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Brief report on distribution of CCT across population

by | Oct 1, 2020 | Manuscripts | 0 comments

Prantik Aditya, Bachelor in Clinical Optometry

Fellow Optom, Dr. Shroff Charity Eye Hospital, Delhi, India

 

The cornea is the clear front surface of the optical system of the eye, which allows light to enter the eye for vision. It provides approximately 65 to 75 percent of the focusing power of the eye. The Central thickness of the cornea can be measured indirectly by Pachymetry (1). The normal central corneal thickness (CCT) is around 510–520 microns (µ) (2). It is measured using optical or ultrasound methods. Thicker corneas are associated with higher intraocular pressures (IOPs) (3) due to increase in resistance to indentation and vice versa in thin corneas. Myopic patient have thinner CCT then hypermetropia and emmetropes patient it may be because of longer axial length following corneo-scleral stretch thus making sclera thinner, and so the corneal stroma will also become thinner in similar way during myopic progression (4).

Other factors such as race, age, sex, obesity, systemic diseases such as Diabetic mellitus, Hypertension, Thyroid also affects CCT (5).

CCT & Race: CCT was assumed to be a constant but subsequently found that it varies (6). In an unpublished study done in our hospital, we found out that the mean CCT of the North Indian Population (sample size: 1635) is 525.27µ. While in Central India it is seen that mean CCT is 514 ± 33µ and in South India is found to be 511 ± 33µ. Among the central Asian the mean CCT was found to be comparable: 552 ± 33µ in Chinese, 540 ± 33µ in Malays, and 540±33µ in Indians was seen in a study (7, 8).

CCT & Gender: Females have shorter axial length and thus steeper cornea than males and so lower CCT than males (9).

CCT & IOP: Increase in CCT leads to increase in IOP also. Previous analyse suggest that every 10 µ change in CCT leads to 0.28unit increase in IOP in Goldmann Applanation Tonometry (10). Glaucoma, the silent killer of eye increased IOP is one of the major cause for it. However increased in IOP cannot always leads to diagnosis of glaucoma.

An increase in collagen fibres and a consequent increase in the thickness of the cornea (11, 12). Corneal endothelial deturgescense mechanism secondarily causing corneal thickening (oedema) can also cause higher IOP (11, 12).

Figure 1:  Performing Ultrasound Pachymetry

CCT & Age: Further CCT also changes with age (13). CCT decreases with age (14) as suggested in earlier studies. According to a study done, a 10-year increase in age would lead to approximately a 7.0 µdecrease in CCT (15). It is because keratocytes, the major cellular components of the cornea stroma decreases with age and that the collagen fibers are broken down as part of the normal aging process and thus resulting in lower CCT with age (16).With aging there is reduced production of aqueous humour which leads to reduction of IOP (17) and structural changes in trabecular meshwork which increases the resistance to aqueous outflow, increasing IOP (18).

CCT & Medication: Patient taking anti-glaucoma medication and topical prostaglandin can also reduce CCT thus making the cornea thin and giving over or under estimated IOP readings (19).

Take Home Message:

Measuring CCT can be an important factor in diagnosing of many corneal related complication as well as glaucoma.

Reference:

  1. Kremer FB, Walton P, Gensheimer G. Determination of corneal thickness using ultrasonic pachometry. Annals of ophthalmology (Birmingham). 1985;17(8):506-7.
  2. Mishima S. Corneal thickness. Surv Ophthalmol. 1968;13:57-96.
  3. Doughty MJ, Zaman ML. Human corneal thickness and its impact on intraocular pressure measures: a review and meta-analysis approach. Survey of ophthalmology. 2000 Mar 1;44(5):367-408.
  4. Divya K, Ganesh MR, Sundar D. Relationship between myopia and central corneal thickness–A hospital based study from South India. Kerala Journal of Ophthalmology. 2020 Jan 1;32(1):45.
  5. Varghese MJ. Familial hypercholesterolemia: A review. Annals of pediatric cardiology. 2014 May;7(2):107.
  6. Shimmyo M, Ross AJ, Moy A, Mostafavi R. Intraocular pressure, Goldmann applanation tension, corneal thickness, and corneal curvature in Caucasians, Asians, Hispanics, and African Americans. American journal of ophthalmology. 2003 Oct 1;136(4):603-13.
  7. Nangia V, Jonas JB, Sinha A, Matin A, Kulkarni M. Central corneal thickness and its association with ocular and general parameters in Indians: the Central India Eye and Medical Study. Ophthalmology. 2010 Apr 1;117(4):705-10.
  8. Vijaya L, George R, Arvind H, Ramesh SV, Baskaran M, Raju P, Asokan R, Velumuri L. Central corneal thickness in adult south Indians: The Chennai glaucoma study. Ophthalmology. 2010 Apr 1;117(4):700-4.
  9. Iyamu E, Memeh M. The association of central corneal thickness with intra-ocular pressure and refractive error in a Nigerian population. Online Journal Of Health And Allied Sciences. 2008 Jan 24;6(3).
  10. Tonnu PA, Ho T, Newson T, El Sheikh A, Sharma K, White E, Bunce C, Garway-Heath D. The influence of central corneal thickness and age on intraocular pressure measured by pneumotonometry, non-contact tonometry, the Tono-Pen XL, and Goldmann applanation tonometry. British Journal of Ophthalmology. 2005 Jul 1;89(7):851-4.
  11. Kniestedt C, Lin S, Choe J, Nee M, Bostrom A, Stürmer J, Stamper RL. Correlation between intraocular pressure, central corneal thickness, stage of glaucoma, and demographic patient data: prospective analysis of biophysical parameters in tertiary glaucoma practice populations. Journal of Glaucoma. 2006 Apr 1;15(2):91-7.
  12. Nemesure B, Wu SY, Hennis A, Leske MC. Corneal thickness and intraocular pressure in the Barbados eye studies. Archives of ophthalmology. 2003 Feb 1;121(2):240-4.

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