Large epithelial plugs may cause more scattering of light, leading to the appearance of visual phenomena such as flares and starbursts – especially in situations such as night driving, where the stark light of car headlights abounds. These dark conditions cause the pupil to dilate, maximizing the amount of scattered light that enters the eye. In cases where large epithelial plugs lead to such aggravating symptoms, patients may seek further surgical treatment to alleviate the symptoms. Increasing altitude can cause partial blindness in people who have undergone<unk> , as discovered by mountaineer Beck Weathers (who had undergone<unk>  during the 1996 Mount Everest disaster. The incisions of<unk>  are used to relax the steep central cornea in patients with myopia. The original technique – consisting of incisions from periphery to center – was called "the Russian technique", while the later advances of performing controlled incision from center to periphery was called "the American technique".<unk>  enjoyed great popularity during the 1980s, and was one of the most studied refractive surgical procedures. Its 10-year data was published as the PERK (Prospective Evaluation of<unk>  study, which proved the onset of progressive hyperopia – often found a decade after the original surgery – is due to continued flattening of the central cornea. A conceptually opposite technique of using hexagonal incisions in the periphery of the cornea is known has hexagonal<unk>  (HK, described by Antonio Mendez of Mexicali, Mexico), which was used to correct low degrees of hyperopia. The idea behind HK was to make six peripheral incisions forming a hexagon around the central cornea to steepen the hyperopic flat cornea and, thereby, focus the rays of light more precisely onto the retina. These incisions can be of two types, either connecting or non-connecting.<unk>  may be performed with different types, numbers, and patterns of incisions. They can have 4, 8, 12, 16 or 32 incisions made in a number of patterns and orientations based on refractive errors, surgeon style and surgeon training. Many of these patients have had additional incisional surgeries like astigmatic<unk>  (AK), where incisions are placed at the steepest points of the cornea in people with astigmatism to relax and transform the cornea to a more spherical shape. Some people have had a combination of intraocular surgeries, such as pseudophakic or phakic implants, along with their<unk> es and many underwent "purse-string" suturing to control over-correction (Dr. Green's lasso suture). Due to the instability of the cornea seen with many age-related pathologies, it may be difficult to address visual acuity satisfactorily in people who have undergone<unk>  surgery but who later develop presbyopia (hyperopia caused by age-related changes in the crystalline lens). In these situations, factors to be considered include: Primary visual factors: :Quantitative: ::Decreased visual acuity (myopia, hyperopia, astigmatism) :Qualitative: ::Irregular astigmatism ::Small optic zone ::Incisions Secondary (associated) visual factors: :Presbyopia :Cataracts :Corneal scars :Corneal instability (thin/ectasia/trampoline effect)
Radial keratotomy
Phototherapeutic keratectomy
Boston keratoprosthesis
Corneal transplantation
Photorefractive keratectomy
Conductive keratoplasty
Laser thermal keratoplasty
Excimer laser trabeculostomy