INSTRUMENTS

Instrumentation For Detecting Corneal Changes From Contact Lens Wear

BY LORETTA B. SZCZOTKA, O.D., M.S. AND
 BETH ANN BENETZ, M.A., C.R.A.
April 2000

Find out which instruments can help you detect corneal changes associated with contact lens wear.

Contact lenses are generally well-tolerated by the majority of patients. Occasionally, patients will present with complaints such as spectacle blur, lens intolerance and decreased wearing time. Corneal changes associated with such symptoms may or may not be detected by examining the cornea with standard clinical instruments, such as the phoropter, slit lamp and keratometer. If subclinical corneal changes are responsible for subtle lens-related complaints, they may be detected only with the use of other equipment. This article highlights some key instruments which can detect symptomatic and pre-clinical corneal changes associated with contact lens wear. Some are advanced forms of technology and are found only in research and teaching institutions. Other instruments are affordable enough to become part of your key devices.

Computerized Videokeratoscopy

Corneal warpage -- Computerized videokeratoscopy (CVK), or corneal topography, is rapidly becoming the gold standard in measuring corneal curvature in most corneal and contact lens practices. Most often, the instruments used in clinical practice are placido-based CVK systems, which employ a series of illuminated annular rings projected onto the cornea. Using the corneal tear film as a mirror, the reflected image of the rings is captured by a digital video camera. The captured image is then subjected to an algorithm to detect and identify the position of the rings relative to the videokeratographic axis. Some thin ring projection systems such as the TMS-2 (Tomey Technology), Humphrey Atlas and Keratron (Optikon 2000) use a peak luminace algorithm where the brightest portion (the center of the ring) is identified as the border. Other wide ring projection systems, such as the EyeSys 2000 (EyeSys/Premier) and CT 200 (Dicon), use a border detection algorithm to detect both edges of each illuminated ring (Fig. 1). Once the borders are detected, a proprietary algorithm (a high-order polynomial equation) is applied to the digital image, which "reconstructs" the corneal curvature. Of the many files that may be created, a table of curvature values and data point locations are calculated. These tables are then used to produce the familiar map of curvatures presented in a color-coded format.

FIG. 1: Example of thick-ring placido system. Each black-white border serves as a separately imaged ring.

In the contact lens practice, corneal topography is extremely helpful in designing and prescribing RGP lenses. Most current generation topographers have efficient and accurate RGP fitting programs. In fact, for nondiseased corneas, I feel comfortable using these programs instead of performing a traditional diagnostic lens fitting. However, corneal topography is even more useful to monitor the stability of the cornea after both short- and long-term lens wear. Subtle corneal steepening or sphericalization that is undetectable with keratometry may characterize early corneal changes from contact lens wear. Serial topography and topographic difference maps generated by a consistent instrument on a given patient over time may reveal such subtle changes.

Occasionally, PMMA and RGP lenses are capable of distorting the corneal surface resulting in transient or permanent corneal warpage. Different forms of corneal distortion can be detected such as the shift from a prolate to an oblate shape, inferior corneal steepening, "smile" impression arcs and variable irregular astigmatism. Most forms of distortion can be traced back to the fit or material of the lens.

Oblate shape -- Long-term flat fitting lenses can permanently shift the normal cornea from a prolate shape to an oblate pattern by flattening of the central cornea and secondarily steepening the periphery. This type of warpage may not be detected with keratometry or manifest refraction if the astigmatism is regular and the patient remains correctable to 20/20. Only corneal topography can reveal the odd shape that may create difficulty in RGP fitting and problem-solving.

Inferior corneal steepening -- Long-term PMMA and low Dk RGP wear and superiorly decentered rigid lenses can cause inferior corneal steeping, giving the appearance of keratoconus (Fig. 2). Even hydrogel lenses can cause severe shape changes, presumably due to corneal hypoxia with secondary tissue swelling.This also gives the appearance of inferior corneal steepening and pseudo-keratoconus (Figs. 3 & 4).

 

 

 

FIG. 3: Hydrogel lens induced corneal swelling and pseudo-keratoconus inferior corneal steepening.

FIG. 4: Resolution of corneal distortion after discontinuing lens wear on same patient in Fig. 3.

Our role is to be able to correctly identify these corneal changes before they create severe visual consequences. Corneal topography simplifies that process and assists in differentiating various contact lens-induced and pathological conditions. Keratoconus, sub-clinical keratoconus (forme fruste), contact lens-induced inferior corneal steepening, a displaced corneal apex and pellucid marginal degeneration all produce similar corneal topography maps that demonstrate superior corneal flattening and marked inferior corneal steepening. Some topography systems (Humphrey Systems, Tomey Technology and Dicon) have introduced keratoconus detection software that attempts to detect and possibly differentiate these conditions. For example, Humphrey Systems' PathFinder Software Module for the Atlas Corneal Topographer differentiates normal, corneal distortion, sub-clinical keratoconus and keratoconus by statistically analyzing three shape indices and comparing them to a normative database. Tomey Technology utilizes an artificial intelligence "expert system" to determine the presence, absence and degree of keratoconus based on the analysis of a series of topographically-derived numerical indices. Dicon uses an analysis of apex localization (Bull's Eye Targeting) utilizing tangential maps to predict the probability of keratoconus suspicion. (Fig. 5)

FIG. 5: Keratoconus detection software on Dicon's CT 200.

