[go: nahoru, domu]
Jump to content

LASIK

From Wikipedia, the free encyclopedia

This is an old revision of this page, as edited by Snafflekid (talk | contribs) at 00:58, 3 February 2007 (→‎Wavefront-guided LASIK: edit for continuity). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

LASIK, an acronym for Laser-Assisted in Situ Keratomileusis, is a form of refractive laser eye surgery procedure performed by ophthalmologists intended for correcting myopia, hyperopia, and astigmatism.[1] The procedure is usually a preferred alternative to photorefractive keratectomy, PRK, as it requires less time for full recovery, and the patient experiences less pain overall. Many patients choose LASIK as an alternative to wearing corrective glasses or contact lenses.

Technology development

The LASIK technique was made possible by Jose Barraquer (Colombia), who around 1960 developed the first microkeratome, used to cut thin flaps in the cornea and alter its shape, in a procedure called keratomileusis.

In 1981, Rangaswamy Srinivasan discovered that an ultraviolet excimer laser could etch living tissue in a precise manner with no thermal damage to the surrounding area. He named the phenomenon Ablative Photodecomposition (APD). Srinivasan and his co-inventors ran tests using the excimer laser and a conventional, green laser to etch organic matter. They discovered that while the green laser produced rough incisions, damaged by charring from the heat, the excimer laser produced clean, neat incisions. In 1983, Srinivasan collaborated with an ophthalmic surgeon to develop APD to etch the cornea[1].

LASIK surgery was developed in 1990 by Lucio Buratto (Italy) and Ioannis Pallikaris (Greece) as a melding of two prior techniques, keratomileusis and photorefractive keratectomy. It quickly became popular because of its greater precision and lower frequency of complications in comparison with these former two techniques. Today, faster lasers, larger spot areas, bladeless flap incision, and wavefront-optimized and -guided techniques have significantly improved the reliability of the procedure as compared to that of 1991. Nonetheless, the fundamental limitations of excimer lasers and undesirable destruction of the eye's nerves have spawned research into many alternatives to "plain" LASIK, including all-femtosecond correction (FLIVC), LASEK, Epi-LASIK, wavefront-guided PRK, and modern intraocular lenses.

Procedure

Preoperative

Patients wearing soft contact lenses typically are instructed to stop wearing them approximately 7 to 10 days before surgery. One industry body recommends that patients wearing hard contact lenses should stop wearing them for a minimum of six weeks plus another six weeks for every three years the hard contacts had been worn. [2] Before the surgery, the patient's corneas are examined with a pachymeter to determine their thickness, and with a topographer to measure their surface contour. Using low-power lasers, a topographer creates a topographic map of the cornea. This process also detects astigmatism and other irregularities in the shape of the cornea. Using this information, the surgeon calculates the amount and locations of corneal tissue to be removed during the operation. The patient typically is prescribed an antibiotic to start taking beforehand, to minimize the risk of infection after the procedure.

Operation

The operation is performed with the patient awake and mobile; however, the patient typically is given a mild sedative (such as Valium) and anesthetic eye drops.

LASIK is performed in two steps. The first step is to create a flap of corneal tissue. A corneal suction ring is applied to the eye, holding the eye in place. The step in the procedure can sometimes cause small blood vessels to burst, resulting in bleeding or subconjunctival hemorrhage into the white (sclera) of the eye, a harmless side effect that resolves within several weeks. Increased suction typically causes a transient dimming of vision in the treated eye. Once the eye is immobilized, the flap is created. This process is achieved with a mechanical microkeratome using a metal blade, or a femtosecond laser microkeratome (procedure known as IntraLASIK) that creates a series of tiny closely arranged bubbles within the cornea.[3] A hinge is left at one end of this flap. The flap is folded back, revealing the stroma, the middle section of the cornea. The process of lifting and folding back the flap can be uncomfortable.

The second step of the procedure is to use an excimer laser (193 nm) to remodel the corneal stroma. The laser vaporizes tissue in a finely controlled manner without damaging adjacent stroma by releasing the molecular bonds that hold the cells together. No burning with heat or actual cutting is required to ablate the tissue. The layers of tissue removed are tens of micrometers thick. Performing the laser ablation in the deeper corneal stroma typically provides for more rapid visual recovery and less pain.

During the second step, the patient's vision will become very blurry once the flap is lifted. He/she will be able to see only white light surrounding the orange light of the laser. This can be disorienting.

