Cct

Medical Hypotheses (2007) 69, 1205–1207

http://intl.elsevierhealth.com/journals/mehy

Central corneal thickness as a risk factor for glaucoma Alireza Mehdizadeh a, Amin Hoseinzadeh a b c

b,*

, Afsoon Fazelzadeh

c

Medical Physics Research Center, Shiraz University of Medical Sciences (SUMS), Iran Medical Physics Research Center, Mashad University of Medical Sciences (MUMS), Iran School of Medicine, Fasa University of Medical Sciences, Iran

Received 5 December 2006; accepted 7 December 2006

Summary Glaucoma is the second leading cause of blindness worldwide. It has many risk factors such as age, race, sex, intraocular pressure, optic nerve changes, corneal thickness, refractive error, systemic diseases, family history and trauma. Central corneal thickness plays an important role in risk stratification among patients in whom glaucoma was of concern. Scleral thickness and central corneal thickness have a moderately positive correlation. Stress is responsible for glaucoma and causes deformations of the retinal layers and the consequent high levels of neural tissue strain. Stress and scleral thickness have inverse relation together and since scleral thickness is related to central corneal thickness positively, stress is related to central corneal thickness inversely. Risk of progression of glaucoma damage has an inverse relation with the central corneal thickness. This hypothesis is provable via two ways: (1) Based on formula of stress, when central corneal thickness decreases, the amount of stress increases inversely. This leads to more interruption of retinal layers and consequent higher levels of neural tissue strain that increases the risk of suffering glaucoma. (2) Variations of central corneal thickness lead to misestimation of intraocular pressure with Goldmann applanation tonometry. Underestimation of intraocular pressure may lead to underdiagnosis and consequently undertreatment of glaucoma. In conclusion central corneal thickness is a powerful predictor for risk of progression of glaucoma in population. So it should be added to routine clinical examinations for screening of glaucoma. Our hypothesis answers the question about relationship between increased susceptibility to glaucoma damage in patients with thinner corneas. c 2007 Elsevier Ltd. All rights reserved.



Introduction Glaucoma is the second leading cause of blindness worldwide [1]. It has many risk factors such as age, * Corresponding author. Tel.: +98 917 7075754; fax: +98 511 8517505. E-mail address: [email protected] (A. Hoseinzadeh).



race, sex, intraocular pressure (IOP), optic nerve changes, corneal thickness, refractive error, systemic diseases, family history and trauma [2]. The issue of central corneal thickness (CCT) has recently become a topic of much interest as studies such as ocular hypertension treatment study (OHTS) [3], have demonstrated convincingly that CCT plays an important role in risk stratification

0306-9877/$ - see front matter c 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.mehy.2006.12.066

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among patients in whom glaucoma was of concern. Several techniques for measurement of CCT have been described such as optical pachymetry, optical interferometry and ultrasound pachymetry [4]. In a recent meta-analysis of the worldwide literature, the average CCT of normal eyes derived from various racial groups with ultrasound pachymetry was 544 lm [5]. Several studies have shown that there are racial variations in mean CCT [6]. Also some other studies have shown that CCT varies among men and women [7]. CCT and scleral thickness (ST) have a moderately positive correlation [8]. So people with thinner corneas also have thinner scleras. As proposed by authors before [9], stress (r) is responsible for glaucoma. Stress causes deformations of the retinal layers and the consequent high levels of neural tissue strain and is calculated by this formula: r ¼ PR=2t;

ð1Þ

where P is the intraocular pressure, R is the inner radius of the eye globe, and t is scleral thickness. So we can see that stress and ST have inverse relation together and since ST is related to CCT positively, stress is related to CCT inversely.

Hypothesis Risk of progression of glaucoma damage has an inverse relation with the CCT. This hypothesis is provable via two ways: 1. Based on formula of stress (1), when CCT decreases, the amount of stress increases inversely. This leads to more interruption (deformation) of retinal layers and consequent higher levels of neural tissue strain that increases the risk of suffering glaucoma. 2. When Goldmann and Schmidt [10] first described the applanation tonometer, they discussed the effect of CCT on IOP as measured by this device. They assumed a corneal thickness of 500 lm and emphasized that; at least theoretically, corneal thickness might influence applanation readings [11]. We now know that CCT varies greatly among the general population, to a degree that impacts the accuracy of Goldmann applanation tonometry (GAT) in daily practice [12]. These variations of CCT lead to misestimation of IOP with GAT. Eyes with thin corneas have an underestimation of IOP, and eyes with thick corneas have an overestimation of IOP [13]. Underestimation of IOP may lead to underdiagnosis and a false sense of security for the physician and patient and conse-

quently undertreatment of glaucoma. So people with thinner CCTs have also more risk of developing glaucoma via this procedure.

Discussion and conclusion In 2005, Jonas et al. [13] designed a clinical study to evaluate whether the amount of glaucomatous damage at the presentation of the patient and the rate of progression of glaucoma during follow-up are related to CCT. They found that at the time of patient referral, the amount of glaucomatous damage correlated significantly with a thin central cornea. But progression of glaucomatous optic neuropathy during follow-up was independent of CCT, suggesting that CCT may not play a major role in the pathogenesis of glaucomatous damage. They explained their findings by this manner: when referred to hospitals, patients with chronic open-angle glaucoma have more advanced glaucomatous optic nerve damage if the cornea is relatively thin than if the cornea is relatively thick. This may be a selection artifact, since patients with thick corneas may be referred earlier because they have falsely higher IOP measurements than patients with thin corneas [13]. Their result is in contrast to the OHTS [3], in which CCT was a significant risk factor for progression of ocular hypertension to primary openangle glaucoma. In OHTS [3], they found that each 40 lm of decreased corneal thickness correlated with a 1.7 greater risk of glaucoma progression. The researchers surmised that the risk of progressive optic neuropathy in a patient with elevated IOP increased from 6% for a cornea with a thickness of 613 lm to 36% for a cornea of 532 lm. Other studies are available that compare the prevalence of glaucoma between different races [14]. They emphasize that average IOPs of blacks and whites are about the same; yet, the incidence of glaucomatous damage is higher in blacks. Several investigators recently provided further evidence that African–American (black) subjects, as a group, tend to have thinner corneas than their white counterparts [12]. A recent study by Aghaian et al. [6] showed African–Americans to have significantly less CCTs than Asians, Caucasians, and Hispanics. These studies show the relationship between increased susceptibility to glaucoma damage in patients with thinner corneas. In conclusion CCT is a powerful predictor for risk of suffering glaucoma in population. So it should be added to routine clinical examinations for screening of glaucoma. Our hypothesis answers the ques-

Central corneal thickness as a risk factor for glaucoma tion about relationship between increased susceptibility to glaucoma damage in patients with thinner corneas.

Acknowledgement The authors thank Dr Steve Jones Scientific Advisor of the Canon Foundation for Scientific Research (7200 The Quorum, Oxford Business Park, Oxford OX4 2JZ, England). Canon Foundation for Scientific Research funded the page charge of this article.

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