What are silicone hydrogel contact lenses?

Silicone hydrogel lenses are often mistakenly referred to as 'silicon' hydrogel. Silicon, however, is a common mineral found in substances such as dust and sand, and can be refined to produce parts for electrical equipment.

Silicone - the component used in silicone hydrogel lenses - is the name attributed to gel-like plastic materials which contain silicon as well as oxygen, carbon and other elements. The flexible properties of silicone make it ideal for use in gas permeable contact lenses, as well as other medical products such as implants and tubing. Although it is soft, it can also be used in producing rigid gas permeable lenses, helping to improve oxygen permeability of these lenses.

Because all contact lenses sit on the eye, they all restrict the amount of oxygen that can reach the eye to some degree. However, if the amount of oxygen reaching the cornea is significantly low - a state referred to as hypoxia - many uncomfortable conditions can occur in the wearer. These include red eyes, swollen corneas and blurred vision; hypoxia can also increase the risk of developing an eye infection.

These eye problems became pronounced during the 1990s, when the numbers of people becoming dependent on wearing contact lenses in place of glasses rose. Eye infections were commonly associated with wearing contact lenses overnight and for days at a time. Silicone hydrogel lenses were designed to combat these problems, allowing the safe and comfortable wear of lenses for short and long periods.

An increased amount of oxygen to the eye is theoretically beneficial for all contact lens wearers. However, many people disregard their optician's instructions for proper lens wear and care. For example, in a study undertaken by Ciba Vision, one fifth of people who wear soft lenses were found to continue wearing them while sleeping, with two thirds of these users wearing lenses that are not suitable for wearing overnight.

There have been some reports of people experiencing symptoms of an allergic reaction when wearing their silicone hydrogel lenses. These symptoms include dry and itchy eyes, lens discomfort, and redness around the eye. This is not a definitive sign of an allergic response, as these symptoms are also common with sensitivity to new lens care products or contact lens induced dry eye (CLIDE). Moreover, there has been no conclusive evidence across America or the UK that silicone hydrogel contact lenses can cause allergic reactions, or in any peer-reviewed medical journals of either country.

However, in some cases the increased silicone in silicone hydrogel lenses can reduce the amount of moisture on the surface of the eye, which causes dryness and itching. This does not occur with all wearers of silicone hydrogel lenses but may develop in those who have even minor pre-existing symptoms of dry eye, making the discomfort seem like an allergic response. However, specially formulated lens care solutions are available that can maintain silicone hydrogel lens hydration throughout the day.

Because more air is enabled to reach the eye by means of silicone hydrogel lenses, some people may also report an increased 'awareness' of their contacts. Many people do not notice their contacts precisely because the lack of Oxygen means the surface of the cornea becomes less sensitive. Therefore, when wearing gas permeable silicone hydrogel lenses, the corneal surface seems to maintain its normal level of sensitivity, allowing the wearer to feel the lens on their eye.

This increased sensitivity was researched in a study completed in 2010 and published in Investigative Ophthalmology & Visual Science. The results were that those fitted with silicone hydrogel lenses, or refitted with them, reported increased sensitivity, as well as increased sensitivity amongst silicone hydrogel wearers when their lens care programs were altered.

Those behind the study have called for further research to be done on how eye surface sensitivity changes with long term usage of silicone hydrogel lenses. They have also suggested that more work can be done into whether corneal and conjunctiva sensitivity may affect the discomfort that wearers of all soft lenses experience towards the end of the day.

It was found that, of the 10,000 patients in the study, those who wore their contacts overnight - in both groups of soft contact lenses - were at a higher risk of developing keratitis. In patients who wore standard hydrogel lenses for continuous periods of time, 0.48 per cent developed NSK, with 0.96 per cent contracting SK; while of those wearing silicone hydrogel contacts continuously, 0.99 per cent developed NSK and 0.20 per cent SK.

Therefore, the study revealed that wearing contact lenses while sleeping causes a significantly higher risk of developing a severe form of keratitis than wearing lenses while awake. However, the study's researchers suggested that for those who choose to wear contacts for continuous periods of time, silicone hydrogel lenses are recommended.

Although from this study silicone hydrogel lenses can be seen to increase the chances of developing non-severe keratitis, it must be pointed out that the study did not show whether those showing signs of NSK had been refitted with silicone hydrogel lenses after contracting a similar infection while wearing regular hydrogel lenses. In such instances, the infection could be caused by their standard hydrogel lenses.

Moreover, when this study was conducted, the only silicone hydrogel lenses available were those designed to be worn for monthly refitting, while standard hydrogel lenses were available for more regular replacement. Now, however, silicone hydrogel contact lenses are available in monthly, two-weekly and even daily disposable formats.

For many opticians, the over-arching benefit of wearing silicone hydrogel contact lenses is the reduction in hypoxia-related symptoms (discomfort associated with a lack of oxygen reaching the cornea), caused by some hydrogel soft lenses. The result has been fewer cases of contact lens-related problems and severe keratitis, caused by wearing contacts continuously.

It is also thought that, for many, the use of highly oxygen-permeable lenses allows them to wear their contacts for longer hours, without experiencing discomfort as they would with standard soft hydrogel lenses. The use of silicone hydrogel materials in these lenses also means that a larger variety of lens designs can be manufactured.

However, everyone's eyes are different, and some people do report more 'awareness' of their lenses with silicone hydrogel contacts. There is even some research to suggest that silicone hydrogel lenses attract certain deposits more than regular hydrogel lenses. Some lens care products may also not be as effective on silicone hydrogel lenses.

Furthermore, in some instances silicone hydrogel lenses have been thought to be associated with a higher chance of developing inflammation of the cornea, as well as some other complications, although the reasons for this are as yet unclear.

Some researchers also say that there is no improvement in end-of-day tiredness of eyes from wearing silicone hydrogel contact lenses, and, in America, the only FDA-approved contact lens for dry eyes in a standard hydrogel lens.

However, as with every form of eyewear there are pros and cons. Therefore, Vision Direct advises that, before making any decisions, you should first consult your options and your requirements with your optician.