By Alice Tassetti, optometrist and student of SAERA
and student of the Expert Certificate in Clinical Optometry
Keratoconus is a progressive disease of the cornea which can lead to blindness as irregular astigmatism increases. Currently, a variety of methods are available for the treatment of keratoconus, and in certain cases, it may be difficult to choose the most appropriate option. This article reviews available hard contact lenses treatment modalities for keratoconus. Various contact lenses methods are available for the treatment of keratoconus in the early stages before cross-linking, intrastromal corneal ring segments, keratoplasty (PK) and transplantation in advanced stages of the disease. To achieve optimum results, it is essential to choose the best option for each individual patient. To review treatment methods for different stages of keratoconus, PubMed and ScienceDirect databases were searched using the keywords “keratoconus”, “contact lens” and “irregular astigmatism”.
I. The Keratoconus
Keratoconus is an axial ectasia of the cornea, generally asymmetric, non-inflammatory. It is bilateral in 96% of cases, it develops in asymmetric way, generally with the diagnosis of delayed disease in the second eye in five years after the first diagnosis. The course of the disease is highly variable, classically starting from puberty until the half of the thirty years and develops for 10-15 years until the fourth or fifth decade of life. The beginning of keratoconus is generally followed by a period of relative stability or very slow progression, which may be interspersed with episodes of more rapid evolution. The end of progression it is also variable: at the time of stabilization of the pathology it can be present an irregular astigmatism (that can be compensated with contact lenses and glasses) and a high thinning of the cornea, protrusion e scars that can be treated by keratoplasty or by other surgical techniques. Progression is initially characterized by a thinning of the central corneal stroma, apical protrusion that determines a curvature of the central curvature and a variable presence of scars. The thinned apex turns down, giving rise to irregular astigmatism, a characteristic of the disease that causes an alteration the quality of vision that can be minimal or high. The incidence of keratoconus varies from 50 to 230 considering a population of 100,000. The prevalence of keratoconus in a normal population has been reported not higher at 0.6%. The prognosis of this condition is unpredictable and its progression is variable; control visits are therefore recommended annual or more frequent. The disease does not cause blindness, but could compromise the quality of life. Most patients with keratoconus (74% of the eyes) can be non-treated surgically, while the remainder (26%) are treated with keratoplasty.
II. Hard contact lenses for treatment of keratoconus
a. Corneal contact lenses: The lenses with total diameter (TD) between 8 and 12.5 mm are considered corneal lenses. With the increase of the diameter of the cone also the diameter of the rear optical zone (BOZD), the radius base and the total diameter must increase, determining a better relationship between the sagittal depth of the cone and the sagittal depth of the optical area of the lens. The determination of the correspondence between these variables can be facilitated through the use of a corneal topographer, with which can be measured the position, size and area of the cone. If the optical area is too large compared to the diameter of the cone, the depth of the contact lens is greater than that of the cone, so that the presence of clearance around the cone may form bubbles in the pre-lens tear film. These conditions can affect visual performance. If the diameter of the optical area of the lens is too small, the depth of the lens will be smaller than the sagittal depth of the cone resulting in instability and lens decentralization. The peripheral area of the lens must flatten more than that of the peripheral flattening of the cornea and in doing so, it will have to ensure an axial lifting of the upper edge compared to that of a lens designed for a normal eye with an average eccentricity value of 0.5-0.6. The eccentricity values for the eyes with keratoconus are included in a range which goes from 0.65 and reaches values above 1.0 in advanced cases.
b.Corneoscleral and semi-scleral permeable gas contact lenses: With the advent of hyper-Dk GP materials, the applicators becomes more confident in the prescription of large-diameter lenses. Lenses with a diameter between 12.9 mm and 13.5 mm can be considered corneo-scleral while the lenses with a diameter between 13.6 mm and 14.9 mm can be considered semi-scleral lenses. For the application of these lenses we have to consider three distinct portions that can be modified in an independent way:
• The corneal portion (BOZD / BOZR),
• The middle-peripheral portion that affects the sclero-corneal junction
• The sclero-conjunctival portion
Corneo-scleral lenses are applied ensuring a support central with a slight apical or corneoconforme clearance, ensuring a clearance above the corneo-scleral junction and alignment in the sclero-corneal area. Semi-scleral lenses are applied in the same way, but with a central clearance greater in order to guarantee a better film turnover tear. If the corneal topographic maps highlights a very large deformed cone (keratoglobal) which can be moved downward (PMD) or irregularly shaped (oblate, as in the case of postkeratoplasty PKP), a very large BOZD is needed and the management oh these cases are characteristic of these lenses.
c. Mini-scleral and scleral GP lenses: Lenses with diameters between 15.0 mm and 18.0 mm can be considered mini-scleral, and with diameters of 18.1 mm over 24 millimeters are considered scleral. The application of these lenses requires knowledge of the profile of the corneo-scleral junction and the scleral topography. These lenses are designed using to get alignment and stabilization on the scleral conjunctiva and lifting on the cornea, without touching the cone. In order to ensure this type of relationship the depth sagittal lens must be greater than that of the cornea. Because with this type of lenses the liquid tear remains under the lens can have an effect therapeutic in case of dry eye, besides masking very large areas of irregularity Corneal. These lenses offer advantages for advanced cases of PMD and keratoglobus and corneal transplants with raised grafts or inclined (or any other deformation due to surgery refractive), and also offer comfort and a better view. For the success of this application similarly to the semi-scleral geometry each of the three zones of the lens must be considered individually.
d. Piggyback applications: The traditional piggyback system, involves the use of a soft lens in which a flare (with diameters between 8.0 and 9.8 mm) has been created in which rigid lens is inserted. The soft lens has a total diameter between 12.5 to 14.5 mm. This king of application has the advantage of guaranteeing, in cases of mild or moderate oval cones, the centering of the contact lens on the pupil and a greater comfort, but has the disadvantage of reducing the oxygen tension under the two lenses to very low levels. With the advent of silicone hydrogel materials there has been a recovery of interest in piggybacking applications. These materials increase the tension of oxygen under the system of two lenses to 34 mm Hg lenses (PMMA lenses and low hydrophily lenses) at 95 mm Hg (with GP and SCL lenses). Another advantages of the piggyback system with Hydrogel silicone lenses is a slight corneal profile modeling and corneal protection when chronic abrasions are present.
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