Keratoconus | What is it, Causes and Treatment

Keratoconus

Written by Dr. Srujana Sahebjada, BS Optom, Ph.D., GCALL.

What is keratoconus?

The cornea acts like the “windshield” for your eye and looks like a dome-shaped window. It helps to focus light on the back of your eye, allowing you to see clearly. Collagen helps to hold your cornea in place, so when it becomes weak, it can’t hold the normally round shape of the cornea. This thins your cornea and will cause it to bulge outwards like a cone (Figure 1) [1]. This condition is called keratoconus (ker-uh-toe-koh-nus).

keratoconus
Figure 1. Comparison of cornea in eyes with and without keratoconus (Source: Dr Barry Belgorod, Concierge Eye Care)

The following sections will briefly outline how common is keratoconus, what causes keratoconus, its signs and symptoms, and some insights into its management and treatment.

How common is keratoconus?

Keratoconus affects both genders and all races worldwide, and its onset typically happens at puberty. The prevalence of keratoconus has risen substantially over the last four decades (affecting 1 in 2000 people in 1986 to 1 in 365 people in 2016) [4] [5]. The prevalence of keratoconus is also greater in children than in adults [6]. Keratoconus is the second most common cause for corneal transplantation globally (18.9%) [2] and as per the Australian Corneal Graft Registry, a keratoconus patient who has corneal transplantation may need up to five transplants throughout their lifetime [3].

What causes keratoconus?

Keratoconus is a complex eye condition [7] most likely caused by a combination of environmental and genetic factors [8]. The influence of genetic and environmental components varies between people with keratoconus, depending on the severity of the disease, the age at which keratoconus develops, and the presence or absence of a family history of keratoconus [8].

Research indicates a genetic predisposition as around 10% of individuals who develop keratoconus have a parent with the disease [1]. Apart from heredity factors, other risk factors include:

  • High exposure to ultraviolet sun rays
  • Excessive eye rubbing or knuckle rubbing [8]
  • History of poorly fitted contact lenses
  • Having genetic conditions such as Down syndrome, Ehlers-Danlos syndrome, osteogenesis imperfecta, retinitis pigmentosa
  • Inflammatory diseases such as allergies, asthma, or atopic eye disease, which can break down the tissue of the cornea from excessive eye rubbing.

What are the common signs and symptoms of keratoconus?

Keratoconus often affects both eyes, but one eye may be worse than the other (asymmetric). Symptoms change over time but can include:

  • Slight blurry and distorted vision
  • Light streaks or double vision (‘ghost’ images)
  • Increased sensitivity to bright light and glare
  • Problems with night driving
  • Redness of the eye, irritation, or headaches associated with eye pain

 

keratoconus
Figure 2. Comparison of vision without keratoconus, with early and advanced keratoconus (Source: Burnett Hood & Tam Optometry, 2021)

These symptoms usually appear in late puberty and progress for 10-20 years. Because of the progressive nature of the disease, the cornea may slowly bulge and cause vision-related problems. You may need to change prescription glasses frequently for this reason. The bulging can sometimes cause tiny cracks in the cornea, which may lead to swelling and a white-looking eye (hydrops). If this occurs, a sudden decrease in vision may be experienced.

Advanced staged keratoconus can also present with:

  • Progressively worse vision for seeing far away objects (near-sightedness) and blurred or distorted vision (irregular astigmatism)
  • Not being able to wear the same contact lenses, as they may not fit properly

Corneal hydrops may result in irreversible vision loss, significant pain, corneal opacity, and sensitivity to light or photophobia. As a result, if this complication is present, corneal transplantation should be done promptly.

If you experience any of these signs and symptoms, schedule an appointment with an eye health professional to get your eyes checked. It is also important to note that the development of eye conditions may even start before symptoms appear, which makes going for regular and timely eye checks that much more essential.

How is keratoconus diagnosed?

