Refraction for Refractive Surgery:

 An accurate refraction is possibly the single most important element in ensuring a good result from any refractive surgery. Prof. Stephen Trokel, who invented PRK, has said that the biggest variable in PRK/LASIK is the refraction. Surgery is not as reversible as glasses and it is best to get it right first time. Some refractive points are:

1. Minimum minus for myopes and maximum plus for hyperopes - do cyclo refraction for the younger hyperope.
2. Auto refractors are very helpful for the axis and strength of the cyl to supplement any retinoscopy.
3. Try and push the optical cyl towards the physical cyl as shown by topography and keratoscopy. Best results are obtained where all these 3 readings are similar. If the optical cyl is wildly different from the physical cyl, then the patient may not be suitable as there will be large torque effects with the laser.

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Surface Laser Care:

1. Haze  is maximal at about 6 weeks post op and is a bigger risk in:

        a) High myopes

        b) Anyone with delayed re-epithelialisation.

In these cases it is probably worth giving a course of steroid drops from about day 7 (epithelialisation should be complete) for about 2 months. I normally use FML The only published paper, from St Thomas's, showed that steroids did not have any effect at all. Many practitioners, however, think that they do make a difference in some patients. Watch for any steroid induced IOP rise and don't forget that the IOP's will be falsely low after PRK, especially when measured with an NCT. Most patients will not need steroid drops.

2. Re-treatment: The 3 pointers for a low risk re-treatment are:

    a) No haze

    b) No loss of BCSVA

    c) Regular topography.

Some patients are primary under treatments and these are also usually low risk and will have all of the above 3 characteristics. They will fail to have an initial hyperopic refraction on the 1 week check-up. Re-treatments are normally done at around 6 months post op.

Re-treatment with Haze: Haze can be lasered or scraped away physically but is very likely to return after a few weeks. There has been about 4 years experience now using Mitomycin-C, a cytotoxic drug. After haze clearance, this is placed on the cornea in a sponge for 2 minutes in a 0.02% dilution and then washed off with copious BSS. 

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Post op check-up's are usually at 1 day, 1 month and 3 months. The best corrected spectacle visual acuity (BCSVA) at day 1 is usually only 1 line down at the most on the pre-op BCSVA. If it is any more than this then you should look for the reason why. Although in LASEK most problems get better with time, this is often not the case with LASIK and problems such as striae, SOS, infection, significant interface debris etc. should be dealt with immediately. The most important follow up appointment is that at day 1. Refractive enhancements are best done at 2-3 months and no later than 6 months.

1. Infection and Sands of the Sahara Syndrome (SOS) - a sterile lamellar keratitis - (from Michael Knorz). SOS 0.4% incidence or less.

   Both present between 12 and 24 hours

   Infection is painful

   SOS is not painful: Graded -

    

   Grade 1 - Partial infiltrate with no topography changes

   Grade 2 - Mild complete infiltrate with no topography changes

   Grade 3 - Complete infiltrate with topography changes

   Grade 4 - Same as 3 + perilimbal injection, flare and cells in AC

    

   In SOS, polymorphs release proteolytic enzymes and lead to melt.

    

   Treatment of SOS:

    

   Grades 1 and 2 Hourly or half-hourly predforte

   Grades 3 and 4 Lift flap and wash out. Then half-hourly predforte. No mechanical debridement as the tissues are very soft

    

   Prevention of SOS : drape lids and use steroids post op. We also clean the microkeratomes with acetone at the beginning of every day to remove any bacterial toxins that have been shown to cause some cases of SOS

    

   Infection: with flap edge infiltrate, don't lift the edge of the flap, it will spread it further. For same reason don't do AC tap. If interface infection then wash out with BSS.

    

    

2. Striae: 0.3% incidence.

   Treat striae right away whether they affect the vision or not, as they will do eventually. Differentiate between epithelial and stromal striae. Kritzinger says that epithelial macrostriae cause stromal microstriae. Treatment is to re-hydrate (one can use hypotonic saline with care on the flap only to straighten out the striae). Distinguish striae, which are parallel and affect the vision, from reticulations, which are not parallel and do not degrade vision. Reticulations are almost universal and due to chord length disparity. Striae should be treated within the first 2 days post op. With a superiorly hinged flap, striae are usually horizontal and are most likely to be in the inferior part of the flap. Lateral flap displacement will cause vertical striae. Very slight peripheral striae can be left, but it is generally better to treat them right away. Minor  flap striae are the main reason for degradation of visual performance with LASIK and there is a narrow line between a good and poor result.

    

    

3. Epithelial Ingrowth: Occurs at 1 to 4 weeks post op. 0.5-1% incidence.

1. at edge of flap. Can be left if only a bit.

2. Central due to cells being introduced at surgery.

Can lead to irregular astigmatism and visual loss and may cause corneal melt and/or FB sensation. Prevention is by avoiding excess irrigation and by suctioning the fornices or keeping them dry. Removed by scraping flap and bed from hinge outwards. Corneal melt occurs at 1-4 months post op. Epithelial ingrowth is more likely following re-treatments, where the flap is "unzipped".

4.     Dry Eyes: 

 

Almost universal due to corneal nerves being cut/lasered - really a neurotrophic keratitis. Dry eye problems are worse after LASIK than PRK. Lasts for about 3/12 with Lasik. Use routine artificial tears about QDS at least in first 3/12. Watch for SPK at check up. Put in temporary or permanent punctal plugs if necessary. Superior hinged flaps may be worse at causing dry eyes because both long ciliary nerves are cut as they enter the cornea from the sides. Silicone punctal plugs are necessary in severe cases. Hyperopes tend to be the worse.

 

 

5.    Regression:  

        Most likely with hyperopes with very steep or flat corneae; 

        (>45.5D or <40D). Most regression is in the first week.       

         Regression also occurs with the higher myopes. 

6.    Subjective symptoms:

        Some people have variable vision during the day in the first few weeks. Glare is generally better than cls or no worse. Halos are not usually a problem in lower myopes but there are problems in some of the higher myopes. (see complications)        

Summary of incidence of complications:

Epithelial ingrowth 0.5 - 1% - more often in flap lift retreatments.

Free Cap 0.01%

Persistent SPK 0.25 - 0.5%(sicca)

Flap oedema 0.01%

Small flap 0.1%

Striae 0.25%

SOS 0.1 to 0.25%

Infection 0.01% ?

Re-treatment rate 2-5% for myopia and 5% for hyperopia.

About 2% have some complication, most minor.