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# New Keratoconus Risk Factors: A Cross-Sectional Case-Control Study

Eloi Debourdeau, Gabriel Planells, Chloe Chamard, David Touboul, Max Villain, Pascal Demoly, Fanny Babeau, Pierre Fournié, Vincent Daien

► To cite this version:

Eloi Debourdeau, Gabriel Planells, Chloe Chamard, David Touboul, Max Villain, et al. New Keratoconus Risk Factors: A Cross-Sectional Case-Control Study. Journal of Ophthalmology, 2022, 2022, pp.6605771. 10.1155/2022/6605771. hal-03930828

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Submitted on 9 Jan 2023

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Distributed under a Creative Commons Attribution 4.0 International License
Research Article

# New Keratoconus Risk Factors: A Cross-Sectional Case—Control Study

Eloi Debourdeau<sup>1,2</sup> Gabriel Planells,<sup>1</sup> Chloe Chamard,<sup>1,2</sup> David Touboul,<sup>3</sup> Max Villain,<sup>1</sup> Pascal Demoly<sup>1,4</sup> Fanny Babeau,<sup>1</sup> Pierre Fournie,<sup>5</sup> and Vincent Daien<sup>1,2,6</sup>

Correspondence should be addressed to Eloi Debourdeau; eloidebourdeau@gmail.com and Vincent Daien; v-daien@chu-montpellier.fr

Received 17 March 2022; Revised 26 July 2022; Accepted 25 August 2022; Published 28 September 2022

Academic Editor: Achim Langenbucher

Copyright © 2022 Eloi Debourdeau et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is
properly cited.

*Purpose*. To evaluate risk factors associated with keratoconus in a monocentric cross-sectional case-control study. *Methods*. This observational study occurred from June 2019 to February 2021 in a university hospital (France). The case group consisted of 195 patients with keratoconus in at least one eye who were followed up by a corneal specialist. The control group consisted of 195 patients without any evidence of keratoconus on slit-lamp examination and corneal topography, who were matched 1:1 to controls by age and sex. Data were collected by a self-completed paper questionnaire before the consultation, and a multivariate logistic regression was performed. *Results*. Multivariate analysis revealed significant associations of keratoconus with family history (odds ratio [OR] = 22.2, *p* < 0.001), rubbing eyes (OR = 10.9, *p* < 0.001), allergy (any kind) (OR = 3.80, *p* < 0.001), smoking exposure (OR = 2.08, *p* = 0.017), and dry eyes (OR = 1.77, *p* = 0.045f). The worst eye was associated with the more rubbed eye (*p* < 0.001) and the more pressed eye during the night according to sleeping position (*p* < 0.041). *Conclusion*. This study confirmed the association between keratoconus and eye rubbing, family history, and allergy. It highlighted the role of pressure on the eyes during sleep. Other less known risk factors such as dry eyes and smoking exposure should be explored in future studies.

**1. Introduction**

Keratoconus is a bilateral and asymmetric corneal disease
[1]. It is characterized by a progressive noninflammatory
ectasia, leading to thinning and protrusion of the cornea,
which assumes a conical shape. It usually results in vision
loss because of refractive errors (irregular astigmatism and
myopia) and corneal opacification. It is a public health issue
because it usually affects young people and leads to social
impairment [2-4]. The disease may progress acutely or be
stable. People at a young age seem to be at a major risk factor
for disease progression [5].

The disease is multifactorial with genetic [6, 7] and
environmental risk factors [8–10]. The genetic contribution
is supported by familial aggregation [11, 12], in particular,
parental consanguinity [13], and monozygotic twins may
have higher ectasia concordance than dizygotic twins [14].
Many genes have been implicated but none alone explains
the genesis of the disease [15].

The most recognized environmental risk factor is eye
rubbing [16–18]. The repeated mechanical pressure on the
cornea causes structural modifications of the collagen,
leading to ectasia [19]. Molecular modifications of tears that
could explain these histologic changes have been reported in

<footnote n="1"><sup>1</sup>Department of Ophthalmology, Gui de Chauliac Hospital, Montpellier F-34000, France</footnote>

<footnote n="2"><sup>2</sup>Institute for Neurosciences of Montpellier INM, Universite Montpellier, INSERM, F-34091 Montpellier, France</footnote>

<footnote n="3"><sup>3</sup>National Center of Keratoconus, CHU Bordeaux, Bordeaux F-33000, France</footnote>

<footnote n="4"><sup>4</sup>Centre Hospitalier Universitaire, Maladies Respiratoires, Hôpital Arnaud de Villeneuve, Cedex 5, Montpellier F-34000, France</footnote>

<footnote n="5"><sup>5</sup>National Center of Keratoconus, CHU Toulouse, Toulouse F-31300, France</footnote>

<footnote n="6"><sup>6</sup>The Save Sight Institute, Sydney Medical School, The University of Sydney, Sydney, NSW 2000, Australia</footnote>
experimental eye rubbings for 60 seconds (increase in levels of metalloproteinases and inflammatory mediators) [20]. This hypothesis is supported by case reports of unilateral or very asymmetric keratoconus when the eye rubbing is unilateral [21–23].

