Laser assisted in-situ keratomileusis-a literature review ( Part 2 )

Abnormal corneal conditions: Pre-operative keratoconus, corneal steepening and keratoglobus can potentially lead to post-operative complications and the loss of vision1, 3, 5-10. Patients with recurrent corneal erosion and corneal dystrophies should also be avoided or approached with caution, as these conditions affect wound healing and can lead to corneal ectasia11-14. The UV radiation from the excimer laser, as well as, the use of steroids post-operatively, can also trigger a recurrence of the viral infection in a patient with a history of herpetic corneal ulcers1, 4.


Patients who are immuno-compromised:
Immuno-compromise, whether due to being on drugs or therapy that suppresses the immune system; or collagen, vascular and auto-immune diseases, has been found to be associated with marginal corneal ulceration following refractive surgery 3,4 , hence immuno-compromised patients are not considered for LASIK.

Abnormal corneal conditions:
Pre-operative keratoconus, corneal steepening and keratoglobus can potentially lead to post-operative complications and the loss of vision 1,3,[5][6][7][8][9][10] .Patients with recurrent corneal erosion and corneal dystrophies should also be avoided or approached with caution, as these conditions affect wound healing and can lead to corneal ectasia [11][12][13][14] .The UV radiation from the excimer laser, as well as, the use of steroids post-operatively, can also trigger a recurrence of the viral infection in a patient with a history of herpetic corneal ulcers 1,4 .

Low pachymetry:
Central corneal thickness of less than 410 µm is inadequate for the LASIK operation as the patient will encounter induced irregular astigmatism, distortion and is at risk of corneal rupture 2,15 .

Pregnant or nursing patients:
LASIK is contra-indicated in patients who are pregnant or nursing, or those expecting to become pregnant within six months following the procedure.This is because of the possibility of fluctuating refractive error resulting from hormonal changes 4, 8, 16.Furthermore, the healing process can be affected during pregnancy and some medications that may be needed, may pose a risk to the unborn or nursing child 8 .

Glaucoma patients:
Eyes with glaucoma, or diagnosed with glaucomatous field losses, would be further compromised during the period of increased intra-ocular pressure induced by the suction ring during the LASIK procedure 1,11,16 .Therefore, patients with this condition cannot be considered for LASIK.

Progressive refractive error:
Patients with progressive myopia or hyperopia are contra-indicated 11 .This is because of possible variation in refractive status following LASIK.

Amblyopia:
Cases of amblyopia are often excluded due to the risk of complications or infections setting in, which may consequently result in a loss of function of the non-amblyopic eye and thus leave the patient visually disabled 8,17 .
Narrow palpebral apertures and small or very deep set orbits: Patients with narrow palpebral apertures and small or very deep set orbits are excluded because the positioning of the microkeratome and suction ring is compromised in these patients 1,11 .

Dry eyes:
Patients exhibiting dry eyes or abnormalities of the precorneal tear film are excluded due to the risk of the tear integrity being further compromised following LASIK 1,18,19 .

Allergy to latex:
Latex gloves are used during the surgery and it comes into contact with delicate ocular tissue, thus patients allergic to latex, are excluded 20 .

Risks and complications
Chisholm 18 reports that complications can occur in up to 5.3% of cases of LASIK correction.These complications can either occur intra-operatively or post-operatively as discussed below.

Intra-operative complications
Intra-operative complications have been attributed to the learning curve that is followed by any new technique, with the incidence of complications expected to decrease as the ophthalmologist becomes more proficient as more surgery is performed 2,11 .The intra-operative complications of LASIK that have been reported include the following:

Pupil bisection:
Pupil bisection occurs if the microkeratome is stopped too early resulting in the hinge of the flap bisecting the pupil.Consequently, there is a very short flap and an inadequate ablation diameter.This complication is often related to poor gear advancement, which may be caused by residual debris on the microkeratome 21 .

Free corneal flap:
Failure in hinge formation, resulting in a free cap, may be due to either the microkeratome not being properly set, or insufficient suction 1,11,21,22 .Often patients with very flat corneas (less than 41 D), which protrude much less into the suction ring, are at a greater risk of developing free flaps 21 .

