The process of the prescriber (usually an optometrist) handing over a patient to the dispensing optician is a vital link in the patient’s continuity of care. It provides an opportunity for the prescriber to convey the results of the examination to the dispensing optician, in front of the patient. More importantly, for the dispensing optician, it is a chance to obtain additional information that may be helpful during the dispensing process.
Recently, the value of the traditional handover has been questioned by some working in the field of optical marketing. As an alternative, or possibly an adjunct to the handover, a pre-examination questionnaire has been suggested. Essentially such a questionnaire is designed to give the patient the opportunity to list what he does not like about his current spectacles before the eye examination and dispensing.
Questions may include: Do you find your spectacles too heavy or the lenses too thick? If you wear progressive power lenses would you like the reading area to be larger or do you want to move your head less when using a computer? Such questions may be interpreted as leading, but they do give practitioners an opportunity to discuss the current spectacle correction with the patient.
Regardless of whether the patient was examined in the practice or elsewhere, the dispensing optician must ensure that the prescription contains all relevant information required to complete the dispensing and the handover process provides an ideal opportunity for the dispensing optician to clarify any areas of concern with the prescriber. Additional and often essential items could include visual acuities, a vertex distance and working distances. If the prescription contains any anomalies or if there is information missing, the dispensing optician has an obligation to contact the prescriber for confirmation.
The process of analysing a spectacle prescription
Many experienced eye care practitioners would argue that the receiving and analysis of a patient’s prescription is the most important aspect of any dispense as it provides the practitioner with an opportunity to talk to the patient. If executed correctly, this initial discussion should answer the following questions:
? Does the patient know why he/she has been prescribed spectacles?
? Does the patient understand the purpose of the prescribed spectacles?
? What are the patient’s expectations?
? Will the prescribed spectacles meet these expectations?
? Is a visual task analysis required?
When presented with a spectacle prescription for dispensing, it is necessary to ensure that it has been written in a clear and unambiguous manner. BS 2738-3:2004+A1:2008 Spectacle lenses – Part 3: Specification for the presentation of prescriptions and prescription orders for ophthalmic lenses provides detailed guidance on the writing of a spectacle prescription.
This publication specifies the information to be included in prescriptions and in prescription orders to enable the correct spectacle lenses for a patient to be supplied. It also specifies the method by which optometric and other information is to be presented. BS 2738-3:2004+A1:2008 gives the following recommendations:
? On all prescriptions and prescription orders, the power of the sphere (spherical power) shall be stated for each eye or lens. If the prescribed lens power exceeds 5.00D, then the distance at which the power was measured should be recorded in the prescription. This should be indicated by giving the vertex distance, in millimetres measured from the corneal apex, in the form, for example, ‘at 12’, following the prescription. This will allow the practitioner dispensing the spectacles (or contact lenses) to make any necessary adjustments to the power ordered. The following information shall be provided on prescriptions and prescription orders for each eye or lens, as applicable:
– The power of the cylinder together with its axis
– The prismatic power together with the prism base setting
– The addition for near vision, ie the near addition
– If clinically necessary, other additional information, such as centration distance, tint, lens form, material or specific lens type
? With regard to cylinder axes, decimal values and the vulgar fraction ½ should not to be used for the orientation of the axis. Values ending in ½, eg 2½ or 7½ should be rounded up or down to the nearest integer. The orientation of cylinder axes is always from 0 (zero) to 180. Only integer values should be used and 180 should be used instead of 0
? For vertical and horizontal prisms, the base setting can be specified using the words ‘up’, ‘down’, ‘in’ or ‘out’ written in full, as applicable, or by using the 360° protractor specified in BS EN ISO 8429, in which case 360 shall be used instead of 0 for the horizontal direction to the right. For oblique prism base settings, a combination of vertical and horizontal components; or stating ‘up at’ or ‘down at’ as applicable, followed by the base setting using a 180° protractor; or using the 360° protractor specified in BS EN ISO 8429 are acceptable methods. As an example, the single oblique prism equivalent to R 3? base down and 4? base in would be specified as ‘R 5? down at 143’ or ‘R 5? at 323’. The degree symbol (°) is not be used after the value indicating the prism base setting. For multifocal and progressive power lenses, unless otherwise stated, the prismatic power is applicable across the whole lens. If a patient requires, or would benefit from, a different prismatic power in a separate near vision prescription, then specific instructions or a separate prescription entry needs to be given
? The details for the right lens should precede the details for the left lens and unless written in full, the letters ‘R’ and ‘L’ should be used to identify the lens data for the patient’s right and left eyes respectively. The letters ‘BE’ should be used to identify data applicable to lenses that are identical for both eyes. The unit of focal power is the dioptre (D) and the unit of prismatic power is the prism dioptre (?). Angles shall be expressed in degrees (º) and distances in millimetres (mm). A zero should be placed before the decimal point if the dioptric power is less than 1.00. For all data relating to focal power, the sign shall be included and two digits shall be used after the decimal point. The orientation of the cylinder axis shall be 0 (zero) to 180 but without the inclusion of the degree symbol (°)
? The use of a line or arrow to indicate the axis direction is an outdated practice which is sometimes misused as a graphical confirmation. Written confirmation, for example, when the right and the left eyes’ axes are (approximately) at right angles to each other, or when one sphere is positive and the other negative, is recommended.
