The amazing thing about PRK is that it works very well to correct vision. And with PRK, there is zero concern about having any issues with a lasik flap. However, PRK does take longer to heal and recover from. And during this healing, there is the potential for haze to develop within the cornea. This haze can cause blurry vision.
Haze after PRK occurs from an overly excited healing process in the cornea. Fortunately, advances in laser technology and the use of a special medication called mitomycin C have dramatically reduced the incidence of haze after PRK. However, certain patients, especially those with high prescriptions will be at higher risk.
To really get an understanding of the haze after PRK, let’s explore what actually causes it, how the surgery has changed to prevent most of it and how different treatments put you at a higher risk.
What Causes Haze?
In order to correct vision, laser energy is applied to the stroma of the cornea. This happens in both lasik and PRK. The stroma is the thickest most rigid part of the cornea. Sitting on top of the stroma is the epithelium.
Epithelium and stroma of the cornea; Image by StemBook (CC BY 3.0) / modified from original
The difference between lasik and PRK is how the stroma is accessed. During lasik, a lasik flap is created to expose the stroma for treatment. During PRK, however, the epithelium is removed instead. The eye is treated with the laser, and then the epithelium regrows over the following days.
After PRK, the cornea must heal up. And haze is actually a result of an exuberant healing response from the cornea.
Not all haze after PRK is the same. There are actually two different types: early haze and late haze.
Early haze
Technically almost everyone may have some degree of early haze. However, this haze is insignificant and may not even be clinically detected in most cases. This isn’t any concerning haze after PRK.
Within the cornea are cells which build the structural framework of the cornea called corneal fibroblasts or corneal keratocytes. Starting about one week after PRK, these cells can start to get a little cloudy. These cells can also start to produce a more disorganized structural framework of the cornea. Both of these two things make the cornea less transparent.
Early haze peaks at around 1-3 months before slowly disappearing over time.
Late haze
Post PRK haze that occurs much later, ie. late haze, is the more concerning type of haze. You can consider late haze as somewhat similar to the scarring process that we have elsewhere on the body.
This particular type of haze can cause the prescription to wear off a little, can cause some glare and when severe enough can cause some blurry vision.
Following PRK, inflammation enters the stroma. This inflammation brings in a variety of cells to try to “repair" the cornea. Eventually this leads to the creation of a particular cell called a myofibroblast.
Remember those corneal keratocytes? Those can also be activated by the laser treatment (especially large laser treatments used to treat high prescriptions) and turn into myofibroblasts as well.
Like in a scar, the myofibroblast attempts to repair the cornea by laying down fibrosis. This causes severe haze in the cornea.
Late haze peaks at approximately 3-6 months after PRK and depending on the severity can take up to years to resolve.
That Sounds Bad!
Yes, having blurry vision from haze after PRK isn’t the best outcome. But fortunately, haze was a bigger issue in the past with PRK than what we experience with today’s treatments.
In fact, the risk of moderate to severe late haze for patients who don’t have very high prescriptions is only about 0.1%. Those with higher prescriptions have a little higher risk at 1%.
As our understanding of the causes of haze increased over the past few decades, we’ve improved our ability to prevent it from occurring.
Quality of laser treatment
The myofibroblast is stimulated by growth factors coming from the natural tears and epithelium. Once the cornea is healed over, the epithelium places barriers to prevent these growth factors from reaching those myofibroblasts. The quicker the epithelium can heal over, the less the risk of haze.
See also How Long Does A Scratched Eye Take To Heal?
Epithelium heals over quicker when the surface is smooth and regular. If the surface is irregular, this delays the complete healing.
Older PRK laser treatments left more of an irregular surface than the lasers we use today. This increased the chance of haze.
Mitomycin C
What if we can prevent those myofibroblasts from forming in the first place? That would work well to prevent late haze. And it was discovered that a special medication called mitomycin C (commonly abbreviated as MMC) can do just that.
Mitomycin C binds to DNA to prevent a cell from reproducing. In fact, because of this ability, mitomycin C is actually used to treat cancer as a chemotherapeutic agent. But mitomycin C is also used effectively in the cornea to prevent extra cells from diving and turning into myofibroblasts.
Immediately after the PRK treatment, mitomycin C is often routinely applied to the stroma before being later washed off. Routine use of this medication has led to a considerable drop in post PRK haze.
Who’s At Risk?
While the chance of haze is very low, there are some patients who will be at a higher risk than others.
- High prescriptions. As mentioned previously, higher prescriptions (such as those greater than -6.00 diopters of prescription) mean more activated corneal cells and higher the risk of developing haze.
- High astigmatism & farsighted treatments. While today’s treatments are very regular, adding a high amount of astigmatism to the treatment or performing a farsighted treatment makes the surface of the cornea more irregular. This can delay complete healing and lead to the development of more haze.
- Younger age. In general, younger individuals have a more robust healing response. Same goes in their corneas.
- Certain conditions such as autoimmune conditions or those prone to develop allergic responses (known as atopy) can have a more intense healing response as well which increases the risk of haze.
- UV light generates microscopic damage within the cornea. This is damage that kicks up the healing response in the cornea. Not ideal after PRK. Thus, it is important to wear sunglasses with UV protection while everything is healing after PRK.
What To Do About Haze
In many cases, not much needs to be done with post PRK haze. This is especially the case if the haze is early or mild. Gradually, mild haze resolves on its own. Keratocytes absorb the abnormal fibrosis to cause the haze to disappear. (similar to how a scar clears up on its own over time). This can take months to sometimes years for the haze to heal up.
Steroid eye drops can potentially help speed up the resolution of mild forms of haze. But steroid eye drops aren’t recommended for long term use because of the risk of cataract and potential increase in eye pressure.
Severe cases of haze may require more manual intervention. This can require manual scraping or using a laser to remove the layer of haze.
Summary
Haze can occur either “early" or “late" after PRK. Early haze is non-significant and doesn’t cause any issues. Late haze, however, can cause some changes in prescription, glare or even blurred vision. Improvements in procedure technique and technology have reduced the chance of haze to very low levels. But certain individuals will be at a little more risk than others.
Like what you just read? Use Social Media?
Stay connected and join the discussion by following Eye Mountain on
Also Check Out: