Modern topographers offer a range of features above and beyond that of standard corneal topography alone. In a workshop session run by Martin Conway (Contamac UK), delegates at the recent Scottish Contact Lens and Ocular Surface Society (SCLOSS) conference undertook four tasks, each showcasing a different topography instrument with the aim of taking measurements and understanding the implications of the different feature sets incorporated into each device.
Station 1 Composite topography (supported by BIB)
Using the latest incarnation of the Medmont E300 corneal topographer, the aim of this station was to demonstrate the increased topographical imaging coverage possible with the use of map compositing. First described in 2006, the use of video compositing for the extension of standard mapping has not previously been available in a commercial machine. A recently introduced software upgrade to the Medmont system (Medmont Studio 5) now offers the ability to image and composite multiple image scans.
For this station, delegates first imaged a patient’s eyes in primary gaze, and then in the four main cardinal directions of gaze (left, right, up and down). The resulting topography scans were then composited using the Medmont Studio Five software’s inbuilt compositing feature. Since the point corresponding to the vertex normal of the central map are present in each of the peripheral maps, the data from these peripheral maps can be added to the edges of the central map to create an ‘enhanced’ topography map that can extend in some cases from limbus to limbus, in both the horizontal and vertical meridians.
The topography map in Figure 1 shows good, but incomplete, imaging coverage obtained using a standard single scan taken in primary gaze only for a particular patient. Figure 2 shows the coverage achieved using an enhanced topography map, composited from five separate maps, for the same patient.
[CaptionComponent="1071"]The coverage can now be seen to extend to the limbus for a full assessment of the corneal topography, and offers increased information about the corneal profile to aid the detection of potential corneal disorders, and also further inform the contact lens fitting process.
Station 2 Imaging of the limbus and beyond (supported by Carleton Optical)
At this station, delegates used the Eaglet-Eye Eye Surface Profiler (ESP) wide-field topographer to image the anterior eye surface to the limbus and beyond. How this information might then be used in the fitting of mini-scleral and scleral lenses was then explored and discussed in detail.
Based on the Maastricht Shape Topographer, this instrument uses an imaging technique known as Double Projector Fourier Profilometry and allows for imaging of the cornea beyond the limbus and out into the sclera to a width of up to 20mm (Figure 3).
[CaptionComponent="1072"]This is achieved with the instillation of fluorescein, which overcomes the inherent limitation to Placido-disc devices due to specular reflection. Unlike anterior segment OCT (AS-OCT) systems, which are also capable of imaging out into the sclera but only produce a two-dimensional (2D) tomograph in one meridian at a time, the ESP instantaneously images in 360° around the eye to produce a full three-dimensional (3D) map of the corneal and corneoscleral topography (Figure 4).
[CaptionComponent="1073"]Using these 3D elevation maps, delegates were then able to see and assess the impact that different scleral and corneoscleral topographies may have on scleral lens fits. Using the onboard NSag and TSag feature (Figure 5), delegates were also able to quantify the difference in sagittal heights between opposing scleral faces. These provide important information to inform the choice and design of first fit lens, and can influence whether a full or mini-scleral lens or even a toric periphery scleral lens might be prescribed.
[CaptionComponent="1074"]Station 3 Tear film assessment (supported by Birmingham Optical Group)
For this station, delegates assessed tear film quality using the Oculus Keratograph 5M instrument. In addition to offering standard Placido-disc type topography measurements, this instrument has a suite of tools which provides an objective assessment of the tear film, including tear meniscus height, non-invasive keratograph break-up time (NIKBUT) and lipid layer stability.
Tear meniscus height is a good indicator of tear volume and heights of less than 0.2mm suggest reduced tear fluid quantity. Assessment of tear meniscus height can be made by matching the illuminated width of a slit-lamp beam set horizontally; however, this technique is not always accurate and can induce reflex tearing rendering the technique invalid. The Keratograph 5M utilises ‘cool’ white-light LEDs to illuminate and image the tear film, thus eliminating this potential source of error and workshop participants were able to make objective measurements at points across the meniscus using the device’s onboard calliper tools (Figure 6).
[CaptionComponent="1075"]Delegates were also able to use the Keratograph 5M for objective assessment and analysis of NIKBUT, which is displayed in graphical form (Figure 7).
[CaptionComponent="1076"]Assessments of lipid layer stability were also made, based on the keratograph’s ability to detect colour fringe differences in this layer of the tear film. The instrument also offers infrared imaging of the meibomian gland (MG) structures to detect morphological changes in the upper and lower eyelids. Using this method, delegates were able to detect partial or complete dropout of MG structures (Figure 8).
[CaptionComponent="1077"]Station 4 Topography maps (supported by Topcon GB)
At this station, delegates first captured the corneal topography of a patient using the combined Aladdin Biometer/Placido Topographer before exploring the difference between axial and tangential topography maps.
Delegates were able to capture axial and tangential topography maps of a keratoconic eye and to assess the differences between the two maps (Figure 9 and Figure 10, respectively).
[CaptionComponent="1078"]It was evident that axial maps generally gave a better overall impression of the corneal topography, but that tangential maps were of more use in detecting and managing corneal disease and also in the management, and assessment of contact lens fitting procedures such as orthokeratology.
[CaptionComponent="1079"]The 2015 SCLOSS Conference will take place on Sunday November 29 in Glasgow.
Further reading
- Franklin RJ, Morelande MR, Iskander DR, Collins MJ, Davis BA. 2006. Combining central and peripheral videokeratoscope maps to investigate total corneal topography. Eye Cont Lens, 32: 27-32.
- Jongsma FH, de Brabander J, Hendriske F, Stultiens BA. 1998. Development of a wide field height topographer: validation on models of the anterior eye surface. Optom Vis Sci, 75(1):69-77.
- Guillon JP and Guillon N. The role oftears in contact lens performance andits measurement, in: Ruben M and Guillon M (eds), Contact Lens Practice,(London: Chapman and Hall Medical), 1994; 453-483.
- Srinivasan S, Menzies K, Sorbara L, Jones L. 2012. Infrared imaging of meibomian gland structure using a novel keratograph. Optom Vis Sci, 89(5):788-794.
Dr Lee Hall is a research optometrist with VisionCare Research