Keratoconus (Cone Shaped Cornea)
The cornea is the clear front surface of the eye. Have you ever wondered about the significance of a Cone-Shaped Cornea is, and how it could affect vision? The word keratoconus comes from the Greek words ‘keras’, meaning cornea, and ‘conus’, meaning cone, which together means ‘cone- shaped’ cornea.
Keratoconus affects all ethnicities and both sexes. The highest rates typically occur in 20- to 30-year-olds, but it is fairly common for it to be found in adolescents. It often develops in the 2nd and 3rd decades of life and tends to progress until the 4th decade. While it is commonly found as an isolated eye condition, it sometimes can coexists with other eye and multi-system diseases.
This disease is more aggressive in children than in adults and can have debilitating consequences for their vision as the condition deteriorates. Given its onset usually during their formative adolescent/adult years keratoconus can fundamentally alter the psychosocial profile of individuals. Even if their visual acuity can be corrected, patients with keratoconus are still likely to experience significant impact on their quality of life (1). The combined visual deterioration and psychological stress may debilitate patients from achieving their academic potential and contributing meaningful to their communities and economy.
Keratoconus is considered a bilateral and asymmetric (i.e. one eye is typically more severely affected than the other) (2-6) eye disease which results in the progressive thinning and steepening of the cornea leading to irregular astigmatism (i.e. irregular contouring of the cornea along its surface, affecting the way in which light enters the eye) and reduced visual acuity (i.e. less “seeing power” for the eye) (7-9). Images may also appear distorted, and the eyes may become more sensitive to glare and light.
Our understanding of the mechanism behind the development of keratoconus remains limited. The interplay between genetic and environmental factors have been associated with the cause and progression of this disease. Keratoconus progresses because of a combination of simultaneously occurring destructive and healing processes.
- Family History and Genetics: It has been estimated that a relative of an individual with keratoconus has up to 67x greater risk of developing keratoconus than an individual with no family history of keratoconus (10). Certain genetic conditions are also known to predispose to Keratoconus, including Down’s Syndrome (11)and Leber Congenitial Amaurosis (12).
- Protein Balance: When the proteins in the cornea are produced in the incorrect proportions compared to a normal health cornea, the cornea may be more susceptible to the damage and coning (13). Equally, when the important proteins (e.g. collagen) in cornea are damaged, often under the oxidative stresses that the cells in the cornea are subjected to, further surface irregularities can follow. A common finding in keratoconus is the loss of collagen in the cornea.
- Environmental Stresses: Persistent eye rubbing has been associated with exaggerating keratoconus, especially those with genetic predisposition (14-16), but stronger evidence from larger studies is required to support this in future studies. It is believed that persistent eye rubbing and hard contact lens wear can trigger the cells of the cornea to undergo their repair mechanisms as a defence to the persistent mechanical contact. These repair mechanisms may change the balance of collagen and other proteins in the cornea to contribute to the progression of Keratoconus (17).
At present, since is no definitive cure for keratoconus, optometrists and ophthalmologists work together to revive the visual acuity and delay the development of the disease. Treatment varies depending on disease severity and progression. Milder cases are typically treated with spectacles. Moderate cases are treated with special types of contact lenses (e.g. softer lenses, hybrid lenses) that are less hostile to the cells in the cornea. It can be particularly difficult to treat keratoconus with contact lenses because of its asymmetrical nature and its ongoing progression.
The best available contact lenses for advanced keratoconus cases are called scleral lenses and require customised fitting. These lens are very expensive and unattainable in South Africa’s public healthcare system – Eyes2Eyes run a specialised programme that raises money and procures specially-fitted scleral lenses for patients with advanced Keratoconus, as solutions in the South African public healthcare system are not currently funded. In recent years, as methods of imaging the front of the eye have improved, scleral lens prescribing has increased (18, 19) including as a first-choice for healthy eyes with ocular surface disease or high regular astigmatism. Severe keratoconus cases that do not resolve with scleral contact lenses may require corneal surgery. These surgeries include corneal-crosslinking, toric intra-ocular lens implantation and transplantation (full thickness penetrating keratoplasty or partial thickness deep anterior lamellar keratoplasty).