Many ocular surface diseases, leading to itching, have been associated with corneal ectasia. Atopy [24, 25] and allergic conjunctivitis [26, 27] are classical diseases, but many other conditions such as dry eyes [28] and blepharitis [29] have been suspected. Computer vision syndrome [30] is a new entity caused by too much screen time resulting in ocular surface irritation. Studies have found that this syndrome could be associated with keratoconus [28].

Sleeping characteristics in keratoconus patients have been explored. Sleeping position (leading to pressure on the eyes) [23, 28], snoring, and sleep apnea [31, 32] could affect the occurrence of keratoconus, but more investigations are needed. Also, we lack knowledge about the role of the characteristics of eye rubbing in the genesis of keratoconus, such as frequency [11], duration [33], intensity [33], time of the day [23], and part of the hand used for rubbing [28].

This study aimed to more precisely characterize risk factors for keratoconus, especially eye rubbing, in a monocentric population in France.

## 2. Materials and Methods

2.1. *Ethics Statement.* A written informed consent was obtained from each patient in accordance with the 1995 Declaration of Helsinki. The study was approved by the medical ethics committee of the University Hospital of Montpellier (Institutional review board no. 18032020).

2.2. *Design.* This study was a cross-sectional monocentric case-control study performed in Gui de Chauliac University Hospital Center, Montpellier, France, from June 2019 to February 2021.

2.3. *Study Population.* All patients underwent bilateral slit lamp examination and corneal topography using Orbscan II (Placido disc and slit-scanning technology, Bausch and Lomb, Rochester, NY) or Pentacam HR (Scheimpflug imagergy, Oculus, Wetzlar, Germany). The primary outcome (keratoconus diagnosis) was determined by two corneal specialists unaware of the patient's risk factors.

The inclusion criterion for the keratoconus group was clinical or topographic keratoconus in at least one eye. Patients who already had a corneal graft for keratoconus were also included.

Except for grafted eyes, each eye with keratoconus was classified from stage I to stage IV using the modified Amsler–Krumeich classification [34, 35]. For each case, we defined which eye was the worst according to the classification. For cases with grafted eyes with no previous topography available in the medical file, we defined the first eye operated as the worst.

The inclusion criterion for the control group was no clinical or topographic keratoconus in any eye. The control

group was sex and age-matched, with a 2-year tolerance, to the case. Controls were patients consulting an ophthalmologist in our center for reasons other than keratoconus mostly for a routine examination.

Exclusion criteria were doubtful topography or slit lamp examination or refusing participation.

2.4. *Data Source.* Each enrolled patient had to complete a paper questionnaire in the waiting room before an ophthalmologist consultation. This questionnaire was created by the National Center of Keratoconus and is currently subjected to a validation study.

The data concerned demographics (age, sex, social status, and dominant hand), medical history (time since keratoconus diagnosis, family history of keratoconus, allergies, eye diseases, dry eye based on OSDI questionnaire), eye rubbing characteristics (frequency, part of the hand used, duration, intensity, time of the day, causes, noise, etc.), sleeping characteristics (regularity, position, snoring or apnea), and environmental exposure (screen time, weather, tobacco exposure, animals, irritant products, etc.).

2.5. *Statistical Analysis.* Continuous statistics are reported as mean ± SD and median (range) for continuous variables. Categorical variables are reported as numbers (percentages). The nonparametric MannWhitney test was used to compare continuous variables and the chi-square test (or Fisher's exact test as appropriate) to compare categorical variables.

Potential factors associated with keratoconus were compared on univariate analysis (chi-square test or Fisher's exact test). Thereafter, a multivariate logistic model was built based on selected parameters from the univariate analysis (level of significance set at *p* < 0.20 for selection). In addition, three procedures for selecting variables (forward, backward, and stepwise) by the minimization of Akaike's information criterion were used to obtain the most appropriate logistic regression model. The absence of multicollinearity has been verified by using the variance inflation factor. Odds ratios (ORs) and 95% confidence intervals (CIs) were estimated. *p* < 0.05 was considered statistically significant, and all statistical tests were two-sided. Statistical analyses were performed with *R* 3.6.1 (Vienna, Austria). No imputation was used for missing values because less than 1% were missing.

