contact lenses

contact lenses The contact lens provides a good example of the lengths to which some humans will go to overcome deficiencies of the visual system. A contact lens is a foreign body and as such causes irritation and pain. In addition there is an excessive flow of tears to wash out the invading object and a strong urge to rub the eye, with the further risk of causing a corneal abrasion. The contact lens wearer has to be strongly motivated to learn to adapt to this irritant and develop the ocular tolerance necessary for comfortable lens wear. Vanity is a powerful driving force, immortalized by Dorothy Parker's malicious ‘Men seldom make passes at girls who wear glasses’. Assuredly there are other advantages with contact lenses in many types of sport, in the rain, and in working in confined and awkward positions, especially if protective goggles are needed as well. An early experiment by Thomas Young in 1801 to eliminate focusing by the cornea involved a convex lens fitted to a short glass socket filled with water which was applied to his eye. He immediately became hypermetropic (long sighted) in that eye, but using his optometer found ‘the same inequality in the horizontal and vertical refractions as without the water’. Interestingly, this showed that his astigmatism was not due to his cornea, which is certainly the commonest cause.

The first recorded use of a protective glass for an eye exposed after removal of the eyelid was in 1887. A blown glass contact lens made from a plaster mould of an eye with keratoconus (conical cornea) was used for optical treatment in 1888. A year later a ground glass contact lens was made to treat an ophthalmologist's own high myopia (short sight).

Scleral lenses covered more than the area of the eye seen between the eyelids, and ground and moulded glass lenses superseded blown glass until that, in turn, was followed by plastic lenses.

The author was present at a lecture by Wichterle in 1963 at the Institute of Ophthalmology, London when he introduced the soft hydrophilic plastic corneal lens. These cost 1 shilling (5p) each to produce, although Wichterle wryly commented that the cost to the patient would bear no comparison. In fact, having passed through expensive phases, daily wear, disposable soft lenses which are thrown away after use cost only £1 each, nearly 40 years later.

The search for new materials to make contact lenses continues, because a number of important constraints have to be met. The lens must allow oxygen and carbon dioxide easy passage between the air and the aqueous humour, the fluid behind the cornea. It must not interfere with the metabolic pump which maintains corneal transparency. The tear flow must be maintained, but not excessive. Adaptive changes occur in corneal sensitivity, so that the lens is better tolerated, but if sensitivity is blunted too far the risk of abrasions and infections increases. A soft lens is more comfortable because it conforms to the shape of the underlying cornea, but then it may not adequately correct any corneal astigmatism.

The most recent new soft contact lens — silicone–hydrogel — has such high oxygen permeability that up to 30 days of continuous (day and night) wear is advocated. Ophthalmologists always view such claims with scepticism because of the risks of low oxygen flow, drying out of the lens, accumulation of protein debris, and infection. If any contact lens is left in place overnight the further barrier to free oxygen exchange between the eye and the atmosphere imposed by the closed lids may be crucial. Daily wear, disposable soft lenses avoid these difficulties, and because each lens is from a sterile pack the armamentarium of small bottles of sterilizing fluids for reusable, hard lenses is banished.

Contact lens fitting has always been an art, necessitating careful consideration of many factors which allow accurate alignment of the optical centres of contact lens and cornea, with the proper thickness of tear film beneath the lens so that it may move slightly in relation to the globe but re-centres itself naturally. Any long-term effect that a contact lens may have on the underlying cornea shape, and thus its refractive power, must be monitored weekly by the patient checking that his spectacles continue to give optimal vision when worn.

Contact lenses can be used most advantageously to correct myopia, but can also be used for hypermetropia and corneal astigmatism. Any astigmatism not corrected by the contact lens (residual astigmatism) is due partly to the patient's own lens within the eye and partly to features of the contact lens itself. Making specially shaped contact lenses to reduce their rotation on the eye can help. Bifocal contact lenses to correct distance and reading vision, so that the obvious bifocal glasses with their ageing implications could be avoided, have not proved successful. Reading glasses for presbyopia still have to be worn over the contact lenses. Another way to overcome this problem is to fit a contact lens for distance viewing to one eye and one focused for reading to the other. There are significant implications for good quality fusion of the images from each eye, in that 3-dimensional vision and depth perception are compromised. This can be important when driving, and spectacles for optimal distance vision should be worn over such contact lenses under these circumstances.

Cosmetic contact lenses fulfil a variety of uses, ranging from different coloured lenses as matching accessories in the fashion industry to films where an opaque scleral lens could be used to simulate a blind or grossly damaged eye.

The new, soft disposable contact lenses can now be used for occasional or social wear, since the long period of adaptation and building up of wearing time with hard contact lenses is no longer necessary.

There are a number of clinical indications for contact lenses, and the irregularly-shaped ‘conical cornea’ is the principal one. Specially made lenses when held temporarily in contact with the eye assist the opthalmologist to obtain detailed, magnified views of otherwise inaccessible areas of the eye, for example in glaucoma patients. The original scleral contact lenses still have a role in treating severely damaged eyes, now that they are made of a highly permeable polymer allowing good gas exchange but giving essential protection.

Peter Fells

See also eyes; refractive errors; squint; vision.