Gas permeable lenses

Gas permeable or ‘GP’lenses for short are an alternative to soft contact lenses. They combine the properties of both rigid and soft lenses and are longer lasting than the highly popular soft lenses.

These lenses are permeable to oxygen which means that they enable a greater amount of oxygen to pass through to the eye compared to soft lenses. This is a definitive advantage as the transmission of oxygen to the eye is vital for its health and normal functioning.

A gas permeable lens is produced from a firmer, durable material which gives them an increased longevity and less risk of tearing or breaking. They also give clearer, crisp vision than soft lenses.

Advantages of gas permeable lenses

Many people prefer gas permeable lenses as they find them easier to handle and insert/remove from their eye. They may take longer to adapt to but when you have become used to them then you will find them as effective as soft lenses.

They sit well inside the eye and help to correct common refractive errors such as long sightedness and short sightedness. They can also be worn by people with astigmatism which includes those people with a high level of this condition.

They are a smaller type of lens which sits within the area of the cornea and is available in a range of styles, corrective powers and designs. The materials used to produce gas permeable lenses are also used in the production of lenses for astigmatism (toric lenses), bifocal lenses and multifocal lenses.

They may not be as popular as soft lenses but there are opticians and/or contact lens practitioners who recommend these instead of soft lenses. They argue that they are better option for correcting faulty vision and are healthier for the eyes as well.

Greater oxygen performance

Gas permeable lenses allow more oxygen to the eye compared to soft lenses although this gap is narrowed since the arrival of silicone hydrogel lenses. Soft lenses may be viewed as easier to use, convenient and comfortable but they do not usually allow the same amount of oxygen to pass to the eye.

The issue of oxygen performance in contact lenses is an important one and something to consider when deciding which lenses to buy.

Long lasting and cost effective

Another plus with gas permeable lenses is their durable nature which means that they last longer than soft lenses which is cost effective as well. Gas permeable lenses are a cheaper option.

Greater resistance to protein deposits

They are more resistant to the build up of deposits such as protein on the lens and are easier to clean as well. They are designed to be re-used so need to be removed and cleaned after each use.

Good alternative to soft lenses

They hold their shape well, are robust and are suitable for people who are unable to wear soft lenses. They are also an option for people with the following eye conditions:

• Keratoconus
• Presbyopia
• Astigmatism (soft lenses do produce the desired result)

They are also worn by people who have undergone a refractive procedure such as laser eye surgery which have not produced the results they wished for.

These lenses are also used during orthokeratology in which a special type of contact lens is worn during the night but removed the following morning. This lens reshapes the cornea during the night to correct a visual problem such as short sightedness.

Orthokeratology is discussed in more detail (see below).

Orthokeratology

This is also known as ‘ortho-k’ for short: it refers to a type of contact lens (usually a gas permeable lens) which is designed to be worn overnight. This alters the shape of the cornea with the aim of correcting a refractive error, e.g. myopia.

This procedure is also used to treat low levels of astigmatism.

These lenses can be worn each night or on alternate nights before being removed in the morning. They are chosen by people who do not want to wear contact lenses or glasses but require some form of visual correction.

But the effects of these lenses are temporary. If these lenses are discontinued for a few days then the cornea returns to its normal shape and prescription.

They are often worn by people who engage in sports such as swimming. This means that they only have to wear contact lenses at night rather than during the day which fits in better with their lifestyle.

Outcome of orthokeratology

It takes around two weeks before the effects of these lenses are noticeable. This modification to your cornea is usually accompanied by a few side effects such as halos and glare which appear as you view an object but these tend to settle down.

However, there are a few people who find that these effects do not completely disappear especially if they have large pupils.

Another thing to bear in mind is that once your cornea has been reshaped to the desired measurement (prescription) then you will need to continue wearing these lenses to maintain this prescription.

Once you stop your cornea will resume its original shape.

Orthokeratology is ideally suited to people who do not want to wear glasses or contact lenses all the time but do not mind doing so for short periods of time, e.g. at night. But they are not a good choice if you do not want to be reliant upon contact lenses at any time.

Laser eye surgery may be a better option in this situation.