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ORIGINAL ARTICLE
Year : 2015  |  Volume : 7  |  Issue : 2  |  Page : 48-52

Corrected visual functions in keratoconus patients with rigid gas-permeable contact lens and its association with cone location


1 Department of Contact Lenses, Faculty of Optometry and Visual Science, Al-Neelain University, Khartoum, Sudan; Program of Optometry, School of Healthcare Sciences, Faculty of Health Sciences, Universiti Kebangsaan Malaysia, Kuala Lumpur, Malaysia
2 Program of Optometry, School of Healthcare Sciences, Faculty of Health Sciences, Universiti Kebangsaan Malaysia, Kuala Lumpur, Malaysia
3 Program of Optometry, School of Healthcare Sciences, Faculty of Health Sciences, Universiti Kebangsaan Malaysia, Kuala Lumpur; Department of Optometry, Faculty of Health Sciences, Universiti Teknologi MARA, Selangor Darul Ehsan, Malaysia
4 Department of Ophthalmology, Faculty of Medicine, Al-Neelain University, Khartoum, Sudan

Date of Web Publication12-Nov-2015

Correspondence Address:
Mustafa Abdu
Faculty of Optometry and Visual Science, Al-Neelain University, Khartoum, Sudan

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DOI: 10.4103/1858-540X.169436

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  Abstract 

Objective: To compare corrected visual functions between spectacles and rigid gas permeable (RGP) lenses, and to evaluate the association of cone location with corrected visual functions with RGP lenses in keratoconus patients. Materials and Methods: Thirty-four eyes from 19 newly diagnosed keratoconus patients were included. All subjects were fitted with hyper Dk (160) RGP lenses. All subjects were divided into two groups following their corneal cone location (central and paracentral). Manifest refraction, best spectacle visual acuity (BSVA) and visual acuity (VA) with an RGP contact lens (RGPVA) using Snellen chart, best spectacle contrast sensitivity (BSCS) and CS with RGP (RGPCS) using Pelli-Robson chart, slit lamp, and topography measurements were conducted. Results: A mean age of all subjects was 26.63 ± 8.58 years. Sixteen eyes (47.1%) have central cones, and 18 eyes (52.9%) have paracentral cones. At baseline, mean refraction was −5.62 ± 3.62 D, mean CA was −6.74 ± 4.75 D, mean best spectacle-corrected VA (BSCVA) was 0.59 ± 0.33, and mean BSCS was 1.54 ± 0.33. Visual functions improved significantly with contact lens wear (P < 0.05). No significant difference was detected between both groups in keratometer readings, mean CA, BSVA, RGPVA, BSCS, and RGPCS (P > 0.05). A significant relation was noted between the mean K and BSCVA, BSCS, RGPCS (P < 0.05), but the insignificant relation was detected between mean K and RGPVA (P > 0.05). No association was found between the cone location and RGPVA or RGPCS (P > 0.05). Conclusion: RGP contact lens is significantly improving visual functions of keratoconus patients. Cone location in keratoconic eyes has no effect on corrected VA or CS and therefore, is not a factor in trial contact lens selection.

Keywords: Cone location, contrast sensitivity, keratoconus, rigid gas-permeable


How to cite this article:
Abdu M, Mohd-Ali B, Mohidin N, Binnawi KH. Corrected visual functions in keratoconus patients with rigid gas-permeable contact lens and its association with cone location. Sudanese J Ophthalmol 2015;7:48-52

How to cite this URL:
Abdu M, Mohd-Ali B, Mohidin N, Binnawi KH. Corrected visual functions in keratoconus patients with rigid gas-permeable contact lens and its association with cone location. Sudanese J Ophthalmol [serial online] 2015 [cited 2021 Jun 19];7:48-52. Available from: https://www.sjopthal.net/text.asp?2015/7/2/48/169436


  Introduction Top


Keratoconus is a noninflammatory degeneration of the central or paracentral cornea. It is characterized by the formation of a localized protrusion (ectasia) and stromal thinning, which is the greatest at the cone apex, and result in increasing irregular astigmatism, which reduces the patient's best corrected visual acuity (VA). [1],[2],[3] The exact etiology of keratoconus is still unknown, but many studies report the association among the condition and atopic diseases, eye rubbing, spring catarrh, genetic inheritance, contact lens wear, and Down's and Marfan's syndromes. [3],[4],[5],[6],[7],[8],[9]

Keratoconus may occur as an isolated entity or as a part of numerous syndromes. Clinically, several characteristic features can be seen using slit lamp in the keratoconic eye such as Munson's and Rizzuti's signs, Fleischer's ring, and Vogt's striae. The clinical, morphological features differ depending on the severity of the condition. Several devices, such as topographers, are currently available for detecting early keratoconus by measuring the anterior corneal contours. Corneal topography is a very useful aid in the diagnosis of keratoconus, especially in the absence of the clinical signs. [10],[11] Many classification schemes for keratoconus have also been proposed. [11],[12],[13] Advanced cases have been morphologically subdivided into round or nipple-shaped cone, oval or sagging cone, and globus. The round cone is smaller (5 mm or less in diameter) and has an apex that lies infer nasally. The oval cone is the most common shape characterized by the displacement of the apex below the midline while the globus form of conical cornea encompasses nearly three quadrants of the corneal surface. [11],[12] Based on the topographic patterns, keratoconus classified as mild, moderate, and advanced. Another scheme of classification divides the conical cornea according to the cone location to the central and paracentral cone. [2],[10],[11],[12],[13]

