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

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

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

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.

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.

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

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

Ocular accommodation for different reading distance and refractive error

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

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

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

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

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

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

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

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

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………………………..

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

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

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

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

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

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

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

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

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

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)

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

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

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

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

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

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

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

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

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

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

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

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

Step 4 Monocular PD

Step 4 Monocular PD Monocular PD 32 29 61

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

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

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

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

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

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

Delivery .. On eye assessment Frame fit Fitting crosses

Delivery .. On eye assessment Distance Vision Near Vision

Delivery Instructions

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

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

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)

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

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