Optical coherence. OCD on OCT

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1 COMPETENCIES COVERED Dispensing opticians: Ocular Examination, Ocular Abnormalities, Low Vision Contact lens opticians: Ocular Examination, Contact Lenses Optometrists: Ocular Examination, Ocular Disease, Contact Lenses CET OCD on OCT by Andrew Keirl BOptom (Hons) MCOptom FBDO Optical coherence tomography (OCT) is a non-invasive imaging technique producing highresolution images, which show the structure of parts of the eye in three dimensions. OCT can be used to image the macula, optic disc and the anterior segment (cornea and anterior chamber angle). With more and more community practices using OCT in primary eyecare, dispensing opticians and support staff may be called upon to take scans of patients on the instruction of an optometrist or ophthalmologist. While it is outside of the scope of practice of the dispensing optician to interpret OCT scans, interpretation of an OCT image is often a case of pattern recognition as common features present with most diseases. Most common data plots produced by the OCT software use a colour coded indicator to show how far the appearance of the image captured is from the norm, which can also aid in the initial screening for certain anomalies. It is therefore interesting and useful for the dispensing optician taking OCT scans to be able to differentiate the normal from the abnormal and, in most cases, it becomes relatively straightforward to recognise the common abnormal ocular conditions that can be visualised using an OCT. It is, however, important to be familiar with the normal anatomical structure of the eye. OCT was first demonstrated in and has been commercially available since around It has rapidly evolved as the only non-invasive diagnostic technique able to provide increasingly detailed and accurate images of the retinal microstructure in vivo. OPTICAL PRINCIPLES All OCT machines work on the principle of interferometry between incident and reflected light. A beam of near infrared light, generated from a superluminescent diode, is directed through the tissue being examined. The wavelength of the incident light dictates the penetration of the beam into the eye. Shorter wavelength infrared will give better resolution images of more anterior structures while a longer wavelength will penetrate further and is more suited for retinal viewing, as is the case with most OCT instruments found in optometric practice. Structures posterior to the retina are not adequately imaged by this technique. The different layers and structures within the tissue will backscatter and reflect this light to various reproducible degrees. The electronics and computer software will then capture and interpret this reflected signal and reconstruct a two- or three-dimensional image based on the signal received. Any obstacle in the optical pathway (such as corneal scarring, cataract, posterior capsular opacification, an accumulation of inflammatory material due to uveitis or asteroid hyalosis) has the potential to block or reduce the strength of the generated image thus affecting the quality of the image captured 2. THE CLINICAL USE OF OCT OCT has several applications in clinical practice, the most common being the assessment of the retina around the macula or assessment of the optic disc. A crosssectional retinal image is produced as the light source passing through the retina scans across the retina, stacking and aligning consecutive axial-scans (A-scans) side by side to produce a two-dimensional transverse-scan (B-scan) 3. An OCT scan covering the whole of the macular region and captured in a few seconds typically consists of over 30,000 A-scans giving enough high resolution to view all the retinal layers. Consecutive B-scans can then be aligned to produce a 3D cross-section of the retina. The image produced resembles that of a histological section, with contrast produced by differences in the refractive index and scattering properties of the different retinal layers. Using an OCT is a bit like performing a vertical biopsy of the retina using laser light rather than a knife! The first OCT instruments introduced in 1996 used a time domain analysis system, whereby the reflected radiation was analysed in terms of any time delay between the reflected light from tissue structures and that from a moveable calibration mirror. A major advance came in 2003 with the introduction of Spectral or Fourier Domain analysis, a complicated analysis process which complements time delay with interpretation of differences in the oscillations within the interference This article has been approved for 1 CET point by the GOC. It is open to all FBDO members, and associate member optometrists. The multiple-choice questions (MCQs) for this month s CET are available online only, to comply with the GOC s Good Practice Guidance for this type of CET. Insert your answers to the six MCQs online at After log-in, go to CET Online. Questions will be presented in random order. 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 August 2015 issue of Dispensing Optics. The closing date is 10 July C Dispensing Optics APRIL 2015