Keratoconus can be diagnosed through a routine eye examination and speaking to your eye doctor about your main vision problems and mentioning your medical and family history [9]. Your doctor will need to look for corneal curvature measurements and test for irregular astigmatism [10].

Children of parent(s) with keratoconus should have annual eye check-ups from the age of 10 to see if they are also developing the condition. Your doctor or optometrist may diagnose keratoconus through the following tests:

  • Slit-lamp examination – directs a light beam on the eye surface and with a microscope, examines and measures your corneal surface to look for corneal shape or size abnormalities.
  • Keratometry – focuses a light beam on your cornea and measures the reflection to determine if your cornea has an irregular shape. They may also use a hand-held keratoscope or ophthalmometer which are other tools that can allow the cornea to be properly examined.
  • Pachymetry – measures the thickness of the cornea.
  • Computerized corneal mapping – maps the cornea through a special computer which projects light rings onto the cornea surface which reflect back and gives information about the shape and structure of the corneal surface.

Early or subclinical keratoconus does not produce any symptoms and thus can go unnoticed both by the person affected or the eye care professional, unless specific eye tests such as corneal topography or corneal tomography (a computer-assisted method for mapping the corneal surface) are performed [11]. The presence of early or subclinical keratoconus may be indicated by abnormal corneal topography, such as inferior-superior localized steepening or an asymmetric bowtie pattern [12-15]. This early detection is essential to perform corneal collagen cross linking (described in treatment section) and prevent the need of corneal transplantation.

Keratoconus-related disorders

There are a variety of conditions that can resemble keratoconus, including pellucid marginal degeneration (thinning and steepening of the outer edges of the cornea), keratoglobus (thinning of the cornea with a globe-shaped or spherical appearance), interstitial keratitis (chronic damage to deep layers of the cornea), and corneal dystrophies (a group of genetic, often progressive, eye disorders which allow for foreign materials to accumulate inside the cornea) [16].

How is keratoconus treated?

The following section will cover the treatment options for keratoconus.

Glasses and/or contact lenses

Treatment for keratoconus depends on the disease severity and progression [17]. In the earliest stages, glasses or soft contact lenses are used to correct blurred and distorted vision. You may need to change your eyeglass prescriptions and/or contacts regularly as the shape of your cornea may change [18]. Most optometrists and ophthalmologists will prescribe contact lenses known as rigid gas permeable (RGP) lenses, which will correct your vision but unfortunately does not stop the progression of keratoconus.

Contact lenses need to be carefully fitted because ill-fitting lenses can cause further damage to your cornea. Frequent eye check-ups and lens changes may be needed to achieve and maintain good vision. Other types of contact lens fitting include “piggybacking”, which simply means to put two different types of contact lenses (one on top of the other) on the eye. One is a soft contact lens (silicone made) and the other (the RGP) is a more rigid fitting lens to better shape the eye. This approach increases wearer comfort because the soft lens acts like a cushioning pad under the rigid gas permeable lens.

Hybrid contact lenses are similar, but the RGP lens has an oxygen permeable centre. Scleral or semi scleral lenses are also more comfortable and stable options, as they apply no pressure to the cornea.

Collagen cross-linking

In the early stage, progressive keratoconus can also be treated by corneal collagen cross-linking [19]. This procedure involves the application of a vitamin B2 (riboflavin) drops to the eye, which is then activated by ultraviolet light for about 30 minutes or less. The treatment causes new collagen bonds to form which can recover and preserve your cornea’s strength and shape to improve your vision. Although it cannot reverse the keratoconus related changes in the cornea, it can halt or slow down further progression.

Implants

With the increasing severity of keratoconus, normal contact lenses may become too uncomfortable to wear. Intacs are clear, arc-shaped corneal inserts that are used to flatten the surface of the cornea which improve vision. They may also allow a better contact lens to fit. These are surgically placed within the peripheral cornea to reshape the front surface of the eye for clearer vision. These implants also have the advantage of being removable and exchangeable. The surgical procedure takes only about 10 minutes.

