Bilateral Progressive Keratoconus
And the importance of e-value in fitting these cases
Key words: Eccentricity, Keratoconus, (R)GP Lenses
Patient KB is a 21-year-old African student. He complained of decreased vision for the last few months with his current spectacles of about one year. His ocular and medical history was unremarkable except for occasional mild, non-specific allergies. He was taking no chronic medication, but occasionally takes an OTC antihistamine. He has never used contact lenses before.
VA and Refraction:
OD -1.25 / -0.50 x 32
OS -3.50 / -0.75 x 44
OD 0.4 -1 (6/15-1)
OS 0.05 (6/120)
OD -2.50 / -0.50 x 5 VA 0.7-2 (6/8.6-2)
OS -8.50 / -6.00 x 85 0.25 (6/24)
Slit lamp Examination (prior to contact lens wear):
OD: A clear cornea with no NaFl staining
OS: A clear cornea with central striae and corneal wrinkling with NaFl stain.
OS: Central Striae
OS: Central Corneal Wrinkling Visible with NaFL
Figure 3.Oculus Keratogragph 4 Overview of right eye
From the topography overview it is evident that there is superior steepening , vertical asymmetry and a greater than normal rate of peripheral corneal flattening (as indicated by the high e-value). This might indicate a typical case of keratoconus. According to the article ‘Keratoconus: A review’ - Romero-Jiménez M, et al: Contact Lens Anterior Eye (2010) - this is probably a stage 2 to 3 keratoconus.
Figuur 4: Oculus Keratogragph 4 Overview of left eye:
A very steep central nipple cone is evident in the topography overview OS. This area is significantly steeper than the superior cornea OD as supported by the clinical slit lamp biomicroscopic findings. There is also a significant rate of peripheral flattening as indicated by the 1.15 e-value. This eye would probably be defined as a stage 3-4 keratoconus, refering to the above mentioned article ‘Keratoconus: A review’. Although we found a superior steepening of the cornea in both eyes, this is a very rare finding. Most cones in keratoconus appear to be inferiorly located.
Aspheric GP lens designs from the Contact Lens Lab of S.A were used to trial fit the less advanced right eye. The first trial lens was fit on flat K, but this lens exhibited too much bearing on the central cornea and decentred along the steeper superior cone. Fitting a slightly steeper base curve achieved the desired 20 microns of tear layer thickness centrally, and the higher e-value allows for better peripheral alignment, which facilitates good movement and tear exchange (Figure 3).
The patient reported good comfort and vision. The slightly smaller 9.5 diameter was chosen to eliminate inferior stand-off.
Final lens :
OD BC 7.90 OZ 7.5 (1.4e) TD 9.5 -2.75 VA 6/5 (1.2)
Figure 5. OD: good corneal alignment
Fitting the left eye proved to be challenging because of the very steep central nipple cone and the very flat periphery. Standard K5 designs resulted in central bubble formation, peripheral horizontal bearing and inferior stand-off. Using smaller diameter designs resulted in poor centration, comfort and vision.
Diagnostic lenses from the Genesis K range were used to trial fit the advanced left eye. The set consists of three different e-values for each specific base curve. Each different e-value corresponds to a different rate of peripheral flattening . This aspheric (e-value specific) lens provides better peripheral corneal alignment, which is especially helpful in cases of keratoconus in which each cornea is unique.
A base curve of 6.3 was chosen with an e-value of 1.2 (this is the higher e-value in the fitting set and was chosen in correlation with the high e-value suggested by the topographer). This trial lens achieved the ideal tear layer thickness over the cone of about 20 microns, good tear exchange during the blink and good horizontal peripheral alignment.
The larger diameter lens resulted in lid attachment (Figure 4), which contributes to good comfort and centration. Although significant inferior pooling is present in the left eye, there is no bubble formation or dislodging of the lens reported.
Final CL RX :
OS BC 6.3 OZ 6.0 (1.2 e) TD 9.8 -14.50 VA 6/7.5 (0.8) Optimum Extreme Blue
Optimum Extreme GP material was used because it has superior Dk (Dk 125) without compromising wettability. High Dk lenses are necessary in high powered and large diameter lenses to maintain good corneal physiology. Aspheric peripheral designs provide better corneal alignment. Aligning the cornea as much as possible, especially in already compromised corneas, is vital to avoid unwanted corneal changes.
Aftercare and Follow Up
This patient frequently followed up during his adaptation period. At the two month progress evaluation, he achieved a maximum comfortable wearing time of 14 hours and good vision is maintained throughout all waking hours of RGP lens wear. No corneal adverse events or physiological problems were noted during slit lamp examination.
Key learning point
An increase in the difference between peripheral flattening relative to the (increase in) central steepening is a classic sign in keratoconus. The amount of flattening is typically referred to as e-value, although p-value and Q-value are also used to describe the same phenomenon. A higher e-value (more relative flattening toward the periphery) is a strong indicator for keratoconus; patients in whom the highest e-value in any part of the eye was 0.8 or higher had a specificity of 98% and a sensitivity of 97% when detecting keratoconus even if the visual acuity was 20/20 (1.0) and slit lamp examination showed no abnormalities in a study by Doa et al Cornea. 1994 Jul;13(4):339-44. Not only is e-value important in detecting keratoconus, it also can be very helpful in designing better keratoconus lenses. Squaring the e-value will provide an indication of the amount of flattening at 30 degrees on the cornea. For example: an e-value of 0.4 roughly represents a 0.16mm flattening in the periphery. However, in keratoconus, as in our case here, this easily increases to more than 0.8. In severe stages of keratoconus this can reach values of 2mm difference between the center and the periphery (with an e-value of 1.4 or higher). Taking these values, the amount of flattening and the e-value into account can be of great value when fitting keratoconic eyes with (R) GP lenses.
BOptom, F.O.A.(S.A), Medpro-X(UNISA), CAS(SA), T.P.A, FIACLE
Bloemfontein, South Africa
Dina Krüger received her optometry degree from the Rand Afrikaans University in Johannesburg, South Africa. After graduating she entered into private practice where she has worked for several practices. She has a special interest in contact lenses and became involved with the writing of the curriculum for the undergraduate Optometry degree since 2003.
Since 2003 she went back to private practice specializing in contact lenses but continued to lecture on a part time basis.
Since 2007 she is the module leader of contact lenses at the University of the Free State and is running the contact lens specialty clinic at the National Hospital in Bloemfontein. She is a fellow of SAOA and IACLE and a member of the BCLA.