As stated in the introduction to Part 16 of this series, the term anisometropia is used to describe the clinical situation that exists when the dioptric values of a patient’s right and left correction are unequal. Anisometropia can give rise to two optical problems. Spectacle magnification and aniseikonia were discussed in Part 16. We now have to address the second and perhaps more problematic issue resulting from anisometropia which is the possibility of unwanted differential prismatic effects. When the eyes of anisometropic subjects rotate to view through points away from the optical centres of a pair of spectacle lenses, different prismatic effects at corresponding points between the two eyes may be experienced. This difference in prismatic effects is known as differential or relative prism and the visual points of concern are usually the near visual points. Differential prismatic effects are therefore of interest when an anisometropic subject reads or performs a task at a close working distance. Due to the existence of large horizontal fusional reserves and smaller horizontal eye movements when reading, differential prismatic effects in the horizontal meridian are rarely a problem. However, if a differential prismatic effect occurs in the vertical meridian, it can create problems by disturbing the patient’s binocular vision.

Differential prismatic effect

There is considerable inter-practitioner and also inter-subject variation in the tolerance for induced vertical differential vision. It is often suggested that vertical differential prism of less than 1? at the near visual points (NVPs) is unlikely to cause problems. However, in clinical practice, many anisometropic subjects with a vertical power difference of over 1.00D never complain of symptoms relating to vertical differential prism. Studies have found some subjects with as much as 5? of vertical differential prism at the NVPs experience no symptoms and have little or no measurable vertical heterophoria.1 This is due to the subject’s ability to ‘soak up’ or adapt to prism. For example, if a 1? base-up prism and a vertical Maddox rod is placed in front of the right eye of an orthophoric subject who views a spot light at 6m, a horizontal streak will be formed below the spot. However, the displacement of the streak image produced by the prism will soon disappear (the line will coincide with the spot) because the subject will adapt to the vertical differential prism produced. However, they re-adapt to their previous fusional state when the differential prism is removed. This prism adaptation is the response of the oculomotor system to the presence of differential prism. Adaptation to differential prism can be fairly rapid but adaptation to prism by a subject does not mean that the patient’s vision is comfortable.

Vertical differential prism and prism adaptation is usually not an issue with single-vision lenses as the patient will simply move his/her head in order to look through the optical centres of the lenses. There is, of course, no prismatic effect at the optical centre of a lens. However, vertical differential prism must be considered in the case of a multifocal lens where the patient has no choice but to look away from the optical centres of the lenses as the NVPs and optical centres do not usually coincide. So exactly how much vertical differential prism can be tolerated? As summarised by Tunnacliffe2 the answer to this question was sought in the 1940s when 47 anisometropic subjects were fitted with two pairs of spectacles, one with and the other without prism compensation (bi-prism lenses). Eighteen subjects reported no difficulty with their uncompensated lenses but, despite prism adaptation, 29 subjects reported that they were more comfortable with the prism-compensated spectacles.3,4 In another study on 50 subjects about 60 per cent of the subjects preferred the prism-compensated lenses.5

[CaptionComponent="2794"]

The effect of unintended vertical differential prism on the binocular contrast sensitivity function (BCSF) was investigated by Tunnacliffe and Williams6 who found that 1? base-down (right eye), and to a lesser extent 0.5? base-down (right eye), caused a drop in BCSF. The reduced contrast sensitivity was less marked in those subjects with a higher vertical fusional reserve, a result which clinical intuition would predict. Although the 1? only caused a 14 per cent drop in BCSF in photopic conditions, it was much more significant at mesopic levels of luminance (25 per cent). Interestingly, the effect was as large with 0.50? in mesopic conditions as it was with 1? in photopic conditions. Although prism adaptation or, more precisely, a tonic adjustment occurred with the longer-term wear of 1? there was no improvement in BCSF. This might have some bearing on the degree of vertical differential prism which should be accepted as a physiological tolerance in completed spectacles.