## 3. Results

3.1. *Participants (Figure 1 and Table 1).* We included 195 patients with keratoconus who were followed up by a corneal specialist, and 195 healthy patients were matched on age and sex. Most participants were men (67%), which is similar to the literature [36]. The mean age was 32 ± 12 years. The mean time since diagnosis was 7 ± 9 years.

In the case population, the mean maximum keratometry ($K_{max}$) was 49.1 ± 5 diopters and the thinnest pachymetry was 441 ± 66 microns. In the control population, these parameters were 43.7 ± 1.5 diopters and 543 ± 37 microns. Eyes
<figure><img src="image_3.png" /><figcaption>FIGURE 1: Flow chart.</figcaption></figure>

with corneal grafts were excluded from these analyses. These parameters were all statistically different (p < 0.001) between the two groups.

Of the 390 eyes with keratoconus enrolled, 152 (39%) eyes were at stage I, 85 (22%) at stage II, 75 (19%) at stage III, 35 (9%) at stage IV, and 41 (11%) with corneal graft according to the modified Amsler-Krumeich classification.

### 3.2. Risk Factors of Keratoconus (Tables 2 and 3 and Figure 2).
The first univariate analysis concerned the whole population of cases and controls. In total, 164 (84%) cases and 66 (34%) controls had a history of eye rubbing (p < 0.001) and 20% (n = 39) of cases had a family history of keratoconus (p < 0.001). Eye dryness was significantly associated with keratoconus (p < 0.001). The only sleeping position associated with keratoconus was the contact of the hand or the forearm with the eyes (p = 0.010). Allergies, in particular asthma and allergic conjunctivitis, were associated with the disease (p < 0.001). Also, cases were more frequently

exposed to smoke (tobacco or cannabis) than controls (p < 0.015).

The parameters selected from univariate analysis for a multivariate logistic model were family history, eye rubbing history, allergy (any kind), dry eyes, smoking exposure, screen time, hand or forearm contact with the eyes during sleeping and snoring, or obstructive sleep apnea (OSA).

The multivariate model confirmed the association of keratoconus with family history (OR = 22.2, 95% CI: 5.88–148; p < 0.001), history of rubbing eyes (OR = 10.9, 95% CI: 6.35–19.3; p < 0.001), allergy (any kind) (OR = 3.80, 95% CI: 2.26–6.54; p < 0.001), smoking exposure (OR = 2.08, 95% CI: 1.15–3.85; p = 0.017), and dry eyes (OR = 1.77, 95% CI: 1.02–3.09; p = 0.045).

### 3.3. Analysis of Rubbing Characteristics (Table 4 and Figure 3).
We analyzed rubbing characteristics in the population of people who rubbed their eyes (164 cases and 66 controls). On univariate analysis of the eye rubbing population,
<table><caption>TABLE 1: Population characteristics.</caption><thead><tr><th></th><th>Cases, N = 195</th><th>Controls, N = 195</th><th>p value<sup>1</sup></th></tr></thead><tbody><tr><td>Sex</td><td></td><td></td><td>&gt;0.99</td></tr><tr><td>Men</td><td>130 (67)</td><td>130 (67)</td><td></td></tr><tr><td>Women</td><td>65 (33)</td><td>65 (33)</td><td></td></tr><tr><td>Age</td><td>32 ± 12; 30 (11–80)</td><td>32 ± 12; 30 (10–79)</td><td>0.93</td></tr><tr><td>Keratometry, diopters</td><td></td><td></td><td></td></tr><tr><td>Kmax (mean right and left)</td><td>49.1 ± 5.0; 48.2 (22.0–68.4)</td><td>43.7 ± 1.5; 43.6 (37.8–47.6)</td><td><strong>&lt;0.001</strong></td></tr><tr><td>Missing</td><td>9</td><td>0</td><td></td></tr><tr><td>Kmax right</td><td>49.3 ± 5.1; 48.2 (41.7–71.3)</td><td>43.7 ± 1.6; 43.8 (37.4–47.9)</td><td><strong>&lt;0.001</strong></td></tr><tr><td>Missing</td><td>21</td><td>0</td><td></td></tr><tr><td>Kmax left</td><td>48.7 ± 5.1; 47.7 (40.4–74.2)</td><td>43.6 ± 1.6; 43.7 (38.2–47.4)</td><td><strong>&lt;0.001</strong></td></tr><tr><td>Missing</td><td>22</td><td>0</td><td></td></tr><tr><td>Pachymetry, microns</td><td></td><td></td><td></td></tr><tr><td>Thinnest (mean right and left)</td><td>441 ± 66; 452 (117–572)</td><td>543 ± 37; 544 (385–620)</td><td><strong>&lt;0.001</strong></td></tr><tr><td>Missing</td><td>10</td><td>0</td><td></td></tr><tr><td>Thinnest right</td><td>444 ± 69; 458 (117–572)</td><td>543 ± 37; 544 (380–632)</td><td><strong>&lt;0.001</strong></td></tr><tr><td>Missing</td><td>22</td><td>0</td><td></td></tr><tr><td>Thinnest left</td><td>444 ± 70; 458 (226–582)</td><td>542 ± 41; 545 (269–622)</td><td><strong>&lt;0.001</strong></td></tr><tr><td>Missing</td><td>21</td><td>0</td><td></td></tr><tr><td>Amsler–krumeich classification</td><td></td><td></td><td></td></tr><tr><td>1</td><td>73 (38)</td><td>79 (41)</td><td>152 (39)</td></tr><tr><td>2</td><td>43 (22)</td><td>42 (22)</td><td>85 (22)</td></tr><tr><td>3</td><td>36 (19)</td><td>39 (20)</td><td>75 (19)</td></tr><tr><td>4</td><td>22 (11)</td><td>13 (6.7)</td><td>35 (9)</td></tr><tr><td>Graft</td><td>20 (10)</td><td>21 (11)</td><td>41 (11)</td></tr><tr><td>Time since diagnosis (years)</td><td>7 ± 9; 4 (0–50)</td><td></td><td></td></tr><tr><td>Intracorneal rings</td><td>13 (6.7)</td><td></td><td></td></tr><tr><td>Cross-linking</td><td>53 (27)</td><td></td><td></td></tr></tbody></table>