2 spectrum of the tissue and reference reflections. This latter method allows significantly faster analysis and offers much greater resolution in any image captured, even allowing the visualisation of individual photoreceptors in some systems. In community optometry, a macular OCT scan is invaluable in the differentiation between wet and dry age-related macular degeneration (AMD), therefore avoiding unnecessary referrals and prioritising those that do. Other clinical benefits of a macular OCT scan include the identification of previously undiagnosed conditions such as vitreomacular traction and epiretinal membranes, which may explain a slight reduction in vision. In addition, OCT is useful in the diagnosis of conditions such as central serous retinopathy (CSR); early detection of diabetic maculopathy and screening for macular oedema following cataract surgery. RETINAL ANATOMY As the images generated by an OCT represent live in vivo histology 4 a sound working knowledge of the structures being imaged is important. However, before we consider abnormal retinal pathology, it is necessary to review the structure of the normal retina, which can be confusing. However, this can be considerably simplified by considering the retinal as a two-layer structure (the neuro-retina and the retinal pigment epithelium (RPE)). The two layers join relatively late in gestation and represent a weakness in structure. Partition of the two is described as a retinal detachment, which is not, as many mistakenly think, the whole retina breaking from the choroid. The neuro-retina includes the nerve fibre layer (NFL), ganglion cells, bipolar cells, and photoreceptors (rods and cones). The photoreceptors convert light into electrical impulses, which are then transmitted to the brain and are the most energy dependent tissue in body. A cross-section through the retina is shown in Figure 1. The RPE, which is in contact with the choroid, recycles material from the photoreceptors (necessary to maintain efficient function), contains pigment (melanin) to stop internal reflections thereby preventing glare inside the eye and pumps water out of the neuro-retina and potential sub-retinal space to keep it dry. The choroid supplies oxygen and glucose to the photoreceptors and the RPE, and has the highest blood flow per unit area of any tissue in the body. Recall what happens when you faint. The retina is always working very hard. Figure 1. The layers of the retina. Modified from Ocular Anatomy And Histology (with permission) The outer retina (supplied with oxygen by the choroid) consists of the RPE and the photoreceptors whereas the inner retina (supplied by the central retinal artery) comprises the nerve fibre layer, ganglion cells and bipolar cells. It is important to note that RPE and photoreceptors must not part company. They act as a single unit and any disruption to this RPE-photoreceptor partnership can result in significant and rapid visual loss. A healthy macular greyscale OCT B- scan is shown in Figure 2, with the different retinal layers identified. Differentiation of the retinal layers is possible due to their varying scattering properties and differences in refractive indices. A false colour image of the same eye is shown on Figure 3. With a false colour image, reflections of a higher intensity are depicted by warm colours (yellow to red), while less intense reflections are depicted by cooler colours (blue to green). Images in greyscale use brighter shading to represent strong reflections from dense structures. Most OCTs allow the user to switch from greyscale to a false colour image, which can help in identifying certain features. As the vitreous is not very dense, it appears black. Similarly, if fluid is present within the retina this will also appear black. There are two well-known areas of high reflection (brighter in a greyscale image and red in a colour image). These are the nerve fibre layer (NFL) and the retinal pigment epithelium (RPE). The NFL is an organised collection of nerve fibres and cytoplasm, which runs laterally along the inner surface Figure 2. A healthy macular greyscale OCT B-scan Dispensing Optics APRIL

