For decades, patients worldwide have been accustomed to their routine eye examinations being conducted with manual instruments. However, Eye Care Practitioners are increasingly embracing new digital technology to make the routine eye examination more accurate and expedient than traditional manual instruments. These digital eye examination systems have enhanced the qualitative and quantitative clinical information substantially. In addition, these high-tech instruments save time and improve patient care significantly.
The digital phoropter is an automated device that works synergistically with a multi-purpose instrument console. The instrument console contains a plethora of tests and optotypes that can be accessed by merely pressing a finger or stylus pen on the touch screen. Lenses, prisms, Jackson crossed-cylinders, duochrome charts etc. can all be accessed within seconds from a single source by merely pressing an icon. Some digital phoropters also enable software upgrades.
Important features that are incorporated include wavefront aberrometers and point-spread function tools to fully reveal the extent of astigmatism or higher-order aberrations in the eye. Point spread function (PSF) uses point spread images, which automatically correct for both low and certain high-order aberrations in the final prescription.
A liquid crystal display (LCD) monitor accompanies the digital phoropter and contrast is no longer an issue, compared to projected charts, were room lights have to be lowered for better contrast on the visual acuity chart. Additional features include patient education information and animations on various spectacle lens designs, eye diseases and optics. These further reinforce the practitioner’s explanations.
For practitioners with carpal tunnel syndrome and arthritis, the digital refraction system allows them to continue practising without worrying about their physical limitations.
At the end of the subjective refraction, the optometrist has the opportunity to dial in the patient’s habitual prescription and save it and to show the patient their old Rx and their new one with just the touch of a button. The goal of the subjective refraction is to achieve clear and comfortable binocular vision and digital refraction supports this in a more refined and efficient manner.
The use of the corneal topographer has expanded from mapping of the cornea’s curvature to evaluation of several specific corneal and ocular surface characteristics. Hence, corneal topography is extremely useful for evaluating tear film nuances and examining characteristics of the cornea such as shape, curvature, power and thickness. Clinicians use the topographical information for assessing the ocular surface and managing various conditions.
The three most popular topographical maps are axial, tangential and elevation. The axial map provides a quick overview of the corneal power. Axial maps are ideal for base curve selection of a corneal gas permeable or soft contact lens, because the average of the central curvature is portrayed. For specific information about the corneal shape and power, other displays will be more helpful. The most sensitive of the power maps are tangential display maps, and as such, they measure power and curvature at individual points on the cornea the most accurately. Tangential maps are beneficial in orthokeratology, especially when evaluating the shape of the peripheral cornea, as this display provides the most accurate peripheral data. The tangential display is also the most sensitive to changes in corneal curvature caused by distortion or warpage of the cornea from contact lens wear. The best option for conveying the true shape of the cornea is the elevation display map. The elevation display map is important when first determining the best lens design to fit on an irregular cornea, specifically when deciding between a corneal or scleral gas permeable lens. Clinical experience shows us that a difference in corneal elevation greater than approximately 325µm between the highest peak and lowest point of elevation will lead to limited success with corneal GP fit stability. A GP lens that is fitted onto a cornea with higher than 325µm level of elevation difference will rock on the eye, fall out of place intermittently and often cause discomfort and visual instability. Scleral lenses will work well here, as well as in other extreme irregularities and ectasias.
Corneal topography can be used to stage diseases, for example, keratoconus. It is invaluable in the diagnosis of pellucid marginal degeneration, Terriens marginal degeneration and corneal dystrophies such as Fuch’s endothelial dystrophy. The management of keratoplasty and corneal lacerations are made easier since the cornea can be followed closely in great detail from epithelium to endothelium throughout the treatment process.
Global pachymetry allows corneal swelling to be monitored, but in active contact lens wear, the primary use of this display is to monitor corneal thickness changes due to contact lens-related hypoxia, especially in scleral lens wearers, as they may be more prone to hypoxic complications. The corneal thickness display map can also be beneficial in ortho-k management, allowing practitioners to monitor corneal thickness changes as tissue is displaced from the centre to the periphery in myopia elimination.
