ABSTRACT

Keratoconus (KC) is a progressive disease that leads to major losses of visual quality and related quality of life. Contact lens (CL) application has a primary place and importance in the correction of the optic problems due to the disease. The corneal changes and increased irregular astigmatism that occur with KC progression necessitate special CL designs and fitting methods. In addition to disease stage, the patient’s lens tolerance also plays a role in the application of CLs in KC patients. With recent advances in materials and design technology, the CLs used in the treatment of KC have developed considerably and there are various types available. In this review, we discuss the wide range of CLs, including rigid and soft lenses, hybrid and scleral lenses, and even custom lens designs, in light of recent scientific advances.

Introduction

Although contact lens (CL) fitting in keratoconus (KC) is a long and complicated process, it can remarkably improve patients’ quality of life, and therefore should be carried out with patience and care. In other words, every well-informed and dedicated effort to apply CLs in patients with KC may be a door to illumination or a magic touch that shapes the future of these individuals, who live in a blurry and somewhat hopeless world.

KC is a progressive, non-inflammatory corneal disease characterized by thinning of the central and often the paracentral inferior cornea, steepening of the corneal curvature, and asymmetry between superior and inferior keratometric values, resulting in irregular myopic astigmatism. Corneal changes are usually bilateral but asymmetric. Unilateral KC has been described at rates of 0.5-4.5%, but with current topography and corneal aberration measurement techniques, it has been reported that changes that may be associated with subclinical KC can also be detected in the apparently normal fellow eye.¹ Age of onset is in the early teens or twenties, with progression continuing until the third or fourth decade, and the rate of progression varies among individuals.² KC is classified as mild, moderate, or severe based on ocular signs and symptoms. In the late stages of the disease, corneal thinning and protrusion increase, higher-order aberrations also increase, and visual acuity deteriorates substantially.³

Because the prevalence of KC is affected by many factors including geographic location and the diagnostic criteria used, different rates have been reported around the world.^4,5 Recent studies indicate that with the use of common diagnostic criteria and advanced technology such as corneal topography, the annual incidence and prevalence of KC may be up to 5-10 times the previous values.⁶ KC affects both sexes. While previous studies indicated no marked difference between the sexes, more recent studies have revealed that KC is more common among males.^6,7

Family history is present in 10-20% of patients, and there are studies suggesting that genetic inheritance plays an important role.⁸ KC is often concomitant with atopy, vernal keratoconjunctivitis, asthma, sleep apnea, mitral valve prolapse, retinitis pigmentosa, Down syndrome, certain noninflammatory connective tissue diseases, and also rigid CL use and eye rubbing, due to their environmental and mechanical effects.^7,8,9,10

Although it is usually difficult to establish causal relationships, some of the conditions associated with KC point to genetic factors, while others indicate recurrent mechanical trauma.⁹ Thus, the point of consensus is that KC has a multifactorial and multigenetic nature and that environmental factors also play an important role in the development of the disease, or in other words, that the disease develops due to genetic predisposition together with these environmental effects.¹⁰ Recent studies have suggested as a possible pathogenic mechanism that cytokines produced in response to epithelial trauma induced by eye rubbing may reduce the durability of the corneal tissue, leading to structural changes that manifest with cone formation.¹⁰ On the other hand, it has been demonstrated in several studies that redox imbalance due to low levels of antioxidant enzymes in the cornea causes increased oxidative stress in keratoconic eyes, and it is now being suggested that tissue destruction caused by the resulting reactive oxygen/nitrogen species may contribute to stromal thinning in KC.¹¹

Each of these facets should be kept in mind when planning treatment, preparing a patient for a lifetime with this chronic disease, and implementing and monitoring optic correction systems, especially CLs.

Recent advances in anterior segment imaging systems have contributed significantly to the early diagnosis and treatment of KC and to the monitoring of pathological changes that occur in KC. Today, advanced computerized corneal topography and tomography systems allow detailed assessment of changes in the anterior and posterior corneal surfaces in KC and changes in corneal thickness that occur as the disease progresses. Furthermore, changes in corneal epithelial thickness are detected with optical coherence tomography, while changes at the cellular level are detected by in vivo confocal microscopy, which enables the follow-up of the natural course of this progressive disease as well as facilitates the guidance, supervision, and close monitoring of treatment response in CL applications.¹²

There is no definitive treatment for KC. However, corneal collagen crosslinking can be performed to alter corneal biomechanical properties and stop or slow the progression of ectasia. Nevertheless, CLs are still necessary to improve vision quality before and after these disease-stabilizing procedures.¹⁰

Because KC is a lifelong disease with a progressive course marked by a gradual decline in the quality of vision, it also seriously impairs quality of life in affected individuals. Survey studies based on the National Eye Institute Visual Function Questionnaire (NEI-VFQ) indicate vision-related impairment of quality of life of patients with KC. However, it was also reported that patients who use CLs have the highest quality of life scores.^13,14,15

The process of increasing vision quality in KC patients is usually complex and closely associated with the rate of disease progression. In the early stages, characterized by regular astigmatism, satisfactory results can be achieved with the use of eyeglasses. In the advanced stages, a wide variety of CL options can be used for irregular astigmatism.¹⁶

In the multicenter and long-term CLEK (Collaborative Longitudinal Evaluation of KC) study, the main results of which were published in 1998, it was reported that only 16% of patients diagnosed with KC used spectacles as a primary optical device, 65-75% used CLs, mostly rigid gas-permeable CLs (RGPCLs), and surgical methods were preferred to treat the 10-20% of patients for whom these methods were not effective.⁹ In their study reviewing treatment methods used in the past 20 years, Mandathara et al.¹⁷ reported that CLs remain the main method for the treatment of KC, and that their use is associated with only reversible complications that do not threaten vision. They also stated that surgical methods such as intracorneal ring segment or phakic intraocular lens implantation can be used to treat patients who are unable to use CLs and that lamellar or full-thickness corneal transplantation may become necessary in case of extreme corneal thinning or scarring, but these surgical interventions can often result in temporary inflammatory responses and permanent sequelae.¹⁷

CL fitting is a long and arduous process for both the patient and the practitioner, and the three main goals of this process are to achieve optimal comfort, increase vision quality, and provide the best possible fit for the irregular corneal structure. Today, a wide array of lens options are used to achieve these goals, including corneal, semi-scleral, and scleral lenses, which differ in diameter, and rigid, soft, and hybrid lenses or piggyback lens systems, which differ in the materials used.^9,13

In this review, we discuss these CLs in terms of visual performance, fitting characteristics, options for changing and combined use to increase success, and potential complications, in light of technological advances and current scientific studies.