Article

STREAMLINE FITS WITH TECHNOLOGY

Advanced technologies make specialty contact lens fitting more efficient.

Managing complex corneal conditions and challenging refractive errors with specialty contact lenses takes a high level of commitment and skill. Fortunately, there are now many advanced technologies available that can make lens evaluation more efficient and can assist in troubleshooting difficult fits. Originally created to detect and monitor disease, these technologies also have tremendous value when applied in specialty lens practice. Investing in some of these technologies will not only help streamline complex contact lens fits, but also will help differentiate and build a specialty lens practice. In this article, we review some of the latest technologies available for diagnosing and monitoring corneal conditions as well as how those technologies can aid in contact lens management.

TEAR FILM ANALYTICS

Contact lens comfort and vision depend on the quantity and quality of the tear film anterior and posterior to the lens. For example, reduced tear breakup time and volume are associated with contact lens discomfort and intolerance.1 Therefore, careful evaluation and management of the tear layer is essential for ocular health and comfort in contact lens patients.

Although the tear film over the corneal and contact lens surfaces can be viewed with sodium fluorescein, it can now also be assessed through non-invasive methods. Currently, reflection-based imaging technologies are available to measure both tear breakup time and stability without directly contacting the eye.

One method of measuring the tear film is through corneal topography systems that project placido disk images over the ocular surface. In this technique, reflection videography of the tear film is recorded, and associated software is used to analyze the tear layer. Physical tear breakup time is viewed as distortion in the placido mire quality over the cornea or the contact lens (Figure 1).

Figure 1. Non-invasive tear breakup analysis. This can be performed with and without the contact lens in place.

Another way to assess tear film stability is through ocular surface interferometry. This technology allows for real-time visualization of the tear film lipid layer to evaluate the dynamic response of the lipids to blinking. Additionally, many instruments use infrared photography to produce high-definition imaging of the meibomian glands (Figure 2).

Figure 2. Infrared photography allows for high-definition imaging of the tear film lipid layer and the meibomian glands. Careful evaluation and management of the tear layer is essential for ocular health and comfort in contact lens patients.

CORNEAL AND SCLERAL IMAGING SYSTEMS

Imaging the cornea is essential to fully manage corneal diseases and conditions. Additionally, these instruments also provide valuable data about the corneal and scleral surface contour, which can aid in contact lens fitting. Currently, both corneal topography and corneal tomography are available to image the corneal surface, while scleral topography and profilometry are capable of imaging both the scleral and corneal surface contour.

Corneal Topography Corneal topography commonly utilizes reflection-based techniques and can be further divided into two categories: placido-based topography and raster photogrammetry. Placido-based topographers project multiple concentric rings onto the corneal surface and analyze the reflected light to measure the anterior corneal curvature. Raster photogrammetry reads curvature by taking multiple triangulated measurements using a projector and two cameras to image the ocular surface. Both of these topography methods depend on tear film mire reflection quality, so images may be difficult to capture if the cornea is severely distorted or in advanced ocular surface diseases.2

Corneal topography allows for imaging of the central and peripheral anterior segment and provides helpful information that aids in contact lens selection during fittings. One value of anterior measurements is the ability to measure the peripheral corneal surface. Eccentricity describes the rate of corneal flattening toward the periphery and is useful in selection of lens design. For example, if a cornea has a higher rate of flattening in comparison to a typical cornea (normal eccentricity value is ~ 0.55), an aspheric lens may be indicated rather than a spherical lens design.

Other data provided that are vital to contact lens design include horizontal visible iris diameter (Figure 3), scotopic and photopic pupil diameters, and angle kappa measurements. These can aid practitioners when they are choosing the correct lens diameter and multifocal design, zone size, and even necessary offset of optics.

Figure 3. Corneal topography of keratoconus. Note the presentation of horizontal visible iris diameter, apex decentration, and eccentricity.

