LASIK

LASIK
From Wikipedia, the free encyclopedia
For the article on the drug used for treatment of hypertension of eye surgery, see Lasix.
LASIK or Lasik (Laser-Assisted in situ Keratomileusis), commonly referred to as laser eye surgery or
laser vision correction, is a type of refractive surgery for the correction of myopia, hypermetropia,
and astigmatism. The LASIK surgery is performed by an ophthalmologist who uses a laser or
microkeratome to reshape the eye's cornea in order to improve visual acuity.[1] For most patients,
LASIK provides a permanent alternative to eyeglasses or contact lenses.[2]
LASIK is most similar to another surgical corrective procedure, photorefractive keratectomy (PRK,
similar to LASIK), and both represent advances over radial keratotomy in the surgical treatment of
refractive errors of vision. For patients with moderate to high myopia or thin corneas which cannot
be treated with LASIK and PRK, the phakic intraocular lens is an alternative.[3][4] As of 2011, over
11 million LASIK procedures have been performed in the United States[5] and as of 2009 over 28
million have been performed worldwide.[6]
Contents
1 History
1.1 Barraquer's early work
1.2 Application of medical laser to refractive surgery
2 Introduction of LASIK
2.1 Patent
3 Implementation in U.S.
4 Further developments
4.1 Experimental techniques
5 Procedures
5.1 Preoperative procedures
5.1.1 Contact lenses
5.1.2 Pre-operative examination and education
5.2 Operative procedure
5.2.1 Flap creation
5.2.2 Laser remodelling
5.2.3 Repositioning of the flap
5.3 Postoperative care
6 Wavefront-guided LASIK
7 Effectiveness
7.1 Patient satisfaction
7.2 Patient dissatisfaction
8 Risks
8.1 Higher-order aberrations
8.2 Dry eyes
8.3 Halos
8.4 Other complications
9 Onset of presbyopia
10 LASIK versus photorefractive keratectomy
11 FDA's position on LASIK
12 See also
13 References
14 External links
History
Barraquer's early work
In the 1950s, the microkeratome and keratomileusis technique were developed in Bogotá,
Colombia, by the Spanish ophthalmologist Jose Barraquer. In his clinic, he would cut thin (one
hundredth of a mm thick) flaps in the cornea to alter its shape. Barraquer also investigated how
much of the cornea had to be left unaltered in order to provide stable long-term results.[7] This work
was followed by that of the Russian scientist, Svyatoslav Fyodorov (1920-2000), who developed
radial keratotomy (RK) in the 1970s and designed the first posterior chamber implantable contact
lenses (phakic intraocular lens) in the 1980s.
Application of medical laser to refractive surgery
In 1980, Rangaswamy Srinivasan, at the IBM Research laboratory, discovered that an ultraviolet
excimer laser could etch living tissue, with precision and with no thermal damage to the surrounding
area. He named the phenomenon "ablative photo-decomposition" (APD).[8] Five years later, in 1985,
Steven Trokel at the Edward S. Harkness Eye Institute, Columbia University in New York City,
published his work using the excimer laser in radial keratotomy. He wrote,
"The central corneal flattening obtained by radial diamond knife incisions has been duplicated by
radial laser incisions in 18 enucleated human eyes. The incisions, made by 193 nm far-ultraviolet
light radiation emitted by the excimer laser, produced corneal flattening ranging from 0.12 to 5.35
diopters. Both the depth of the corneal incisions and the degree of central corneal flattening
correlated with the laser energy applied. Histopathology revealed the remarkably smooth edges of
the laser incisions."[9]
Together with his colleagues, Charles Munnerlyn and Terry Clapham, Trokel founded VISX USA
inc.[10] Marguerite B. MacDonald MD performed the first human VISX refractive laser eye surgery
in 1989.[11]
Introduction of LASIK
Patent
A number of patents have been issued for several techniques related to LASIK. For example, Samuel
E. Blum filed a patent application in 1982. Later, on 20 June 1989, Gholam A. Peyman was granted a
US patent for LASIK (US4840175). It was,
"A method and apparatus for modifying the curvature of a live cornea via use of an excimer laser.