Data are (%) or mean (SD)/median (range); the ¹chi-square test; and the Wilcoxon rank sum test.

keratoconus was associated with frequency of rubbing “often, 1–10 times a day” (OR = 8.12, 95% CI: 4.13–16.6; $p < 0.001$) and “very often ≥10 times a day” (OR = 52.1, 95% CI: 10.4–951; $p < 0.001$), duration “≥15 sec” (OR = 2.74, 95% CI: 1.16–7.57; $p = 0.032$), “hard” intensity of rubbing (OR = 3.06, 95% CI: 1.28–8.20; $p = 0.017$), and rubbing noise (OR = 1.97, 95% CI: 1.06–3.77; $p = 0.036$). Other associated factors were using knuckles for rubbing (OR = 2.46, 95% CI: 1.35–4.59; $p = 0.004$) and the back of the hand (OR = 2.97, 95% CI: 1.19–9.05; $p = 0.032$) but index fingertips were less often used (compared to other types of friction) as compared with controls (OR = 0.34, 95% CI: 0.18–0.62; $p < 0.001$).

On multivariate analysis, keratoconus remained associated with rubbing “often, 1–10 times a day” (OR = 9.48, 95% CI: 4.62–20.6; $p < 0.001$) and “very often ≥10 times a day” (OR = 57.3, 95% CI: 11.0–1,059; $p < 0.001$). The only way of rubbing significantly associated with multivariate analysis was protective: “use of fingertips” (OR = 0.27, 95% CI: 0.13–0.55; $p < 0.001$).

3.4. “More Advanced Side” Analysis (Table 5). The side of the worst eye was not linked to the dominant hand used for rubbing ($p = 0.43$) but was associated with the preferential eye rubbed ($p < 0.001$) and the side with more mechanical pressure during the night according to the sleeping position ($p = 0.041$).

## 4. Discussion

The main point of our study is that it supports the mechanical etiology of keratoconus. According to the literature, we found a strong association between eye rubbing and multivariate analysis [11, 28, 36, 37]. The association between the preferential eye rubbed and the worst diseased eye confirms a strong link. New risk factors have been described: tobacco, probably due to the dryness and the induced rubbing and the way of rubbing the eye. Using knuckles and the back of the hand for rubbing was associated with keratoconus, with less use of the index fingertips for rubbing as compared with controls; knuckles and the back of the hand expose the eye to harder mechanical power by direct bone contact. The main rubbing characteristic leading to keratoconus in the eye rubbing population was high frequency rather than intensity or duration of rubbing.

The worst diseased eye was associated with more eye rubbing and more pressed eyes during the night, according to the sleeping position. Mazharian et al. [23] also found that patients with unilateral or highly asymmetric keratoconus exhibited homolateral eye rubbing and slept on the same side.