In addition to lens selection, anterior corneal maps are helpful in monitoring and troubleshooting orthokeratology fits3 (Figure 4) and may be used to check the on-eye alignment of multifocal contact lenses.4

Figure 4. Subtractive map showing the effect of orthokeratology treatment. The treatment zone is well centered and homogenous.

Additionally, some corneal topographers come equipped with software programs that can assist virtually with contact lens fitting. This allows selection from a list of preloaded lens brands and will simulate the fluorescein pattern of the lens over a patient’s corneal map. Virtual adjustments to the lens parameters can be made, and subsequent changes in the fluorescein pattern can be viewed.

Corneal Tomography Different from corneal topography, corneal tomography is a projection-based imaging technology that generates a three-dimensional reconstruction of the anterior segment using a series of optical cross-sections (Figure 5)

Figure 5. Tomographic corneal sections of keratoconus.

Whereas placido disc imaging is limited to the anterior central and peripheral cornea, corneal tomography can map both the anterior and posterior corneal surfaces. Additionally, this technology allows complete pachymetric evaluation of the cornea. Depending on the instrument, either optical slit-scan or rotating Scheimpflug imaging are used to gather data for projection-based tomography. Elevation-based corneal tomography is more precise in comparison to placido-based imaging because it does not rely on the quality of the reflection off of the anterior cornea.2

Corneal posterior elevation maps are a sensitive indicator for detection of early keratoconus or other ectatic disease states. In fact, mapping the posterior surface has become the gold standard when ruling out the presence of forme fruste keratoconus during refractive laser consultations.5 In addition to monitoring disease, posterior corneal analysis can be helpful in the management of specialty lens patients by providing information about potential visual acuity that can be expected with contact lenses (Figure 6). With increasing central posterior elevation, visual potential through a specialty lens decreases.6

Figure 6. Tomographic elevation maps of keratoconus (upper and lower right). Posterior elevation maps are helpful in providing information about potential visual acuity that can be expected with contact lenses. An increase in elevation is associated with degradation in visual acuity.

Additionally, one device has added the capability to merge multiple Scheimpflug-based scans to map beyond the cornea and create profilometry data (Figure 7).

Figure 7. Tomographic profilometry data derived from multiple scans.

Profilometry Also of interest in specialty contact lens fitting, scleral topographers or profilers are now available to assist scleral lens fitting and design. These profilers map both the cornea and sclera, providing information about the contour and depth of the ocular surface as well as whether toricity is needed in the landing zone of the scleral lens design. Currently, raster photogrammetry is being used to measure scleral curvature in profilometry devices for scleral lens fittings.

One fluorescence-based topographer captures a single image of the cornea and sclera in primary gaze. The system projects from two directions a line pattern on the front surface of the eye in which fluorescein acts as diffusing medium. Each scan provides more than 350,000 measure points across a 20mm area. With another device, images are taken in primary and/or in the up- and down-gaze positions to create a three-dimensional model of the eye (Figure 8). The acquired corneal and scleral curvature measurements and height data can be used to select a lens from the available designs in the topography software program. After lens selection, the instrument generates the initial lens diameter based on the size of the cornea as well as the suggested sagittal depth of the lens and the amount of toricity in the landing zone of the lens7,8 (Figure 9).

Figure 8. Scleral topography allows for imaging of both the corneal and scleral contour. This information is valuable for initial scleral contact lens selection and troubleshooting lens design.

FIgure 9. Profilometry derived 3D model of the eye with a virtual scleral lens.

The ability to map scleral contour has produced peer-reviewed data showing that very few patients have spherical scleral contour; the vast majority of patients have scleral shapes that could be described as symmetrically toric, asymmetrically toric, or completely asymmetric.9

ANTERIOR SEGMENT OPTICAL COHERENCE TOMOGRAPHY

Anterior segment optical coherence tomography (AS-OCT) imaging has numerous clinical and research applications related to the tear film, cornea, conjunctiva, sclera, and ocular adnexa in addition to evaluation of soft, GP, and hybrid contact lenses. The following will review how OCT can be used in a modern specialty lens practice.

