August 2019

ASCRS NEWS

A review of “Comparison of effect of eye rubbing in keratoconus and healthy eyes using Scheimpflug analysis and a dynamic bidirectional applanation device”


by Meleha Ahmad, MD, Pujan Dave, MD, Inas Aboobakar, MD, Ishrat Ahmed, MD, and Michael Boland, MD


Michael Boland, MD

Associate residency
program director
Wilmer Eye Institute
Johns Hopkins University
School of Medicine
Baltimore


Wilmer Eye Institute residents, from left: Pujan Dave, MD, Meleha Ahmad, MD, Ishrat Ahmed, MD, and Inas Aboobakar, MD
Source: Wilmer Eye Institute

Keratoconus is the most common corneal ectasia and one of the leading indications for corneal transplantation in developed countries.1 It is characterized by thinning of the central cornea, which leads to myopia, irregular astigmatism, and potentially significant visual impairment.2 In addition to genetic predisposition, environmental factors are thought to contribute to disease development, with eye rubbing being the strongest known environmental risk factor.3 The precise mechanism whereby eye rubbing contributes to keratoconus development remains unknown, though possible explanations include progressive epithelial thinning, increasing concentration of inflammatory mediators, increased corneal temperature, and changes to keratocytes in response to mechanical trauma.4 In this article, the authors seek to further investigate the role of eye rubbing in keratoconus by assessing corneal biomechanical changes in keratoconus eyes compared to healthy eyes using the Ocular Response Analyzer (ORA, Reichert Technologies). They also assess changes in corneal tomography as measured by Scheimpflug imaging in response to eye rubbing in keratoconus patients and controls.

Study summary

Henriquez et al. undertook a comparative study to compare the effect of a specific eye rubbing procedure on various corneal and anterior segment metrics in eyes with and without keratoconus. The study consisted of 30 healthy eyes (mean age 27.2 years) and 31 eyes with keratoconus (mean age 27.8 years), with one eye per subject randomly selected. The healthy eyes had best corrected visual acuity of at least 20/20 without significant refractive error (<1.5 D), and the eyes with keratoconus had signs of disease on clinical exam as well as on corneal topography and a history of eye rubbing. Subjects with Vogt striae, a non-clear cornea, contact lens use within 3 months, and topical medication use were excluded.
Eye rubbing was performed by a researcher masked to the underlying study group. The researcher used the right index finger to apply 5–6 Newtons (about 1.25 pounds) of force while moving their finger in a clockwise, circular motion over a closed eyelid with the subject in primary gaze. This was performed twice, 1 minute at a time, with a 5-second break in between. Tomography and corneal biomechanics were measured using a Scheimpflug camera and the ORA. These tests were performed less than 1 minute apart before, immediately after, 7 minutes after, and 14 minutes after standardized eye rubbing.
Corneal imaging was used to measure a variety of parameters and found three that were statistically different in eyes with keratoconus but not in healthy eyes. Immediately after eye rubbing, there was an increase in posterior corneal astigmatism (p=0.007, +0.14 D), decrease in anterior chamber volume (p<0.001, –5.1 microL), and a decrease in Goldmann-correlated IOP (p<0.001, –1.61 mm Hg). The decrease in IOP was the only change that persisted at 7 minutes (p=0.004, –1.33 mm Hg). Healthy eyes showed an increase in the anterior steepest keratometry (p=0.01, 0.07 D) immediately after eye rubbing, while keratoconus eyes did not. Five keratoconus eyes had Scheimpflug analysis performed at a mean of 8.8 months after eye rubbing, at which time 2 eyes had signs of progression. Finally, the group also correlated pre-eye rubbing metrics to these variables that exhibited statistically significant changes to eye rubbing to determine whether there was any positive correlation (i.e., if any metrics could predict changes due to eye rubbing). The only positive correlation was between pre-eye rubbing maximum keratometry and change in posterior astigmatism (r=0.37, p=0.043).