Concerning the sleeping position in the total population, the only statistically positive result was contact with the hand or forearm on the eyes during sleep on univariate analysis only. We did not confirm any correlation with the prone, supine, and side sleep positions or screen time as found in
<table><caption>TABLE 2: Risk factors of keratoconus and univariate analysis.</caption><thead><tr><th></th><th>Cases, N = 195</th><th>Controls, N = 195</th><th>p value<sup>1</sup></th></tr></thead><tbody><tr><td><strong>Rubbing eyes</strong></td><td></td><td></td><td><strong>&lt;0.001</strong></td></tr><tr><td>Active</td><td>108 (55)</td><td>66 (34)</td><td></td></tr><tr><td>Never</td><td>31 (16)</td><td>129 (66)</td><td></td></tr><tr><td>Past: stopped</td><td>56 (29)</td><td>0 (0)</td><td></td></tr><tr><td>Eye rubbing history (past or active)</td><td>164 (84)</td><td>66 (34)</td><td><strong>&lt;0.001</strong></td></tr><tr><td><strong>Family history</strong></td><td>39 (20)</td><td>2 (1.0)</td><td><strong>&lt;0.001</strong></td></tr><tr><td>Dry eyes</td><td>81 (42)</td><td>19 (9.7)</td><td><strong>&lt;0.001</strong></td></tr><tr><td>History of eyes irritative acne treatment (retinoid)</td><td>6 (3.1)</td><td>7 (3.6)</td><td>0.78</td></tr><tr><td>Screen time per day</td><td></td><td></td><td>0.10</td></tr><tr><td>&lt;4 hr</td><td>82 (42)</td><td>98 (50)</td><td></td></tr><tr><td>&ge;4 hr</td><td>113 (58)</td><td>97 (50)</td><td></td></tr><tr><td><strong>Sleeping data</strong></td><td></td><td></td><td></td></tr><tr><td>Snoring or OSA</td><td>41 (21)</td><td>27 (14)</td><td>0.065</td></tr><tr><td>Missing</td><td>0</td><td>1</td><td></td></tr><tr><td>Regular sleep</td><td>112 (61)</td><td>111 (57)</td><td>0.44</td></tr><tr><td>Missing</td><td>11</td><td>0</td><td></td></tr><tr><td>On the back</td><td>50 (26)</td><td>47 (24)</td><td>0.73</td></tr><tr><td>On the side</td><td>131 (67)</td><td>122 (63)</td><td>0.34</td></tr><tr><td>Head in the pillow</td><td>44 (23)</td><td>51 (26)</td><td>0.41</td></tr><tr><td>Hand or forearm on the eye</td><td>20 (10)</td><td>7 (3.6)</td><td><strong>0.010</strong></td></tr><tr><td><strong>Exposure data</strong></td><td></td><td></td><td></td></tr><tr><td>Dry condition or air conditioning</td><td>46 (24)</td><td>61 (31)</td><td>0.083</td></tr><tr><td>Missing</td><td>0</td><td>1</td><td></td></tr><tr><td>Smoking (tobacco or cannabis)</td><td>59 (30)</td><td>38 (20)</td><td><strong>0.015</strong></td></tr><tr><td>Missing</td><td>0</td><td>1</td><td></td></tr><tr><td>Animals</td><td>61 (31)</td><td>68 (35)</td><td>0.43</td></tr><tr><td>Missing</td><td>0</td><td>1</td><td></td></tr><tr><td><strong>Allergy data</strong></td><td></td><td></td><td></td></tr><tr><td>Allergy (any kind)</td><td>124 (64)</td><td>78 (40)</td><td><strong>&lt;0.001</strong></td></tr><tr><td>Missing</td><td>2</td><td>0</td><td></td></tr><tr><td>Asthma</td><td>39 (20)</td><td>15 (7.7)</td><td><strong>&lt;0.001</strong></td></tr><tr><td>Skin allergies</td><td>29 (15)</td><td>23 (12)</td><td>0.37</td></tr><tr><td>Allergic rhinitis or hay fever</td><td>53 (27)</td><td>40 (21)</td><td>0.12</td></tr><tr><td>Allergic conjunctivitis</td><td>78 (40)</td><td>26 (13)</td><td><strong>&lt;0.001</strong></td></tr><tr><td>Angioedema</td><td>7 (3.6)</td><td>1 (0.5)</td><td>0.068</td></tr><tr><td>Drug allergies</td><td>12 (6.2)</td><td>19 (9.7)</td><td>0.19</td></tr><tr><td>Food allergies</td><td>15 (7.7)</td><td>10 (5.1)</td><td>0.30</td></tr></tbody></table>

OSA = obstructive sleep apnea. Data are n (%); the ¹chi-square test; and the Fisher's exact test.

the Moran et al. study [28], nor any already suspected associations with sleep apnea [31, 32], perhaps because of a lack of controls included. The sleeping position could be a risk factor for the pathogenesis of keratoconus itself or for the progression of the disease. The genesis of keratoconus is linked to an anatomical predisposition to thin corneas that are less resistant to mechanical stress. Keratoconus, therefore, could only occur in certain patients who have a predisposition and who apply a mechanical constraint on their eyes during sleep or eye rubbing.

