Refractive Errors and Laser Eye Surgery

Laser eye surgery – many have heard about it, and those among us who wear glasses have dreamed of having it ourselves. But despite being a glamorous solution to refractive errors like long- and short-sightedness, it remains a mystery, even with its huge implications for ophthalmology.

Laser eye surgery, also known as laser vision correction, is an umbrella term for different surgeries that reduce refractive errors using laser treatment. In normal vision, light entering the eye converges at a certain point on the retina (the part of the eye that detects light) - this is called the focal point. Refractive errors are disorders caused by eyeball length being too long or short, so the focal point occurs in front of, or behind the retina. In myopia (short-sightedness) the eyeball is too long, which means the focal point occurs in front of the retina instead of on it. On the other hand, in hyperopia (long-sightedness), the eye is too short, which means the focal point is behind the retina. Normal vision, myopia and hyperopia are all shown in the diagram below. 

The most common forms of laser eye surgery are LASIK (laser assisted in-situ­ keratomileusis), SMILE (small incision lenticule extraction) and PRK (photorefractive keratectomy). Although they are different surgeries, they all work under the simple principle of reshaping the cornea. The cornea sits at the front of the eye and is responsible for most of the refraction that occurs as light enters the eye. Therefore, reshaping it will cause light to refract differently in order to balance out the effects of myopia or hyperopia. In myopic patients, laser eye surgery removes the central corneal tissue, flattening the corneal curvature  and reducing its refractive power. This pushes the point of focus backwards and thus closer to the retina. Conversely, in hyperopic patients, laser eye surgery removes peripheral tissue, increasing the curvature of the cornea. This allows it to refract light more, bringing the point of focus forward and towards the retina.

Although similar in their overall effect, each procedure differs in how it goes about reshaping the cornea. In LASIK, a flap is made from corneal surface tissue, called the epithelium, using a microscopic blade called a keratome. This exposes the underlying corneal tissue, the stroma, which is strategically vaporized using a laser to flatten the cornea. This laser fires for less than a picosecond (10-12 s), ensuring that only one layer of corneal tissue is vaporized at a time. After this has been done, the flap is closed and remains in place by natural adhesion until it heals.

PRK operates in a similar manner except that instead of creating a flap to access deeper tissues, the laser is used to vaporize the corneal epithelium directly to access deeper corneal tissues. In SMILE, instead of a flap being created, a small hole is cut out, allowing the laser to penetrate straight to deeper corneal tissues. The laser then cuts out a disk-shaped section of corneal tissue (called a lenticule) which is then removed through the incision.

 Compared to cataract and glaucoma surgeries, practiced as far back as the 1740s and 1860, laser eye surgery is a novel procedure, discovered only in 1987 and still being perfected. Because of this, there is still a lot of uncertainty surrounding the procedure. However, even in this short timespan, laser eye surgery has advanced so much, starting from PRK, to LASIK and now the up-and-coming SMILE surgery. One thing remains certain; with the current rate of progress in laser eye surgery, a future where long- and short-sightedness are no longer a problem looks clearer than ever.

Written by Omar Haque


References:

Barrett, K, Barman, S, Brooks, H and Yuan, J (2019). Ganong's Review of medical physiology. 26ed. New York. McGraw-Hill Medical.         

The Eye Clinic of Florida (2021). Laser Vision Correction – LASIK and PRK [internet].  [Last accessed 16 Oct 21]. Available from: https://seebetterflorida.com/lasik/

Moshirfar M, Bennett P, Ronquillo Y. Laser In Situ Keratomileusis. [Updated 2021 Jul 31]. StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. [Last accessed 16 Oct 21] Available from: https://www.ncbi.nlm.nih.gov/books/NBK555970/

Salim, S. Glaucoma and Refractive Surgery (2020). Review of Ophthalmology [internet]. Available from: https://www.reviewofophthalmology.com/article/glaucoma-and-refractive-surgery

Optegra (2019). Facts And Figures About Laser Eye Surgery [internet]. [Last accessed 17 Oct 21] Available from: https://www.optegra.com/eye-health-blog/laser-eye-surgery/facts-and-figures-about-laser-eye-surgery/

Image 1: The process of LASIK surgery (The Eye Clinic of Florida, 2021)

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