Currently manufactured excimer lasers use a eye tracking system that follows the patient's eye position up to 4,000 times per second, redirecting laser pulses for precise placement within the treatment zone. The energy of each pulse is usually in the milliwatt range [4] Typically, each pulse is on the order of 10–20 nanoseconds. After the laser has reshaped the stromal layer, the LASIK flap is carefully repositioned over the treatment area by the surgeon, and checked for the presence of air bubbles, debris, and proper fit on the eye. The flap remains in position by natural adhesion until healing is completed.

Postoperative

Patients are usually given a course of antibiotic and anti-inflammatory eye drops. These are discontinued in the weeks following surgery. Patients are also given a darkened pair of goggles to protect their eyes from bright lights and protective shields to prevent rubbing of the eyes when asleep.

Higher-order aberrations

Higher-order aberrations are visual problems not captured in a traditional eye exam which only tests for acuteness of vision. Severe aberrations can effectively cause significant vision impairment. These aberrations include starbursts, ghosting, halos, double vision, and a number of other post-operative complications listed below.

Concern has long plagued the tendency of refractive surgeries to induce higher-order aberration not correctable by traditional contacts or glasses. The advancement of LASIK technique and technologies has helped reduce the risk of clinically significant visual impairment after the surgery. One of the major discoveries was the correlation between pupil size and aberrations:[2] Effectively, the larger the pupil size, the greater the risk of aberrations. This correlation is the result of the irregularity between the untouched part of the cornea and the reshaped part. Daytime post-lasik vision is optimal, since the pupil is smaller than the LASIK flap. But at night, the pupil may expand such that light passes through the edge of the LASIK flap into the pupil which gives rise to many aberrations. There are other currently unknown factors in addition to pupil size that also affect higher order aberrations.

In extreme cases, where ideal technique was not followed and before key advances, some people could suffer rather debilitating symptoms including serious loss of contrast sensitivity in poor lighting situations.

Over time, most of the attention has been focused on spherical aberration. LASIK and PRK tend to induce spherical aberration, because of the tendency of the laser to undercorrect as it moves outward from the center of the treatment zone. This is really only a significant issue for large corrections. There is some thought if the lasers were simply programmed to adjust for this tendency, no significant spherical aberration would be induced. Hence, in eyes with little existing higher order aberrations, "wavefront optimized" lasik rather than wavefront guided LASIK may well be the future.

Wavefront-guided LASIK

Wavefront-guided LASIK[3] is a variation of LASIK surgery where, rather than apply a simple correction of focusing power to the cornea (as in traditional LASIK), an ophthalmologist applies a spatially varying correction, guiding the computer-controlled eximer laser with measurements from a wavefront sensor. The goal is to achieve a more optically perfect eye, though the final result still depends on the physician's success at predicting changes which occur during healing. In older patients though, scattering from microscopic particles plays a major role and may exceed any benefit from wavefront correction. Hence, patients expecting so-called "super vision" from such procedures may be disappointed. However, while unproven, surgeons claim patients are generally more satisfied with this technique than with previous methods, particularly regarding lowered incidence of "halos", the visual artifact caused by spherical aberration induced in the eye by earlier methods.

Complications

A subconjunctival hemorrhage is a common and minor post-LASIK complication.

The incidence of refractive surgery patients having unresolved complications six months after surgery has been estimated from 3%[4] to 6%.[5] The following are some of the more frequently reported complications of LASIK[4][5]:

Complications due to LASIK have been classified as those that occur due to preoperative, intraoperative, early postoperative, or late postoperative sources:[8]

Preoperative sources of complications

Intraoperative complications

  • The incidence of flap complications has been estimated to be 0.244%.[9] Flap complications (such as displaced flaps or folds in the flaps that necessitate repositioning, diffuse lamellar keratitis, and epithelial ingrowth) are common in lamellar corneal surgeries[10] but rarely lead to permanent visual acuity loss; the incidence of these microkeratome-related complications decreases with increased physician experience.[11][12] This risk is further reduced by the use of IntraLasik and other non-microkeratome related approaches.
  • A slipped flap (a corneal flap that detaches from the rest of the cornea) is one of the most common complications. The chances of this are greatest immediately after surgery, so patients typically are advised to go home and sleep to let the flap heal.
  • Flap interface particles are another finding whose clinical significance is undetermined.[13] A Finnish study found that particles of various sizes and reflectivity were clinically visible in 38.7% of eyes examined via slit lamp biomicroscopy, but apparent in 100% of eyes using confocal microscopy.[13]