Discussion

Eye rubbing is thought to be a significant factor in the development of keratoconus and has also been shown to increase the progression of keratoconus once it develops.5 However, no study to date has investigated the effect of eye rubbing on mechanical parameters of the cornea in eyes with a prior diagnosis of keratoconus. The present study is the first to evaluate the effect of eye rubbing on biomechanical parameters of the anterior segment in patients with keratoconus. Further understanding of these factors might help us to understand the effect of eye rubbing in keratoconus progression.
After a standardized session of eye rubbing, keratoconus eyes were found to have a significant, but transient, increase in posterior corneal astigmatism and decrease in anterior chamber volume, which was not present in healthy eyes after eye rubbing. In addition, keratoconus eyes had a significant decrease in IOPG after standardized eye rubbing, which persisted at least 7 minutes after eye rubbing had been completed. These findings are interesting because they provide further physiologic evidence that keratoconus corneas might be weaker than healthy corneas. In addition, it may provide further insight into the pathogenesis of this disease.
One major strength of this study is the inclusion of a control group that was similar in age and sex distribution. A lack of change in the posterior corneal astigmatism and AC volume in healthy eyes lends further credence to the changes observed in those with keratoconus. However, inclusion of information on baseline characteristics would have been extremely helpful to further put these findings into context. It would have been interesting to compare baseline refractive error, axial eye length, and central corneal thickness in the keratoconus and healthy group, to see how comparable these two groups are.
The main limitation of this study is the fact that measurements were done in a group that already displayed signs of keratoconus. The findings may therefore not reflect corneal biomechanics that may lead to keratoconus but instead only those present once disease is manifest. An additional limitation is the baseline variability in anterior chamber volume and IOPG in both the healthy and the keratoconus groups, as well as posterior corneal astigmatism in the keratoconus group. This variability may be masking significant differences between time points for the variables emphasized in this study. The table in the study reveals a wider range for several variables in the keratoconus group, including anterior astigmatism, suggesting additional variables may be identified in a sufficiently powered study.
Future studies, if adequately powered, would also allow for additional stratification based on patient characteristics, including severity of astigmatism, refractive error, baseline IOPG, and even the presence of genetic mutations. These analyses could identify factors that increase the risk of keratoconus progression in the setting of eye rubbing in specific keratoconus populations.
It was noted in the study that 2 of the 5 keratoconus eyes that had follow-up Scheimpflug analysis demonstrated disease progression. However, further description of parameters of those two eyes that progressed were not provided. It would be interesting to monitor more subjects over time to help identify factors that predict progression of keratoconus and, in particular, whether response to eye rubbing in keratoconus eyes could provide any additional barometer of likelihood of progression in these patients.
The study results show that eye rubbing affects keratoconus eyes in a different way than healthy eyes and further postulate that specifically the changes in the posterior cornea may explain why eye rubbing is considered a risk factor for development of keratoconus.

I invited the Wilmer residents to review this August JCRS study that seeks to compare and quantify corneal biomechanical changes caused by eye rubbing in patients with and without keratoconus.

—David F. Chang, MD
EyeWorld Journal
Club Editor


Contact information

Boland
: boland@jhu.edu

References

1. Gordon-Shaag A, et al. The genetic and environmental factors for keratoconus. BioMed Res Int. 2015;2015:795738.
2. Sugar J, Macsai MS. What causes keratoconus? Cornea. 2012;31:716–9.
3. Davidson AE, et al. The pathogenesis of keratoconus. Eye. 2014;28:189–95.
4. McMonnies CW. Mechanisms of rubbing-related corneal trauma in keratoconus. Cornea. 2009;28:607–15.
5. McMonnies CW. Abnormal rubbing and keratectasia. Eye Contact Lens. 2007;33:265–71.

A review of “Comparison of effect of eye rubbing in keratoconus and healthy eyes using Scheimpflug analysis and a dynamic bidirectional applanation device” A review of “Comparison of effect of eye rubbing in keratoconus and healthy eyes using Scheimpflug analysis
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