Poor flaps:
Poor flaps include those that are perforated, thin or both.A perforated flap results from inadequate suction pressure, a malfunctioning microkeratome, blade imperfections or by buckling of steep corneas during the microkeratome pass [21][22][23] .

Corneal perforation:
Corneal perforation is the most significant potential intra-operative complication, which may occur if the depth plate was improperly inserted or not in place at all 11,24 .This complication can be very serious since the pressure in the eye is being maintained at 65 mmHg and may lead to lesions in the iris, lens or an explosive haemorrhage 24 .

Bleeding of neovascular vessels:
Prior to surgery, many LASIK patients who were previous contact lenses wearers, could have developed superior, and sometimes inferior micropannus 11,23 .If the area of neovascularization is involved in the flap formation, then it could lead to bleeding, which is difficult to control.

Decentration:
Decentration can occur because of poor fixation during surgery, often due to the patient looking down to look away from the surgeon, or squeezing his/her eyes shut, which induces Bell's phenomenon 1,25 .Poor alignment of the optics within the laser delivery system can also result in decentration 26 .Decentration will result in the induction of irregular astigmatism and thus lead to ghosting, distortion and night glare 11 .Prismatic effect can also be induced and consequently there will be a reduction of the best corrected visual acuity.

Post-operative complications
Post-operative complications refer to those complications that occur during the healing period and include the following:

Displaced flaps:
A displaced flap usually occurs within the first 24 hours and is often related to significant ocular trauma post-operatively 27 .Flap dislocation can, however, occur later and thus open up the cornea to epithelial in-growth and infection 28 .Patients, therefore, need to be aware of the inherent risk of traumatic flap dislocation or loss.Fortunately, the flap remains hinged and very rarely becomes free.However, it could result in infection and in permanent striae formation.

Corneal infection:
Corneal infection can occur due to poor steril-ity of the equipment, or compromise of the epithelium 3,[29][30][31] .In some cases, corneal infection has occurred when bandage contact lenses were used without antibiotic coverage 3 .

Epithelial defects:
Epithelial defects can occur especially along the incision edge of the flap.Proliferation of the epithelium irregularly at the edge of the flap can lead to epithelial in-growth 32 being triggered off when there is a poor adherence of the flap to the underlying stroma and thus poor apposition of the flap 22 .Central epithelial defects may occur due to epitheliotoxicity to the topical anesthetic and/or because of dehydration 11 .Another reason could be the implantation of epithelial cells during the surgery secondary to the microkeratome mechanically dragging epithelial cells into the lamellar interface 33 .These defects can result in the patient being very photophobic and experiencing excessive tearing and pain.Further consequences of these epithelial defects may be infection, melting of the edge of the flap, astigmatism being induced and scarring, which will affect vision as the in-growth invades the visual axis 27, 33 ,34 .

Diffuse Lamellar Keratitis:
Diffuse lamellar keratitis (DLK), usually referred to as "Sands of Sahara", seems to be caused by contaminants on the microkeratome blade, hence it is emphasised that the blades be cleaned thoroughly before the surgical procedure 35 .It occurs as a result of interface debris or bacterial toxins triggering off an inflammatory reaction, and thus anterior chamber activity 18 .DLK has also been linked to residue of cleaning solutions on the surgical instruments 36 .

Corneal ectasia:
Corneal ectasia, which refers to progressive thinning and weakening of the central cornea, has been reported when the residual corneal thickness is less than 250 µm 27, 37-39 .Other risk factors have been identified as forme fruste keratoconus, a preoperatively thin cornea or due to a unexpectedly thick flap [40][41][42] .

Flap wrinkles or folds:
Wrinkles or folds within the flap can occur with slight movements of the flap, and will result in distortions and irregular astigmatism if not corrected 18,21 .

Night glare:
Night glare is often experienced as a result of the pupil diameter exceeding the optical zone created.Patients complain of haloes, ghosting and decreased vision at night 11 .

Retinal changes:
There have been cases of retinal haemorrhages or retinal detachment reported following LASIK 22,43,44 .However, it has been suggested that any retinal haemorrhage may be due to some pre-existing pathology 15 .Vitreo-retinal interface changes may, however, play a role in macular changes following LASIK 45 .

