Heads you win, tails you lose

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1 4 dispensingoptics September 2014 Heads you win, tails you lose By Andrew Keirl BOptom (Hons) MCOptom FBDO CompetencIes covered: Dispensing opticians: Ocular Examination, Contact Lenses, efractive Management Contact lens optician: Ocular Examination, Contact Lenses Optometrists: Ocular Examination, Contact Lenses, Optical Appliances In this CET article, based on a lecture given by the author at the 2014 ABDO Conference, Andrew Keirl considers some of the optical advantages and disadvantages of spectacle lenses and contact lenses. When considering the optical differences between spectacle lenses and contact lenses, the following factors need to be considered: The correction of ametropia Magnification, retinal image size and visual acuity Field of view Accommodation and convergence Binocular vision and anisometropia The correction of ametropia Both spectacle lenses and contact lenses are, of course, effective at correcting refractive errors. These include myopia, hypermetropia and astigmatism. However, contact lenses, particularly rigid gas permeable (GP) contact lenses, are better for correcting astigmatism induced by irregular corneas than other forms of correction. Irregular corneas can occur in patients with keratoconus, keratoplasty and in patients who have undergone refractive surgery. When fitted with an GP contact lens, the tear lens that is formed between the back surface of the contact lens and the front surface of the cornea fills in the irregularities of the corneal surface, producing a more regular refracting surface. Some binocular vision problems are easily managed using spectacle lenses. However, binocular vision problems are difficult to manage using contact lenses. In cases of high myopia and hypermetropia, the apparent size of the eyes and surround can be a concern to the patient when corrected with spectacle lenses. These magnification and minification effects, of course, do not occur with contact lenses. Patients often change from a spectacle to a contact lens correction and vice versa. Both modes of correction are usually effective in producing in-focus retinal images. However, there are some differences between these two modes, most of which are associated with the position of the correction. In order to correct a refractive error, a distance spectacle or contact lens correction needs to form an image of an object at the far point of the eye. However, due to the vertex distance, the far point will lie at slightly different distances from the two types of correcting lens. This means that the powers of the spectacle lens and the contact lens required to correct a particular eye will therefore be different. Using a hypermetropic eye as an example, the geometry relating the far point of an ametropic eye and the correcting lens is shown in Figure 1. When a hydrogel contact lens is fitted to an eye, the lens drapes over the This article has been approved for 1 CET point by the GOC. It is open to all FBDO members, including associate member optometrists. The multiple-choice questions (MCQs) for this month s CET are available on page 10 and online. Insert your answers to the six MCQs on the inserted sheet or online at After log-in, go to CET Online. Please ensure that your address and GOC number are up-to-date. The pass mark is 60 per cent. The answers will appear in the January 2015 issue of Dispensing Optics. The closing date is 12 December C-37002

2 Continuing Education and Training Geometry relating the far point of an ametropic eye and the correcting lens Patient and practice management A hypermetropic eye The far-point M lies behind the eye eye and follows the curvature of the anterior ocular surface. This implies that the tear lens formed between the contact lens and the cornea (if indeed there is one) will have zero power, and the ametropia is corrected by the back vertex power (BVP) of the hydrogel contact lens. As the vertex distance of a contact lens is zero, we can in most cases assume that in order to correct a patient s ametropia, the BVP of a hydrogel contact lens will be close to the patient s ocular refraction (K). In other words F CL = K. If the power of the correcting spectacle lens along with its vertex distance is known, K can be calculated using first principles as shown in Figure 1 or using the expression: F sp K = 1-(dFsp ) where d is the vertex distance in metres. However, when an GP contact lens is placed on the eye, the back surface of the contact lens maintains its shape and a tear lens of predictable form and power is formed between the rigid contact lens and the cornea (Figure 2). The patient s ametropia is, therefore, corrected with a contact lenstear lens system and the BVP of the GP contact lens will not (unless an afocal tear lens is formed) be the same as the patient s ocular refraction. The contact lenstear lens system formed when a GP contact lens is placed on an eye means that three elements are involved in the formation of the final retinal image: the contact lens, tear lens and the eye. The vergence that actually corrects the patient s refractive error is the vergence leaving the back surface of the tear lens L 4 (Figure 3). When