References
- Yung M, Mannis MJ. Chapter 12 – Psychology of Keratoconus. In: Izquierdo L, Henriquez M, Mannis M, editors. Keratoconus. New Delhi: Elsevier; 2023. p. 169-76.
- Nichols JJ, Steger-May K, Edrington TB, Zadnik K. The relation between disease asymmetry and severity in keratoconus. Br J Ophthalmol. 2004;88(6):788-91.
- Burns DM, Johnston FM, Frazer DG, Patterson C, Jackson AJ. Keratoconus: an analysis of corneal asymmetry. Br J Ophthalmol. 2004;88(10):1252-5.
- Jones-Jordan LA, Walline JJ, Sinnott LT, Kymes SM, Zadnik K. Asymmetry in keratoconus and vision-related quality of life. Cornea. 2013;32(3):267-72.
- Chopra I, Jain AK. Between eye asymmetry in keratoconus in an Indian population. Clin Exp Optom. 2005;88(3):146-52.
- Zadnik K, Steger-May K, Fink BA, Joslin CE, Nichols JJ, Rosenstiel CE, et al. Between-eye asymmetry in keratoconus. Cornea. 2002;21(7):671-9.
- Li X, Rabinowitz YS, Rasheed K, Yang H. Longitudinal study of the normal eyes in unilateral keratoconus patients. Ophthalmology. 2004;111(3):440-6.
- Zadnik K, Barr JT, Gordon MO, Edrington TB. Biomicroscopic signs and disease severity in keratoconus. Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study Group. Cornea. 1996;15(2):139-46.
- Kennedy RH, Bourne WM, Dyer JA. A 48-year clinical and epidemiologic study of keratoconus. Am J Ophthalmol. 1986;101(3):267-73.
- Wang Y, Rabinowitz YS, Rotter JI, Yang H. Genetic epidemiological study of keratoconus: evidence for major gene determination. Am J Med Genet. 2000;93(5):403-9.
- Mathan JJ, Gokul A, Simkin SK, Meyer JJ, Patel DV, McGhee CNJ. Topographic screening reveals keratoconus to be extremely common in Down syndrome. Clin Exp Ophthalmol. 2020;48(9):1160-7.
- Elder MJ. Leber congenital amaurosis and its association with keratoconus and keratoglobus. J Pediatr Ophthalmol Strabismus. 1994;31(1):38-40.
- Yam GH, Fuest M, Zhou L, Liu YC, Deng L, Chan AS, et al. Differential epithelial and stromal protein profiles in cone and non-cone regions of keratoconus corneas. Sci Rep. 2019;9(1):2965.
- Lindsay RG, Bruce AS, Gutteridge IF. Keratoconus associated with continual eye rubbing due to punctal agenesis. Cornea. 2000;19(4):567-9.
- Sahebjada S, Al-Mahrouqi HH, Moshegov S, Panchatcharam SM, Chan E, Daniell M, et al. Eye rubbing in the aetiology of keratoconus: a systematic review and meta-analysis. Graefes Arch Clin Exp Ophthalmol. 2021;259(8):2057-67.
- Yeniad B, Alparslan N, Akarcay K. Eye rubbing as an apparent cause of recurrent keratoconus. Cornea. 2009;28(4):477-9.
- McMonnies CW. Mechanisms of rubbing-related corneal trauma in keratoconus. Cornea. 2009;28(6):607-15.
- Vincent SJ. The rigid lens renaissance: A surge in sclerals. Cont Lens Anterior Eye. 2018;41(2):139-43.
- Woods CA, Efron N, Morgan P. Are eye‐care practitioners fitting scleral contact lenses? Clinical and Experimental Optometry. 2020;103(4):449-53.
