Astigmatic Keratotomy (AK) can be viewed as a modified form of Radial Keratotomy (RK).  Regular astigmatism occurs when the cornea's shape is like a football, more curved in one direction than the other.  Light entering the cornea focuses in more than one point within the eye resulting in blurry and distorted vision.  Astigmatism is often found in combination with nearsightedness and farsightedness and can be both regular and irregular in form.

To treat astigmatism, the cornea must be made spherical, like a basketball.  This is done by making several incisions in the steepest part of the cornea causing it to relax and become more round.  Astigmatic Keratotomy is often performed in combination with other refractive procedures.

The Surgical Experience

The patients refraction is determined and full eye examination is performed (including topographies, keratometry, and pachymetry). The cylinder is expressed in (or transposed to) the + form (because the incision is made perpendicular to the + axis).

Then 90 and 0 axes are marked on the cornea using a gentian violet marker with the cornea anaesthetized with topical Proparacaine and the patient seated at the slit lamp and fixating on a distant object. Pre-op sedation (usually sublingual Ativan 1.0mg) may be given if desired.

The patient is reclined on the surgical chair and is positioned under the operating microscope. The center of the pupil is marked and the appropriate areas on the cornea to be incised are marked. The diamond blade is adjusted to 90% of the central pachymetry value for corneal incisions. For limbal incision, .6mm setting is used.

A variation of the radial keratotomy procedure can be used to correct astigmatism.   This variation involves making transverse cuts to the cornea to correct myopia.   These transverse cuts work by relaxing areas of the cornea so they bulge out slightly and heal with a reduced curvature.  In the case of astigmatism, where one meridian has more curvature than the other, the transverse cuts serve to reduce the curvature of one meridian so it heals in symmetry with the other.

The incision(s) are made according to the nomogram. Any debris is washed out of the incision, topical antibiotic (usually Ciloxan) is instilled, the eye is patched and the patient is discharged.

Post Operative Care

Usually little more than a scratchy eye sensation is experienced during the first day and after that nothing.  Antibiotic drops (usually Ciloxan) are used 4 times a day until the epithelium is healed.  The refraction may fluctuate for about 1 month and at that time enhancement may be performed if necessary.  A conservative approach to surgery is best to avoid over correction.

Complications of Astigmatic Keratotomy (AK)

1. Surgical perforation of the cornea.
   If a micro perforation occurs during the incision, the procedure may be completed later. If a micro perforation occurs, a suture may be required to close. However in AK, or Limbal Relaxing Incisions (LRI), perforation is very uncommon because of the relative thickness of the cornea. Theoretically damage to the iris and lens could occur in a perforation, but this is extremely rare.

2. Infection.
   If the incision becomes infected and particularly if there is a perforation, the eye is at risk of endophthalmitis (infection involving the structures inside the eye). This condition requires intensive treatment with topical, local, and systemic antibiotics. There is a risk of complete loss of vision following endophthalmitis. The incidence of serious infection is about 1 in 5000 cases. Full sterile precautions are taken and prophylactic antibiotic eye drops are used to minimize the risk of infection.

3. Over correction and under correction.
   The exact outcome is always uncertain due to variability in individual healing response so that some patients may be overcorrected or under corrected. Further surgery to reverse the over correction or under correction may be necessary.

4. Instability of refraction.
   In the immediate post op period there is fluctuation up until about 2 months. Occasionally a hyperopic shift will occur over several years. Changes in atmospheric oxygen may cause the cornea to change and increasing myopia may occur. This is reported in mountaineers. These changes are more noticeable in Radial Keratotomy than in Astigmatic Keratotomy.

5. Weakening of the cornea.
   Obviously if the cornea has several deep incisions, it may be more prone to rupture if a direct blow is received. This has been reported in Radial Keratotomy but not in simple AK.
   

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In spite of the above mentioned complications, the chance of having surgery without complications is 99%. If complications do develop, treatment is possible and the final result should be excellent.

Astigmatism cannot easily or predictably be corrected fully.  About one third of those who have surgery to correct the irregularity find that their eyes regress to a considerable degree and only a small improvement is noted.  Another third find that the astigmatism has been significantly  reduced but not fully corrected.  The remaining third have the most encouraging results with the most or all of the desired correction achieved.

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