carry out RET on RE
why is duochrome target test based on?
based on principle of longitudinal chromatic aberration - shorter wavelength ( green light ) refracted more than longer wavelength light ( red )
what happens when viewing targets on red background ?
the longer red wavelengths are relevant . the eye need to accommodate slightly to bring the target into focus
what happens when viewing targets on green background ?
the shorter green wavelengths are relevant the target would become more blurred with accommodation .
how to correct the spherical error in retinoscopy ?
. look at the meridian with the slowest with movement or fastest against movement
. this is the meridian with the tendency towards hyperopia/away from myopia ( needs highest positive power or lowest negative power )
. correct this meridian with a spherical lens
how to correct the cylinder error in RET ?
. should observe an against movement so you can use minus cyl
. slow against = high powered negative cylinder
. fast against = lower powered negative cylinder
why is there no need to check for ‘ with ‘ movement in the right eye ?
. if you got reversal in right eye - the patient’s refractive error is over plus as we haven’t took into account working distance- patient’s vision is blurry so this acts as fogged lens
why do we take working distance into account when conducting RET ?
- you have conducted RET from a working distance which gives the light entering the eye some divergence
- the divergence disappears when the eye views a distant object, taking the focal point in front of retina
- reduce positive power / increase negative power equivalent to the working distance to take focal point back to retina
what to do after RET ?
subjective refraction - light ON
- adjust your estimate to allow for the working distance
- check VA monocularly and record it - not always good
- do subjective refraction RE first , with LE occluded
- when RE subjective refraction is complete, occlude the RE and do subjective on LE