Anisometropia:
Anisometropia and aniseikonia, can occur due to different refractive outcomes in both the eyes.This may result in the patient becoming amblyopic, experiencing diplopia and having poor depth perception 46 .

Overcorrection:
Variations in the healing response in each patient, together with an incorrect evaluation of pre-operative refractive error, or incorrect set up of computer software, can limit the accuracy of the surgical outcome relative to the attempted correction 1 .This can result in the patient being overcorrected for example, a myope becoming hyperopic or a hyperope becoming more hyperopic.Early presbyopes and presbyopic patients will experience poor vision at near if they become mildly hyperopic from an originally myopic state.This may necessitate a hyperopic LASIK procedure, the use of some other refractive surgery e.g.laser thermokeratoplasty 1 , or by optical means.

Undercorrection:
The cause of undercorrections following LASIK are similar to those of overcorrections and include spontaneous regression, inaccurate refractive error evaluation, incorrect interpretation of nomograms and the incorrect set up of the computer software 1,18,47 .Undercorrections occur frequently in patients with higher refractive errors 5 .These patients will complain of poor distance vision especially at night.Enhancements will thus be required and in some cases other types of refractive surgery may need to be considered 11,18 , or optical correction with spectacles or contact lenses.

Regression:
Regression refers to the post-operative refractive error slowly going back to the original refractive error.It occurs more often in the higher myopes and has been attributed to irregular re-epithelialization and hyperepithelialization, particularly at the edge of the ablated area 48 .Regression has also been linked to a "remodelling" of the stroma, even though, the deeper stromal is supposed to be relatively inert in the process of wound healing 49 .An increase in 10 µm epithelial thickness will result in 1 D of regression 50 .When regression occurs it necessitates an enhancement depending on the residual corneal thickness 21 .

Blurred vision:
Transient blurred vision may occur in the first 12 to 24 hours due to small epithelial defects or stromal oedema, but is expected to rarely last longer than about 48 hours 11 .This corneal haze, from the oedema, has been attributed to increased reflectivity of the anterior stromal keratocytes that are involved in the healing process 51 .

Optical aberrations:
Corneal surface irregularities have been reported to induce significant optical aberrations 52 .Optical aberrations may be perceived as haloes, ghost images, shadows and slight distortions.These optical aberrations may or may not interfere with normal visual acuities, but will affect the optical quality of the post-operative image 51,53 .

Discomfort:
Any discomfort will usually occur during the first 24 hours.The lack of pain has been attributed to the fact that neither the epithelium, nor the ciliary muscle, has been traumatized 1 .However, patients can experience a foreign body sensation and an orbital ache due to the use of the lid speculum.Pain may also sometimes be an indication of a displaced flap or an epithelial defect 37 .

Photophobia:
Patients may be transiently more sensitive to sunlight following surgery, which has been attributed to scattering of light by the edematous or irregular cornea 11 .

Conjunctival changes:
An occasional conjunctival haemorrhage following conjunctival edema and injection, may be seen initially in the first 12 to 24 hours 11 .The haemorrhage occurs due to the application of the suction ring.This condition must be treated as it can affect visual performance following surgery, if it gets into the flap interface 59 .

Loss of corneal sensitivity:
Loss of corneal sensitivity has been associated with various refractive surgical procedures including LASIK.During LASIK most of the stromal nerve trunks are cut, with only those in the hinge being spared 60 .Thus, the cornea is compromised when this sensory capability is drastically reduced.This is particularly important since normal corneal sensitivity is essential for normal corneal structure and function 61 .

Conclusion
The goal of refractive surgery is to provide the patient with the best possible visual acuity with minimal surgery.Even though ophthalmologists are principally involved in performing it, the role of the optometrist with regards to refractive surgery includes unbiased advice, as well as pre-and post-operative care and research into the relevant procedures 62,63 .It is acknowledged that the optometrist, equipped with facts provided by relevant research, will be capable of assisting the patient to make an educated decision with regards to refractive surgery.An optometrist can also be able to provide post-operative management and advise the patient on long-term implications.In this way, optometry can become actively involved in the growing field of refractive surgery to ensure the visual well-being of the patient.