performing a paraxial ray-trace through a contact lenstear lens system, it is often assumed that a thin air gap exists between each element, which can simplify the calculation of surface powers and vergences (Figures 2 & 3). When fitting an GP contact lens to a patient, it is important to determine the likely magnitude of the tear lens and how it varies as the back optic zone radius (BOZ) is changed, as this helps to determine the required contact lens BVP and aids fit evaluation. The following rules of thumb are often used by contact lens practitioners: The tear lens power increases by D for each 0.05 mm that the BOZ is steeper than the cornea The tear lens power increases by D for each 0.05 mm that the BOZ is flatter than the cornea. The tear lens formed by an GP contact lens is shown in Figure 4. Magnification, retinal image size and visual acuity In practice, myopic patients are often told that they may obtain slightly better distance vision with their contact lenses than with their spectacles. Hypermetropic patients may be told the opposite. These potential differences in acuity are caused by differences in spectacle magnification when comparing correction with spectacles and contact lenses. Spectacle magnification is the ratio of the retinal image size in the corrected ametropic eye, compared to the retinal image size in the same eye when uncorrected. h c SM = hu For thin lens systems and model eyes where the entrance pupil coincides with the cornea or the reduced surface spectacle magnification is given by: K SM = Fsp When considering hydrogel contact lenses, as the tear lens is usually considered to be plano and the contact lens thin, we can assume that the power of a hydrogel contact lens is equal to the eyes ocular refraction. The spectacle magnification produced by hydrogel contact lens is therefore: K SM cl = Fcl = Unity When comparing the distance visual acuity obtained with spectacle lenses and contact lenses, the usual assumptions made are summarised in Table 1. k = f SP - d K = 1 k F SP d M Hypermetropia: F CL > F SP Myopia: F CL < F SP f SP Figure based on abbettsjalie Clinical Visual Optics (Elsevier 2007) Figure 1: Geometry relating the far point of an ametropic eye and the correcting lens The contact lenstear lens system Thin air gap GP Contact lens Liquid lens Thin air gap Figure 2: The contact lenstear lens system Light from a distant object The contact lenstear lens system F 1 F 2 n L 1 L 1 L 2 L 2 = L 3 t F 3 F 4 n L 3 L 4 L 4 t GP CL Tear-lens L 2 = BVP of CL in air L 4 = ocular refraction K Figure 3: The contact lenstear lens system Figure 4: The tear lens formed by an GP contact lens The monocular static visual field for a right eye k M F SP 60 degrees UP 75 degrees down 100 degrees Temporal 60 degrees NASAL Figure 5: The monocular static visual field for a right eye Continued overleaf

3 Spectacle frame 6 dispensingoptics September 2014 Apparent field of View: Empty frame Ametropia Spectacles CL Aperture size 2y Myope SM < 1 SM 1 Fitting distance s Hypermetrope SM > 1 SM > 1 < Specs Table 1: Spectacles vs. CLs: assumptions regarding visual acuity 2 = Apparent field of view When considering contact lens systems, the entrance pupil is normally taken to be 3mm behind the corneal surface and both the contact lens and the tear lens have a thickness. In such cases, we are not therefore able to simply state that the spectacle magnification provided by a contact lens system is equal to unity. For a contact lens tearlens system, the spectacle magnification produced is the product of the power factor and the shape factor, the expression for which is: SM = where: 1 1-aF v Fv is the BVP of the contact lenstear lens system (L 4 in Figure 3); F eq is the equivalent power of the lens system; and a is the distance from the back vertex of the lens system to the entrance pupil of the eye. For spectacles a = vertex distance + 3mm and for contact lenses a = 3mm as d = 0. For a contact lens tearlens system, the equivalent power is given by: L 2 x F v F eq F eq = L 1 x x x L 2 L 3 L 3 L 4 As an example, a patient is corrected using a spectacle lens of power DS at a vertex distance of 12mm. The back surface power is -2.00D, centre thickness 6mm and refractive index The spectacle magnification produced by this correction would be in the region of 25 per cent. If the same patient is fitted with a GP contact lens, where the contact lens centre thickness is 0.3mm and the tear lens thickness is 0.1mm, the spectacle magnification produced would be approximately five per cent. Even though the L 4 thicknesses of both the GP contact lens and the tear lens are small compared to the thickness of the spectacle lens, the steep curves involved mean that the spectacle magnification produced is significant. Field of view The normal monocular static visual field for a right eye is shown in Figure 5. Assuming that there is no relevant pathology, this would be the monocular static field of view enjoyed by an emmetrope or a contact lens wearer (assuming that the contact lens or its optic zone is not very small in diameter). With reference to the field of view provided by a spectacle lens, the static field of view is the total area visible through the lens. It is usually expressed