surgery. participants who developed new visual symptoms after surgery, most often developed halos. up to 40 percent of participants with no halos before lasik had halos three months following surgery. up to 28 percent of participants with no symptoms of dry eyes before lasik, reported dry eye symptoms at three months after their surgery. less than 1 percent of study participants experienced "a lot of" difficulty with or inability to do usual activities without corrective lenses because of their visual symptoms (halos, glare, et al.) after lasik surgery. participants who were not satisfied with the lasik surgery reported all types of visual symptoms the questionnaire measured (double vision/ghosting, starbursts, glare, and halos). the fda's director of the division of ophthalmic devices, said about the lasik study "given the large number of patients undergoing lasik annually, dissatisfaction and disabling symptoms may occur in a significant number of patients". also in 2014, fda published an article highlighting the risks and a list of factors and conditions individuals should consider when choosing a doctor for their refractive surgery. contraindications not everyone is eligible to receive lasik. severe keratoconus or thin corneas may disqualify patients from lasik, though other procedures may be viable options. those with fuchs' corneal endothelial dystrophy, corneal epithelial basement membrane dystrophy, retinal tears, autoimmune diseases, severe dry eyes, and significant blepharitis should be treated before consideration for lasik. women who are pregnant or nursing are generally not eligible to undergo lasik. people with large pupils (e.g. due to taking medications or in the younger age group) may be particularly prone to symptoms such as glare, halos, starbursts, and ghost images (double vision) in dim light after surgery. because the laser can only work on the inner section of the cornea, the outer rim is left unaffected. in dim lighting, a patient's pupils dilate and may be predisposed to optic aberrations due to refractive asynchrony of the two regions with regards to the incoming light. further research since 1991, there have been further developments such as faster lasers; larger spot areas; bladeless flap incisions; intraoperative corneal pachymetry; and "wavefront-optimized" and "wavefront-guided" techniques which were introduced by the university of michigan's center for ultrafast optical science. the goal of replacing standard lasik in refractive surgery is to avoid permanently weakening the cornea with incisions and to deliver less energy to the surrounding tissues. more recently, techniques like epi-bowman keratectomy have been developed that avoid touching the epithelial basement membrane or bowman's layer. experimental techniques "plain" lasik: lasek, epi-lasik, wavefront-guided prk, advanced intraocular lenses. femtosecond laser intrastromal vision correction: using all-femtosecond correction, for example, femtosecond lenticule extraction, flivc, or intracor), keraflex: a thermobiochemical solution which has received the ce mark for refractive correction. and is in european clinical trials for the correction of myopia and keratoconus. technolas femtec laser: for incisionless intracor ablation for presbyopia, with trials ongoing for myopia and other conditions. lasik with the intralase femtosecond laser: early trials comparing to the lasik with microkeratomes for the correction of myopia suggest no significant differences in safety or efficacy. however, the femtosecond laser has a potential advantage in predictability, although this finding was not significant. comparison to photorefractive keratectomy a systematic review that compared prk and lasik concluded that lasik has shorter recovery time and less pain. the two techniques after a period of one year have similar results. a 2017 systematic review found uncertainty in visual acuity, but found that in one study, those receiving prk were less likely to achieve a refractive error, and were less likely to have an over-correction than compared to lasik.