3 Continuing Education and Training analysis (compared to a normative database) along with disc, cup and neuroretinal rim measurements, which are useful in the detection and monitoring of glaucoma. Pachymetry and anterior chamber visualisation is also a feature of most instruments. Figure 3. A false colour image of Figure 2 of the retina towards the disc and, combined with the internal limiting membrane and posterior hyaloid, gives off a high reflection on OCT and most machines use this reflection as part of their means of measuring NFL thickness. The RPE is an organised monolayer of cells at the outer retina and, together with their apical cellular pigmentation and Bruch s membrane at the basal surface, produce a high level of reflection, which is useful in detecting the outer retinal boundary. With age, there is a build-up of deposits of the waste products of photoreception on top of Bruch s membrane which, by the age of 60 years, are usually large enough to cause depigmentation of the overlying RPE revealing them as small, discrete white lesions or drusen. The OCT is easily able to locate these at the outer retina. Exudates, lipid leakage common in diabetic eye disease, may appear similar using ophthalmoscopy but the OCT highlights their position towards the middle layers of the neuroretina, clearly anterior to drusen. The retinal layers visible on OCT are not simply reflective bands but are cells consisting of nuclei, cell bodies and processes. Cell nuclei generally have a low level of reflectance and this is seen in the retina at the level of the ganglion cell nuclei, the inner nuclear layer and the conerich outer nuclear layer around the fovea. An interesting normal anatomical feature can be observed in the foveal region, as the outer segments of photoreceptors appear to become oedematous (darker). This is a normal feature of the fovea and represents the elongation of cone photoreceptors to enable closer packing and hence provide high visual acuity and is indicated using the red arrow in Figure 2 and the white arrow in Figure 3. The photoreceptor integrity line is the junction between inner and outer segments (IS and OS) of the photoreceptors. It is barely visible in histological sections but due to the difference in the refractive indices of the inner and outer segments of the photoreceptors, it is highly prominent with OCT and a well demarcated IS/OS junction suggest good photoreceptor function. Retinal thickness varies over the macular region with the thinnest area being at the very bottom of the central foveolar pit (the umbo). There is a wide range of retinal thicknesses in the normal population and retinal thickness is reported to vary according to several factors including age, axial length, ethnicity and gender 5,6,7. The average thickness of a normal macular is around 200 microns. A retinal thickness of more than 250 microns is often described as a thick retina and is usually due to leakage whereas a retinal thickness of less than 150 microns is usually due to atrophy and can be described as thin. However, it can be difficult to assess retinal function on thickness alone. While OCT is good at showing swelling due to leakage, a fundus fluorescein angiogram (FFA) is still needed for showing blockage of blood vessels. Retinal pathology (as seen using OCT) can be identified in and associated with specific layers. For example, diabetic retinopathy and retinal vein occlusion are associated with the inner retina (retinal circulation), whereas AMD and central serous retinopathy (CSR) are associated with the outer retina (choroidal circulation). Retinal surface pathology (mechanical problems) includes vitreomacular traction and epiretinal membrane. However, retinal pathology can involve more than one layer. For example a full thickness macular hole involves all layers and a lamellar macular hole usually involves the surface and inner retina. Most OCTs will permit a scan of the optic disc and will provide a nerve fibre DISEASES It is not the intention of this article to provide a detailed clinical discussion and description of eye diseases but to provide a brief overview of common abnormal ocular conditions, which may be encountered in practice where the use of the OCT is of particular value. AGE-RELATED MACULAR DEGENERATION AMD is a major cause of ocular morbidity