Jimenéz et al invested the effect that decentred spectacle lenses had on depth perception by evaluating stereopsis using random-dot stereograms.7 The decentred lens of course induced an unwanted prismatic effect. The results showed that variations in fusional convergence due to increments of decentration reduced stereopsis in observers. These effects were noticeable with both vertical and horizontal prismatic effects but were more noticeable with induced vertical prism.

In clinical practice, patients prescribed with single-vision lenses can adjust their head position and/or any reading material position in order to view through points on the lens that are closer to the optical centres of the lenses and therefore reduce any vertical differential prismatic effect. Multifocal wearers cannot do this and prism-compensation may need to be considered in order to provide comfortable vision when reading. The question now is should we be dispensing more prism-compensated lenses and how can we identify patients who may benefit from prism-compensation? The measurement of vertical fusional reserves along with aligning prism by the optometrist in the consulting room can be helpful. For a first-time presbyopic patient, if a vertical slip is present when viewing a near Mallett unit through the NVPs of single-vision lenses, but not when viewing a distance Mallett unit through the optical centres, then prescribing prism-compensated multifocal lenses for near may be helpful. The same principle can be applied to multifocal lens wearers who are anisometropic and are wearing uncompensated bifocal lenses. The need for a vertical aligning prism measured through the bifocal segment where none exists for distance may indicate that prism compensation may be required. In addition, the effect of compensating for vertical differential prism can be easily investigated by holding up the appropriate neutralising prism while the patient is observing near print through the NVPs of the lenses. The size of the near print must be the smallest that the patient can manage. If the patient reports that near vision is more comfortable when the prism is in place then prism compensation may be indicated.

The use of a Fresnel prism (Figure 1) means that the effect of prism compensation on binocular vision and visual comfort can be judged by the patient before permanent lenses are dispensed. Fresnel prisms are thin polyvinyl chloride sheets, about 1mm thick which can be cut to fit over the reading area of a spectacle lens. Since they reduce acuity by about one to two rows of letters on a Snellen chart, they are generally used as a temporary solution. However, they are perfect for experimentation. A trial can be run in which the patient does not know when prism compensation is occurring. This is done by fitting the prisms so the bases are both up (or down) and then one up and one down for a similar period of time. In the former case the prisms cancel each other out and in the latter, the vertical differential prism is neutralised. The patient reports on the effects during the two periods. If the patient is symptom-free in the prism-compensated period, but not during the uncompensated trial, then the practitioner can confidently dispense a prism-compensated lens.

[CaptionComponent="2795"]

Calculation of prismatic effect

The calculation of differential (relative) prismatic effect sometimes causes confusion. As stated above, it is simply the difference in prismatic effect at corresponding points away from the optical centres on a pair of lenses. Or put another way, how much more (or less) prism there is at a point on one lens compared to the same point on the other. So, if the prismatic effect at corresponding points on a pair of lenses was 1.00? base up in the right eye and 3.00? base up in the left, the differential prismatic effect is 2.00? base up in the left eye. In other words, at these particular points away from the optical centres, the left lens is producing 2.00? more base-up prism than the right, or, the right lens is producing 2.00? more base-down prism than the left. This is where the preferred term ‘relative’ can be applied. Differential prismatic effect must always be stated using a value for the differential prism, the base direction and an eye, for example, ‘the vertical differential prismatic effect is 2.00? base up in the left eye’. As another example, if the prismatic effects at corresponding points away from the optical centres on a pair of lenses were 2.00? base up in the right eye and 2.00? prism base-down in the left, the differential prismatic effect would be 4.00? base up in the right eye or 4.00? base down in the left. Relatively speaking, the right eye has more base-up prism (4.00?) than the left or the left eye has more base-down prism (again, 4.00?) than the right. It can be seen that the calculation of differential prismatic effect is the opposite of prism splitting. If a patient was prescribed 4.00? base up in the right eye this would usually be ordered as 2.00? base up in the right eye and 2.00? base down in the left. Exactly the same right and left prisms would be ordered if the patient was prescribed with 4.00? base down in the left eye. With regard to the calculation of horizontal differential prismatic effect, it is also useful to think of this as the opposite of prism splitting. If a patient was prescribed with 4.00? base out in the right eye this could be ordered as 2.00? base out in the right eye and 2.00? base out in the left. The original 4.00? is the differential prism which of course could also be prescribed in the left eye (again, 4.00? base out) with the prism being split in exactly the same way. The differential prism is therefore 4.00? base out in either eye. The word ‘either’ is always used when stating horizontal differential prismatic effect.