Early postoperative complications

  • The incidence of diffuse lamellar keratitis (DLK)[6], also known as the Sands of Sahara syndrome, has been estimated at 2.3%.[14] When diagnosed and appropriately treated, DLK resolves with no lasting vision limitation.
  • The incidence of infection responsive to treatment has been estimated at 0.4%.[14] Infection under the corneal flap is possible. It is also possible that a patient has the genetic condition keratoconus that causes the cornea to thin after surgery. Although this condition is screened in the preoperative exam, it is possible in rare cases (about 1 in 5,000) for the condition to remain dormant until later in life (the mid-40s). If this occurs, the patient may need rigid gas permeable contact lenses, Intrastromal Corneal Ring Segments (Intacs),[15] Corneal Collagen Crosslinking with Riboflavin[16] or a corneal transplant.
  • The incidence of persistent dry eye has been estimated to be as high as 28% in Asian eyes and 5% in Caucasian eyes.[5] Nerve fibers in the cornea are important for stimulating tear production. A year after LASIK, subbasal nerve fiber bundles remain reduced by more than half.[17] Some patients experience reactive tearing, in part to compensate for chronic decreased basal wetting tear production.
  • The incidence of subconjunctival hemorrhage has been estimated at 10.5%.[14]

Late postoperative complications

  • The incidence of epithelial ingrowth has been estimated at 0.1%.[14]
  • Glare is another commonly reported complication of those who have had LASIK.[18]
  • Halos or starbursts around bright lights at night are caused by the irregularity between the lasered part and the untouched part. It is not practical to perform the surgery so that it covers the width of the pupil at full dilation at night, and the pupil may expand so that light passes through the edge of the flap into the pupil.[19] In daytime, the pupil is smaller than the edge. Modern equipment is better suited to treat those with large pupils, and responsible physicians will check for them during examination.
  • Late traumatic flap dislocations have been reported 1–7 years post-LASIK.[20]

Other

Although there have been a number of improvements in LASIK technology[21][22][23] , a large body of conclusive evidence on the chances of long-term complications is not yet in place. Also, there is a small chance of complications, such as slipped flap, corneal infection, haziness, halo, or glare. The procedure is irreversible.

The incidence of macular hole has been estimated at 0.2%[7] to 0.3%.[24]

The incidence of retinal detachment has been estimated at 0.36%.[24]

The incidence of choroidal neovascularization has been estimated at 0.33%.[24]

The incidence of uveitis has been estimated at 0.18%[25]

Although the cornea usually is thinner after LASIK because of the removal of part of the stroma, refractive surgeons strive to maintain a minimum thickness in order to not structurally weaken the cornea. Decreased atmospheric pressure at higher altitudes has not been shown to be extremely dangerous to the eyes of LASIK patients. However, some mountain climbers have experienced a myopic shift at extreme altitudes.[26][27] There are no published reports documenting diving-related complications after LASIK.[28]

Laser in situ keratomileusis increases higher order wavefront aberrations of the cornea.[29][30] Glasses do not correct higher order aberrations.

Microfolding has been reported as "an almost unavoidable complication of LASIK" whose "clinical significance appears negligible".[13]

Factors affecting surgery

The cornea typically is avascular because it must be transparent to function normally. Its cells absorb oxygen from the tear film. Low oxygen-permeable contact lenses reduce the cornea's absorption of oxygen, which sometimes results in the growth of blood vessels into the cornea, a process known as corneal neovascularization. This can cause a mild increase in inflammation and healing time and some discomfort during the surgery because of augmented bleeding. Although some contact lenses, notably modern RGP and soft silicone hydrogel lenses, are made of materials with higher oxygen permeability that help reduce the risk of corneal neovascularization, patients considering LASIK are cautioned to avoid overwearing their lenses. It is usually recommended that contact lens use be discontinued several days or weeks before the LASIK procedure.

A 2004 Wake Forest University study found that LASIK results are affected by heat and humidity, both during the procedure and in the two weeks before surgery.[31]

Patient satisfaction

Various surveys have been performed to determine patient satisfaction with LASIK. These surveys have found most patients to be satisfied, with anywhere from 92–98% of respondents describing themselves as satisfied.[18][32][33][34]

Some patients with poor outcomes from LASIK procedures report that they have significantly reduced quality of life due to vision problems, and have expressed concerns over the way that LASIK is marketed to potential patients.[35] Patients who have suffered LASIK complications have published websites[36] [7]in an effort to educate the public about the risks, and to provide forums[37][38]where prospective and past patients can discuss the surgery.