Non-invasive tear break-up scores can be measured. We can gauge the quality of the natural tear film and see how it is affected by contact lens wear. Similarly, the surface wettability of the lens can be indirectly evaluated by using tear break-up displays when taking topography over the top of a contact lens. Some instruments allow for video recording broken down by frames per second. This allows practitioners to dynamically evaluate changes in the tear film quality as patients blink.
Contact lens fitting software can simulate a contact lens on the ocular surface allowing the practitioner to empirically order lenses that incorporate the entire shape of the cornea, including peripheral eccentricities, to provide a more customised contact lens fit. These programs are effective at predicting the fit of most contact lenses as well.
In addition to the cornea, these instruments are capable of taking photographs of the iris and pupil. Practitioners can use these photographs to measure pupil size and centration. When fitting lenses for presbyopia and myopia control, this feature allows practitioners to match pupils to the optics of the lens. From this, practitioners can also calculate the HVID and glean information about eyelid placement in relation to the cornea and pupil.
Meibography can be done using some topography systems to show changes in meibomian gland dysfunction (MGD).
Aberrometry can also be used on many topography systems to troubleshoot visual dissatisfaction with spectacles and contact lenses. If a patient has non-specific visual complaints, taking these measurements with and without spectacles and contact lenses in place can help determine which modality is most appropriate. If an excess amount of aberration exists prior to contact lens wear, the patient will be best suited for RGP lenses. Aberrometry taken over a contact lens will show how well a lens is able to correct higher-order aberrations.
OPTICAL PATH DIFFERENCE (OPD) SCAN
Most OPD scan machines combine wavefront aberrometry, corneal topography, autokeratometry, autorefraction, and pupillometry. The wavefront aberrometer provides assessment of visual acuity and quality of vision that goes beyond standard refraction and keratometric data. The autorefractor provides accurate refractions under mesopic and photopic condition. This is important for planning orthokeratology, myopia control, refractive surgery or analysing common refractive problems. The autokeratometer, in addition to standard keratometric measures such as simulated K, also provides novel corneal surface descriptors, such as average pupil power and effective central corneal power, which can aid in calculating IOL power for eyes after refractive surgery. The pupillometer measures photopic and mesopic pupil size and documents the shape of the pupil under these conditions, which may alter refraction and affect orthokeratology and surgical planning. The deviation of the visual axis from the pupil centre is also displayed. This is useful for laser refractive surgery and, in particular, lens surgery. Corneal and internal (combined lenticular and back surface cornea) components indicate each individual axis: corneal, internal, and total. This is useful when planning cataract surgery with toric IOLs. Wavefront-derived astigmatism data (Zernike S2) are provided for the overall eye, cornea, and internal components. This information is particularly useful in corneas that have non-orthogonal astigmatism or an asymmetric bowtie (eg, forme fruste keratoconus). Internal wavefront astigmatic information is useful for verifying correct toric IOL alignment postoperatively. The overall, corneal, and internal point spread functions (PSFs) and simulated letter “E” help the ECP to objectively evaluate visual quality and the source of a visual problem.
In addition to the overview summary, a number of more focused summary screens, customisable to the ECP’s preferences, can be called up to assist with particular tasks and it serves the ECP as a guide for clinical decision-making. These include contact lens summary, cataract, toric IOL, optical quality, and white-to-white summaries. Pages with multiple maps for comparison can also be called up.lens surgery. Corneal and internal (combined lenticular and back surface cornea) components indicate each individual axis: corneal, internal, and total. This is useful when planning cataract surgery with toric IOLs. Wavefront-derived astigmatism data (Zernike S2) are provided for the overall eye, cornea, and internal components. This information is particularly useful in corneas that have non-orthogonal astigmatism or an asymmetric bowtie (eg, forme fruste keratoconus). Internal wavefront astigmatic information is useful for verifying correct toric IOL alignment postoperatively. The overall, corneal, and internal point spread functions (PSFs) and simulated letter “E” help the ECP to objectively evaluate visual quality and the source of a visual problem.
