Lasik can effectively correct up to +4.00 dioptres (400 degrees) of farsightedness. Farsightedness is the optical opposite of myopia (nearsightedness). It is caused by a shorter than normal eyeball length or flatter than normal cornea curvature.

However, it is important not to confuse normal farsightedness with an aging condition called presbyopia or middle-age farsightedness. This is a different condition that afflicts those past the age of 35 years. Presbyopia is caused by the reduced ability of the eye to focus for near as a person grows older. Lasik will NOT do away with the need to wear reading glasses for this condition. In fact, those 35 years and above may find that after Lasik, while they can now see perfectly well for distance, reading glasses may be required for reading (see also Q33 & Q34).

-Under or over-correction:
Under or over-correction of refractive power can occur despite the surgeon's best effort. This is due to individual variation in the healing process and in tissue density. Over- correction is less well accepted than under-correction. While an under-corrected eye can still see well for near without lenses, a significantly over-corrected eye will need glasses to see well for both near and distance. For this reason, most doctors will deliberately aim for a slight under-correction, especially when correcting patients with high myopia.

-Cornea flap complications:
The Lasik procedure may be aborted because of problems encountered when creating the cornea flap. These include an incompletely cut flap, a completely sheared off flap or a flap with a button-hole defect. Post-operatively, wrinkles may develop within the flap or the flap may become dislodged. All these flap complications can cause optical aberrations that are difficult to correct with glasses.

There is also a category of post-op flap interface disorders. This includes Diffuse Lamellar Keratitis which is caused by trapped foreign body debris or toxins and an epithelial ingrowth syndrome caused by migration of surface epithelial cells into the undersurface of the flap. In the severe case, the flap needs to be re-lifted, cleaned and repositioned. While severe cornea flap complications are rather uncommon, they can lead to loss of best corrected visual acuity.

- Loss of Best Corrected Visual Acuity:
This means seeing worse after the operation than before despite the best spectacles or contact lens correction. A common cause would be irregular astigmatism arising from de-centred or uneven ablation. The uneven bumps on the surface of the cornea that result from uneven ablation is also known as "central islands".

Loss of best corrected visual acuity can also occur from excessive cornea scarring, cornea haze and flap related complications. Usually, vision improves with time as further healing occurs. Severely affected patients may suffer a permanent drop in their best corrected visual acuity. Fortunately in most cases, vision loss is limited only to difficulty in reading the bottom few lines of the visual acuity chart.

- Equipment malfunction:
Both the microkeratome and the excimer laser are manufactured and maintained according to the manufacturers' exacting specifications. However, it is still possible that the machines may malfunction, requiring the procedure to be stopped before completion. In some instances, this could result in loss of best corrected vision.

- Contact Lens Wear Intolerance:
There is the risk that should you need to wear contact lenses after the procedure, they may not fit comfortably. This is due to changes in the cornea shape after the procedure that affect the way the contact lens sits on it.

- Dry Eyes:
Dryness of the eyes and recurrent cornea surface epithelial cell erosions are other common but non-sight-threatening problems that can occur after Lasik surgery. Nonetheless, they can be a source of frequent irritation.Therefore, frequent instillations of lubricating eye drops are recommended during the first few weeks post-operatively.

- Eye Infection:
Severe infection that cannot be controlled by antibiotics is one of the most feared complication of any eye surgery. Fortunately the incidence is rare. It is estimated to occur in less than 1 in 5,000 eyes. When it occurs, it can cause severe cornea scarring, vision loss and even loss of the eye. Therefore, it is important to keep the eye clean, avoid going to crowded, dirty places and to apply the prescribed antibiotic eye drops diligently in the immediate post-op period.

- Glaucoma:
In some eyes, the long term use of the anti-inflammatory steroid eye drops commonly prescribed to control scarring can cause glaucoma.

- Retinal Detachment:
Retinal detachment may occur if predisposing lesions, such as retinal tears and areas of retinal thinning, are not detected and treated.

Yes. This complication is also known as loss of best corrected visual acuity. It means that you are not able to see as well after surgery as you did before surgery, even with the best spectacles or contact lens correction given. It most commonly arise from cornea related problems such as corneal haze or scarring, decentred or uneven laser ablation and cornea flap complications. Fortunately, the likelihood of this happening is less than two per cent and there is a fair chance of further vision improvement with time. Loss of best corrected vision occurs is usually limited to difficulty in seeing clearly the bottom few lines of the visual acuity chart.

Other attributes of vision may become affected. Foremost of these is a decrease in contrast sensitivity. This means that even though you may have normal 6/6 vision, you have decreased ability to discriminate between shades of grey, especially under dim lighting. This may cause objects to appear fuzzy or greyish. There may also be complaints of excessive light sensitivity, glare, haloes or ghosting. Haloes are "rings"around lights. Ghosting is the phenomenon of seeing a second false image next to the real one. Most of these vision complaints will alleviate with time but in some cases, they may persist indefinitely and affect the overall quality of vision.