Safety and efficacy

The reported figures for safety and efficacy are open to interpretation. In 2003, the Medical Defence Union (MDU), the largest insurer for doctors in the United Kingdom, reported a 166% increase in claims involving laser eye surgery; however, the MDU averred that these claims resulted primarily from patients' “unrealistic expectations” of LASIK rather than “faulty surgery”.[39] A 2003 study reported in the medical journal Ophthalmology found that nearly 18% of treated patients and 12% of treated eyes needed retreatment.[40] The authors concluded that “higher initial corrections, astigmatism, and older age are risk factors for LASIK retreatment.”

In 2004, the British National Health Service's National Institute for Health and Clinical Excellence (NICE) considered a systematic review of four randomized controlled trials[41][42] before issuing guidance for the use of LASIK within the NHS.[43] Regarding the procedure's efficacy, NICE reported, "Current evidence on LASIK for the treatment of refractive errors suggests that it is effective in selected patients with mild or moderate short-sightedness" but that "evidence is weaker for its effectiveness in severe short-sightedness and long-sightedness." Regarding the procedure's safety, NICE reported that "there are concerns about the procedure's safety in the long term and current evidence does not appear adequate to support its use within the NHS without special arrangements for consent and for audit or research." Leading refractive surgeons in the United Kingdom and United States, including at least one author of a study cited in the report, believe NICE relied on information that is severely dated and weakly researched.[44][45]

On October 10, 2006, WebMD reported that a statistical analysis revealed the risks of infection due to contact lens wear is greater than the risk of infection from LASIK.[46] Daily contact lens wearers have about a one in 100 chance of developing a serious lens-related eye infection over 30 years of use, and a one in 2,000 chance of suffering significant vision loss as a result. The researchers calculated the risk of significant vision loss due to LASIK surgery to be closer to one in 10,000 cases.