OCT instruments now have additional software that allows mapping of the corneal epithelial layer. Knowledge of epithelial thickness patterns in normal and abnormal cases is valuable in monitoring corneal conditions. Early detection of corneal conditions through OCT epithelial mapping offers the opportunity to provide better care and treatment choices for specialty lens patients.

The epithelium does not have a homogenous depth over Bowman’s layer in those who have stromal irregularities, such as patients who have corneal ectasia or who have had myopic laser refractive surgery. Rather, the epithelium compensates for these stromal irregularities by becoming thicker over areas in which the stroma is depressed and thinner where the stroma protrudes more anteriorly.10 As a result, the anterior cornea may appear smoother on typical corneal topography compared with the underlying anterior stroma. Thus, assessment based on topography and total corneal pachymetry with no knowledge of epithelial depth may be misrepresentative of corneal conditions. In fact, epithelial thickness mapping can help practitioners identify early keratoconus. Overall, epithelial thickness appears more irregular and increased in ectatic corneas, with the thinnest areas of epithelium over the keratoconic protrusion11,12 (Figure 10).

FIgure 10. OCT pachymetry maps of keratoconus. The thinning of the epithelium matches location with apical thinning.

Larger-diameter sclerals are designed to vault the corneal apex and limbus and to align with the underlying sclera. Sclerals should be fitted in a manner that induces minimal adverse changes to the underlying ocular tissues.

AS-OCT imaging of scleral lenses on-eye (Figure 11) provides valuable clinical information when used in addition to slit lamp evaluation. AS-OCT imaging has allowed greater understanding of corneal, conjunctival, and scleral anatomy and is a valuable tool that aids lens fitting and troubleshooting.

Figure 11. AS-OCT is able to image the corneal vault and haptic landing of a scleral contact lens over the ocular surface.

Excessive corneal clearance of scleral lenses may contribute to corneal edema,13 and low corneal clearance may also result in adverse ocular changes during scleral lens wear, such as unintentional corneal touch following lens settling and excessive lens suctioning.14 Scleral lens vault over the central and peripheral cornea can be evaluated and measured through OCT imaging. Nevertheless, corneal clearance data from OCT imaging, while very close, is not truly accurate because anterior segment thickness measurements are affected when imaging through a contact lens due to optical distortion.

Regardless, OCT imaging provides repeatable estimates of the scleral lens vault over the corneal surface and can be used as an adjunct to slit lamp evaluation of the lens.15 In addition to measuring scleral lens vault over the corneal surface, AS-OCT imaging can also be used to quantify conjunctival and scleral compression during and following scleral lens wear. Tissue compression may affect tear exchange, comfort during lens wear, and the extent of conjunctival staining following lens removal.16

MICROSCOPY

Corneal Confocal Microscopy This type of microscopy provides images of the different structural cellular layers of the cornea. Comparable to histology-quality images, this instrument provides viewing of the corneal epithelium, Bowman’s layer, stroma, Descemet’s membrane, and the corneal endothelium. Confocal microscopy is a non-invasive technique and allows for real-time imaging of the living cornea; it has been used to investigate numerous corneal diseases. This technology can, therefore, be used in the detection and management of infectious corneal conditions such as Acanthamoeba keratitis, corneal dystrophies, and ectasias as well as to monitor corneal transplants.17 Most recently, confocal microscopy has been used to evaluate dry eye in contact lens wear. An increase of both inflammatory and immune mediating cells were found on confocal microscopy imaging in contact lens wearers who had symptomatic discomfort from dry eye.18

Specular Microscopy The endothelial layer acts as a fluid barrier and metabolic pump, maintaining the normal cornea transparency and thickness. If this function is lost, the cornea can swell, resulting in edema and transparency loss. Careful evaluation of the endothelium through specular microscopy is valuable in monitoring the corneal response in both corneal and scleral GP lens wearers as well as in soft lens wear (Figure 12).