OCULAR COHERENCE TOMOGRAPHY (OCT) SCAN
OCT is a non-contact and non-invasive diagnostic technique, able to provide images of the corneal and retinal microstructure. OCT generates cross-sectional or three-dimensional images by utilising low coherence interferometry to detect and measure the depth and magnitude of reflected light. A two-dimensional, cross-sectional retinal image is produced as the light source scans across the retina, stacking and aligning consecutive axial-scans (A-scans) side by side to produce a two-dimensional transverse scan (B-scan). Basically, OCT often reveals structural defects that are difficult to identify ophthalmoscopically or by slit lamp biomicroscopy.
Anterior OCT is used to assess the cornea, anterior chamber angle and crystalline lens. Corneal thickness measurements are useful when considering eligibility for refractive surgery, diagnosis and management of corneal diseases, interpretation of intraocular pressure measurements, and aftercare of contact lens wearers. OCT reveals details that are not visible with standard optical instruments.
Anterior segment OCT can aid in the assessment of a contact lens fit, making it possible to accurately assess the edge alignment and the central fit, even enabling measurement of the space between the corneal surface and the contact lens. Use of anterior segment OCT in both small-diameter rigid lenses and larger-diameter scleral lenses has been shown to improve contact lens fitting, resulting in less contact lens intolerance and increased patient satisfaction among others.
Assessment of tear film quantity is an integral part of a dry eye work up. An insufficient tear volume results in ocular discomfort and compromised ocular surface health. Many of the methods used to determine tear volume are invasive and induce reflex tearing, resulting in an overestimation of basal tear flow and volume. The inferior tear meniscus comprises seventy-five to ninety per cent of the total tear volume and is a good indicator of overall tear volume. Typically, gross measurement of the tear meniscus in practice has relied upon manipulation of the height of the slit lamp beam. Anterior segment OCT offers a non-invasive method for the imaging and measurement of tear meniscus height, with good levels of repeatability.
Gonioscopy has been the traditional tool in anterior chamber angle evaluation; however, assessment is very subjective. Posterior synechiae and depth of corneal laceration, corneal grafts, sands of Sahara, corneal neovascularisation and Fuch’s endothelial dystrophies are easily visible on OCT. Post-surgery, OCT can be used to determine the patency of an iridotomy.
Posterior OCT is used to assess the vitreous, vitreoretinal interface and retina. Posterior retinal OCT is a useful diagnostic tool for: evaluating structural integrity of posterior pole, examining the extent of retinal defects and abnormalities of the retina and its sub layers. A wide range of scanning patterns is available to allow the ECP to select a scan that suits the retinal region and ocular pathology.
The benefits of macular OCT involve AMD detection and monitoring. They also include identification of persistent vitreo-macular traction and epi-retinal membranes. These may explain a slight reduction in vision, which may have gone undiagnosed without the use of OCT. Moreover, the OCT facilitates diagnosis of conditions such as central serous retinopathy (CSR); early detection of diabetic maculopathy and screening for macular oedema post cataract surgery.
In addition to 3D macular scanning, additional macular scan protocols can provide further information on the macular region.
The normal vitreoretinal interface is characterised by:
- optically clear vitreous
- highly reflective nerve fibre layer (NFL)
- V-shaped fovea is normal; however, a widened foveal contour signifies abnormal foveal thinning.
OCT interpretation is based on the following key questions:
- How does the vitreoretinal interface appear?
- What is the foveal contour like?
- Is retinal architecture altered?
- Is the uniformity of the RPE/Choriocapillaris Complex layer disrupted?
Differentiation of the retinal layers is possible due to their varying scattering
properties and differences in optical densities. With a colour image, large reflections are depicted by warm colours (yellow to red), while smaller reflections are depicted by cooler colours (blue to green). Images in greyscale utilise brighter shading in place of warmer colours. As the vitreous is not very dense, it appears black. Similarly, if fluid is present, this will also appear black. Conversely, structures including the retinal nerve fibre layer (RNFL) and the retinal pigment epithelium (RPE), are much denser; therefore they appear brighter (or red in a colour image). The practitioner who is new to interpreting OCT images, should take heed of a few normal anatomical features that can be confusing and cause unnecessary concern. For example, blood vessels are highly reflective; as a result they cause a shadow to fall underneath them in the OCT scan, due to blocking of the infrared OCT signal. This can also occur with dense vitreous floaters, which will cast a shadow across all retinal layers of the OCT scan. Also, as you move over the foveal region of an OCT scan, the outer segments of photoreceptors appear to become oedematous. The temperature thickness plot gives a representation of the retinal thickness over the scan area, with thicker areas appearing as warmer colours, and thinner areas as cooler colours. Observation of the temperature thickness plot provides a quick method for establishing whether the retinal architecture is normal over the macular region.