In addition to the information given in BS 2738-3:2004+A1:2008 it is usual for the cylinder powers and the cylinder axes to be the same for distance and near. However, on rare occasions it is possible for the patient to require different cylinder axes (resulting from cyclo-rotations at near) and different cylinder powers (for example, near vision effectivity error with high powered cylinders) at near when compared to distance. It is also usual to expect that any prescribed reading addition will be the same for the right and left eyes.
In summary:
? All lens powers should be written using two decimal places
? Cylinder axes should be written without the use of the degree symbol
? A vertex distance should be included for lens powers that exceed 5.00D
? If the prescription is for distance and near spectacles, the cylinder axes and powers should be identical for distance and near
? Any prescribed near addition should be the same for both eyes
? Prism base directions should be compatible.
Patient expectations
Before any decision as to a suitable lens or frame can be made, the dispensing optician will want to ascertain various facts about the patient’s lifestyle, their expectations and the likely use of any new spectacles. This is where a pre-examination/dispensing questionnaire may be useful. Points of interest may include:
? What types of spectacles does the patient currently use?
? When do they use them and for what?
? Has the patient encountered previous difficulties including intolerance to PPLs or aspheric lenses?
? Their vocation, hobbies and other activities
? Why do they want new spectacles (apart from any prescription change)?
? How will new spectacles fit into their lifestyle?
? Are there any specific requirements such as protection from radiation or mechanical hazards?
As part of this process a more detailed visual task analysis can often prove helpful in determining the type of correction required. Obvious examples include single vision, bifocal, progressive, enhanced reading or maybe a combination of pairs, with the aim of meeting all of the patient’s visual requirements and not just ‘selling’ them a pair of spectacles. Discussing these issues at the onset enables the dispensing optician to make a more informed decision.
Spectacle frames
Once a decision has been reached about the general type of appliance to be dispensed, the process of frame selection can begin. Spectacle frames were discussed in Parts 2, 3 and 4 of this series. This article will now consider the processes of frame and facial measurement, specification and initial fitting (setting up) of the frame.
Initial fitting
During the frame selection and dispensing process it may be necessary to undertake preliminary frame adjustment, sometimes to simply enable the patient to visualise how the finished spectacles will look and feel.
These initial adjustments can be summarised as:
? For a metal frame with pads on arms, splay angle, frontal angle and distance between pad centres (Figure 1). Using pad adjustment pliers to place the pad surfaces flat against the sides of the nose and ensuring the bridge bar is not in contact with skin (Figure 2). This will also confirm if the overall bridge size is appropriate
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? Head width (Figure 3) confirms the overall frame size is suitable for the patient’s head dimensions
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? Pantoscopic angle. Using protective pliers (Figure 4) to ensure the pantoscopic angle is set to approximately 10° and that the lower rims of the frame do not sit in contact with the patient’s cheeks
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? Face-form angle (Figure 5). This will differ from frame to frame with approximately 5° being common. The right and left face-form angles can also differ
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? Length to bend (Figure 6). The overall side length is long enough to allow for approximately 35mm of drop behind the ear. Length to bend can of course be affected by the face form angle. If the face-form angle is adjusted to be flatter, the length of bend will need to be increased.
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Setting up at this stage also provides the dispensing optician with confirmation that the frame will be suitable for the specific lens type (particularly if a multifocal lens is being dispensed) in respect of overall horizontal and vertical lens size. In the author’s opinion, practitioners should have an appropriate and consistent system for frame adjustment. The first item to be assessed and adjusted should be the bridge, followed by the lugs, sides and end tips.
Initial measurements
For some frame types, precise measurement and specification of the frame in relation to the final fitting is required in advance. A good example of this is when ordering a handmade plastics regular bridge frame where the following measurements would need to be taken in advance.
? Horizontal and vertical lens size
? Crest height
? Apical radius
? Distance between rims at 10mm and 15mm below the crest
? Frame temple width
? Frame head width
? Bridge projection
? Joint height
? Total length of side
? Angle of let back
? Length to bend
? Length of drop
? Downward angle of drop.
The terms given in the above list were explained in Part 3 of this series.
