• Giant papillary conjunctivitis  
• Corneal abrasion 
• Corneal neovascularisation 
• Corneal oedema 
• Corneal ulcer

All contact lenses are still foreign bodies to the eyes, they can and sometimes do give rise to eye problems. However, these complications are fairly uncommon and easily remedied. The incidence of these complications from lens wearing can be prevented if they are utilized properly, in terms of proper lens fitting, appropriate wearing schedule and stringent lens hygiene. Wearers should view the warning signs and symptoms seriously. Consult your eye-care practitioner immediately if prolonged red-eye, eye discomfort, reduced vision, sensitivity to light and eye discharge develops.
 

Giant papillary conjunctivitis(GPC) is the most common contact lens related problem.

It appears as numerous tiny swelling on the inside surface of the eyelids, particularly on the upper tarsal plate. 
The most common underlying cause is an allergic reaction to the lens protein deposit, lens material or solution. Although it's not sight-threatening, the itchiness, increased lens awareness, sticky discharge and reduced vision make lens wearing unbearable. 

Once GPC occurs, it's best to discontinue contact lens wear until the signs and symptoms have resolved and your practitioner has given you green light to resume lens wear. The recurrence of this condition is not uncommon. When resume NEW lens wear, you are advised to pay particular attention on lens maintenance, replace your lenses more frequently, or consider switching to disposable or RGP contact lenses. 
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Corneal abrasion may occur from a tiny particle (for example sand or some airborne debris) getting under the lens. This is far more common with RGP than soft lenses. There is a varying degree of pain or discomfort and a feeling of foreign body sensation. 

 It may result from wearing an RGP lens with an edge defect or a soft lens with an edge tear. Often does not require medical treatment. If deeper corneal layer is affected or the abrasion is over a large area, immediate medical treatment is needed. 
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Corneal neovascularisation is the ingrowth of abnormal blood vessel into the cornea from the limbus (junction of cornea and eye-white).The cornea normally has no blood vessels. Contact lens wear slightly reduce the oxygen deliver to the cornea, when lens wear is prolonged for days at a time or a lens that significantly limit the oxygen supply to the cornea, the cornea responds to this chronic oxygen deprivation by growing new abnormal blood vessels. 

Further progression involves ingrowth of larger vessels accompanied by increasing amount of connective tissue into the transparent cornea. This fibrovascular scar is called Pannus, if unchecked it can grow over the pupil region of the cornea. 

The occurrence of neovascularisation requires immediate lens change to allow sufficient oxygen supply to the cornea, by using of higher oxygen transmissibility lens material and stop extended wearing schedule. 
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Corneal oedema (swelling), like neovascularisation, related to insufficient oxygen to the cornea. Improperly used extended wear lenses are the most likely cause. If detected early and remedial action taken, the cornea will most likely without complications. 

There are often no symptoms. In some cases, wearer may experience hazy vision, haloes around lights and pain upon removal of the lenses . Allowing the condition to continue can cause breaks on the corneal surface and lead to corneal infection and permanent scarring of the cornea. 

Prevention is the best treatment. Regular follow-up examinations can detect oxygen deprivation and microscopic cornea changes before they become problematic. Replacing contact lenses as recommended and refrain from over-night lens wear is necessary to maintain normal eye health. 
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Corneal ulcer is the most devastating contact lens complication. The responsible micro-organisms to this complication may be bacteria, fungi or parasitic amoeba. 
Wearing a lens without proper cleaning and disinfection, small break or abrasion on the cornea as a result of foreign body or excessive corneal stress, have the greater likelihood for infectious micro-organisms to cause corneal infection. The risk is greater in soft lens wearers and those wearing lenses on extended wear basis. 

Symptoms of acute eye pain, foreign body sensation, eye discharge and a red-eye should warn the wearer to remove the lens and seek advice immediately from your practitioner. Delay in treatment of this condition can lead to corneal scarring or corneal perforation in extreme case. 

Prevention is to: 

• Stop extended lens wearing to minimize the possibility of break on the cornea.
• Maintain stringent lens hygiene.
• Use only recommended lens solutions as your practitioner's direction.