as an angular measure and is defined as the maximum angular extent of vision obtainable through the lens. Factors affecting the field of view of a spectacle lens include the aperture size, lens power and vertex distance. To obtain the maximum field of view, whatever the size of the aperture might be, the spectacle lens should be fitted as close to the eyes as the lashes permit. There are two other terms that are used when discussing the field of view of a spectacle lens. These are the real and apparent fields of view. The apparent field of view (Figure 6) is the angle subtended by the empty frame aperture at the eye s centre of rotation, whereas the real field of view is the field of view obtained when a spectacle lens is glazed into the frame. When comparing the real and apparent fields of view, it is important to note that the static real field of view provided by a positive spectacle lens is less than the apparent field of view implied by the empty spectacle frame (Figure 7). This means that hypermetropes suffer from a decrease in field of view and there will be an area around the edge of a lens from which no light can enter the eye (a ring scotoma). Figure 6: The apparent field of view Field of view: Positive lens 2 = eal field of view 2 = Apparent field of view is the image of Angular loss of field or scotoma Figure 7: eal field of view (positive lens) Field of view: Negative lens 2 = eal field of view 2 = Apparent field of view is the image of y Angular increase in field Figure 8: eal field of view (negative lens) Ocular accommodation - Spectacles When an eye is corrected by a spectacle lens A oc = K - L 2 where K is the ocular refraction and L 2 is the vergence arriving at the eye from the near object. The ocular refraction K = D which is what the eye needs to see measured at the eye 13 m L 1 = 3.00 D mm The vergence of the light arriving at the eye from the near object L 2 is D The ocular accommodation is therefore D y K = L 2 = The eye is receiving D from the near object and therefore has to accommodate to make up the difference Figure based on abbettsjalie Clinical Visual Optics (Elsevier 2007) Figure 9: Calculation of ocular accommodation (hypermetropia) Ocular accommodation - Spectacles The same principles apply for myopia When an eye is corrected by a spectacle lens A oc = K - L 2 where K is the ocular refraction and L 2 is the vergence arriving at the eye from the near object. -13 m K 1 = D mm L = K 2 = The vergence of the light arriving at the eye from the near object L 2 is D Compared to K the eye is receiving an additional D from the near object and therefore has to accommodate neutralise this excess Note that a myope The has ocular to accommodate accommodation less than is an therefore emmetrope for a Dgiven near object. Figure based on abbettsjalie Clinical Visual Optics (Elsevier 2007) Figure 10. Calculation of ocular accommodation (myopia) Continued overleaf

4 8 dispensingoptics September 2014 Ocular accommodation Contact lenses For an eye corrected by a contact lens the required ocular refraction K is zero However, the static real field of view provided by a minus spectacle lens is greater than the apparent field of view implied by the empty spectacle frame (Figure 8). Myopic subjects therefore benefit from an increase in field of view, but there will be an annular area around the lens periphery where objects will be seen in diplopia. As an example, a round lens, 48mm in diameter is fitted at a distance of 25mm from the centres of rotation of the eyes for a 10.00D myope and a 10.00D hypermetrope. The apparent static field of view produced will be The real static field of view produced in the myopic case will be (an increase) whereas the real static field of view produced in the hypermetropic case is 71.5 (a decrease). Bearing in mind the assumptions made above, a contact lens wearer will have the widest field of view. Accommodation and convergence The ocular accommodation (A oc ) is the accommodation required to neutralise negative vergence arising from a near object measured in the plane of the eye. The ocular accommodation is calculated simply by comparing the subject s ocular refraction (K) with the vergence arising from a near object measured in the plane of the eye (L 2 ). A OC = K - L 2 As an example, a subject, corrected for distance vision with a +5.00D spectacle lens fitted at a vertex distance of 15mm, views an object placed 13m from the spectacle plane. As the ocular refraction (K) for this subject will be +5.40D, the required ocular accommodation is +3.34D (Figure 9). A second subject again corrected for distance vision but this time with a -5.00D spectacle lens fitted at vertex distance of at 15mm views the same near object. In this case, the ocular refraction (K) is -4.65D and the ocular accommodation +2.49D (Figure 10). An emmetrope (or a contact lens wearer) viewing the same near object would have to accommodate by +2.87D for the eye to form a clear image on the retina (Figure 11). The myopic patient will therefore need to accommodate more when wearing contact lenses than when wearing spectacles (which could be an issue for the emerging presbyope). The opposite is true for the hypermetropic patient as hypermetropes will