4. Non- tolerance Flashcards
Lens non-tolerance meaning?
Intolerance to a particular lens type, material, lens form or Rx.
Physiological non-tolerance meaning?
Irregular symptoms that fail to fit any pattern. Apparent determination to simply ‘not get on’ with the new specs.
If Px is physiologically rejecting the specs, how to eliminate actual lens intolerance?
Adopt a diplomatic approach to finding a mutually acceptable solution. Vital to retain Px confidence.
2 types of non-tolerance?
Lens and psychological
Single vision lenses with correct RX, what other factors can cause single vision non-tolerance?
- Optical centration errors
- Change in lens form or material
- Incorrect frame fitting
- Cosmesis
Optical centration error results in?
Unwanted prismatic effect- Prism increases with errors in centration or power of lens.
Binocular fusion system has getter tolerance for horizontal or vertical errors?
Horizontal
Impact of horizontal vs vertical mis-centered lenses?
Horizontal mis-centered les cause stress to visual system over a period of time. Vertical mis- centered lens cause immediate stress to the visual system.
2 ways of confirming prism in a lens?
- Mark optical centers with focimeter using marking tools and ask px to wear the lens and see if px is looking through the optical centers.
- Mark px pupils on the lens when worn then focimeter through those points to read off the prism.
Change in base curve from what px was using before can lead to?
Can lead to distortions
How is lens fitting checked?
Single and bifocal lenses
Single visions: Is lens pitching on nose, uncomfortable, red marks, pads need adjustment?
Bifocals: Check add power- sufficient? Check fitting (seg height, choice and inset)
How is lens fitting checked- Progressive lenses?
Is near power sufficient?
Check frame depth- to ensure px has maximum add.
Check frame fitting- fitting cross heights, mono PDs and lens design
How much vertical mis-centration is not tolerated by the eyes?
Not more than 1 diopter over a long period of time.
Aspherical lenses pose a get risk of?
Potential single vision lens intolerance
What factors affect spectacle magnification (SM)?
Surface power, lens thickness and refractive index
Spectacle magnification (SM) directly affects?
Retinal Image Size
In anisometropic px’s change in relative image size between RE & LE leads to?
Leads to breakdown in binocular fusion and non-tolerance
Change in material- px will complain of?
Lack of clearly & defined sharpness to colour fringing around high contrast objects and blur for low contrast objects
How is v value picked?
Comparing suitable lenses and by discussing the period of adaptation required.
Pxs vision through a lens is perfect, but they still complain of poor comfort or frame keeps slipping/ contact marks, why?
Incorrect frame fitting or inappropriate frame selection
Failure to consider any vertex distance results in?
Over or under correction of final lens power
Vertex distance depends on?
Pantoscopic tilt, pad angles, head width, temple width and length
Adjusting pantoscopic tilt can make small adjustments in?
Small adjustments in vertical positioning of optical centres
Symptoms of bifocal non-tolerance
- Jump
- Reduced field of view when reading
- Segment inappropriately placed
- Unsatisfactory intermediate vision
Benefit of flat-top design in minus lenses and round designs in plus lenses?
Neutralizing some prism created by the main lens when wearer glances downwards.
How to reduce non-tolerance in Px’s that previously wore bifocals?
Examine previous bifocals worn by Px- seg size and position should be the same as previous lenses.
Non-tolerance in anisometropic patients?
Issue of differential prism- if this is not resolved at near vision point risk of intolerance
Non-tolerance in anisometropic patients solutions?
- Different diameter round segment bifocals
- Split bifocals
- Prism controlled bifocals
- Cemented or boned bifocal segments
- Slab on/ slab off
How is TCA calculated?
TCA = p/V (p =cF)
What is constringence?
Abbe number/ v value
What axis is considered when calculating vertical DPE?
Value along 90 meridian
When is TCA value a reason for concern?
When TCA > 0.1. Px likely to notice the effects
Relationship between refractive index and v value?
As refractive index increases, v value decreases.
What is required to calculate jump?
Radius in cm
Add power
And ans will be BD (because jump causes image to move up)
What is anisometropia? S
Significant difference in Rx between both eyes