Bespoke options are available (for example TD Tom Davies) but most frames dispensed will be pre-assembled and ‘off-the-peg’, so many of these measurements are not required. However, a few measurements still remain relevant to most dispensings: Length to bend (Figure 6) is the distance from the dowel point to the ear point. If 35mm is added to this measurement the practitioner will obtain, in most cases, the overall length of the side. This can be important if you require the laboratory to shorten or extend the supplied side length and as mentioned above the required length to bend may be affected by the face-form angle. The ability to reduce the length of a metal side is an essential skill in modern dispensing as the total length of side supplied is often too long. The length of the side on a plastic frame can also be shortened if the practice has the necessary equipment. However, in such cases it may be necessary to employ the services of a specialist frame repairer. Head width (Figure 3) can be measured using specially designed callipers or a combination of the standard facial and frame rulers. Consider taking this measurement when you know that a considerable alteration is going to be required. It is also a useful measurement to take in the dispensing of a rimless frame when the horizontal lens size is to be increased. All relevant measurements should be recorded on the patient’s record so the frame can be fully adjusted prior to collection. This means that on collection, any adjustments required will be to fine tune the fit only and not to perform major surgery in front of the patient!
Vertex distance
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For prescriptions with a power exceeding ±5.00D, the vertex distance of the trial frame or phoropter must be specified on the prescription. The vertex distance is the measurement from the corneal apex to visual point on the lens. Once the chosen frame has been set up, the dispensing optician must measure this distance using vertex distance callipers (Figure 7) or a ruler (Figure 8) with the patient wearing the new frame. Alternatively, the digital measuring devices now available also measure vertex distance. Failure to adjust the lens power when the vertex distance has been altered causes an effectivity error which may result in blurred vision and a possible non-tolerance. A number of methods to calculate the adjustment required in the final prescription order are available, the most common being vertex distance/back vertex power compensation charts, tables or wheels. Alternatively, the following expressions can be used.
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If the vertex distance is decreased:
Fold
Fnew =
1- (dFold)
If the vertex distance is increased:
Fold
Fnew =
1+ (dFold)
In both expressions, d (in metres) represents the change in vertex distance and not the vertex distance itself.
Examples:
? -8.00DS at 10mm
Dispensed at 14mm
Vertex distance increased by 4mm
Rx required is -8.25DS
? +8.00DS at 10mm
Dispensed at 14mm
Vertex distance increased by 4mm
Rx required is +7.75DS.
It is important to note that in cases of astigmatism each principal power must be compensated and not the cylinder. It is also important to remember that the vertex distance also affects spectacle magnification and field of view as a shorter vertex distance will reduce spectacle magnification and increase the real field of view of a spectacle lens.
? Part 13 will discuss the selection and fitting of spectacle lenses.
Further reading
Fowler C and Latham Petre K (2001). Spectacle Lenses: Theory and Practice Butterworth Heinemann Oxford UK.
Jalie M (1984). Principles of Ophthalmic Lenses 4th edition The Association of British Dispensing Opticians London UK.
Jalie M (2008). Ophthalmic Lenses & Dispensing 3rd Edition Butterworth Heinemann Oxford UK pp 75-87.
Norville Optical (2012) Prescription Companion.
Ophthalmic Lens Availability (2014). The Association of British Dispensing Opticians London UK.
Tunnacliffe A H (2003). Essentials of Dispensing, 2nd Edition ABDO.
? Andrew Keirl is an optometrist and dispensing optician in private practice, associate lecturer in optometry at Plymouth University, ABDO principal examiner for professional conduct in ophthalmic dispensing, ABDO practical examiner and external examiner for ABDO College
Model answers
(Model answers are in bold text)
1 When dispensing high plus powers which of the following is not a reason for keeping the vertex distance as small as possible?
A To maximise the field of view
B To reduce the retinal image size
C To reduce convergence demand
D To reduce the centre thickness of the lens
2 Which of the following is not a reason for correctly centring a spectacle lens?
A To position the lens where the paraxial prescription is most effective.
B To reduce unwanted differential prismatic effects.
C To control the centre thickness of the lens.
D To reducing the possibility of the formation of ghost images.
3 Which of the following lenses is bi-aspheric in form?
A Zeiss Lantal
B Hoya Nulux EP
C Essilor Lineis
D Hoya Nulux
4 When dispensing for high myopia and assuming that all other parameters are equal, which of the following products is likely to produce a lens with the thinnest edge substance?
A Essilor Lineis
B Hoya Eyry
C Zeiss Tital
D Zeiss Lantal
5 Which of the following statements is most correct?
A The Super Lenti lens is available in both resin and glass materials.
B The use of high refractive index materials will always result in a heavier finished lens.
C The Rodenstock Lentilux lens is manufactured using a resin material.
D When dispensing medium to high plus lens, the uncut size used is of no consequence.
6 Which of the following statements is most correct?
A If a high powered plus lens with a spherical back vertex power is manufactured with a convex prolate ellipsoidal surface, aberrational oblique astigmatism will be reduced.
B If a high powered plus lens with a spherical back vertex power is manufactured with a convex prolate ellipsoidal surface, both aberrational oblique astigmatism and distortion will be reduced.
C Ring scotoma and the Jack-in-the-box effect are not eliminated with polynomial aspheric designs.
D The lens of choice for a high powered plus lens of spherical back vertex power is one that incorporates an atoroidal surface.