Dispensing 5 - Non Tolerance Flashcards
(40 cards)
what is lens non-tolerance?
intolerance to a particular lens type, material, lens form or prescription
what is the difference between lens non-tolerance and psychological non-tolerance?
lens non tolerance is an actual intolerance to an aspect of the lens whereas psychological is where the symptoms are irregular and fail to fit any pattern hence there is not usually a physical problem more the problem is due to how the patient feels
what are potential reasons for psychological non-tolerance?
-patient feels like they were forced into the purchase
-realised they didnt want to spend that much for their glasses
-changed their mind about the style of frame chosen
-third party made an unfavourable comment about their new specs
-spectacle lens solution chosen does not fit into patient’s lifestyle
-spectacles fail to meet the pxs expectations
what are two types of non tolerance?
lens and psychological
how do you prevent either form of non-tolerance
-carefully consider optical, functional and cosmetic properties of the spectacles
-carefully consider any changes recommended compared to the pxs previous spectacles
-take a methodical problem solving approach
what is the general order or steps when dealing with a non tolerance case in single vision spectacles?
- focimeter and confirm sphere/cyl/axis
- Focimeter and confirm prism (ground in or intentional/unintentional shift in
optical centre) - Compare base curve (use a lens clock)
- check coatings and tints
- check frame fitting
- consider px adaptation times/ educate on how to use their lenses
- recheck considering cycloplegic refraction, any change in cylinder and pathological refractive change (e.g. cataract)
what is the general order or steps when dealing with a non tolerance case in bifocals?
- focimeter and confirm sphere/cyl/axis
- Focimeter and confirm prism (ground in or intentional/unintentional shift in
optical centre) - Compare base curve (use a lens clock)
- check near add power and whether it is sufficient for the patient’s requirements
- check fitting in relation to segment height and inset and check segment choice
- check coatings and tints
- check frame fitting
- consider px adaptation times/ educate on how to use their lenses
- recheck considering cycloplegic refraction, any change in cylinder and pathological refractive change (e.g. cataract)
what is the general order or steps when dealing with a non tolerance case in progressives?
- focimeter and confirm sphere/cyl/axis
- Focimeter and confirm prism (ground in or intentional/unintentional shift in
optical centre) - Compare base curve (use a lens clock)
- check near add power and whether it is sufficient for the patient’s requirements
- check frame depth to ensure patient can achieve the maximum add and check fitting (fitting cross heights and mono pds) and choice of lens design
- check coatings and tints
- check frame fitting
- consider px adaptation times/ educate on how to use their lenses
- recheck considering cycloplegic refraction, any change in cylinder and pathological refractive change (e.g. cataract)
what could cause single vision non-tolerance assuming the Rx is correct?
o Optical centration errors
o Changes in lens form and/or material
o Incorrect frame fitting or inappropriate frame selection
o Cosmesis
o changing the lens from spherical to aspherical or vice versa
o changes in base curve
o spec mag
o material
o eye size
What could cause optical centration errors in non tolerance lenses?
-unwanted prismatic effect
-Binocular fusion system has a greater tolerance for horizontal errors than vertical errors.
-if they need prism/ have the wrong prism D
-calculating differential prismatic effect
-patient not having had enough time to adapt
how can unwanted prismatic effect cause optical centration error?
As prism increases with error in centration/ power of the lenses
What’ s the difference between horizontally mis-centred lenses and vertically mis-centred lenses?
Horizontally mis-centred lenses can cause the visual system to become
stressed over a period of time whereas vertically mis-centred lenses can cause the visual system to be stressed immediately
according to Tunnacliffe and Williams (1985) what did they say to compare how photopic conditions and mesopic conditions affect visual function?
“In photopic conditions one prism-dioptre causes a significant drop in sensitivity at all spatial frequencies and in mesopic conditions as
little as a half prism-dioptre has a similar effect.”
what is a common change in lens form?
spherical to aspherical lenses?
why does changing from spherical to aspherical lenses cause lens - intolerance?
because, a wearer that has adapted to the off-axis performance of a spherical lens will often find the
sudden correction of the off axis when they are switched to an aspheric lens form (or vice versa) difficult to adapt to
how can changes in base curve cause single vision non-tolerance?
Steeper base curves (6 to 8 base) such as those found in many wraparound style sunglass lenses can also lead to intolerance as the spec mag and off axis vision can be hard to adapt to
how can you summarise the formula for spectacle magnification (SM)
the product of the shape
factor [ 1 / (1 - (t/n * F1) ) ] and power factor [ 1 / (1 – (d * Fv) ) ] where:
o t = centre thickness
o d = BVD
o Fv = BVP
o F1 = front surface power
what variables affect the degree of spectacle magnification?
-centre thickness
-bvd
-bvp
-front surface power
for spectacle magnification, what symptoms may Px complain about and why? what do you need to tell the patient
-objects such as the ground appear closer or further away,
although many patients do eventually adapt to this change.
-as SM of a lens directly affects retinal image size
- is important to
emphasise the need for patience while the adaptation process occurs.
why may anisometropic patients be non-tolerant to SM?
because a change in the relative image sizes between the right
and left eyes may lead to a breakdown in binocular fusion
how can changes in material cause single vision non-tolerance?
because changes in refractive index may give rise to complaints ranging from a lack of clearly defined sharpness to colour fringing around high contrast objects and blur around low contrast objects so careful consideration of the v-values when comparing suitable lenses and by discussing the period of adaptation often required.
why can eye size cause single vision non-tolerance?
because the larger the eye size, the greater the oblique astigmatism causing symptoms of clear central vision and increasing degrees of blur and disorientation as oblique
gaze increases
what are symptoms of incorrect frame?
poor comfort, contact marks and constant slipping
why can incorrect frame fitting cause lens non-tolerance?
because initial symptoms can be fixed with adjustments but failure to consider any vertex distance adjustment can result in an over or under correction of the final lens power