The live cornea has a thin layer removed therefrom, leaving an exposed internal surface thereon.
Then, either the surface or thin layer is exposed to the laser beam along a predetermined pattern to
ablate desired portions. The thin layer is then replaced onto the surface. Ablating a central area of
the surface or thin layer makes the cornea less curved, while ablating an annular area spaced from
the center of the surface or layer makes the cornea more curved. The desired predetermined pattern
is formed by use of a variable diaphragm, a rotating orifice of variable size, a movable mirror or a
movable fiber optic cable through which the laser beam is directed towards the exposed internal
surface or removed thin layer."[12]
The patents related to so-called broad-beam LASIK and PRK technologies were granted to US
companies including Visx and Summit during 1990-1995 based on the fundamental US patent issued
to IBM (1983) which claimed the use of UV laser for the ablation of organic tissues. In 1991, J.T. Lin
was granted a US patent (US5520679) for a new technology using a flying-spot for customized
LASIK that has been used worldwide. The first US patent (in 1993) using an eye-tracking device to
prevent decentration in LASIK procedures was granted to S. Lai.
Implementation in U.S.
The LASIK technique was implemented in the U.S. after its successful application elsewhere. The
Food and Drug Administration (FDA) commenced a trial of the excimer laser in 1989. The first
enterprise to receive FDA approval to use an excimer laser for photo-refractive keratectomy was
Summit Technology (founder and CEO, Dr. David Muller).[13] In 1992, under the direction of the
FDA, Greek ophthalmologist Ioannis Pallikaris introduced LASIK to ten VISX centres. In 1998, the
"Kremer Excimer Laser", serial number KEA 940202, received FDA approval for its singular use for
performing LASIK.[14] Subsequently, Summit Technology was the first company to receive FDA
approval to mass manufacture and distribute excimer lasers. VISX and other companies
followed.[14]
The excimer laser that was used for the first LASIK surgeries by I.Pallikaris
Pallikaris suggested a flap of cornea could be raised by microkeratome prior to the performing of
PRK with the excimer laser. The addition of a flap to PRK became known as LASIK.
Further developments
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. However, use of the excimer laser risks damage to the retina and optic nerve. The goal
of refractive surgery is to avoid permanently weakening the cornea with incisions and to deliver less
energy to the surrounding tissues.
Experimental techniques
"plain" LASIK: LASEK, Epi-LASIK,
Sub-Bowman's keratomileusis (thin flap 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.[15] and is in European clinical trials for the correction of myopia and keratoconus.[16]
Technolas FEMTEC laser: for incisionless IntraCOR ablation for presbyopia,[17] with trials ongoing
for myopia and other conditions.[18]
IsoVision : It's a surgical technique that can completely correct near and distant vision of each eye
(Hypertropia, Myopia, Emmetropia and Astigmatic associated with Presbyopia). Developed since
2008 by the French ophthalmologist Frédéric HEHN.[19]
Procedures
Play media
Video of a complete LASIK-treatment
The planning and analysis of corneal reshaping techniques such as LASIK have been standardized by
the American National Standards Institute, an approach based on the Alpins Method of astigmatism
analysis. The FDA website on LASIK states,
"Before undergoing a refractive procedure, you should carefully weigh the risks and benefits based
on your own personal value system, and try to avoid being influenced by friends that have had the
procedure or doctors encouraging you to do so."[20]
The procedure involves creating a thin flap on the eye, folding it to enable remodeling of the tissue
beneath with a laser and repositioning the flap.
Preoperative procedures
Contact lenses
Patients wearing soft contact lenses are instructed to stop wearing them 5 to 21 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 have been worn.[21] The cornea is avascular because it must be transparent to function
normally. Its cells absorb oxygen from the tear film. Thus, low-oxygen-permeable contact lenses
reduce the cornea's oxygen absorption, sometimes resulting in corneal neovascularization--the
growth of blood vessels into the cornea. This causes a slight lengthening of inflammation duration
and healing time and some pain during surgery, because of greater bleeding. Although some contact
lenses (notably modern RGP and soft silicone hydrogel lenses) are made of materials with greater
oxygen permeability that help reduce the risk of corneal neovascularization, patients considering
LASIK are warned to avoid over-wearing their contact lenses.
