1. Prescribing Guidelines for Bifocals and PALs
2nd COE advancing optics and ophthalmic dispensing, 9th July 2016
Gauri Shankar Shrestha, MOPTOM, FIACLE
Assistant Professor
Maharajgunj Medical Campus
IOM , TU

2. What is bifocal
A lens designed for presbyopes having two focal points that one for distance and the other for near
Executive bifocals
D-segment bifocals
Kriptok bifocals

3. Age is not absolutely a part of presbyopic correction
Who needs bifocals?
Inadvertently PRESBYOPES
Accommodative ESOTROPIA in children
Aphakia
Myopia
High AC/A ratio and esotropia-
Accommodative dysfunctions
Bifocals in pre-presbyopia
Old age vision- physiological diminution of accommodative focusing power tha results from the natural loss of elasticity of both the lens capsule and the lens substance
Accommodative iertia
CNS diseases-encephalitis
Systemic diseases-diabetes
Eye diseases-untreated glaucoma
Medications-atropine like compound
trauma
Age is not absolutely a part of presbyopic correction

5. Performance of bifocal
Vertical placement of the optical center of the segment
Pantoscopic tilt and optical axis
Lateral placement of the optical center of the segment
Segment inset
The correct amount of segment inset depends on- distance IPD,stop distance,fixation distance,the power of the distance correction in the horizontal meridian.

6. Performance of bifocal
Differential displacement (Image Jump)
Kriptok, D-segment, Executive,
Resultant displacement at the reading level
Hyperopia- kriptok
Myopia-D-segment, executive
The correct amount of segment inset depends on- distance IPD,stop distance,fixation distance,the power of the distance correction in the horizontal meridian.
The bifocal segment should be selected so that differential displacement at the reading level is eliminated. This is done by choosing a bifocal with a segment pole located at the reading level. The straight top fused bifocal(segment pole 5mm below the segment top)satisfies this criterion.
The bifocal segment should be selected so that the total displacement at the reading level is zero or as near zero as possible.This can be done by selecting a bifocal with a segment that provides prismatic effect opposite that provided by the distance lens.For eg a. for a minus distance Rx,the BD prismatic effect at the reading level can be opposed by the BU prismatic effect of an Executive style segment. b. for a plus distance RX,the BU prismatic effect of the distance power can be opposed by the BD prismatic effect of an Ultex segment or can be minimised in round Kryptok segment.
Displacement due to distance portion displacement due to near portion and trade off between these two.

7. Does age is the reliable guide to onset of presbyopia?
Amplitude of accommodation may vary individually
Proportion of available accommodation that the patient can use comfortably (Accommodative reserve)
Yes to some extent/ certainly not…….
Onset of presbyopia is influenced by
The amplitude of accommodation
Proportion of accommodation that can be used comfortably
The refractive error
Working distance

9. Refractive error Amount Full/ partial correction
Method of correction (spectacle/contact lens)
Accommodative stress is a significant factor in the progression of myopia
Bifocal suppress myopia
Need to have clinical confirmation
Spare from burden unless specifically indicated
useful
Correct myopia moderately in early pre/presbyopic age
It is best left uncorrected unless sharp acuity is demanded
Increasing myopia in older presbyopes receds
Reading range
Loss of intermediate vision
Need to idamentify latent hyperopia
Amplitude of accommodation recedes earlier
undercorrected astigmatism/improper astigmatism axis causes early onset of presbyopia

10. Ocular accommodation for different reading distance and refractive error

11. Working distance Demand by job Eg, a farmer, a watch maker, a reader
Limit due to arm length, height, and posture
Clinically, presbyopia depends on individual own specific visual requirements
People with identical refractive error and amplitude of accommodation may need presbyopic addition at different chronological age

12. Determine ADD (+) Corrective lens at distance are accurate & Balanced
Identify a patients habitual working distance
Measure amplitude of accommodation
Range of near vision (Don’t make bifocal too strong )
Eg Amp A= 3:00D, WD 33cm, Add= +1.50, Range of near vision: ????
Identify latent hyperopia/over-corrected myopia

13. Bifocals for children
A child is binocular at distance with full cycloplegic correction but demonstrates a persistent esotropia at near
Executive bifocal/large D-segment (35mm)
Place segment top as high as the lower age of the pupil

14. Bifocals in children
<5 year
6-7 year
> 8 year

15. Anisometropia
Anisometropes may manage distance vision with or without correction without complaint
But, reading level, vertical anisophoria may be significant due to induced differential prismatic effect
Vertical imbalance may also be related to facial asymmetry

16. Bifocal in Anisometropia
Dissimilar bifocal segment in for the two eyes.
Asymmetric Decentration of distance OC or near bifocal segment

17. tips More desk work, short arm length- raise segment
avoid in selecting big frames and aviator shaped frames
Picturize a detailed work conditions including specific working height and distances
Previous glass history (success/failure)
Proud posture= lower seg top

18. If a patient has been comfortable wearing a less- than-ideal bifocal segment, it is neither necessary nor desirable to change it.

