Uncorrected myopia (short-sightedness) is a major public health challenge restricting employment opportunities, affecting the learning ability of children and limiting the quality of life of affected individuals. Further to this, high degrees of myopia create an increased risk of retinal detachment and possibly, visual impairment and blindness. Recent reviews of the epidemiology of myopia reveal that the prevalence and incidence of myopia have been increasing, especially in Asian populations where myopia has reached epidemic proportions . The number of myopes in the world is estimated to grow from 1.6 billion currently to a staggering 2.5 billion by 2020 .
Development of Myopia
Myopia occurs when light is focussed in front of the retina  due to the eyeball being either too long or the cornea having increased curvature as compared to the standard eye. Consequently, distant objects appear blurred while near objects are visually clear. Infant eyes undergo a process of emmetropization whereby both the average amount and the variance in the distribution of refractive errors are reduced . The refractive status shifts from hyperopia and astigmatism to emmetropia (zero refractive error). Failure to achieve emmotropization results in myopia, which can be classified into two groups according to the magnitude of myopia i.e. low to moderate degree of myopia (referred to as simple myopia of the magnitude -0.5 to -6.0 dioptres) and high or pathological myopia of the magnitude i.e. greater than 6.0 dioptres .
Causes of Myopia
In 1885, Fuchs stated that “acquired myopia is the result of several factors, among which prolonged work requiring close vision takes the first place” . More than a century later, we cannot fully explain the cause of myopia, although the rapid increase in its prevalence in the past century provides some clues about its aetiology . Myopia has traditionally been considered to be a genetically pre-determined refractive condition, though more recent indications are that environmental factors such as excessive near work have an effect on the development and progression of myopia .
Risk factors for the onset and progression of myopia include :
- Genetic predisposition or family history;
- Premature and low birth weight individuals;
- Cumulative effect of excessive near work.
Myopia can be broadly classified into 2 categories :
1. Physiological - simple myopia not linked to any notable degenerative fundus changes;
2. Pathological - progressive or degenerative myopia usually leading to abnormal lengthening of the eyeball and risk of retinal detachment .
Conventional methods of correcting myopia have been single vision spectacles and contact lenses . Increasingly, bifocal lenses and progressive lenses are being used in myopia control. In younger patients, bifocal lenses are occasionally used to slow the progression of myopia .
Soft contact lenses have been used extensively to correct simple myopia, with rigid gas permeable lenses used for high myopia . While, patients desire comfort, clarity of vision, and prolonged contact lens wear when contact lenses are used as a refractive correction, practitioners require maintenance of the integrity of the eye with minimal complications that jeopardize vision or ocular health . For 30 years, contact lens research focused on the need for high oxygen-permeable (Dk) soft lens materials as this was seen as necessary to maintaining corneal integrity over the long term. High Dk silicone hydrogel contact lenses, made available in 1999, met this need, allowing as much as seven times more oxygen to pass through to the cornea . The availability of highly oxygen permeable contact lenses has therefore increased the tolerance and safety of contact lens wear. These improvements, together with those in lens design and maintenance products have reduced but not eliminated the risks of infection, inflammation, other reactions associated with contact lens wear .
Orthokeratology uses rigid lenses to reshape the cornea and thus alter the refractive error and is enjoying a revival in popularity due to the advancements in lens design and the improvement in RGP material. Ortho-k can have both a corrective and preventive/control effect in childhood myopia. However, there are substantial variations in changes in eye length among children and there is no way to predict the effect for individual subjects .
Increasingly refractive surgery has become an option for vision correction. In myopia the laser flattens the central cornea and brings the focal point back on to the retina. Currently Laser in situ keratomileusis (LASIK) is the most commonly performed refractive surgery. According to Andrew Velazquez, “Of current refractive procedures, LASIK offers the least discomfort and the most rapid postoperative healing and visual improvements. Epi-LASIK, photo refractive keratectomy [PRK], and laser epithelial keratomileusis [LASEK] are usually reserved for patients whose corneas are too thin or shaped in a way that rules out LASIK” .
Other surgical corrective options are clear lens extraction, implantable contact lens (between the iris and lens) intracorneal lens implants (within the cornea) and intracorneal ring segments (plastic arcs in the peripheral cornea) (Fig 1).
Fig. 1: Intracorneal Rings - plastic inserts placed in the cornea which flatten the central cornea to correct low degrees of myopia (sourced from www.isecmalaysia.com).
The use of pharmaceutical agents, as a result of costs and discomfort, has been limited. Pharmaceutical agents such as atropine have been used with relative success in some parts of the world in reducing the progression of myopia, though the bulk of evidence from wellconducted studies shows that overall, most therapies for myopia have small treatment benefits that last for a relatively short period of time or have significant side effects .
Future prospects: Contact Lenses
The solution to the reduction of infection from contact lenses and the development of more affordable contact lenses remain elusive. Although the ocular health benefits of silicone hydrogel lenses have increased the length of time lenses can be worn overnight, the risk of infection is similar to that found with other soft lens types, and overnight wear remains a higher risk factor for infection than daily wear, regardless of lens material . Future contact lens research will focus on gaining a better understanding of the way in which contact lenses interact with the corneal surface, upper eyelid, and the tear film, and the lens-related factors contributing to infection and inflammatory responses . Further the development of affordable and highly oxygen permeable contact lens materials that meet the criteria for ocular health are critical to the growth of the contact lens market .
The corneal onlay project of the Vision CRC in Australia is developing a permanent correction for refractive errors with an implantable contact lens. A synthetic lens is surgically placed under the superficial layer of corneal tissue (the epithelium), altering the corneal curvature. The lens is designed to provide stable vision for at least five years, while being a reversible procedure in which no permanent functional or structural damage is done to the eye.
That Arrest, Slow or Control the Progression Of Myopia The ability to focus images on the peripheral retina may influence emmetropisation. A novel optical device that controls image formation in this region may be a major breakthrough in eliminating or slowing the progression of myopia .
The elevation of uncorrected refractive error in the blindness prevention agenda has increased the focus on myopia and the management thereof. Achieving greater access and efficiency in the management of myopia is thus a critical challenge for eye care professionals.