Also, on multivariable analysis, we confirmed classical known associations such as family history [11, 12, 36], dry eyes [28], and allergy [38] at rates similar to the literature.

A 2010 meta-analysis considered that the most important risk factors for keratoconus were eye rubbing, positive family history, allergy, asthma, and eczema [9]. The latter factor was the only one not found in our study, perhaps because we analyzed “skin allergies” in general and not

eczema in particular. In addition, allergic conjunctivitis, which is a classical risk and disease severity factor [26], was also positive in our univariate analysis (p < 0.001).

We found an association between smoking and keratoconus but other studies usually did not [36, 39]. An Australian study in 2021 found a correlation between severity and smoking cigarettes on univariate analysis, which was not confirmed in the multivariate model [40]. Different hypotheses may explain the association between smoking and keratoconus. The dysfunction of the meibomian glands and the dry syndrome induced by smoking could increase friction. The increase in rubbing could also be explained by the personality type of anxiety more frequent in this population. Eye rubbing would decrease the stress felt by the patient via the stimulation of the Vagus X nerve by the oculocardiac reflex.

We did not explain this association and other studies should be conducted to confirm this.
<table><caption>TABLE 3: Risk factors of keratoconus and multivariate logistic regression.</caption><thead><tr><th rowspan="2">Characteristic</th><th colspan="3">Univariate analysis</th><th colspan="3">Multivariate analysis</th></tr><tr><th>OR</th><th>95% CI</th><th>p value</th><th>OR</th><th>95% CI</th><th>p value</th></tr></thead><tbody><tr><td>Family history</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>No</td><td>—</td><td>—</td><td></td><td>—</td><td>—</td><td></td></tr><tr><td>Yes</td><td>23.4</td><td>7.02-145</td><td><strong>&lt;0.001</strong></td><td>22.2</td><td>5.88-148</td><td><strong>&lt;0.001</strong></td></tr><tr><td>Eye rubbing history</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>No</td><td>—</td><td>—</td><td></td><td>—</td><td>—</td><td></td></tr><tr><td>Yes</td><td>10.5</td><td>6.54-17.4</td><td><strong>&lt;0.001</strong></td><td>10.9</td><td>6.35-19.3</td><td><strong>&lt;0.001</strong></td></tr><tr><td>Allergy (any kind)</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>No</td><td>—</td><td>—</td><td></td><td>—</td><td>—</td><td></td></tr><tr><td>Yes</td><td>2.77</td><td>1.84-4.20</td><td><strong>&lt;0.001</strong></td><td>3.80</td><td>2.26-6.54</td><td><strong>&lt;0.001</strong></td></tr><tr><td>Dry eyes</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>No</td><td>—</td><td>—</td><td></td><td>—</td><td>—</td><td></td></tr><tr><td>Yes</td><td>2.45</td><td>1.58-3.83</td><td><strong>&lt;0.001</strong></td><td>1.77</td><td>1.02-3.09</td><td><strong>0.045</strong></td></tr><tr><td>Smoking exposure (tobacco or cannabis)</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>No</td><td>—</td><td>—</td><td></td><td>—</td><td>—</td><td></td></tr><tr><td>Yes</td><td>1.86</td><td>1.16-2.99</td><td><strong>0.010</strong></td><td>2.08</td><td>1.15-3.85</td><td><strong>0.017</strong></td></tr><tr><td>Screen time per day</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>&lt;4 hr</td><td>—</td><td>—</td><td></td><td>—</td><td>—</td><td></td></tr><tr><td>≥4 hr</td><td>1.43</td><td>0.96-2.13</td><td>0.083</td><td>1.54</td><td>0.91-2.61</td><td>0.11</td></tr><tr><td>Hand or forearm on the eye (sleeping position)</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>No</td><td>—</td><td>—</td><td></td><td>—</td><td>—</td><td></td></tr><tr><td>Yes</td><td>3.04</td><td>1.31-7.90</td><td><strong>0.014</strong></td><td>—</td><td>—</td><td></td></tr><tr><td>Snoring or OSA</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>No</td><td>—</td><td>—</td><td></td><td>—</td><td>—</td><td></td></tr><tr><td>Yes</td><td>1.61</td><td>0.95-2.77</td><td>0.082</td><td>—</td><td>—</td><td></td></tr></tbody></table>

CI = confidence interval, OR = odds ratio, and OSA = obstructive sleep apnea. Data are n (%) or odds ratio (OR) and 95% confidence interval (CI). Odds ratios were calculated by univariate and multivariate logistic regression. The multivariate model was created with the variables with *p* < 0.20 in the univariate analysis with a method of eliminating variables by using the corrected Akaike information criterion. A generalized variance inflation factor was used to check for multicollinearity.