OCT has become increasingly popular for real-time quantitative evaluation of retinal thickness, due to its ability to detect the inner and outer retinal boundaries to a high degree of accuracy, automatically producing a retinal value. We can easily differentiate macular holes, lamellar holes and pseudo holes. Macular thickness is most commonly analysed and presented on the Early Treatment Diabetic Retinopathy Screening Study (ETDRS) grid, where the patient’s retinal thickness is compared to that of a normative database.
COMMON RETINAL PATHOLOGIES THAT CAN BE IDENTIFIED USING OCT
Dry AMD, characterised by drusen within the macular region accounts for up to ninety per cent of all cases of AMD. On OCT examination, drusen appear as focal, hyper-reflective elevations of the RPE, disrupting the typically straight and smooth RPE layer. Development of choroidal neovascularisation (CNV) is the hallmark of wet AMD, a stage found in approximately 10 per cent of all AMD cases. CNV on OCT examination typically presents as increased reflectivity of the RPE, often associated with irregular RPE elevation. Leakage of these new blood vessels causes development of fluid, which appears as dark spaces within B-scan. Fluid may be classified as intra-retinal when it is found above the photoreceptors, sub-retinal when it forms below the photoreceptors but above the RPE, or sub-RPE, when it forms below the RPE. In cystoid macular oedema, which is also associated with diabetes and branch retinal vein occlusion, intra-retinal fluid forms characteristic cystic spaces. A sign of persistent vitreo-macular traction is another common observation in macular OCT scans, where vitreo-macular traction is seen as a thin, moderately reflective band, which is pulling on the retina in an incomplete v-shaped posterior vitreous detachment (PVD).
If vitreoretinal traction has resulted in significantly reduced vision, metamorphopsia and photopsia, referral should be made to an ophthalmologist for consideration for treatment with vitrectomy or ocriplasmin.
FUNDUS AUTOFLUORESCENCE (FAF)
The FAF is a non-invasive method to evaluate the RPE without contrast dye. The function is helpful for detecting early stage retinal disorders. A flurophore is a component of a molecule which causes a molecule to fluoresce. The major source of fundus autofluorescence is lipofuscin of the RPE. Lipofuscin accumulates as a byproduct of phagocytosis of the photoreceptor’s outer segment. Essentially, FAF is a method of metabolically mapping the RPE and has been developed as a tool to evaluate the RPE during aging and ocular disease. The hallmark of aging is the accumulation of lipofuscin granules in the cytoplasm of the RPE cells. FAF adds to the understanding of retinal diseases.
OCT AND GLAUCOMA DIAGNOSIS
A prime characteristic of glaucoma is the loss of retinal ganglion cells (RGCs), which leads to nerve fibre loss and optic nerve changes. The macular region contains a high concentration of more than fifty per cent of RGCs, which can be quantified relatively easily. In addition, the macular region is the primary location of glaucomatous damage in the disease’s early stage. OCT pays a considerable emphasis on glaucoma diagnosis. Glaucoma screening tools include several macular and disc scans. Also, OCT automatically outlines the optic nerve head, optic cup, and disc borders similar to the mental estimations by clinicians, but then also calculates more objective measurements such as optic disc area and neuroretinal rim area in addition to the classic clinician-subjective average and vertical cup-to-disc ratios. The calculated optic nerve head (ONH) parameters are then compared to a normative database. An overview of the key glaucoma screening/diagnostic indicators are populated and then generated as a report.
Eye Care Practitioners aspiring to acquire high-tech equipment, should consider those that provide that qualitative and quantitative information on the refraction, ocular biometry, ocular media, anterior segment and posterior segment. Certainly, embracing high-technology adds value to everyday clinical practice. Practitioners are encouraged to attend workshops and continuing practitioner development lectures to appreciate these new technologies. Also, visit the various optical instrument companies, where support staff will guide you on their equipment.