and irreversible vision loss in high income countries, accounting for over half of blind and partial sight certifications in the UK 8. If a patient presents with suspected AMD, a fundal examination (using a binocular indirect ophthalmoscopy technique, for example, a slit-lamp and fundus viewing lens) and OCT will provide a great deal of information towards deducing the nature of the AMD as both the wet and dry types are easily distinguished using macular OCT scans. The most common type of AMD is the dry or atrophic type, which accounts for up to 90 percent of all cases of AMD 9. Dry AMD is characterised in its early stages by drusen within the macular region (Figure 4). When examined using an OCT macular scan, drusen appear as focal, hyper-reflective elevations of the RPE, disrupting the usually straight and smooth appearance of the RPE with no obvious fluid (Figure 5). The disease usually develops slowly and patches of retina thin and atrophy, possibly becoming flecked with pigment. The retinal thinning is clearly detectable by OCT. Figures 4 and 5 are images of the same eye. Figure 4. Drusen in the macular region as seen using digital retinal imaging. This is characteristic of dry AMD 24 Dispensing Optics APRIL 2015

4 Figure 6. Wet AMD as seen using digital retinal imaging. Note the haemorrhages and grey appearance of the macular area Figure 5. Drusen as seen using a macular OCT scan. Note the lumpy-bumpy hyper-reflective appearance of the RPE Wet (exudative or neovascular) AMD (Figures 6 and 7) is associated with the development of choroidal neovascularisation (CNV) and is found in approximately 10 per cent of all AMD cases. CNV is considered to be a key clinical sign in wet AMD and describes new (weak and leaky) vessels growing into the sub-rpe space caused by serous fluid leaking through Bruch s membrane from the choroid. RPE elevation is indicative of wet AMD and a cross-section of the retina using OCT shows this bulging, often in the absence of any significant change seen with traditional ophthalmoscopy. Moving the RPE away from the choroid results in ischaemia (reduced blood supply from the choroid), and this is the trigger for new vessels to grow as a response. At this stage, vision is distorted and is often described by the patient as waviness or shimmering. When the new vessels leak and burst, blood enters the sub-rpe space and causes a dramatic drop in vision. Dark spaces within the scan represent fluid leaking from the choroid. The fluid appears dark because it has a low reflectance. When fluid or blood is found above the photoreceptors due to breaks in the neuroretina it is classified as intra-retinal. Figures 6 and 7 are images of the same eye. VITREOMACULAR TRACTION Vitreomacular traction (Figure 8) has a very characteristic appearance with macular OCT scans and may be seen in asymptomatic patients. However, it can cause a reduction in visual acuity and produce distortion on an Amsler grid. Vitreomacular traction results from changes in the consistency of the vitreous (causing it to shrink away from the retina except at points of vitreoretinal attachment) and is seen as a thin, moderately reflective band that pulls on the retina in an incomplete v-shaped posterior vitreous detachment (PVD). Traction on the retina may lead to the formation of cysts, distortion of the retina and macular holes. If the patient is relatively asymptomatic and vitreomacular traction is an incidental finding, referral to secondary care is not required and patients should be advised to self-monitor with an Amsler grid. However, if vitreomacular traction has resulted in reduced vision and the patient has symptoms of concern, referral should be made for possible treatment by vitrectomy or Ocriplasmin injection. Hospitals may recommend immediate or urgent referral in these cases. EPIRETINAL MEMBRANE Epiretinal membranes (Figure 9) are often seen during examination of the fundus but OCT is particularly useful in confirming diagnosis and for purposes of documentation. An epiretinal membrane occurs when a fibrous layer forms along the internal limiting membrane (ILM). This membrane often shrinks and the retina under the membrane begins to wrinkle or pucker. It is sometimes described as cellophane maculopathy. On macular OCT, an epiretinal membrane appears as a hyper-reflective Figure 7. Wet AMD as seen using a macular OCT scan. Note the increase in macular thickness, disruption to the RPE/photoreceptors and pockets of fluid Figure 8. Vitreomacular traction as seen using an OCT scan Dispensing Optics APRIL