If a patient’s correction is either spherical, or astigmatic with the principal meridians vertical and horizontal then Prentice’s rule (P = cF) can be used to calculate the prismatic effect at a point away from the optical centre of the lens. As an example consider the single-vision prescription:

Right -5.00DS

Left -3.00/-1.00 x 90

The powers along the vertical meridians will be -5.00D in the right eye and -3.00D in the left eye. Assuming that the NVPs are located 10mm below and 4mm inwards from the optical centres of the lenses, the vertical prismatic effects at the NVPs will be 5.00? base down in the right eye and 3.00? base down in the left eye. In this simple example the vertical differential prismatic effect is 2? base down in the right eye (or 2? base up in the left). Is this amount of vertical differential prism likely to cause the patient visual problems?

[CaptionComponent="2796"]

The answer here is ‘probably’ but this depends on the patient’s vertical fusional reserves which are usually in the range of 2? to 4?. As a general rule 1? is usually stated as the tolerance for induced vertical differential prismatic effect.8 However, some patients will tolerate more than this and some less. 1.5? has also been suggested as a maximum tolerable difference.9 Induced vertical differential prism of less than 1? measured at the near visual points (NVPs) is unlikely to give rise to symptoms of asthenopia. However, even though some patients can adapt to vertical differential prism, adaptation does not mean that a patient is necessarily visually comfortable with uncompensated vertical differential prism. The research undertaken by Tunnacliffe and Williams6 showed that 0.5? of vertical differential prism adversely affects the BCSF. Potential symptoms of uncompensated vertical differential prism include blur, diplopia, ‘shadowing’ of print or ‘print running together’ and headache/eyestrain.

In the above example the horizontal prismatic effects at the NVPs are 2.00? base in (0.40 x 5.00) for the right eye and 1.6? base in (0.40 x 4) for the left. The horizontal differential prismatic effect is therefore 3.60? base in either eye.

When calculating the prismatic effects at a point on a lens the base directions are determined by inspection of Figures 2 to 5. A positive spherical lens can be visualised as shown in Figures 2 and 4. At points away from the optical centre, the lens will act as a prism and the bases of these prisms will face the optical centre of the lens. At any point below the optical centre of the lens the prismatic effect will be base up and at any point above the optical centre of the lens the prismatic effect will be base down. For both right and left lenses at any point inwards from the optical centres of the lenses the prismatic effect will be base out and at any point outward from the optical centres of the lenses the prismatic effect will be base in. A negative spherical lens can be visualised as shown in Figures 3 and 5. At points away from the optical centre, the lens will act as a prism and the bases of these prisms will face away from the optical centre. At any point above the optical centre of the lens the prismatic effect will be base up. At any point below the optical centre of the lens the prismatic effect will be base down. For both right and left lenses at any point inwards from the optical centres of the lenses the prismatic effect will be base in and at any point outward from the optical centres of the lenses the prismatic effect will be base out.

[CaptionComponent="2797"]