Pre-operative examination and education
In the United States, the FDA has approved LASIK for age 18 and over.[22] More importantly the
patient's eye prescription should be stable for at least one year prior to surgery. The patient may be
examined with pupillary dilation and education given prior to the procedure. Before the surgery, the
patient's corneas are examined with a pachymeter to determine their thickness, and with a
topographer, or corneal topography machine,[1] to measure their surface contour. Using low-power
lasers, a topographer creates a topographic map of the cornea. The procedure is contraindicated if
the topographer finds difficulties such as keratoconus[1] The preparatory process also detects
astigmatism and other irregularities in the shape of the cornea. Using this information, the surgeon
calculates the amount and the location of corneal tissue to be removed. The patient is prescribed
and self-administers an antibiotic beforehand to minimize the risk of infection after the procedure
and is sometimes offered a short acting oral sedative medication as a pre-medication. Prior to the
procedure, anaesthetic eye drops are instilled. Factors that may rule out LASIK for some patients
include large pupils, thin corneas and extremely dry eyes.
Operative procedure
Flap creation
A soft corneal suction ring is applied to the eye, holding the eye in place. This 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 causes a transient dimming of vision in the treated eye. Once the eye is
immobilized, a flap is created by cutting through the corneal epithelium and Bowman's layer. This
process is achieved with a mechanical microkeratome using a metal blade, or a femtosecond laser
that creates a series of tiny closely arranged bubbles within the cornea.[23] 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 sometimes be uncomfortable.
Laser remodelling
The second step of the procedure uses all laser lasik an excimer laser (193Â nm) to remodel the
corneal stroma. The laser vaporizes the tissue in a finely controlled manner without damaging the
adjacent stroma. No burning with heat or actual cutting is required to ablate the tissue. The layers
of tissue removed are tens of micrometres thick. Performing the laser ablation in the deeper corneal
stroma provides for more rapid visual recovery and less pain than the earlier technique,
photorefractive keratectomy (PRK). During the second step, the patient's vision becomes blurry,
once the flap is lifted. They will be able to see only white light surrounding the orange light of the
laser, which can lead to mild disorientation. The excimer laser uses an 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. Typical pulses are around 1 millijoule (mJ) of pulse energy in
10 to 20 nanoseconds.[24]
Repositioning of the flap
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 care
Patients are usually given a course of antibiotic and anti-inflammatory eye drops. These are
continued in the weeks following surgery. Patients are told to rest and are given dark eyeglasses to
protect their eyes from bright lights and occasionally protective goggles to prevent rubbing of the
eyes when asleep and to reduce dry eyes. They also are required to moisturize the eyes with
preservative-free tears and follow directions for prescription drops. Occasionally after the procedure
a bandage contact lens is placed to aid the healing, and typically removed after 3-4 days. Patients
should be adequately informed by their surgeons of the importance of proper post-operative care to
minimize the risk of complications.[25]
Wavefront-guided LASIK
Wavefront-guided LASIK[26] is a variation of LASIK surgery in which, rather than applying a simple
correction of only long/short-sightedness and astigmatism (only lower order aberrations as in
traditional LASIK), an ophthalmologist applies a spatially varying correction, guiding the computer-
controlled excimer 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 that occur during healing and other factors that may have to do with the
regularity/irregularity of the cornea and the axis of any residual astigmatism. Another important
factor is whether the excimer laser can correctly register eye position in 3 dimensions, and to track
the eye in all the possible directions of eye movement. If a wavefront guided treatment is performed
with less than perfect registration and tracking, pre-existing aberrations can be worsened. In older
patients, scattering from microscopic particles (cataract or incipient cataract) may play a role that
outweighs any benefit from wavefront correction. Therefore, patients expecting so-called "super
vision" from such procedures may be disappointed.[27][28][29][30]
One method of planning and analyzing the results of LASIK surgery (as well as cataract/IOL, corneal,
and other refractive surgical procedures) uses vector analysis, and in the case of LASIK, may be
combined with a wavefront-guided approach.[27][28][29][30] Many people undergoing LASIK have
preexisting astigmatism, which may be regular or irregular, and is caused by some combination of
external (corneal surface) and internal (posterior corneal surface, human lens, fluids, retina, and
eye-brain interface) optical properties. In some patients, the external optics may have the greater
influence, and in other patients, the internal optics may predominate. Importantly, the axes and
magnitudes of external and internal astigmatism do not necessarily coincide, but it is the
combination of the two that by definition determines the overall optics of the eye.