Smile patterns (impression arcs) -- RGP-induced arcuate corneal shape changes are most often located in the inferior one-third of the cornea and can implicate a sub-optimal fitting relationship. In traditional RGP fitting, a flattened arcuate compression ring usually signifies unintended corneal molding from the inferior edge of a superiorly decentered RGP. Just outside the lens edge, an arcuate zone of steepening appears, best viewed on an instantaneous map (Fig. 6). Secondary clinical signs may include spectacle blur and increased myopia from associated central corneal steepening. This may suggest intermittent contact lens adhesion, and an attempt should be made to flatten the posterior curves for improved corneal shape stability.

FIG. 6: Inferior arcuate "smile" topography 
pattern from a superiorly displaced RGP lens.

Ultrasound Pachymeter

Corneal thickness -- The ultrasonic pachymeter has replaced the optical pachymeter for quickly measuring corneal thickness in clinical practice. Because it's so easy to use, ancillary staff may operate it proficiently with only modest training. Its small size makes it portable and more convenient than the optical pachymeter. Each reading produced by an ultrasonic pachymeter represents an averaged series of values computed by the instrument, therefore interobserver and intersession variability is minimized.

Corneal thickness is measured when the probe tip is placed within 10 degrees of the perpendicular at any location on the cornea (Fig. 7). Ultrasonic waves are emitted and then detected by the probe. The computation of corneal thickness is based on the speed with which sound travels through the cornea. The readings are usually accurate within five to 10 µm, but typically at least three measurements are taken at each corneal location, and the middle or most frequent number is chosen.

FIG. 7: Ultrasonic pachymetry.

In the contact lens practice, ultrasonic pach-ymetery has two primary uses: assessment of corneal swelling in extended wear patients and measurement of corneal thickness during a refractive surgery consultation to ensure adequate corneal thickness exists prior to considering a myopic laser procedure.

Corneal swelling -- It's a well-established finding that with no lens wear, corneas swell an average of four percent during eye closure. In extended wear with traditional hydrogel lenses, an average of nine to 10 percent corneal swelling has been documented. Most clinicians don't routinely practice pachymetry, and low levels of edema are difficult or impossible to visualize with the slit lamp. The following list correlates levels of stromal edema that are detectable with a slit lamp:

• Corneal striae are typically observed at approximately seven to eight percent swelling.
• Corneal folds are seen with 11 to 12 percent swelling.
• More than 20 percent corneal swelling is associated with a loss of transparency.

However, it's important to detect low levels of edema when prescribing and following patients in extended wear because all patients' oxygen requirements are not alike. The swelling response can range from less than two percent to more than 17 percent . Therefore, a large range of individual needs for oxygen makes it difficult to predict which patients will succeed in extended wear lenses. Some practitioners use the corneal swelling response as an indicator of successful extended wear candidates. O. David Solomon, M.D., uses ultrasonic pachymetry (Corneo-Gage II, Sonogage, Inc.) to measure corneal thickness changes after overnight wear of contact lenses as a "stress test" for extended wear lenses. It's a provocative test used to identify a cornea at risk for significant oxygen deprivation following one night of extended wear. If more than five percent swelling occurs, they are discouraged from continuing in extended wear. In a 1996 study by Dr. Solomon published in The CLAO Journal, any eye with more than 13 percent corneal swelling definitely failed in extended wear with traditional hydrogels. Therefore, this test provides an easy, objective and accessible measure of corneal physiology that can be used to provide a screening tool. If more than five percent corneal swelling exists, you may want to consider hyper Dk silicone hydrogels for extended wear or other refractive options.

Refractive surgery consultations -- Any contact lens practitioner must be ready to accept patient requests for refractive surgery advice and potential consultations. A portable, ultrasound pachymeter is a good device to have on hand to screen these patients prior to considering LASIK. The average corneal thickness is 550µm, and at least 250µm of tissue must remain under the flap for safety after surgery. The microkeratome typically creates a flap 160µm thick. The excimer laser removes approximately 12µm of tissue for each diopter of correction. Therefore, the refractive error dictates how much tissue must be removed to achieve the desired result. These calculations are critical for all patients above 10.00D of myopia because their corneal thickness may not be sufficient to safely perform the procedure. The ultrasound unit can save patients from undergoing a lengthy consultation elsewhere if they are not suited for the procedure in the first place. Remember the attempted ablation depth (12µm per diopter of correction), plus the flap thickness (160µm), plus the minimal remaining stromal bed thickness (250µm) must all equal less than the patient's central pachymetry measurement.