<figure><img src="image_4.png" /></figure>

1 2 5 10 20 50 100

<caption>FIGURE 2: Forest plot of multivariate analysis of risk factors in the general population. Graphical representation of the odds ratio of the risk factors of keratoconus in the general population from the multivariate linear regression.</caption>

Our study has some weaknesses and is prone to bias. First, the data were obtained from a questionnaire. Hence, there is a risk of reporting bias depending on the precision of the questions, the knowledge of patients, and their willingness to cooperate. For example, the fact that cases

were aware of the association between keratoconus and eye rubbing may have influenced their response. Keratoconus is an acquired disorder, and we selected our control group with normal topographic and clinical criteria at the time of inclusion. Because of no follow-up,
<table><caption>TABLE 4: Risk factors of keratoconus in the eye rubbing population and multivariate logistic regression.</caption><thead><tr><th rowspan="2">Characteristic</th><th rowspan="2">Cases N = 164</th><th rowspan="2">Controls N = 66</th><th colspan="3">Univariate analysis</th><th colspan="3">Multivariate analysis</th></tr><tr><th>OR</th><th>95% CI</th><th>p value</th><th>OR</th><th>95% CI</th><th>p value</th></tr></thead><tbody><tr><td><strong>Rubbing frequency</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Once/day maximum</td><td>97 (59)</td><td>17 (26)</td><td>—</td><td>—</td><td></td><td>—</td><td>—</td><td></td></tr><tr><td>Often, 1–10 times/day</td><td>32 (20)</td><td>48 (73)</td><td>8.12</td><td>4.13–16.6</td><td><strong>&lt;0.001</strong></td><td>9.48</td><td>4.62–20.6</td><td><strong>&lt;0.001</strong></td></tr><tr><td>Very often, &gt;10 times/day</td><td>35 (21)</td><td>1 (1.5)</td><td>52.1</td><td>10.4–951</td><td><strong>&lt;0.001</strong></td><td>57.3</td><td>11.0–1,059</td><td><strong>&lt;0.001</strong></td></tr><tr><td><strong>Rubbing duration</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>&lt;15 sec</td><td>34 (22)</td><td>6 (9.2)</td><td>—</td><td>—</td><td></td><td>—</td><td>—</td><td></td></tr><tr><td>≥15 sec</td><td>121 (78)</td><td>59 (91)</td><td>2.74</td><td>1.16–7.57</td><td><strong>0.032</strong></td><td></td><td></td><td></td></tr><tr><td><strong>Intensity of rubbing</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Superficial</td><td>59 (36)</td><td>32 (48)</td><td>—</td><td>—</td><td></td><td>—</td><td>—</td><td></td></tr><tr><td>Moderate</td><td>62 (38)</td><td>27 (41)</td><td>1.15</td><td>0.61–2.18</td><td>0.67</td><td></td><td></td><td></td></tr><tr><td>Hard</td><td>43 (26)</td><td>7 (11)</td><td>3.06</td><td>1.28–8.20</td><td><strong>0.017</strong></td><td></td><td></td><td></td></tr><tr><td><strong>Rubbing noise</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>No</td><td>92 (56)</td><td>48 (73)</td><td>—</td><td>—</td><td></td><td>—</td><td>—</td><td></td></tr><tr><td>Yes</td><td>71 (44)</td><td>18 (27)</td><td>1.97</td><td>1.06–3.77</td><td><strong>0.036</strong></td><td></td><td></td><td></td></tr><tr><td><strong>Type of rubbing</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td><strong>Index fingertips</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>No</td><td>103 (63)</td><td>24 (36)</td><td>—</td><td>—</td><td></td><td>—</td><td>—</td><td></td></tr><tr><td>Yes</td><td>61 (37)</td><td>41 (63)</td><td>0.34</td><td>0.18–0.62</td><td><strong>&lt;0.001</strong></td><td>0.27</td><td>0.13–0.55</td><td><strong>&lt;0.001</strong></td></tr><tr><td><strong>The palm of the hand</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>No</td><td>138 (84)</td><td>54 (82)</td><td>—</td><td>—</td><td></td><td>—</td><td>—</td><td></td></tr><tr><td>Yes</td><td>26 (16)</td><td>11 (17)</td><td>0.89</td><td>0.41–2.01</td><td>0.77</td><td></td><td></td><td></td></tr><tr><td><strong>Knuckles</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>No</td><td>73 (45)</td><td>44 (67)</td><td>—</td><td>—</td><td></td><td>—</td><td>—</td><td></td></tr><tr><td>Yes</td><td>91 (55)</td><td>21 (32)</td><td>2.46</td><td>1.35–4.59</td><td><strong>0.004</strong></td><td></td><td></td><td></td></tr><tr><td><strong>Thumb and index clamp</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>No</td><td>141 (86)</td><td>56 (85)</td><td>—</td><td>—</td><td></td><td>—</td><td>—</td><td></td></tr><tr><td>Yes</td><td>23 (14)</td><td>9 (14)</td><td>0.91</td><td>0.40–2.22</td><td>0.83</td><td></td><td></td><td></td></tr><tr><td><strong>Back of the hand</strong></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>No</td><td>132 (80)</td><td>59 (89)</td><td>—</td><td>—</td><td></td><td>—</td><td>—</td><td></td></tr><tr><td>Yes</td><td>32 (20)</td><td>6 (9)</td><td>2.97</td><td>1.19–9.05</td><td><strong>0.032</strong></td><td></td><td></td><td></td></tr></tbody></table>