If the cylinder axis of an astigmatic lens is oblique, the calculation of prismatic effect is more problematic and can involve the use of graphical constructions and complicated formulae. However, most textbooks on ophthalmic lenses contain useful tables of prismatic effects for use with astigmatic lenses. Such a table has been included in the 2014 edition of Ophthalmic Lens Availability (ABDO). In addition, a useful programme called the Optometric Toolbox (Figure 6) can be downloaded free of charge from Thomson Software Solutions. The toolbox enables the practitioner to instantly calculate the prismatic effect at any point on any single-vision lens both spherical and astigmatic. All the practitioner then has to do is work out the differential prismatic effect. There are two graphical methods in common use for finding the prismatic effect at a point away from the optical centre on an astigmatic lens. One considers the lens as a pair of crossed-cylinders and the other the lens as a sphere and a separate plano-cylinder. The first (crossed-cylinder) method can be used to find the prismatic effect at a point on a lens and also the decentration required to produce a given amount of prism. The second method which considers an astigmatic lens as a sphere and a separate plano-cylinder is used to find the prismatic effect at a point on a lens and was first published by Jalie.10,11 The prismatic effect produced by the sphere is found by using Prentice’s rule. A simple construction using a ruler and a protractor is used to find the prismatic effect produced by the cylinder. The prismatic effect produced by the sphere is then added to the prismatic effect produced by the cylinder to find the total prismatic effect at the given point. This method will be illustrated using the following example (Figure 7):

[CaptionComponent="2798"]

Find the vertical and horizontal prismatic effects at a point 10mm down and 4mm inwards from the optical centre of the lens Right -6.00/-2.50 x 30.

First, construct an origin using 90 and 180 meridians and locate and mark the nasal side of the lens. Draw the cylinder axis along its prescribed axis direction (30º). Using a suitable scale, for example, 1cm = 1mm, mark the position of the visual point, R, on the construction. In this example it will be 10mm down and 4mm in from the origin. Drop a perpendicular from R to the cylinder axis meeting it at P. Since we need to find the vertical and horizontal prismatic effects we must resolve PR into vertical and horizontal components, PQ and QR. PQ is the effective decentration of the plano-cylinder in the vertical meridian and QR is the effective decentration of the plano-cylinder in the horizontal meridian. By measurement, PQ is 9.2mm and QR is 5.3mm. The base direction of the prismatic effect at R is determined by noting the position of P with respect to R. If the cylinder is positive, P represents the position of the prism base. If the cylinder is negative, P represents the position of the prism apex and the prism base will of course lie in the opposite direction. In this example, P lies above and temporal to R (up and out relative to R). When the cylinder is negative in sign, as in this example, P represents the position of the prism apex so the prism base direction is down and in. The horizontal prismatic effect at R due to the cylinder is given by QR (cm) x Fcyl. The vertical prismatic effect at R due to the cylinder is given by PQ (cm) x Fcyl. So in this example the prism due to the cylinder is:

PH = 0.53 x 2.50 = 1.32? base in

PV = 0.92 x 2.50 = 2.30? base down

The prism due to the sphere only is:

PV = 1.00 x 6.00 = 6.00? base down

PH = 0.40 x 6.00 = 2.40? base in

[CaptionComponent="2799"]

The base directions for the prismatic effect produced by the sphere will be obtained using Figures 4 and 5. The total prismatic effects 10mm below and 4mm inwards from the optical centre of the lens found by simple addition. These are:

8.30? base down and 3.72? base in

When the author was a student dispensing optician in the late 1970s at what was fondly referred to as ‘City College’ this method of finding the prismatic effect at a point on an astigmatic lens was affectionately known as Jalie’s fag-packet method as it was simple enough to be performed on the back of a fag (cigarette) packet. However, students should note that the use of graph paper is preferred!

Summary

Although students are happy to state that ‘according to British Standards, patients can tolerate 1? of vertical prismatic effect, whether at the centration points or differential (relative) prism as the wearer looks up or down through the lenses’, to the best of the author’s knowledge, and following consultations with colleagues, there is no agreed physiological tolerance for vertical differential prism. Although manufacturing tolerances for mounted spectacle lenses are published (ISO 21987:2009) there appears to be no reference to a tolerance in relation to the differential prismatic effect induced at corresponding points on a pair of correctly manufactured and correctly centred spectacle lenses in BS, ISO or CEN publications. It is interesting to note that Tunnacliffe and Williams6 suggested that a vertical differential prism of 1? might be too great an amount for a working physiological tolerance if a 25 per cent drop in the BCSF is clinically significant.