Specular Microscope

The development of the specular microscope in the 1970s allowed clinicians to observe the corneal endothelium of contact lens wearers. Specular microscopy is an in-vivo technique of viewing the corneal endothelium. It's a standard method for determining cell loss and changes in cell size (polymegethism) or cell shape (pleomorphism) with aging, and also following the cornea after topical therapies, contact lens wear, cataract surgery, corneal preservation, corneal transplantation, laser iridotomy and trabeculoplasty, and keratorefractive procedures. Imaging the endothelium traditionally is performed by a contact specular microscope designed to output to either video or film. Until recently, noncontact specular microscopes have not provided sharp images of sufficient magnification for reliable cell density determination or morphometric analysis.

One technique for determining endothelial cell density, the fixed-frame method, involves placing a grid on the video screen or print and manually counting cells within the grid or using a comparative counting panel. Polymegethism and pleomorphism are determined subjectively. Several computer-assisted systems for endothelial cell analysis reduce the subjectivity inherent in this method; they also allow for a quantitative analysis of cell morphology including cell area, coefficient of variation, figure coefficient and percentage of hexagonal cells. With these systems, cell density either is calculated from mean cell area or by utilizing a fixed-frame or variable-frame analysis program.

The non-contact specular microscopes now available have incorporated semi-automated, computer-assisted cell analysis. The non-contact systems are easy to operate with minimal training, allow short procedure time compared to contact methods of endothelial imaging and enable multiple fields of study to be imaged. The microscope doesn't touch the patient's cornea; thus, patient comfort is excellent. The non-contact microscopes also remove the risk of contamination and abrasion from the contact probe. A wide age range of normal subjects, including young children, can be examined without difficulty. Yet scarred, edematous or ectactic corneas that sometimes present a challenge to image with contact methods often are impossible to image with the noncontact specular microscopes.

The major function of this device is to evaluate the short- and long-term response to contact lens wear. For corneal transplant patients, it can be used to evaluate the health of the donor endothelium. It's also useful in predicting which patients can undergo anterior segment surgery without eventual corneal decompensation.

Short-term response to contact lens wear-- In the late 1970s, Holden and Zantos, due to specular microscopy, first described an endothelial bleb response that developed immediately after the insertion of lenses in healthy, unadapted lens wearers. The endothelial response is characterized by the appearance of small, nonreflective, blister-like eruptions (blebs) scattered over the endothelial mosaic that peaks about 25 minutes after lens insertion (Fig 8). Endothelial bleb formation from lens wear may represent changes in the contour of the posterior endothelial cell membranes associated with cell swelling. It's likely an effect of rapid tissue adaptation to carbon dioxide and pH changes. The bleb response can be visualized with all contact lens types, although hyper Dk materials induce the least response, signaling less stress to the cornea.

FIG. 8 (left): Endothelial bleb response in a normal, non-contact lens wearer 15 minutes after contact lens insertion.

FIG. 9 (right): Endothelial polymegethism.

Long-term resonse to contact lens wear-- Specular microscopy is perhaps most beneficial in detecting endothelial morphologic changes. Endothelial polymegethism is one of the model manifestations of chronic hypoxia (Fig. 9). Polymegethism usually is accompanied by pleomorphism, and both have been documented with the use of PMMA and low Dk contact lenses. The effects can be somewhat reversed after discontinuing lens wear or refitting to high Dk lenses; however, persistent endothelial cell changes can occur with long-term contact lens wear. Polymegethism has been correlated with a disturbance in endothelial cell function, resulting in significant abnormalities in the barrier and pump functions of the corneal endothelium. Clinically, concerns lie in the potential for future endothelial cell compensation in long-term low Dk contact lens wearers as they are subjected to aging or surgical intervention.

Other Instruments

Other instruments, used mainly in the research setting, include the confocal microscope and the fluorophotometer, which both show promise in assessing new findings associated with extended wear. For example, the fluorophotometer has been used to assess epithelial cell permeability during extended wear, and it has shown that wearing low Dk lenses results in increased epithelial cell permeability, signaling a decrease in the epithelial barrier function.

The confocal microscope allows real-time, in-vivo, sequential optical sectioning imaging at the cellular level of the cornea. Many types of confocal microscopes exist, such as the basic research laser scanning microscopes and the clinical diagnostic white light microscopes. They have the ability to focus illuminating light and the microscope's objective on precisely the same plane within the specimen being observed. The objects above and below the focal plane are not imaged, making it possible to obtain very high X, Y and Z axis resolution. These types of instruments are capable of imaging cells, organisms and other anomalies within a transparent cornea which can be digitally analyzed, enhanced and archived. In extended wear contact lens clinical research, this instrument has been used to evaluate epithelial cell desquamation as a function of lens Dk/l, to examine stromal keratocyte density in lens wearers and to identify bacteria in patients diagnosed with contact lens-related keratitis. However, it has yet to be found in abundance in the average corneal practice and is mainly reserved as a research tool. 

To receive references via fax, call (800) 239-4684 and request document #59. (Have a fax number ready.)

Dr. Szczotka is an assistant professor at Case Western Reserve University Dept. of Ophthalmology and Director of the Contact Lens Service at University Hospitals of Cleveland.

 

Beth Ann Benetz is Senior Instructor at Case Western Reserve University. She serves as Director of Ophthalmic Photography for University Ophthalmologists, Inc., University Hospitals of Cleveland and Case Western Reserve University.