19. Intermediate vision loss
Low myopes needing presbyopic add
Undercorrected myopes needing full correction and near addition
Aged 50+ patient having to increase near ADD
occupation needing intermediate vision correction
Bifocal may be unsatisfying………………………..

22. Intermediate Vision (a) Bifocal (b) PAL Advantages
As the slide opposite shows, the presbyopic patient is able to comfortably and quickly obtain clear vision for intermediate viewing by using the intermediate portion of the continuous power progression in the PAL.
Continuous support to the eye’s accommodation
The progressive lens is able to mimic the eye’s pre-presbyopic ability to focus on all points between the far point and the near point. If a presbyopic patient uses a bifocal lens design, there will be an abrupt shift as the eye’s gaze switches from distance to near viewing, as the visual axis travels across the segment line.
If single vision readers are used, then continuous vision is possible, but only for near and some intermediate distances, depending on the eye’s amplitude of accommodation. No distance vision is possible.
(a) Bifocal
(b) PAL

23. PAL Clinicians- knowledge and awareness
Hard design /soft design
Multi design
Short corridor
Intermediate near
Internal design
Opticians- Proper fitting techniques
Public- Lens benefits

24. Why Use PALs? Feature Benefit
Uninterrupted vision from distance to near
No visible line
No jump in vision from distance to near
Better vision as intermediate is clear
Looks like single vision
Lighter/thinner than SV
Looks better
More natural vision
More visual comfort
Confidence in mobility

25. Physiological Considerations
Typical dioptric power (D) for clear viewing of objects
Physiological Considerations
A successful optical correction depends on an accurate assessment of the patient’s visual requirements, the physiology of the eye’s performance for various viewing distances, and a knowledge of the advantages and limitations of the ophthalmic lens design.
Paramount to prescribing the optimum correction is a lens that is well-matched to the eye’s ability to focus objects at various distances onto the fovea (see Module 1). In addition, for objects which fall onto the peripheral retina (see diagram opposite), the patient does not see them clearly due to the drop-off in retinal acuity. In this region, it is important that the progressive lens is able to maintain the ability of the wearer to locate objects in space, perceive their form, and detect their movements.
For the normal pre-presbyope, spectacles are usually prescribed to correct refractive error. Sometimes the pre-presbyopic patient will require a modified correction, such as when visual demand at nearer distances is dictated by vocational or lifestyle requirements. The determination of intermediate and near correction is a routine component of the eye examination when a presbyopic patient is involved.
However, ophthalmic lenses have various physiological implications that should be considered by both patient and practitioner when spectacles are prescribed.
Distance
Intermediate
Near

26. Usable Field of View
Usable Field of View
Lens design
Reading habit
The usable field of view is comprised of head and eye movements as shown here for the horizontal plane.
Usable field of view
The usable field of view (sometimes called the field of gaze) is comprised of eye rotations and head movements which combine to allow the patient to locate objects in the field of view. This is depicted in the diagram opposite.
The comparative head and eye movements through progressives will depend on the lens design (for example, extent of aberration areas) and the patient’s own reading characteristics and habits. It is generally desirable for the lens design to minimize head movements. Smaller head movements will reduce the peripheral “swimming” effect of progressive lenses. The swim effect is related to the degree of binocularity achieved through the lenses as discussed below.
The blending of the power variations in progressive lenses allows the patient to comfortably view objects at near and intermediate distances.
Posture in near vision
Progressive lenses allow the presbyope to comfortably locate reading material with near-natural head and eye movements. This enables the patient to read with a more comfortable posture.
Progressive lenses are well accepted by most patients. However, a few patients may be disturbed by the astigmatism and visual aberrations in the lens periphery, and may not adapt to progressive lenses (see below).
Head movement
Eye rotation