[CaptionComponent="2800"]

So the question that practitioners need to ask themselves when dealing with anisometropic prescriptions is: ‘Should I compensate?’ In order to answer this question the practitioner ideally needs the following clinical information:

• What is the patient’s binocular status and visual acuities?

• What are the patient’s fusional reserves?

• Is the patient suppressing?—? What is the degree of anisometropia?

• Has the patient previously tolerated differential prism?

• Is there likely to be an improvement in the vision with prism compensation?

• Does the patient require a single-vision or a multifocal lens?

• Would a balance lens be appropriate?

The methods for prism compensation will be discussed in detail in Part 18.–

Model answers

(The correct answer is in bold text)

1 The generally accepted tolerance for vertical differential prismatic effect is:

A 00?

B 00?

C 00?

D 00?

2 A subject is to be dispensed with the prescription right -1.25/-1.75 x180 and left -0.50/-0.50 x180. Assuming that the near visual points are 10 mm below the distance optical centres, what will be the vertical differential prismatic effect?

A 00? base down in the right eye

B 00? base down in the left eye

C 00? base up in the right eye

D 00? base down in the right eye

3 A subject is to be dispensed with the prescription right +1.25/+1.75 x 90 and left +0.50/+0.50 x180. When wearing this prescription the subject rotates his eyes to look through near visual points 10 mm below and 5 mm in from the optical centres of the lenses. What will be the horizontal differential prismatic effect?

A 75? base in right eye (or 1.75? base in left eye)

B 75? base out right eye (or 1.75? base out left eye)

C 25? base out right eye

D 10.25? base out left eye

4 Which of the following statements is incorrect?

A Unwanted vertical differential prismatic effect can adversely affect the binocular contrast sensitivity function.

B Unwanted vertical differential prismatic effect can adversely affect stereopsis.

C Subjects who adapt to unwanted vertical differential prismatic effect are always visually comfortable.

D Potential symptoms of uncompensated vertical differential prism the “shadowing” of print or “print running together”.

5 Which of the following devices is the least useful in helping practitioners identify patients who may benefit from prism-compensation?

A The Mallett Near Vision Unit

B A Fresnel prism

C The Maddox rod

D Loose or trial case prisms

6 What percentage of patients may benefit from the use of prism compensated lenses?

A 30

B 40

C 50

D 60

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.

Ophthalmic Lens Availability (2014). The Association of British Dispensing Opticians London UK.

Tunnacliffe A H (2003). Essentials of Dispensing. 2nd Edition ABDO.

References

1 Henson DB, North R. Adaptation to prism induced heterophoria. Am J Optom Physiol Opt, 1980; 57, 129-137.

2 Tunnacliffe. Essentials of Dispensing 2nd edition, Association of British Dispensing Opticians 1995, p84.

3 Ellerbrock VJ and Fry GA. Effects induced by anisometropic corrections. Am J Optom Arch Amer Acad Optom, 1942; 19, 444-459.

4 Ellerbrock VJ and Fry GA. Further study of effects induced by anisometropic corrections. Am J Optom Arch Amer Acad Optom, 1942; 25, 430-437.

5 Elvin FT. The results of prescribing vertical prisms from the Turville test. Am J Optom Arch Amer Acad Optom, 1954; 31, 308-314.

6 Tunnacliffe A H and Williams AT. The effect of vertical differential prism on the binocular contrast sensitivity function. Ophthal Physio Opt, 1985; Vol 5 No4 pp417-424.

7 Jimenéz et al. Changes in Stereoscopic Depth Perception Caused by Decentration of Spectacle Lenses. Optom Vis Sci, 2000; 77:421–42.

8 Duke Elder (1970). Ophthalmic Optics and Refraction, Vol 5, Henry Kimpton, London.

9 Bennett. Emsley and Swaine’s Ophthalmic Lenses, Volume 1, Hatton Press, 1968 p212.

10 Jalie M. An analysis of near prismatic effects – part 2. Optician, 1965; 149.

11 Jalie M. Ophthalmic Lenses & Dispensing 3rd Edition Butterworth Heinemann Oxford UK, 2008 pp 51-53.

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