5 Continuing Education and Training Figure 9. An epiretinal membrane as seen using an OCT scan. Note the pockets of fluid that have formed beneath the retinal surface Figure 10. Central serous retinopathy as seen on an OCT scan layer at the very inner aspect of the retina (the bright white line on the innermost surface of the retina in Figure 9). In the early stages, they usually cause only a mild reduction in visual acuity. However, as the condition progresses, membrane formation may lead to metamorphopsia and more severe visual impairment. Most epiretinal membranes have an associated PVD. As with vitreomacular traction, if an epiretinal membrane is an incidental OCT finding and the patient asymptomatic referral is not required. Treatment, if required, might include vitrectomy and a membrane peel. CENTRAL SEROUS RETINOPATHY Central serous retinopathy (CSR) typically affects men in the 20 to 50 year old age group and has been tentatively linked to stress and individuals with a Type A personality 10. Such individuals are often ambitious, high achievers, rigidly organised, sensitive, impatient, take on more than they can handle, anxious and proactive. Presentation is usually unilateral and the exact aetiology of CSR is unknown. In CSR, fluid accumulates between the neural retina and the RPE (Figure 10). The OCT is very good at detecting this type of fluid due to its low level of reflectivity. Symptoms include blurred vision and distortion and a hypermetropic shift is often detected during a refraction, which is caused by the change in shape of the retina. CSR may or may not be visible on fundus examination and often resolves without intervention. Generally, the visual prognosis is good and patients suffer no significant permanent visual loss. The patient represented in Figure 10 presented with a sudden loss of vision in the left eye. The visual acuity on presentation was 6/60. Figure 11 shows the same patient one month later. The scan is now normal and VA had returned to 6/6. The patient was not referred to the hospital eye service. DIABETIC RETINOPATHY Diabetic retinopathy, a major cause of blindness among the working population in the UK 11, is characterised by damage to Figure 11. The patient represented in Figure 10 one month later. The scan is now normal blood vessels in the retina with stages ranging from mild non-proliferative retinopathy through to severe nonproliferative and ultimately proliferative retinopathy. A routine eye examination will often reveal the early signs of diabetic retinopathy, which may include retinal pathology (haemorrhages, exudates and swelling) and changes in refractive error due to lenticular shape change. The OCT is useful in assisting diagnosis as a macular scan may reveal subtle oedema leading to retinal thickening, which may have been difficult to detect by fundus examination. A macular scan may also show hyper-reflective hard exudates located within the inner aspect of the retina, which appear as highly reflective intraretinal spots. Drusen, associated with dry AMD occur in the outer retina. A macular OCT scan of a diabetic patient is shown in Figures 12a and 12b. Note the pockets of fluid (dark areas) and hard exudates in the inner retina. MACULAR HOLE Typically, macular holes are idiopathic and affect elderly, late to middle-aged females and result from vitreous traction on the macular area. They are not usually associated with retinal detachment but can form after blunt trauma. The mean age of onset is 65 years but onset in patients as young as the third decade has been reported. They cause a painless impairment of central vision (a central scotoma) which is often noticed when the fellow eye is closed. Treatment is by vitrectomy, which is often followed by strict postoperative facedown posturing for several weeks. A full thickness macular hole is shown in Figure 13. Note the subretinal fluid surrounding the hole and the loss of RPE/photoreceptors. The patient represented in Figure 13 had gross 26 Dispensing Optics APRIL 2015

