Keratoconus and other Ectatic (thinning) corneal disorders
Keratoconus; literally means conical cornea, is a clinical term used to describe a condition in which the cornea assumes a conical shape because of a non inflammatory disease process of thinning and protrusion. Keratoconus is usually bilateral and involves the central two thirds of the cornea. The disease process results in mild to marked impairment of visual function (Feder, Gan 2011). Despite considerable research, the etiology of keratoconus remains unclear. In the epithelium, the basal layer normally degenerates and breaks down leading to the dissolution of the Bowman’s layer. As the epithelium and the stromal layers come closer to each other, structural and cellular alterations occur leading to scarring, corneal thinning and bulging In the stroma, fibrils are compacted and lose their arrangement. Normal fibroblasts, keratocytes and collagen lamellae numbers decrease; however, the endothelium is normally unaffected.
The cornea is responsible for approximately for approximately two-thirds of the refracting power of the eye (Smith, Atchison 1997), therefore changes in corneal morphology (shape) result in changes in optical performance. The downward displacement of the corneal apex in keratoconus is particularly devastating to retinal image quality because it induces high order aberrations (HOA) which cannot be effectively compensated for with standard spectacle or soft lens corrections (Marsack, Rozema et al. 2013). As keratoconus progresses, the vision, even with the best possible spectacle correction may deteriorate because only low order optical aberrations, like myopia hyperopia and regular astigmatism, are amenable to correction with spectacles and soft contact lenses. As the level of HOA increases the vision becomes progressively degraded. At this stage rigid Gas Permeable (RGP) contact lenses are usually fitted, because unlike soft contact lenses which wrap the cornea and transpose the HOA to their front surface, RGP lenses maintain their rigid, highly regular shape and neutralise corneal irregularities by the tear lens behind its back surface. The lens becomes the new, regular front surface of the cornea, and vision is therefore restored. Even in the presence of the gold standard RGP contact lens corrections, HOA levels remain elevated compared with normal eyes, demonstrating that available methods of correction do not always fully restore an optimal retinal image quality in eyes with keratoconus (Negishi, Kumanomido et al. 2007). Corneal scarring due to keratoconus disease progression occurs in approximately 30% of eyes (Barr, Zadnik et al. 2000). By inducing forward and backward scatter as well as absorption scarring alters the image forming properties of the eye thus detrimentally affects visual quality.
See the end of this page for publication references
Barnard and Levit Centre of Excellence for the management of the irregular cornea disorders.
Doctor Levit specialises in the management of the visual symptoms of keratoconus and other corneal ectatic disorders with contact lenses, he is the inventor and designer of a specialist contact lens design and correction system for keratoconus called the ALK™ which are manufactured and distributed all over the world by Ultravision CLPL. Doctor Levit has many years of experience of vision correction of ectatic corneal disorders like keratoconus, keratoglobus, pellucid and Terrien's marginal degeneration and other aquired irregular corneal disorders which require specialist contact lens treatment. He has been engaged in specialist contact lens work both in the hospital eye service and private practice since 1995.
The centre of excellence for the management of the irregular cornea disorders was established at Barnard and Levit in 2012. The idea is to provide a collaborative referral centre for long term management and care of patients with these chronic progressive disorders. The collaboration with consultant corneal surgeons enables the provision of required care at the appropriate time and by the appropriate specialist.
The main role of the Barnard and Levit practice is to provide the optical vision correction as well as the expertise and technology to diagnose, monitor and appropriately manage the patients with these disorders in collaboration with corneal surgeon colleagues .
Although spectacle lenses may provide adequate visual quality to patients with very mild forms of corneal disorders, contact lenses are the main modality in the optical correction and visual restoration of corneal irregularity disorders. Doctor Levit specialises in fitting the widest range of speciality contact lenses to the extent that all levels and severities of corneal disorders may be treated with the most appropriate type of contact lenses for the individual patient and eye.
As mentioned in the introduction the gold standard of optical restoration of the irregular cornea are rigid gas permeable (RGP) lenses. The delivery of a regular, rigid, optically transparent surface to the irregular front of the cornea, neutralises the corneal distortions by the generation a tear medium with its new, regular front surface sculpted by the back surface of contact lens.
That regular rigid surface may be delivered by a wide range of contact lens types and designs;
* Standard RGP lenses.
* Speciality RGP lenses.
- Standard Corneal lenses.
- Large Corneal lenses.
- Small Corneal lenses.
- Large diameter; Simi-limbal and Limbal lenses.
- Large diameter Semi-scleral and Scleral lenses.
* Standard Hybrid (RGP centre soft periphery) lenses.
* Speciality Hybrid (RGP centre soft periphery) lenses.
* Speciality Soft contact lenses.
-Spherical speciality soft contact lenses.
-Toric (astigmatism correcting) soft contact lenses.
Each modality has advantages and disadvantages and might be more or less appropriate for the individual idiosyncrasies of each eye and patient's requirements.
Please contact reception for further information and to bok an appointment. If you were referred by another specialist please remember to bring all relevat information, including contact lenses and spectacle prescriptions with you.
BARR, J.T., ZADNIK, K., WILSON, B.S., EDRINGTON, T.B., EVERETT, D.F., FINK, B.A., SHOVLIN, J.P., WEISSMAN, B.A., SIEGMUND, K. and GORDON, M.O., 2000. Factors associated with corneal scarring in the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study. Cornea, 19(4), pp. 501-507.
FEDER, R.S. and GAN, T.J., 2011. Noniflamatory ectatic disorders. In: KRACHMER JH, MANNIS MJ, HOLLAND EJ, ed, Cornea. 3 edn. St Louis: Mosby Elsevier, pp. 865-878.
MARSACK, J.D., ROZEMA, J.J., KOPPEN, C., TASSIGNON, M.J. and APPLEGATE, R.A., 2013. Template-based correction of high-order aberration in keratoconus. Optometry and vision science : official publication of the American Academy of Optometry, 90(4), pp. 324-334.
NEGISHI, K., KUMANOMIDO, T., UTSUMI, Y. and TSUBOTA, K., 2007. Effect of higher-order aberrations on visual function in keratoconic eyes with a rigid gas permeable contact lens. American Journal of Ophthalmology, 144(6), pp. 924-929.
SMITH, J. and ATCHISON, D.A., 1997. The eye and visual optical instruments. 1 edn. Cambridg: Cambridge University Press.