References

  1. ^ "LASIK." Aetna InteliHealth Inc. Accessed October 18, 2006.
  2. ^ http://www.usaeyes.org/lasik/faq/lasik-pupil-size.htm
  3. ^ http://www.iroc.ch/wir_publikation.html
  4. ^ a b Council for Refractive Surgery Quality Assurance. "The most common complications of refractive surgery.". ComplicatedEyes.org.
  5. ^ a b Albietz JM, Lenton LM, McLennan SG. "Dry eye after LASIK: comparison of outcomes for Asian and Caucasian eyes." Clin Exp Optom. 2005 Mar;88(2):89–96. Cite error: The named reference "Albietz" was defined multiple times with different content (see the help page).
  6. ^ Mirshahi A, Schopfer D, Gerhardt D, Terzi E, Kasper T, Kohnen T. "Incidence of posterior vitreous detachment after laser in situ keratomileusis." Graefes Arch Clin Exp Ophthalmol. 2006 Feb;244(2):149-53. Epub 2005 Jul 26. PMID 16044328.
  7. ^ a b Arevalo JF, Mendoza AJ, Velez-Vazquez W, Rodriguez FJ, Rodriguez A, Rosales-Meneses JL, Yepez JB, Ramirez E, Dessouki A, Chan CK, Mittra RA, Ramsay RC, Garcia RA, Ruiz-Moreno JM. "Full-thickness macular hole after LASIK for the correction of myopia." Ophthalmology. 2005 Jul;112(7):1207–12. PMID 15921746.
  8. ^ Majmudar, PA. "LASIK Complications". Focal Points: Clinical Modules for Ophthalmologists. American Academy of Ophthalmology. September, 2004.
  9. ^ Carrillo C, Chayet AS, Dougherty PJ, Montes M, Magallanes R, Najman J, Fleitman J, Morales A. "Incidence of complications during flap creation in LASIK using the NIDEK MK-2000 microkeratome in 26,600 cases." J Refract Surg. 2005 Sep-Oct;21(5 Suppl):S655-7. PMID 16212299.
  10. ^ http://www.lasikinstitute.org/Intraoperative.html
  11. ^ http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=99153623&dopt=Citation
  12. ^ http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=10811084&query_hl=9
  13. ^ a b c Vesaluoma M, Perez-Santonja J, Petroll WM, Linna T, Alio J, Tervo T. "Corneal stromal changes induced by myopic LASIK." Invest Ophthalmol Vis Sci. 2000 Feb;41(2):369-76. PMID 10670464.
  14. ^ a b c d Sun L, Liu G, Ren Y, Li J, Hao J, Liu X, Zhang Y. "Efficacy and safety of LASIK in 10,052 eyes of 5081 myopic Chinese patients." J Refract Surg. 2005 Sep-Oct;21(5 Suppl):S633-5. PMID 16212294.
  15. ^ http://www.usaeyes.org/faq/subjects/intacs.htm
  16. ^ http://www.usaeyes.org/lasik/faq/c3-r.htm
  17. ^ Lee BH, McLaren JW, Erie JC, Hodge DO, Bourne WM. "Reinnervation in the cornea after LASIK." Invest Ophthalmol Vis Sci. 2002 Dec;43(12):3660–4. PMID 12454033.
  18. ^ a b Tahzib NG, Bootsma SJ, Eggink FA, Nabar VA, Nuijts RM. "Functional outcomes and patient satisfaction after laser in situ keratomileusis for correction of myopia." J Cataract Refract Surg. 2005 Oct;31(10):1943–51. PMID 16338565.
  19. ^ http://www.usaeyes.org/lasik/faq/lasik-pupil-size.htm
  20. ^ http://www.journalofrefractivesurgery.com/showAbst.asp?thing=12869
  21. ^ http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=16552654&query_hl=27&itool=pubmed_docsum
  22. ^ http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=16378639&query_hl=27&itool=pubmed_docsum
  23. ^ http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=16329381&query_hl=27&itool=pubmed_docsum
  24. ^ a b c Ruiz-Moreno JM, Alio JL. "Incidence of retinal disease following refractive surgery in 9,239 eyes." J Refract Surg. 2003 Sep-Oct;19(5):534-47. PMID 14518742.
  25. ^ Suarez E, Torres F, Vieira JC, Ramirez E, Arevalo JF. "Anterior uveitis after laser in situ keratomileusis." J Cataract Refract Surg. 2002 Oct;28(10):1793–8. PMID 12388030.
  26. ^ http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=11738908&query_hl=3
  27. ^ http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=12553606&query_hl=3
  28. ^ http://www.scuba-doc.com/diveye.htm
  29. ^ Yamane N, Miyata K, Samejima T, Hiraoka T, Kiuchi T, Okamoto F, Hirohara Y, Mihashi T, Oshika T. "Ocular higher-order aberrations and contrast sensitivity after conventional laser in situ keratomileusis." Invest Ophthalmol Vis Sci. 2004 Nov;45(11):3986–90. PMID 15505046.
  30. ^ Oshika T, Miyata K, Tokunaga T, Samejima T, Amano S, Tanaka S, Hirohara Y, Mihashi T, Maeda N, Fujikado T. "Higher order wavefront aberrations of cornea and magnitude of refractive correction in laser in situ keratomileusis." Ophthalmology. 2002 Jun;109(6):1154–8. PMID 12045059.
  31. ^ Walter KA, Stevenson AW. "Effect of environmental factors on myopic LASIK enhancement rates." J Cataract Refract Surg. 2004 Apr;30(4):798–803. PMID 15093641.
  32. ^ Saragoussi D, Saragoussi JJ. "[Lasik, PRK and quality of vision: a study of prognostic factors and a satisfaction survey.]" J Fr Ophtalmol. 2004 Sep;27(7):755-64. PMID 15499272.
  33. ^ Bailey MD, Mitchell GL, Dhaliwal DK, Boxer Wachler BS, Zadnik K. "Patient satisfaction and visual symptoms after laser in situ keratomileusis." Ophthalmology. 2003 Jul;110(7):1371–8. PMID 12867394.
  34. ^ McGhee CN, Craig JP, Sachdev N, Weed KH, Brown AD. "Functional, psychological, and satisfaction outcomes of laser in situ keratomileusis for high myopia." J Cataract Refract Surg. 2000 Apr;26(4):497–509. PMID 10771222.
  35. ^ http://www.lasikinfocenter.net/Webpages/Deceptive%20Marketing%20Practices%20Webpage.html
  36. ^ http://www.lasikeyesurgerywebsite.com
  37. ^ http://www.lasikflap.com
  38. ^ http://www.lasermyeye.org
  39. ^ http://news.bbc.co.uk/2/hi/health/2937512.stm
  40. ^ http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=12689897&query_hl=5
  41. ^ http://www.nice.org.uk/pdf/ip/233overview.pdf
  42. ^ http://www.nice.org.uk/pdf/ip/Finalreport%20010605.pdf
  43. ^ http://www.nice.org.uk/pdf/2004_51_launchLASIK.pdf
  44. ^ http://www.prnewswire.co.uk/cgi/news/release?id=136786
  45. ^ http://escrs.com/Publications/Eurotimes/05january/pdf/regmatters.pdf
  46. ^ http://www.webmd.com/content/article/128/117072.htm

See also

Retrieved from "https://en.wikipedia.org/w/index.php?title=LASIK&oldid=105229929"