6 Figures 12a and 12b. A macular OCT scans of a diabetic patient (the same patient) anisometropia, was an early presbyope (age 48) and a hydrogel contact lens wearer. A compromise prescription had been given in the right eye to assist near vision. The patient had a history of right retinal detachment surgery and mixed lens opacities were noted in the right eye at the last eye examination 11 months previously. The patient attended for a contact lens check complaining of blurred vision in the right eye. The patient s visual acuity was 6/30 (previously 6/7.6) and an overrefraction achieved a VA of 6/19. The patient was dilated and a full thickness macular hole was found using a macular OCT scan. Interestingly she did not fit the usual profile of a potential macular hole patient. Figure 14 shows the same patient one month after treatment. Sometimes what appears to be a full thickness macular hole with ophthalmoscopy, though perhaps with less impact on vision, is shown by OCT to be less deep, or more like a depression. This helps distinguish lesions such as lamellar holes (Figure 15) or pseudoholes from the full thickness hole. Management of these lesions may be different. GLAUCOMA Glaucoma, a progressive optic neuropathy, is an area where OCT could be of great benefit within an optometric practice. Around two per cent of white Europeans older than 40 years have chronic open angle glaucoma (COAG), rising to almost 10 per cent in people older than 75 years. The prevalence may be higher in people of black African or black Caribbean descent, or those who have a family history of glaucoma 12. It is usually associated with increased intraocular pressure (IOP) and subsequent damage to the retinal nerve fibres with a corresponding loss of visual field. Traditionally the diagnosis of glaucoma includes an optic nerve head examination, measurement of IOP, pachymetry (measurement of corneal thickness and correlated with IOP), gonioscopy (assessment of the structures that form the anterior chamber angle) and visual field assessment. As an adjunct to these investigations, OCT can be used to detect and measure NFL thickness accurately around the disc (Figure 16). The scan can also measure the size of the disc and cup, cup volume and thickness of the neuroretinal rim. The data collected is compared with an age-matched normative database. The thickness is measured around a ring or annulus just outside the disc and should be thickest inferiorly, then superiorly, nasally and thinnest temporally (the ISNT rule). If this thickness ring is rolled out as a flat graph, it has a characteristic twin peaks appearance (as shown in Figure 16) and variations in this shape are significant. NFL analysis has high specificity, which means it should be reliable in confirming that a patient is normal (negative). However, a high specificity can generate a number of false negatives and this is important if other clinical parameters such as raised IOP, normal central corneal thickness (CCT) or a repeatable visual field defect have indicated possible glaucoma. It may be that the greatest value of OCT in optometric practice will be as an aid to the early detection of glaucoma and screening of family members with routine regular follow-up 13. Many modern OCTs can measure the thickness of the ganglion cell layer, thinning of which is thought to be an Figure 13. A full thickness macular hole Figure 14. The patient represented in Figure 13 one month after treatment Dispensing Optics APRIL

7 Continuing Education and Training Figure 15. A lamellar hole Figure 16. A nerve fibre and optic head analysis using an OCT scan early sign of glaucoma way ahead of the appearance of visual field loss. THE ANTERIOR EYE Most OCTs have the ability to scan and assess the cornea, anterior chamber and angle. With a good quality scan, one can distinguish the five corneal layers. Measurement of the corneal thickness (pachymetry) is straightforward and the OCT will take numerous scans around the central corneal region and assess overall corneal thickness. Again this can be compared to a normative database. The non-contact pachymetry possible with OCT compares well with the established contact ultrasound technique, and is likely to be preferred by patients. Pachymetry is useful if a patient s IOPs are outside (higher than) the normal range as a thicker than normal cornea can result in a higher IOP measurement. The normal CCT is around 550 microns. Figure 17 shows OCT pachymetry resulting in a thicker than normal cornea while Figure 18 shows a keratoconic cornea with an area of marked inferior corneal thinning indicative of keratoconus. The 46-year-old male patient represented in Figure 17 had been referred to the hospital eye service (HES) on several occasions by different practitioners for raised IOP. His IOPs were evidently always in the mid-20s but there were never any other indications of glaucoma. The patient was discharged from the HES on each occasion. His IOPs on this occasion were 24mmHg in both eyes. The OCT scan (Figure 17) revealed thick corneas (CCT of 634 