Spectacles are the initial form of the optical management of keratoconus, especially in its early stages. However, the degree and irregularity of astigmatism increases as the condition progresses. Contact lenses can provide a regular refractive surface, thus eliminating much of irregular astigmatism. For mild or moderate irregularities, soft or soft toric contact lenses can be used, but severe irregularities require rigid gas-permeable (RGP) contact lenses to provide optimum vision by forming a new regular optical surface. [1],[2],[14],[15],[16] Numerous RGP designs have been developed for keratoconus such as Super cone™, Rose K™, and some other alternatives such as hybrid lenses. [14],[16]

Fitting the keratoconic eye with contact lens is one of the big challenges to many of the practitioners due to irregular corneal shape, and the difficulties in the selection of the suitable lens design. Many techniques were demonstrated to achieve acceptable fitting. [17],[18] In this study, we compared corrected visual functions (VA and contrast sensitivity [CS]) between spectacles and RGP contact lenses. In addition, we evaluated the association of corneal cone location with RGP contact lens corrected VA and CS in keratoconus patients.


  Materials and methods Top


This study adhered to the tenets of Declaration of Helsinki and was approved by the Ethical Committee of Universiti Kebangsaan Malaysia. A total of 19 newly diagnosed keratoconus patients were referred to the contact lens clinic at Faculty of Health Sciences, Universiti Kebangsaan Malaysia for contact lens fitting between May 2011 and May 2013. Careful routine eye examination was carried among these subjects, which includes general and ophthalmic history, retinoscopy, best spectacle-corrected VA (BSCVA), best RGP contact lens corrected VA (RGPVA), best spectacle CS (BSCS), and best CS with RGP lenses (RGPCS) using Snellen chart and CS using Pelli-Robson chart. Clinical signs (such as Vogt's striae, Munson's sign, Fleicher's ring, and corneal scar) were evaluated using slit lamp biomicroscope. Corneal topography and corneal astigmatism (CA) were measured using OPD-Scan II ark-10000 Corneal topographer (Nidek CO., Ltd, Japan). The patients with corneal scar, other ecstatic disorders other than keratoconus, and any other ocular abnormalities were excluded from this study.

Following Nejabat et al., [19] corneal cone location was classified to central cone (if the steepest area located in central 3 mm zone) and paracentral cone (if the steepest area falls out of the 3 mm zone) [Figure 1] and [Figure 2]. Furthermore following to the mean keratometric reading (mean K), the patients were divided into mild (mean K <45D), moderate (mean K between 45 and 52D), and severe (mean K more than 52D).
Figure 1: Topographical map showing the central cone location

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Figure 2: Topographical map showing the paracentral cone location

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RGP contact lenses (Rose K2™ and Rose K2IC™ from Menicon, Japan) were used in this study, and the trial lens was selected according to the flat Sim K reading. The white light from the slit lamp was used to evaluate the lens centration and movement, and the fluorescein patterns of the fitting were assessed with cobalt blue light from the same device. The fitting protocol of this study followed the fitting guidelines for keratoconus and irregular corneas set by Menicon Pty. Ltd., Japan. The lens parameters were illustrated in [Table 1].
Table 1: Lens specifications

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Measurements of VA and CS were conducted during contact lens delivery sessions, after 30 min of adaptation to contact lens wear. Statistical analysis software IBM SPSS 20 (IBM Corp., Armonk, NY, USA) was used to evaluate the data obtained. Sample demographics were analyzed using frequencies and simple descriptive tests. The results were analyzed using paired sample t-test, independent sample t-test, Pearson's correlation, and Pearson's Chi-square test. The value of P < 0.05 was considered as statistically significant result.


  Results Top


A total of 19 subjects with keratoconus (34 eyes) participated in this study. All subjects had previously never worn contact lenses. All subjects' age ranged from 15 to 47 years (mean 26.63 ± 8.58 years) of whom 11 (57.9%) were males while 8 (42.1%) were females. Based on the cone location, the eyes were divided into a central cone (16 eyes 47.1%) and paracentral (18 eyes 52.9%) cone groups. According to the mean K readings, only 1 eye (3%) was classified as mild keratoconus (mean K less than 45D), 16 eyes (47%) were found to be at moderate stage (mean K between 45 and 52D), and 17 eyes (50%) were reaching the advanced stage (mean K more than 52D) of the condition. Twenty-one eyes (61.8%) were fitted with Rose K2™ RGP lenses while 13 eyes (38.2%) were fitted using Rose K2 IC™ lenses.