27. Binocular Vision
While engaged in visual task, presbyopic correction should ensure normal BSV
Optical design should ensure image of right eye and left eye fall on CRP
PAL design should ensure a symmetrical power progression & compliment image size
Binocular Vision
When engaged in a visual task such as reading at near, it is important that the presbyopic correction is able to maintain normal binocular vision. To obtain normal binocular perception of the two eyes, the optical design should ensure that the images produced by the right and left lenses form on corresponding retinal points.
Corresponding retinal points. When the patient's gaze is lowered for reading at near, the eyes naturally converge to maintain a fused, single binocular image (see diagram opposite). The PAL design should ensure that the power progression follows this path of convergence downwards. Thus, the progressive meridian is oblique to follow the downgaze path of the visual axes; the two meridians follow a V-shape, as shown in the diagram opposite.
For ease of effort in fusing images in all directions of gaze, the right and left lenses should incorporate approximately equal vertical prism on either side of the power progression path.
Similar images. To achieve optimum binocular fusion, the images formed in both eyes by the PALs should be similar in all directions of gaze. This means that the power variations found at corresponding points on the right and left lenses should be complementary, thus ensuring that any image effects are approximately equal in the two eyes. If the image sizes are mismatched, the patient can experience swim effects on head movement.
If the image sizes are mismatched, the patient can experience swim effects on head movement

28. Peripheral (Extra-Foveal) Vision
When object imaged on periphery of retina. The physiological factors becomes important:
Locating an object in space
Perceiving the object’s form
Detecting the object’s movement
Optical clarity is less of an issue
When an object is imaged in the periphery of the retina, the physiological factors that become important are:
Locating an object in space
Perceiving the object’s form
Detecting the object’s movement
The optical clarity becomes less of an issue due to the fall-off in retinal acuity (see Module 1).
Space and form perception
This is influenced by how prism is distributed in the progressive lens. (As discussed previously in Modules 6 and 7, prismatic effects are an inherent feature of ophthalmic lenses.) The patient’s visual comfort with PALs can be affected by slight distortions of vertical and horizontal lines. These distortions arise from prismatic effects associated with the progression of optical power in the PAL. The asymmetric design is better in this regard. This is discussed below under “Progressive Lens Design”.
Movement perception
The perception of movement of objects when using a PAL should be as natural as possible to avoid patient discomfort. Movement perception occurs over the entire retina. Hence, the prismatic variations across the whole lens should be smooth and gradual.
The patient’s visual comfort with PALs can be affected by slight distortions of vertical and horizontal lines

30. So, understand the Principal parameter
Size of distance & near area
Type and intensity of aberration
Depth & usable width of corridor

31. Patient selection for PAL
Who are good candidate??
Those who require add power for certain task but prefer edge not visible
Presbyope complaining image jump
Emerging presbyopes
Person needing trifocal
Head mover ???
Pseudophakes
Aphakes wearing contact lens and pseudophakes

32. Patient selection for PAL
Who are Poor candidate??
Having motion sickness
Eye mover ???
Satisfied with bifocal
High add requirement(3.00D)
Significant vertical muscle imbalance
Anisometropia (>3Ds)

33. What should we keep in mind?
Understand experience with PG
Highlight limitations of PG
Explain feature/benefits of Rx
Refer to “change in vision” when refitting into different design
Listen to the wearer’s needs
Restate the wearer’s needs

34. Fittings are always tailored to the wearer’s needs
Fitting tips...
Fittings are always tailored to the wearer’s needs

35. Patient’s Requirements & Expectations
Step 1
Identification of
Patient’s Requirements & Expectations

36. Useful tip Judge the motivation
Highlight the advantages, but mention the limitations as well

37. Step 2. Frame Selection Frame shape Frame depth Frame size Good Shape
Dispensing Progressive Lenses
1. Frame selection
Frame shape: Progressive lenses are best suited to regular shapes, that is, round, oval and square and close variations of these. They are least suited to aviator style frames which cut off much of the reading zone. It is important that the shape allow the full use of all zones on the lens.
Frame depth: All progressive lens manufacturers specify a minimum distance from the fitting cross to the bottom of the frame. This is also to allow full use of the near zone. For most progressive lenses this distance is between 18 and 23 mm, measured from the fitting cross to the inner rim directly below the fitting cross. It is important to note, though, this is not the point of reference for measuring the height of the fitting cross. We will consider this issue later. The frame chosen must at least allow for the manufacturer’s minimum depth. There is generally no recommended minimum height above the fitting cross, however, common sense should be used to avoid selecting a frame where the fitting cross is too close to the top of the frame. Some manufacturers do suggest a minimum of 12 mm.
Frame size: Most frames are of adequate size for progressives, with the possible exception of depth as described above. The common problem with frame size is choosing a frame that is too large. The smaller the frame the more of the areas of astigmatic power that are removed from the lens. That is, the larger the frame the more of the areas of astigmatism that will be present in the finished spectacles. Fortunately, progressives are ideally suited to the modern regular frame shapes.
Frame shape
Frame depth
Frame size
Good Shape
Good Depth
Good Size
Bad Shape
Insufficient Depth
Too Large