Corneal transplant­

Some cases of keratoconus have corneal scarring, extreme corneal thinning, poor vision with the strongest prescription lenses, or an inability to wear any type of contact lenses. In these people, corneal transplantation surgery might be needed [21]. Depending on the curvature of the cornea and the disease severity, options include:

In a full-cornea transplant (also called penetrating keratoplasty), a donor cornea replaces the person’s damaged cornea. Vision usually remains blurry for about three to six months after the transplant, and you must take medication to avoid transplant rejection. Glasses or contact lens are usually necessary to provide the clearest vision after transplant surgery. Cornea transplant usually has a high success rate however some complications that may arise include graft rejection, infection, poor vision, and astigmatism.

A partial-cornea transplant is categorised into procedures called deep anterior lamellar keratoplasty (DALK), descement’s stripping automated endothelial keratoplasty (DSAEK), and descemet’s membrane endothelial keratoplasty (DMEK) as illustrated in Figure 3 below. In a DALK, DSAEK and DMEK, unhealthy layers of the cornea are replaced with layers of healthy donor tissue and only a small area of the cornea is disturbed, often resulting in a more stable outcome.

keratoconus
Figure 3. Types of corneal grafts and corneal transplantations (Source: Contact Lens Spectrum)

Complications from keratoconus

Treatment of keratoconus may result in poor vision, infection, corneal transplant rejection and/or corneal graft-host junction thinning.

Recovery from keratoconus

With early diagnosis and prompt intervention using corneal cross-linking, you may retain your visual function with contact lenses. The prognosis for corneal transplantation is excellent and you may be able to return to an active and normal lifestyle. Your visual rehabilitation will include use of contact lenses, and you may need long term maintenance therapy with steroid medications.

Progression and/or recurrence of keratoconus even after corneal transplant surgery has been reported but it is not clear how common or to what extent this may occur [22].