need to accommodate more when wearing spectacles than when wearing contact lenses, which is a distinct benefit of fitting contact lenses to hypermetropes over 40 years of age. Convergence is defined as the movement (rotation) required from the primary position, for the eyes to fixate an object point on the mid-line (Figure 12). Both convergence and accommodation should be equal for normal binocular vision, and convergence can be expressed in degrees, prism dioptres or using the metre angle. The base in effect of minus lenses (Figure 13) means that myopes corrected with spectacle lenses converge less than emmetropes or contact lens wearers. However, the base out effect of plus lenses (Figure 14) means that hypermetropes corrected with spectacle lenses converge more than emmetropes or contact lens wearers. For the two subjects in the above examples, assuming that the lenses are centred for distance vision, the distance PD is 66mm and the centres of rotation of the eye lie 27mm behind the spectacle plane, the convergence required to view the near object would be 4.65 or 8.14Δ for the myopic subject and 5.97 or 10.46Δ for the hypermetropic subject. If the subject was an emmetrope or was corrected using contact lenses (Figure 15), the convergence required to view the same near object would be 5.23 or 9.16Δ. The above examples show that the myopic patient will need to converge more when wearing contact lenses than when wearing spectacles, and the hypermetropic patient will need to converge more when wearing spectacles than when wearing contact lenses. This correlates with the examples comparing accommodation. So, to summarise the differences between mm The vergence of the light arriving at the eye from the near object L is D Ocular accommodation A oc = D The contact lens wearer has to accommodate more than the spectacle lens wearer to view the same near object Note: Opposite to the hypermetrope! Figure based on abbettsjalie Clinical Visual Optics (Elsevier 2007) Figure 11: Calculation of ocular accommodation for a contact lens wearer PD PD Figure 12: The definition of convergence h D spectacle lens h l l Convergence required is 4.65 or 8.14 s = convergence = convergence h = PD Figure 13: Calculation of convergence (myopia) h D spectacle lens s l l Convergence required is 5.97 or Figure 14: Calculation of convergence (hypermetropia) h Contact lens wearer l + s Convergence required is 5.23 or 9.16 = convergence h = PD = convergence h = PD Figure 15: Calculation of convergence for a contact lens wearer h

5 Continuing Education and Training accommodation and convergence demands when the same subject is corrected with spectacles and contact lenses: Myopia: Accommodation and convergence, more with contact lenses than spectacles Hypermetropia: Accommodation and convergence, less with contact lenses than spectacles. So, when changing from contact lenses to spectacles (and vice versa) the accommodation:convergence ratio is only minimally disturbed. Binocular vision For some binocular vision anomalies, contact lenses offer advantages over spectacles. However, for other binocular vision anomalies, contact lenses are contraindicated so a knowledge of the orthoptic status of a patient is important before they are fitted with contact lenses. In cases when an optometrist carries out the eye examination and a contact lens optician in the same practice performs the contact lens fitting, this is not likely to be a problem as long as the patient s binocular vision status is communicated to the contact lens optician by the optometrist. This is particularly important if the patient s interest in contact lenses is made clear before the eye examination. In cases where the contact lens fitting is separated from the eye examination, as a precaution, the contact lens optician may wish to carry out the following essential investigations (or seek the assistance of an optometric colleague): History: Double vision, a turning eye, a lazy eye, eye muscle surgery? Symptoms: Eyestrain, headaches, blurring or diplopia associated with a visual task? Accurate measurement of current spectacles to detect prism or decentration Cover test at distance and near Ocular motility. The above tests are particularly important in patients with high myopia and in those who are fitted, or are potentially going to be fitted, with monovision or multifocal contact lenses. If any of these essential investigations reveal suspicious findings, additional investigations such as fixation disparity or dissociation tests may be appropriate. If the eye examination and contact lens fit are performed in the same practice, the above tests may have been included in the eye examination. However, this should not be assumed and it is good practice to check this with the examining optometrist. If a patient requires a prismatic correction (or decentration to give a required prismatic effect) and this cannot be replicated in contact lenses, contact lens wear is contraindicated. Orthoptic indications for contact lenses Optical problems associated with the spectacle correction of some refractive errors are minimised in contact lens wear because the contact lens moves with the eye. Potential problems include off-axis aberrations (particularly in