38. Step 3
Frame Adjustment
Check and Adjust before taking facial measurements
Adjust when delivering spectacles

39. Step 3 Frame Adjustment Back vertex Distance - shorter
Reduced BVD gives
wider field of view

40. Increase Pantoscopic Tilt increases near field of view
Step 3
Frame Adjustment
Pantoscopic Tilt - 10 to 12 degrees
Increase Pantoscopic Tilt
increases near field of view

41. Step 3
Frame Adjustment
Facial wrap - must not be too flat

42. Adjustment for frame tilt
Dispensing Progressive Lenses
2. Frame adjustment
The frames will need to be adjusted twice for progressive lens patients.
First, when the measurements are being taken. It is not possible to take accurate facial measurements, particularly height, if the frames are not sitting exactly how they will when the patient wears the finished spectacles.
Second, on final delivery of the spectacles. Good adjustment is critical not only for comfort but also to ensure that the lenses work to best effect.
The normal frame adjustment procedures should be followed when adjusting the frame. This involves starting from the front of the frame and working back since any changes made to the frame front will affect the length to bend.
Start with the facial bow and horizontal alignment of the frame, followed by the let back (the angle of the temples to the front when looking from above), the pantoscopic tilt, the length to bend and, finally, the angle down and anatomical bend.
Adjustment for frame tilt

43. Adjustment for pantoscopic tilt
Dispensing Progressive Lenses
1. Frame selection
The next three frame requirements are all closely related in that they all affect the field of view through the various zones and they will be discussed further in the following section on frame adjustment:
Back vertex distance: The best frames for progressive lenses are those that can sit comfortably with a relatively small back vertex distance.
Pantoscopic tilt: The frame you select should also sit comfortably with a reasonable pantoscopic tilt. That is, a pantoscopic tilt of at least 10° - 12°.
Facial wrap: The frame you select for progressives must not be too flat in its facial wrap.
Adjustment for pantoscopic tilt

44. Useful tip Prefer half rimmed metal frames with adjustable nose pads
Back vertex distance- minimum
Pantoscopic tilt- 10 to 12 degrees
Facial wrap- It must follow the line of the face
Level of the frame- frame should sit squarely on the face
Temple length- optimum length to minimize sliding
Frame with adjustable nose pad

45. Step 4
Monocular PD

46. Step 4
Monocular PD
Monocular PD 32 29 61

47. Step 5
Marking the lens insert to align fitting cross with pupillary reflex

48. PD measurement should compliment the marking lens insert
Step 6
Remark and confirm Monocular PD
PD measurement should compliment the marking lens insert

49. Accommodate adequate and usable segments
Step 7
Fitting height
Accommodate adequate and usable segments

50. Useful tip Natural head posture Sit at the same level as the patient
Check binocularly
Use a fine marker

51. Step 8 Checking the cut out Ensure adequate and usable field of vision
Selection of proper diameter of fitting lens

52. Delivery ……… Markings Distance power & prism Addition
Fitting cross heights & PDs

53. Delivery ..
On eye assessment
Frame fit
Fitting crosses

54. Delivery ..
On eye assessment
Distance Vision
Near Vision

55. Delivery Instructions

56. Useful tip Use positive language Remove the markings at the end
Maintain a record of the order forms

57. Tips to successfully adapt patient to PAL
Prescribe an extra +0.25D add of all progressives, but never Over Do Add
long progressive corridor
Large frame size
Make sure lenses are not fit too low
Explain the possible adaptation problem in before hand

58. Tips to successfully adapt patient to PAL
Learn about the progressives you are dispensing with
Hard design vs. soft designs
Multi-design
Short corridor
Intermediate near
Internal design
Do not get overwhelm with all the conflicting technical claims (optima width, corridor length, size of reading area, distortion zones)

59. Tips to successfully adapt patient to PAL
Arrive at a practical and acceptable solution to the patients presbyopic problems
technical specification is only a asset,
Rather important issues are
nature and variety of near work,
working area,
working distance,
change in Rx,
patients habit, intelligence, attitude, personality and behavior

60. Do not change any aspect of a patient’s glasses unless you have a specific reason to do so and have taken the time to discuss the potential benefits and the potential problems with the patient
Thank You