References

  1. S. Sahebjada et al., “Evaluating the association between keratoconus and the corneal thickness genes in an independent Australian population,” Investigative ophthalmology & visual science, vol. 54, no. 13, pp. 8224-8228, 2013.
  2. T. Röck, K. U. Bartz-Schmidt, and D. Röck, “Trends in corneal transplantation at the University Eye Hospital in Tübingen, Germany over the last 12 years: 2004 – 2015,” (in eng), PLoS One, vol. 13, no. 6, p. e0198793, 2018, doi: 10.1371/journal.pone.0198793.
  3. “The Australian Corneal Graft Registry. 1990 to 1992 report,” (in eng), Aust N Z J Ophthalmol, vol. 21, no. 2 Suppl, pp. 1-48, May 1993.
  4. Y. M. Wang and C. C. Pang, “Molecular genetics of keratoconus: clinical implications,” in Ocular Surface Diseases: Some Current Date on Tear Film Problem and Keratoconic Diagnosis: IntechOpen, 2019.
  5. D. A. Godefrooij, G. A. de Wit, C. S. Uiterwaal, S. M. Imhof, and R. P. Wisse, “Age-specific Incidence and Prevalence of Keratoconus: A Nationwide Registration Study,” (in eng), Am J Ophthalmol, vol. 175, pp. 169-172, Mar 2017, doi: 10.1016/j.ajo.2016.12.015.
  6. E. Chan et al., “Prevalence of Keratoconus Based on Scheimpflug Imaging: The Raine Study,” (in eng), Ophthalmology, vol. 128, no. 4, pp. 515-521, Apr 2021, doi: 10.1016/j.ophtha.2020.08.020.
  7. B. J. McComish et al., “Association of Genetic Variation With Keratoconus,” JAMA Ophthalmology, vol. 138, no. 2, pp. 174-181, 2020, doi: 10.1001/jamaophthalmol.2019.5293.
  8. S. Sahebjada et al., “Eye rubbing in the aetiology of keratoconus: a systematic review and meta-analysis,” Graefe’s Archive for Clinical and Experimental Ophthalmology, pp. 1-11, 2021.
  9. L. T. Nordan, “Keratoconus: diagnosis and treatment,” International ophthalmology clinics, vol. 37, no. 1, pp. 51-63, 1997.
  10. G. Castro-Luna and A. Pérez-Rueda, “A predictive model for early diagnosis of keratoconus,” BMC ophthalmology, vol. 20, no. 1, pp. 1-9, 2020.
  11. Y. Li, W. Chamberlain, O. Tan, R. Brass, J. L. Weiss, and D. Huang, “Subclinical keratoconus detection by pattern analysis of corneal and epithelial thickness maps with optical coherence tomography,” (in eng), J Cataract Refract Surg, vol. 42, no. 2, pp. 284-95, Feb 2016, doi: 10.1016/j.jcrs.2015.09.021.
  12. S. Sahebjada, J. Xie, E. Chan, G. Snibson, M. Daniel, and P. N. Baird, “Assessment of anterior segment parameters of keratoconus eyes in an Australian population,” (in eng), Optom Vis Sci, vol. 91, no. 7, pp. 803-9, Jul 2014, doi: 10.1097/opx.0000000000000295.
  13. F. Abolbashari, N. Mohidin, S. M. Ahmadi Hosseini, B. Mohd Ali, and S. Retnasabapathy, “Anterior segment characteristics of keratoconus eyes in a sample of Asian population,” (in eng), Cont Lens Anterior Eye, vol. 36, no. 4, pp. 191-5, Aug 2013, doi: 10.1016/j.clae.2013.01.005.
  14. S. Emre, S. Doganay, and S. Yologlu, “Evaluation of anterior segment parameters in keratoconic eyes measured with the Pentacam system,” (in eng), J Cataract Refract Surg, vol. 33, no. 10, pp. 1708-12, Oct 2007, doi: 10.1016/j.jcrs.2007.06.020.
  15. D. P. Piñero, J. C. Nieto, and A. Lopez-Miguel, “Characterization of corneal structure in keratoconus,” Journal of Cataract & Refractive Surgery, vol. 38, no. 12, pp. 2167-2183, 2012.
  16. Y. S. Rabinowitz, “Keratoconus,” Survey of ophthalmology, vol. 42, no. 4, pp. 297-319, 1998.
  17. V. Jhanji, N. Sharma, and R. B. Vajpayee, “Management of keratoconus: current scenario,” British Journal of Ophthalmology, vol. 95, no. 8, pp. 1044-1050, 2011.
  18. K. D. Andreanos, K. Hashemi, M. Petrelli, K. Droutsas, I. Georgalas, and G. D. Kymionis, “Keratoconus treatment algorithm,” Ophthalmology and therapy, vol. 6, no. 2, pp. 245-262, 2017.
  19. S. L. Watson et al., “Save Sight Registries Keratoconus; Tracking the outcomes of corneal cross-linking for Keratoconus from routine clinical practice across Australia and New Zealand,” Investigative Ophthalmology & Visual Science, vol. 58, no. 8, pp. 3523-3523, 2017.
  20. W. S. Al-Tuwairqi, U. L. Osuagwu, H. Razzouk, A. AlHarbi, and K. C. Ogbuehi, “Clinical evaluation of two types of intracorneal ring segments (ICRS) for keratoconus,” International ophthalmology, vol. 37, no. 5, pp. 1185-1198, 2017.
  21. C. Henein and M. A. Nanavaty, “Systematic review comparing penetrating keratoplasty and deep anterior lamellar keratoplasty for management of keratoconus,” Contact Lens and Anterior Eye, vol. 40, no. 1, pp. 3-14, 2017.
  22. A. C. Ferdi, V. Nguyen, D. M. Gore, B. D. Allan, J. J. Rozema, and S. L. Watson, “Keratoconus natural progression: a systematic review and meta-analysis of 11 529 eyes,” Ophthalmology, vol. 126, no. 7, pp. 935-945, 2019.

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