Scleral Contact Lenses
What are scleral lenses?
Scleral lenses are large diameter rigid gas permeable lenses. Whilst standard or conventional rigid gas permeable (RGP’s) lens diameters range from between 8mm to 10mm scleral contact lenses generally range from 14mm to over 20mm in diameter. Conventional RGP’s sit directly on and are supported exclusively by the cornea (the clear dome of tissue that overlies the coloured part of the eye or iris) and do not extend beyond the limbus (the junction between the cornea and the white of the eye). Corneoscleral lenses are supported by both the cornea and sclera (the white part of the eye that forms the outer wall of the eye). Scleral contact lenses are referred to as “scleral” as despite covering the cornea the lens doesn’t actually touch the cornea but instead vaults the cornea and extends onto the scleral.
Why are scleral lenses useful?
Both corneoscleral and scleral lenses offer certain advantages over corneal lenses. First of all, large-diameter lenses may be more comfortable for patients than corneal lenses. The cornea is one of the most highly sensitive tissues in the body. The conjunctiva (soft, clear tissue that lies over the sclera) is much less sensitive than the cornea. So, lenses that rest primarily or exclusively on the conjunctiva may induce less sensation than smaller lenses that rest upon the cornea.
In some individuals, corneal tissue is damaged for example is Neuropathic Dry Eye. Scleral lenses trap a reservoir of fluid behind the lens. This fluid protects the cornea, and may even allow it to heal in some cases. Corneal lenses can become decentered, and may even become dislodged. Since scleral and corneoscleral lenses extend under the upper and lower lids, and therefore rarely dislocate.
Who could potentially benefit from scleral lenses?
Patients with irregular corneas, patients with conditions that affect the tear film, and patients with refractive error (nearsightedness, farsightedness, or astigmatism) who are unable to wear other forms of correction could benefit from scleral lenses.
Conditions such as keratoconus and pellucid marginal corneal degeneration cause irregularity in the surface of the eye. Surgery (keratoplasty, refractive surgery) can also lead to corneal irregularity. If the cornea is not smooth, vision will not be easily correctable with spectacles or most soft contact lenses. Scleral lenses mask this irregularity and allow for clearer vision by providing a smooth front surface through which light can enter the eye.
Some patients have disorders that affect the quality or quantity of tears that help to keep the eye’s surface smooth and healthy. Dry eye syndrome, graft vs. host disease, Sjogren’s syndrome, Stevens Johnson syndrome, neuropathic dry eye and neurotrophic keratopathy are examples of such conditions. Some inflammatory conditions, including limbal stem cell deficiency and ocular cicatricial pemphigoid, also cause serious damage to the front surface of the eye. Patients who cannot close their lids completely may also experience problems with the health of the surface of the eye. The fluid reservoir trapped beneath a scleral lens may improve comfort for these patients, and may allow the corneal surface to heal.
Patients who need visual correction, but are unable to wear other contact lenses, may find larger-diameter lenses more comfortable than other lens designs. Corneoscleral lenses may be particularly useful for these patients.
Are scleral lenses new?
In short no. Scleral lenses were actually the first contact lenses described in medical literature in the late 1800’s. However, due to manufacturing challenges and because of the lack of oxygen transmission through early lenses, scleral lenses did not become popular at that time. During the course of the next century, corneal rigid gas permeable lenses and soft contact lenses became increasingly popular.
Fairly recent advances in lens technology and corneal topographers (a machine allowing highly detailed, accurate and 3D measurements of the cornea to be obtained) have induced somewhat of a renaissance in scleral contact lens fitting. Corneal rigid gas permeable lenses, hydrogel lenses and silicone hydrogel soft contact lenses cannot be used to solve some of the problems caused by corneal irregularity and eye surface disease and have thus led to the proliferation of the use of scleral contact lenses.