When treating a patient with preexisting astigmatism, most wavefront-guided LASIK lasers are
designed to treat regular astigmatism as determined externally by corneal topography. In patients
who have an element of internally induced astigmatism, therefore, the wavefront-guided astigmatism
correction may leave regular astigmatism behind (a cross-cylinder effect). If the patient has
preexisting irregular astigmatism, wavefront-guided approaches may leave both regular and
irregular astigmatism behind. This can result in less-than-optimal visual acuity compared with a
wavefront-guided approach combined with vector planning, as shown in a 2008 study.[31] Thus,
vector planning offers a better alignment between corneal astigmatism and laser treatment, and
leaves less regular astigmatism behind on the cornea, which is advantageous whether irregular
astigmatism coexists or not.
The "leftover" astigmatism after a purely surface-guided laser correction can be calculated
beforehand, and is called ocular residual astigmatism (ORA). ORA is a calculation of astigmatism due
to the noncorneal surface (internal) optics. The purely refraction-based approach represented by
wavefront analysis actually conflicts with corneal surgical experience developed over many
years.[30]
The pathway to "super vision" thus may require a more customized approach to corneal astigmatism
than is usually attempted, and any remaining astigmatism ought to be regular (as opposed to
irregular), which are both fundamental principles of vector planning overlooked by a purely
wavefront-guided treatment plan.[30] This was confirmed by the 2008 study mentioned above, which
found a greater reduction in corneal astigmatism and better visual outcomes under mesopic
conditions using wavefront technology combined with vector analysis than using wavefront
technology alone, and also found equivalent higher-order aberrations (see below).[31] Vector
planning also proved advantageous in patients with keratoconus.[32] Additional discussion of
wavefront-guided LASIK can be found here.
No good data can be found that compare the percentage of LASIK procedures that employ wavefront
guidance versus the percentage that do not, nor the percentage of refractive surgeons who have a
preference one way or the other. Wavefront technology continues to be positioned as an "advance"
in LASIK with putative advantages;[33][34][35][36] however, it is clear that not all LASIK
procedures are performed with wavefront guidance.[37]
Still, 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. A meta-analysis of eight trials showed a lower
incidence of these higher order aberrations in patients who had wavefront-guided LASIK compared
to non-wavefront-guided LASIK.[38] Based on their experience, the United States Air Force has
described WFG-Lasik as giving "superior vision results".[39]
Effectiveness
In 2006, the British National Health Service's National Institute for Health and Clinical Excellence
(NICE) considered evidence of the effectiveness of laser surgery and issued guidance stating
"current evidence suggests that photorefractive (laser) surgery for the correction of refractive errors
is safe and efficacious for use in appropriately selected patients."[40]
Patient satisfaction
Surveys of LASIK find rates of patient satisfaction between 92 and 98 percent.[41][42][43] In March
2008, The American Society of Cataract and Refractive Surgery published a patient satisfaction
meta-analysis of over 3,000 peer-reviewed articles from international clinical journals. Data from the
prior 10 years revealed a 95.4 percent patient satisfaction rate among LASIK patients.[44]
Patient dissatisfaction
Some patients with poor outcomes from LASIK surgical procedures report a significantly reduced
quality of life because of vision problems or physical pain associated with the surgery. A small
percentage of patients may need to have another surgery because their condition is over-corrected
or under-corrected. Some patients need to wear contact lenses or glasses even after treatment.[45]
Patients who have suffered LASIK complications have created websites and discussion forums where
prospective and past patients can discuss the surgery.