At baseline, mean spherical equivalent found was −5.62 ± 3.62D (range: 0.75 to −16.50D), mean of unaided VA (in decimal) was 0.18 ± 0.16 (range: 0.01-0.70) while mean BSCVA was found to be 0.59 ± 0.33 (range: 0.05-1.20) and mean spectacle contrast sensitivity (SPCS) test results was 1.54 ± 0.33 Log CS (range: 0.30-1.95). With the RGP lenses, the mean best contact lens VA RGPVA was detected as 0.98 ± 0.21 (range 0.50; 1.20), while the mean contrast sensitivity (RGPCS) was 1.78 ± 0.19 Log CS (range 1.35; 1.95). The results were summarized in [Table 2].

An independent sample t-test indicated that there were no statistically significant differences between the central cone and paracentral cone groups in values of Sim K1, Sim K2, mean K, and CA (P > 0.05). Furthermore, no significant differences were detected between BSCVA, SPCS, and the spherical equivalent of refraction in both groups (P > 0.05). The analysis also showed no significant difference in RGPVA and RGPCS between both the groups, P > 0.05. [Table 3] illustrates the means and significant levels between the two groups according to the values of Sim Ks, VAs, CS, and the spherical equivalent of refraction.
Table 2: Mean of vision, refractive status, and topographic data

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Table 3: Means and the significant levels of parameters between study groups

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Paired sample t-test indicated that there was a statistically significant difference between BSCVA (M = 0.59, standard deviation [SD] = 0.33) and RGPVA (M = 0.99, SD = 0.21), t (33) = −7.26, P < 0.001. A significant difference was also detected between SPCS (M = 1.58, SD = 0.25) and RGPCS (M = 1.78, SD = 0.20), t (23) = −3.88, P = 0.001, two-tailed. Statistical analysis using bivariate Person's correlation indicated a negative and moderate correlation between BSCVA and mean K, r (32) = −0.556, P = 0.001. However, no associations were detected between RGPVA and mean K, P > 0.05. On the other hand, a negative but strong correlation was found between SPCS and mean K, r (28) = −0.686, P < 0.001, and moderate negative correlation was detected between RGPCS and mean K, r (31) = −0.421, P = 0.04. Pearson's Chi-square test (with α = 0.05) was used to evaluate the association between cone location and contact lens design. The analysis yielded statistically nonsignificant results (χ2 [2, n = 34] = 1.82, P > 0.05).


  Discussion Top


The contact lens fitting process for keratoconus is often long and difficult for both the patient and the fitter; in addition, it is time-consumption. The first side needs to achieve good vision and comfort with the lens, whereas the fitter needs to find an adequate and suitable lens fit that does not affect and compromise the anterior eye surface and to improve patient's quality of life. [15],[20],[21] Many designs of contact lenses were recently developed to manage the condition; some taking hard nature, and the other manufactured with soft nature to improve lens tolerance. [16] Several studies have been reported a successful correction for keratoconus using different types of RGP lenses in term of marked improving in VA. [22] In the present study, we found that VA and CS of keratoconus patients improved significantly with RGP lenses as compared to spectacles. The results support similar findings from earlier works. [1],[11],[12] It is hypothesized that RGP contact lenses masked most of corneal aberrations induced by keratoconus by replacing the irregular corneal surface with regular refractive surfaces of the RGP lenses and liquid "tear lens" that help to improve vision. [23] Previous researchers have also been suggested that RGP lens design and fitting strategies help to facilitate good visual outcomes in keratoconus patients. [23],[24] The Rose-K™ lens designs used in this study have different curves across the back surface of the lens, and a decreasing optic zone as the base curve steepens which help with the lens alignment on the corneal surface. It is possible that the lens design together with the fitting guide used in this study contributed to the improvement of visual functions of the patients.

The results of this study also showed insignificant differences in topographic and visual functions parameters measured between eyes with central cone and those with paracentral cone location. Nejabat et al. [19] found that keratoconus patients with central cone have higher values in the topographic parameters (Sim K1, Sim K2, and mean K) as compared to patients with paracentral cones. They postulated that the higher central readings are due to cone position on the visual axis as compared to patients with the paracentral cone, where the cone located outside the visual axis and has less effect on K readings. The discrepancies in the present study were probably due to smaller sample size. However, the present results agreed with Nejabat et al. [19] findings which showed insignificant difference in RGPVA and RGPCS between both the groups. The nonparametric analysis in the present study also illustrated no significant association between cone location and lens designs. This indicates that cone location has no effects on corrected visual functions of keratoconus patients.


  Conclusion Top


RGP contact lens wears significantly improved visual functions of keratoconus patients than spectacles. Cone location on keratoconic eyes has no association with the corrected visual functions.

Acknowledgment

Research grants from UKM (INDUSTRI-2011-030) and Menicon Pty. Ltd., Japan.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

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    Figures

  [Figure 1], [Figure 2]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]


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