microns in the right eye and 636 in the left), which were probably the cause of the raised IOPs. Once more, there were no other indications of glaucoma. The patient was not referred to the HES on this occasion. Gonioscopy is the traditional method of viewing the structures that form the anterior chamber angle. However, the technique is invasive and is not easy to master. An OCT scan of the anterior chamber angle is a useful alternative if the van Herrick technique* indicates a narrow anterior chamber angle. Figure 19 shows an eye with a shallow anterior chamber angle. Note the narrow gap between the back surface of the cornea and the front surface of the iris (iridocorneal angle). The patient was referred to the HES for prophylactic YAG laser iridotomy. Sometimes a patient presents with what is believed to be a very narrow angle when assessed by van Herrick, but revealed by OCT to have a nice patent and open angle. Unnecessary referrals can therefore be avoided. SUMMARY OCT has provided eyecare practitioners with a new awareness in the visualisation and understanding of retinal diseases, glaucoma detection and corneal assessment. It is a non-invasive method of capture of accurate and reproducible information and should allow the practitioner to provide eyecare with an increased degree of confidence in his/her diagnostic abilities. Thanks to OCT, it is now possible to see what lies beneath the retinal surface. However, OCT is a tool of which there are many available and it must of course be used in conjunction with these other tools along with a thorough history and symptoms. This article is not intended to give an in-depth clinical description of eye diseases but the author hopes that it has been of some interest to the reader. Figure 17. Pachymetry using an OCT scan. The normal central corneal thickness (CCT) is around 550 microns Figure 18. Pachymetry in keratoconus. Note the thin inferior corneal thickness (blue region) 28 Dispensing Optics APRIL 2015

8 11. Broadbent D M, Scott J A, Vora J P et al. Prevalence of diabetic eye disease in an inner city population: the Liverpool diabetic eye study. Eye : NICE Guidelines CG85 Glaucoma: Diagnosis and management of chronic open angle glaucoma and ocular hypertension Mahon G and McCrudden J. Optical coherence tomography (OCT) - is it simply pattern recognition? Optometry in Practice : Figure 19. A shallow anterior chamber angle as imaged using an OCT * The Van Herrick technique performed using a slit-lamp bio-microscope during a routine eye examination, is used to grade the depth or openness of the anterior chamber angle. REFERENCES 1. Huang D, Swanson E A, Lin C P, et al. Optical coherence tomography. Science : Drexler W and Fujimoto J G. State-ofthe-art retinal optical coherence tomography. Prog. Retin. Eye Res : Costa R A, Skaf M, Melo L A et al. Retinal assessment using optical coherence tomography. Prog. Retin. Eye Res : Swanson E A, Izatt J A, Hee M R, et al. In vivo retinal imaging by optical coherence tomography. Opt. Lett : Song W K, Lee S C, Lee E S, et al. Macular thickness variations with sex, age, and axial length in healthy subjects: a spectral domain-optical coherence tomography study. Invest. Ophthalmol. Vis. Sci : Lam DS, Leung KS, Mohamed S, et al. Regional variations in the relationship between macular thickness measurements and myopia. Invest. Ophthalmol. Vis. Sci : Kelty PJ, Payne JF, Trivedi R H, et al. Macular thickness assessment in healthy eyes based on ethnicity using Stratus OCT. Invest. Ophthalmol. Vis.Sci : Bunce C, Xing W, Wormald R. Causes of blind and partial sight certifications in England and Wales: April 2007 March Eye : Klein R, Klein B E, Linton KL. Prevalence of age-related maculopathy. The Beaver Dam Eye Study. Ophthalmology 1992; 99: Bennett G. Central serous retinopathy. Br. J. Ophthalmol : RECOMMENDED READING Bruce J, Chew c and Bron A. Lecture Notes: Ophthalmology 10th ed. Blackwell 2007 Hiscox R. Discover what lies beneath. Optometry Today April Hiscox R. What you need to know about OCT - Parts 1, 2 & 3. Optician November 2014, December 2014 and January Pipe D and Rapley L. Ocular Anatomy and Histology 3rd ed. ABDO Jackson T. Moorfields Manual of Ophthalmology. Elsevier 2008 ACKNOWLEDGEMENTS David Pipe and Linda Rapley for Figure 1 and their permission to modify it for the purpose of this article and also for their comments on parts of the text. Bill Harvey for his helpful comments on an earlier version of this article. 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. Dispensing Optics APRIL