Figure 12. Careful evaluation of the endothelium through specular microscopy is valuable in monitoring the corneal response in both corneal and scleral GP lens wearers as well as in soft contact lens wear.

Cellular changes such as polymegethism, pleomorphism, and reduced cell density have occurred in long-time wearers of low-Dk lens materials.19 Moreover, those who have received corneal transplants may need to be monitored more closely for stress from hypoxia when wearing contact lenses.13

ABERROMETRY

Residual visual distortion from higher-order aberrations (HOAs) may still limit the vision of a patient who has been fitted in a specialty contact lens. In such cases, further assistance for troubleshooting the contact lens parameters is available through measurement of HOAs with wavefront aberrometry.

Interestingly, incorporation of wavefront-guided corrections into contact lenses has been attempted with some success.20 Take, for example, a patient who has irregular astigmatism wearing three different scleral lenses with varying front-surface eccentricity values but otherwise identical optical power and back-surface parameters. Note that the low- (Figure 13A) and high-eccentricity (Figure 13B) optics produced larger amounts of aberrations, while the mid-eccentricity optics (Figure 13C) allowed for reduced total and HOAs. The result was 20/30 visual acuity with the low-eccentricity value, 20/25+ visual acuity with the mid value, and 20/30 visual acuity with the high value. With the mid-value lens, this patient noted significantly improved quality of vision.

Figure 13. Measurement of higher-order aberrations (HOAs) through aberrometry can be used to improve residual visual distortion from HOAs in contact lenses. Measurements of HOAs were taken in a patient wearing a front-surface scleral lens that incorporated low-, mid-, and high-eccentricity. Note that the low- (A) and high-eccentricity (B) optics produce larger amounts of aberrations, while the mid-eccentricity (C) lens allowed for reduced total aberrations and HOAs. With the mid-value lens, this patient noted significantly improved quality of vision.

TELEHEALTH COMMUNICATION

Managing complex corneal conditions through specialty lenses requires an exceptional level of expertise. Although controversial, telehealth communication resources are now available to help practitioners streamline care of specialty contact lens patients.

One available resource allows practitioners to directly communicate with patients to better manage their care. The objective for this technology is to check in with patients prior to their follow-up exams and to complete non-office-related tasks online; this potentially improves compliance and decreases chair time.

Additionally, there are other communication resources that connect practitioners with manufacturer consultants to troubleshoot specialty lens design (Figure 14). This allows real-time contact lens assessment with consultants while a patient is still in your chair. The objective of this technology to make the fitting process of contact lenses more efficient.

Figure 14. Telehealth communication resources connect practitioners with manufacturer consultants to troubleshoot specialty lens design.

SUMMARY

Contact lens fitting is influenced by ocular surface anatomy and physiological health. In turn, contact lenses can physically alter the shape of the ocular surface and influence ocular surface physiology. Beyond correcting for refractive error, contact lenses—especially therapeutic specialty lenses such as sclerals—are used to treat conditions such as dry eye, keratoconus, Sjögren’s syndrome, and others. It is important to not only monitor the effectiveness of the treatment but also to ensure that the lenses themselves are doing no harm.

Instruments such as corneal topographers have obvious application in specialty contact lens fitting, whereas others such as OCT have been adapted to meet the needs of practitioners fitting specialty lenses. As treatment regimens advance, the features of some instruments that may have gone unnoticed are becoming more important. For example, extrapolated tear film analysis available from corneal topographers informs about dry eye treatments and contact lens fits alike.

Some techniques, including specular microscopy and scleral topography, are of particular interest for specialty lens practitioners. Not only do such instruments elevate the capabilities of the practitioners to effectively manage ocular surface disease, they serve as differentiators that set advanced fitters apart from general practitioners.

Finally, emerging technologies such as telehealth communication allow practitioners to more closely monitor their patients and also provide better treatments through real-time consultations. Keeping up-to-date with the latest and greatest challenges allows eyecare practitioners to provide cutting-edge contact lens-based treatments. And, when used well, these technologies also improve outcomes for their patients. CLS

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