In 1999, "Surgical Eyes" was founded in New York city as a resource for patients with complications
of LASIK and other refractive surgeries by RK patient Ron Link. "Surgical Eyes" has since been
superseded by the "Vision Surgery Rehab Network" (VSRN).[46][47][48][49][50]
Risks
On October 10, 2006, WebMD reported that statistical analysis revealed that the infection risk for
contact lens wearers is greater than the infection risk from LASIK.[51] Daily contact lens wearers
have a 1-in-100 chance of developing a serious, contact lens-related eye infection in 30 years of use,
and a 1-in-2,000 chance of suffering significant vision loss as a result of infection. The researchers
calculated the risk of significant vision loss consequence of LASIK surgery to be closer to 1-in-10,000
cases.
Morris Waxler, a former FDA official who was involved in the approval of LASIK, has subsequently
criticized its widespread use. In 2010, Waxler made media appearances and claimed that the
procedure had a failure rate greater than 50%. The FDA responded that Waxler's information was
"filled with false statements, incorrect citations" and "mischaracterization of results".[52]
Higher-order aberrations
The term "higher-order aberrations" are visual problems that require special testing for diagnosis
and are not corrected with normal spectacles (eyeglasses). These aberrations include 'starbursts',
'ghosting', 'halos' and others.[53] Some patients describe these symptoms post-operatively and
associate them with the LASIK technique including the formation of the flap and the tissue
ablation.[54] The advancement of the LASIK technology has reduced the risk of clinically significant
visual impairment after surgery. There is a correlation between pupil size and aberrations.[55] This
correlation may be the result of irregularity in the corneal tissue between the untouched part of the
cornea and the reshaped part. Daytime post-LASIK vision is optimal, since the pupil size is smaller
than the LASIK flap. However, at night, the pupil may dilate such that light passes through the edge
of the LASIK flap which gives rise to aberrations. LASIK and PRK may induce spherical aberration if
the laser under corrects as it moves outward from the centre of the treatment zone, especially when
major corrections are made. Others propose that higher order aberrations are present
preoperatively.[56] They can be measured in micrometers (µm) whereas the smallest laser beam
size approved by the FDA is about 1000 times larger, at 0.65Â mm. In situ keratomileusis effected at
a later age increases the incidence of corneal higher-order wavefront aberrations.[57][58] These
factors demonstrate the importance of careful patient selection for LASIK treatment.
A subconjunctival hemorrhage is a common and minor post-LASIK complication.
Dry eyes
Some people experience dry eyes following surgery.[59] Although it is usually temporary it can
develop into dry eye syndrome.[60]
Underlying conditions with dry eye such as Sjögren's syndrome are considered contraindications to
Lasik.[61]
Treatments include artificial tears, prescription tears and punctal occlusion. Punctal occlusion is
accomplished by placing a collagen plug in the tear duct, which normally drains fluid from the eye.
Some patients complain of ongoing dry eye symptoms despite such treatments and dry eye
symptoms may be permanent.[62]
Halos
Some post-LASIK patients see halos and starbursts around bright lights at night[63][64][65] At
night, the pupil may dilate to be larger than the flap leading to the edge of the flap or stromal
changes causing visual distortion of light that does not occur during the day when the pupil is
smaller. The eyes can be examined for large pupils pre-operatively and the risk of this symptom
assessed.