OR = odds ratio and CI = confidence interval. Data are n (%) or odds ratio (OR) and 95% confidence interval (CI). Odds ratios were calculated by univariate and multivariate logistic regression. The multivariate model was created with the variables with *p* < 0.20 in the univariate analysis with a method of eliminating variables by using the corrected Akaike information criterion. A generalized variance inflation factor was used to check for multicollinearity.

<figure><img src="image_5.png" /><figcaption>FIGURE 3: Forest plot of multivariate analysis of risk factors in the eye rubbing population. Graphical representation of the odds ratio of the risk factors of keratoconus in the eye rubbing population from the multivariate linear regression.</figcaption></figure>
<table><caption>TABLE 5: Analysis of eye for more advanced disease.</caption><thead><tr><th rowspan="2">Laterality</th><th colspan="2">Eye with more advanced disease</th><th rowspan="2">p value<sup>1</sup></th></tr><tr><th>Left, N = 88</th><th>Right, N = 107</th></tr></thead><tbody><tr><td>Dominant hand</td><td></td><td></td><td>0.43</td></tr><tr><td>Left</td><td>12 (14)</td><td>19 (18)</td><td></td></tr><tr><td>Right</td><td>76 (86)</td><td>88 (82)</td><td></td></tr><tr><td>Preferential eye rubbed</td><td></td><td></td><td><strong>&lt;0.001</strong></td></tr><tr><td>No preferential rubbing</td><td>59 (67)</td><td>65 (61)</td><td></td></tr><tr><td>Left</td><td>20 (23)</td><td>10 (9.3)</td><td></td></tr><tr><td>Right</td><td>9 (10)</td><td>32 (30)</td><td></td></tr><tr><td>Press on the eye during sleeping relative to the sleeping position</td><td></td><td></td><td><strong>0.041</strong></td></tr><tr><td>Both or none</td><td>59 (67)</td><td>60 (56)</td><td></td></tr><tr><td>Left</td><td>14 (16)</td><td>12 (11)</td><td></td></tr><tr><td>Right</td><td>15 (17)</td><td>35 (33)</td><td></td></tr></tbody></table>

Data are *n* (%). ¹ the chi-square test.

some controls could have exhibited keratoconus during the study, which could imply selection bias. We controlled that bias by excluding all patients with doubtful corneal topography.

## 5. Conclusion

Our study confirmed the most important risk factors for keratoconus: eye rubbing, family history, and allergy. It allows for more understanding of the eyes rubbing habits of our patients by detailing many of the rubbing characteristics associated with the genesis of corneal ectasia. We also found that the sleeping position could play a key role in the pathophysiology of keratoconus. We recommend that all patients be systematically screened for inappropriate sleeping positions to identify the use of night-time eye protection if necessary. These elements are crucial in care because we can use them to advise our patients and help them avoid their habits. Indeed, helping people to stop eye rubbing could be an effective treatment on its own to stop the progression of ectasia. In addition, it can be a target for primary prevention by educating the population to not adopt these behaviors.

## Data Availability

The dataset analyzed during the current study is available from the corresponding author on reasonable request.

## Ethical Approval

This study was approved by the ethics committee of Montpellier University Hospital (Institutional review board no. 18032020) in accordance with the Declaration of Helsinki and all participants have given their written informed consent.

## Conflicts of Interest

The authors declare that there are no conflicts of interest.

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