high ametropia) and prismatic effects. Improving the clarity of the optical image by the use of contact lenses may improve sensory fusion, which might improve the orthoptic status. The most commonly encountered refractive error where there are marked orthoptic advantages to wearing contact lenses is anisometropia. The two optical problems associated with anisometropia are differential prismatic effects and aniseikonia. When considering aniseikonia in both spectacle and contact lenses correction, it is necessary to calculate the relative spectacle magnification produced (the ratio of the retinal image size in the corrected ametropic eye compared with the retinal image size in the standard emmetropic eye) for a given distant object. It is also interesting to consider any differences in aniseikonia if the patient s refractive error is axial or refractive in origin. In axial ametropia, spectacles are theoretically better if the anisometropia is axial in origin as any aniseikonia will be less and binocular vision will be more comfortable. The opposite is true if the anisometropia is refractive in origin, as correction with contact lenses will result in the right and left retinal images being the same size. However, this theoretical prediction, known as Knapp s law, was disproved by research that revealed that contact lenses reduce aniseikonia in all forms of anisometropia 1. In addition, refractive (non-strabismic) anisometropes are likely to achieve their best binocular visual acuity and stereoacuity when wearing contact lenses as opposed to spectacles 2. efractive correction without patching can improve the bestcorrected acuity in an amblyopic eye and this therapeutic effect may be enhanced with contact lenses. It is important to remember that patients with pure anisometropic amblyopia (no strabismus) can respond to treatment at almost any age. Correction of motor deviations with contact lenses Some cases of decompensated heterophoria or strabismus can be treated with a refractive correction using either spectacles or contact lenses. For example, accommodative esotropia can be corrected using plus lenses and a decompensating exophoria can be corrected using minus lenses. The incorporation of a prismatic correction is limited with contact lenses but it is possible to work base-down prism on GP and hydrogel contact lenses. Horizontal prism can be incorporated into complex scleral designs. The Igel toric hydrogel lens from UltraVision CLPL can include up to 2Δ with the prism base in any direction. The lens is stabilised using dynamic stabilisation. Orthoptic contraindications for contact lenses Monovision: Contact lenses are well suited to monovision because of the lack of differential prismatic effects. However, the resulting monocular blur is dissociating and monovision is contraindicated in patients whose binocular status is easily compromised as decompensation can potentially occur. Visual compromise: Occasionally, a visual compromise is deemed acceptable because of the cosmetic advantages of contact lenses, and Continued overleaf

6 10 dispensingoptics September 2014 the patient may be happy to live with slight blur in one eye. Again, this blur could cause binocular vision to decompensate in certain cases. High myopia: Base-in prism with a spectacle lens centred for distance when reading can be helpful in cases of near exophoria. A similar effect occurs with high hypermetropes who have a near esophoria. However, these prismatic effects are lost when contact lenses are fitted. Superior oblique palsies: Decompensation can occur if the patient is forced to fixate in the field of action of the weak muscle, ie. to look down and in. Alternating vision multifocals are contraindicated in such cases and this applies to both contact lenses and spectacles. ecent cases from practice Miss B: a 28-year-old student veterinary nurse complaining of near vision and display screen problems at the end of the day. Her prescription was -3.00D right and left and she was fitted with continuous wear silicone hydrogel contact lenses. A binocular vision assessment showed a marked exophoria at both 6m and 40cm (greater at near) with poor recovery at 40cm along with low base out fusional reserves, and a small vertical deviation. 3Δ of base in aligning prism and 0.50Δ of vertical prism were indicated with the Mallett Near Vision Unit. The patient wanted to continue with the continuous wear modality, so over-spectacles were prescribed for reading and display screen use with a low minus prescription (to stimulate accommodation and therefore convergence) along with the required aligning prism. Mr T: an 18-year-old college student fitted with daily wear monthly disposable hydrogel toric contact lenses six months ago. His prescription was right x 60 and left x 160. At his last eye examination six months ago he displayed a moderately well controlled esophoria and required 2Δ base out (left eye) in his spectacles. However, he is now complaining of horizontal diplopia at distance. A binocular vision assessment revealed a left esotropia. Without the 2Δ base out, the esophoria had broken down into an esotropia, and 14Δ base out (left eye) was required to give binocular single vision with both spectacles