Features can aid with surgical care, contact lens fits, dry eye and more
Traditionally, eye care practitioners have used topographical measurements to assess the shape of the corneal surface to aid in contact lens fits, for refractive surgery screenings, determining the diagnosis of keratoconus and selecting IOLs. That said, this technology offers novel features you may not yet have tried.
Here, I review the topography technology available and discuss some of these features. While not inclusive, this is a list of topography features I found valuable in my practice setting.
REVIEWING THE TECHNOLOGY
The first modern topographers were Placido disc, which work by projecting a concentric annular light source onto the corneal surface and capturing reflected light to measure corneal curvature and irregularities of the anterior cornea, such as contact lens-induced corneal warpage. Placido disc topographers depend on good tear film quality for accuracy.
Other modern topographers use Scheimpflug or scanning-slit technology to provide an assessment of the anterior and posterior cornea, as well as the rest of the anterior segment.
Scheimpflug technology uses rotating, or dual, cameras to capture optical sections of the cornea and lens and can image the iris as well.
Scanning-slit topographers project two vertical scans through 40 optical slits at fixed angles to enable the practitioner to analyze curvatures at the anterior and posterior corneal surface.
Some modern topographers combine these technologies with Placido disc to enhance the accuracy of anterior cornea measurements.
|Corneal transplant status||Z94.7|
|Difficulty fitting contact lens||Q13.4|
|Hereditary corneal dystrophies||H18.50-H18.59|
|Peripheral corneal degeneration||H18.461-H18.469|
|Unspecified corneal scar and opacity||H17.9|
Some of the novel features of this technology:
- Densitometry. This feature of Scheimpflug topographers allows O.D.s to grade cornea and crystalline lens opacification on a scale from 1 to 100. Thus, it enables O.D.s to follow the degree of cataract formation in an objective fashion over time and to relate visual decline to a hard number that they can discuss with patients.
Also, optometrists can use this feature to follow Fuchs’ corneal dystrophy prior to surgical referral and observe for improvement of the condition on medical therapy or after Descemet’s Stripping Endothelial Keratoplasty.
Further, densitometry can help O.D.s determine the main contributor to vision reduction when both corneal disease and cataract are involved, as optometrists can evaluate the different density maps for the cornea and lens to see which site demonstrates the highest density and, thus, is the major cause of the problem.
Finally, the feature can capture the location and depth of corneal scars and opacities, in that it presents scatter data over a curved plane that is interpolated between the anterior and posterior corneal surfaces, making it a useful tool for assessing the depth and position of scattering phenomena that may occur within the cornea.
- Anterior chamber depth. With this function, available on Scheimpflug and scanning slit devices, O.D.s can easily explain narrow angle anatomy or phacomorphic glaucoma changes to a patient before referral for a YAG PI or cataract extraction. After surgery, optometrists can show the patient the improvement in their anatomy.
- Pachymetry. This feature, available on scanning-slit and Scheimpflug devices, can help O.D.s follow edema in contact lens abusers or when concerned about hypoxia in lower DK or scleral lens wearers. It can show patients improvement on lens modality change.
- RGP and scleral contact lens fits. Optometrists can upload design-specific software for corneal RGP lenses, available on Placido disc and Scheimpflug devices, to simulate an RGP fit on a patient’s cornea without ever using a fit set!
Specifically, the software allows the O.D. to adjust parameters, such as base curve, and observe the estimated fluorescein pattern to aid in an empirical fit. Also, optometrists can record sagittal depth, horizontal visible iris diameter and profiles of the scleral contours to help them determine a fit.
- LASIK referral and surgery planning. Scheimpflug and scanning-slit topographers can measure residual stromal bed under the flap, which allows a referring doctor to estimate whether there is enough stroma for re-treatment.
Also, some LASIK surgeons have adapted topography-guided LASIK — all devices have this capability — due to its ability to normalize or smooth out the cornea at the time of surgery, providing improved night vision and reduction of halos.
Further, some surgeons are using topographers “off-label” for retreatment of 20/unhappy patients after primary LASIK surgery, as it can correct for decentration and enlarge treatment zones.
- Detailed dry eye disease (DED) documentation. A Scheimpflug topographer offers diagnostic imaging to document TBUT, as well as meibomian gland appearance via infrared imaging. This can help guide DED treatment decisions and show an objective measurement of response to treatment. The TBUT function can also help the O.D. compare the surface wettability of a patient wearing a contact lens vs. without contact lens wear.
Further, following meibography over time can assist in determining meibomian gland drop-out associated with contact lens wear.
- Anterior segment photography. This feature, available on Scheimpflug topographers, enables optometrists to capture basic anterior segment photos to document pathology, such as corneal scars, pterygium and conjunctival hyperemia, and observe for change over time — great for patient education.
- Aberrometry. Optometrists, via any type of topographer, can troubleshoot vague vision complaints in contact lens patients. In addition, measuring aberrometry over a multifocal contact lens can help O.D.s select a multifocal contact lens that has optics that align with the patient’s pupil. That said, these measurements do not represent the total aberrations of the eye, only the contribution of the corneal surface, necessitating an aberrometer.
Also, aberrometry measured without, then with, a contact lens in place can show how well a soft lens can smooth out higher-order aberrations.
MAKE THE MOST OF IT
To determine whether you are aware of all your topographer can do for you, ask your sales rep to come in and retrain you and your staff. Also, ask whether there are any updates and they have any online tools — all of which can help you make sure you are maximizing the use of your device.
If you are looking to purchase a topographer, use this article to ask the sales team whether their topographer has any of the features discussed that you feel may help you in your practice.
The greatest pleasure of being an optometrist is making a positive impact on patients. These additional features available on modern topographers can help you to achieve that. OM
Special thanks to Kenneth A. Lebow, O.D., F.A.A.O., for reviewing this column.