Complications due to LASIK have been classified as those that occur due to preoperative,
intraoperative, early postoperative, or late postoperative sources:[66] According to the UK National
Health Service complications occur in fewer than 5% of cases.[59]
Other complications
flap complications - The incidence of flap complications is about 0.244%.[67] 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[68] but rarely lead to permanent
loss of visual acuity. The incidence of these microkeratome-related complications decreases with
increased physician experience.[69]
"slipped flap"Â - is a corneal flap that detaches from the rest of the cornea. The chances of this are
greatest immediately after surgery, so patients typically are advised to go home and sleep to let the
flap adhere and heal. Patients are usually given sleep goggles or eye shields to wear for several
nights to prevent them from dislodging the flap in their sleep. A short operation time may decrease
the chance of this complication, as there is less time for the flap to dry. .
"Flap interface particles"Â - are a finding whose clinical significance is undetermined.[70] Particles
of various sizes and reflectivity are clinically visible in about 38.7% of eyes examined via slit lamp
biomicroscopy and in 100% of eyes examined by confocal microscopy.[70]
Diffuse lamellar keratitis (or DLK)Â - an inflammatory process that involves an accumulation of white
blood cells at the interface between the LASIK corneal flap and the underlying stroma. It is known
colloquially as "sands of Sahara syndrome" because on slit lamp exam, the inflammatory infiltrate
appears similar to waves of sand. The USAeyes organisation reports an incidence of 2.3% after
LASIK.[71][72] It is most commonly treated with steroid eye drops. Sometimes it is necessary for the
eye surgeon to lift the flap and manually remove the accumulated cells. DLK has not been reported
with photorefractive keratectomy due to the absence of flap creation.
Infection - the incidence of infection responsive to treatment has been estimated at 0.4%.[72]
Ectasia - post-LASIK corneal ectasia[73] is a condition where the cornea starts to bulge forwards at
a variable time after LASIK, causing irregular astigmatism. The condition is similar to keratoconus.
If this occurs, the patient may need rigid gas permeable contact lenses, Intrastromal corneal ring
segments,[74]corneal collagen cross-linking with Riboflavin,[75] or a corneal transplant. A
significant risk factor is forme fruste keratoconus, a genetic condition that predisposes the cornea to
thinning after surgery. Although this condition is screened for in the preoperative examination, it is
possible for the condition to appear later in life (the mid-40s). When cross-linking is performed only
after the cornea becomes distorted, vision remains blurry even though the disease is stabilised. As a
result, combining corneal collagen cross-linking with LASIK ('LASIK Xtra') aims to strengthen the
cornea at the point of surgery and may be useful in cases where a very thin cornea is expected after
the LASIK procedure. This would include cases of high spectacle power and patients with thin
corneas before surgery. Definitive evidence that the procedure can reduce the risk of corneal ectasia
will only become available a number of years later as corneal ectasia, if it happens, usually occurs in
the late post-operative period. Some study show that combining LASIK with cross-linking adds
refractive stability to hyperopic treatments and may also do the same for very high myopic
treatments.[76][77]
subconjunctival hemorrhage - A report shows the incidence of subconjunctival hemorrhage has
been estimated at 10.5%.[72]
Corneal scarring - or permanent problems with cornea's shape making it impossible to wear
contact lenses.[45]
epithelial ingrowth - the incidence of epithelial ingrowth has been estimated at 0.1%.[72]
traumatic flap dislocations - Cases of late traumatic flap dislocations have been reported up to
seven years after LASIK.[78]
Glaucoma - After LASIK, measurements of intraocular pressure (used to diagnose and treat
glaucoma) can be more difficult. The changes also affect the calculations used to select the correct
intraocular lens implant for cataract surgery. This is known as "refractive surprise." Preoperative,
operative and postoperative measurements can assist.