and contact lenses. The patient was referred for orthoptic and ophthalmological assessment. eferences 1. Winn B, Ackerley G, Brown C A, Murray FK, Prais J, St John MF (1988) educed aniseikonia in axial anisometropia with contact lens correction. Ophthalmic and Physiological Optics 8: Edwards KH (1979) The management of ametropia and anisometropic amblyopia with contact lenses. Ophthalmic Optician 8: Acknowledgment Figures 1, 9, 10 and 11 are based on and adapted from abbettsjalie Clinical Visual Optics (Elsevier 2007). Further reading Evans BJW (2005) Eye Essentials: Binocular Vision, Elsevier, Oxford, UK. Keirl AW, Christie C (2007) Clinical Optics and efraction: A Guide for Optometrists, Dispensing Opticians and Contact Lens Opticians, Elsevier, Oxford, UK. abbetts B (2007) Bennett & abbetts Clinical Visual Optics, Elsevier, Oxford, UK. The figures in this article can also be veiwedprinted as a handout. Click the pdf icon just to the left of 'Take Test' on the CET Online page of the ABDO website. 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, and External Examiner for ABDO College. n

7 Continuing Education and Training Multiple choice questions (MCQs) Heads you win, tails you lose 1. Which statement is correct? a. The back vertex power of a contact lens for a particular patient will always be equal to the patient s ocular refraction b. GP contact lenses are optically useful when fitting patients with irregular corneas c. When fitting a patient with an GP contact lens, a positive tear lens will be produced if the back surface of the contact lens and the front surface of the cornea have the same radius of curvature d. When fitting a myopic patient with a hydrogel contact lens, the back vertex power of the contact lens will be greater than the patient s spectacle refraction 2. An over-refraction is performed following the fitting of a patient with an GP trial contact lens. The result of the over-refraction is -0.50DS more than the expected value. Which statement is correct? a. The over-refraction indicates that the lens is too steep and a lens with a BOZ 0.10mm flatter than the trial lens should be considered b. The over-refraction indicates that the lens is too flat and a lens with a BOZ 0.10mm steeper than the trial lens should be considered c. The over-refraction indicates that the lens is too steep and a lens with a BOZ 0.20mm flatter than the trial lens should be considered d. Based on the over-refraction result alone, the result indicates that the fit of the contact lens is correct 3. Which statement is correct? a. The size of the retinal image formed in an eye corrected by a contact lens will always be the same as the size of the retinal image formed in the same uncorrected eye b. Spectacle magnification is the ratio of the retinal image size in the corrected ametropic eye compared with the retinal image size in the standard emmetropic eye for a given distant object c. The tear lens formed when an GP contact lens is placed on an eye can affect spectacle magnification produced d. The power factor is of no consequence when calculating the spectacle magnification produced by contact lens tear lens system 4. Which statement is correct? a. The term real field of view relates to the field of view produced by an empty spectacle frame b. Hypermetropic subjects benefit from an increase in field of view compared to myopes, and there will be an area around the edge of a lens from which no light can enter the eye c. Myopic subjects suffer from a decrease in field of view compared to hypermetropes, but there will be annular area around the lens periphery where objects will be seen in diplopia d. A contact lens with a small overall diameter, or small diameter optic zone, can potentially affect the field of view experienced by a contact lens wearer 5. Which statement is correct? a. Compared to correction with spectacles, myopic subjects will require more accommodation and convergence when corrected with contact lenses b. A positive spectacle andor contact lens correction can be helpful in cases of a near exophoria c. A negative spectacle andor contact lens correction can be helpful in cases of a near esotropia d. Prismatic correction cannot be incorporated into contact lenses 6. Which statement is correct? a. In cases of anisometropia, contact lenses reduce aniseikonia only if the anisometropia is axial in origin b. In cases of anisometropia, contact lenses reduce aniseikonia only if the anisometropia is refractive in origin c. Contact lenses reduce aniseikonia in all forms of anisometropia d. efractive anisometropes are not likely to achieve their best binocular visual acuity and stereoacuity when wearing contact lenses as opposed to spectacles The deadline for posted or faxed response is 12 December The module code is C Online completion after member log-in go to CET online After the closing date, the answers can be viewed on the 'CET Online' page of To download, print or save your results letter, go to 'View your CET record'. If you would prefer to receive a posted results letter, contact the CET Office or cet@abdocet.infoman.org.uk Occasionally, printing errors are spotted after the journal has gone to print. Notifications can be viewed at on the CET Online page