Retinal detachment: the incidence of retinal detachment has been estimated at 0.36 percent.[79]
Choroidal neovascularization: the incidence of choroidal neovascularization has been estimated at
0.33 percent.[79]
Uveitis: the incidence of uveitis has been estimated at 0.18 percent.[80]
for climbers - Although the cornea usually is thinner after LASIK, because of the removal of part of
the stroma, refractive surgeons strive to maintain the maximum thickness to avoid structurally
weakening the cornea. Decreased atmospheric pressure at higher altitudes has not been
demonstrated as extremely dangerous to the eyes of LASIK patients. However, some mountain
climbers have experienced a myopic shift at extreme altitudes.[81][82]
corneal keratocytes - There is a report of decreases in numbers of corneal keratocytes (fibroblasts)
after LASIK.[83]
Late postoperative complications - A large body of evidence on the chances of long-term
complications is not yet established and may be changing due to advances in operator experience,
instruments and techniques.[84][85][86]
Onset of presbyopia
Myopic (nearsighted) people who are close to the age (mid- to late-forties) when they will require
either reading glasses or bifocal eyeglasses may find that they still require reading glasses despite
having undergone refractive LASIK surgery. Myopic people generally require reading glasses or
bifocal eyeglasses at a later age than people who are emmetropic (those who see without
eyeglasses), but this benefit may be lost if they undergo LASIK. This is not a complication but an
expected result of the physical laws of optics. Although there is currently no method to completely
eradicate the need for reading glasses in this group, it may be minimized by performing a variation
of the LASIK procedure called "slight monovision." In this procedure, which is performed exactly like
distance-vision-correction LASIK, the dominant eye is set for distance vision, while the non-dominant
eye is set to the prescription of the patient's reading glasses. This allows the patient to achieve a
similar effect as wearing bifocals. The majority of patients tolerate this procedure very well and do
not notice any shift between near and distance viewing, although a small portion of the population
has trouble adjusting to the monovision effect. This can be tested for several days prior to surgery by
wearing contact lenses that mimic the monovision effect. Recently, a variation of the laser ablation
pattern called PresbyLASIK, has been developed to reduce or eliminate dependence on reading
glasses while retaining distance vision.
LASIK versus photorefractive keratectomy
There have been a number of studies comparing Lasik to photorefractive keratectomy, or PRK. Lasik
is associated with decreased inflammation and quicker recovery, but at the cost of decreased
expression of nerve growth factor (NGF) at the surgical bed.[87] PRK, on the other hand, has longer
recovery time with more postoperative discomfort and irritation, but rates of ocular dryness are
less,[88][89][90] as only the epithelium is denuded in this approach. Lasik, on the other hand,
involves creation of a corneal flap.[91] The medical literature is not uniform, however, regarding
their comparative effects on ocular dryness,[92] although the majority of research supports PRK as
causing less reduction in the tear film. Surgical technique has improved with time, and a more
recent study has shown no difference in ocular dryness in Lasik or PRK at 12 months, as compared
to pre-operative baseline, although interval assessments at months 1, 3 and 6 did show dryness in
both groups.[93] Quantitative changes occurring at the eye surface are more pronounced with Lasik,
but more irritation, pain and eyelid sticking are felt with PRK, which could be related to increased
denervation with Lasik.[87][94] A Systematic review compared LASIK and PRK and concluded that
LASIK may probably have faster recovery time and lower pain in comparison to PRK, however
results may be similar in both beyond a period of one year.[95]
FDA's position on LASIK
Since October 2009, the FDA has had LASIK under a Quality of Life Collaboration Project (LQLCP).
LQLCP is a government partnership among the FDA, the Department of Defense, and the National
Eye Institute. This project examines patient-reported outcomes (PROs) following LASIK.[96] This
investigation was supposed to be concluded in 2012; however, no conclusions have been reached
yet.[97]
On December 6, 2011, FDA posted a video on FDA's YouTube channel and FDA's LASIK web site to
help explain the risks of LASIK and other important information to potential patients. The video
includes images of what certain visual symptoms may look like to patients experiencing them.[97]
See also
Samuel E. Blum
Automated lamellar keratoplasty
Femtosecond laser intrastromal vision correction
Refractive error
Wavefront
References
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External links
What is LASIK? -- Food and Drug Administration
Laser Eye Surgery -- United States National Library of Medicine
Retrieved from "http://en.wikipedia.org/w/index.php?title=LASIK&oldid=618455342"
http://en.wikipedia.org/wiki/LASIK