LASIK and high myopia
High myopia is a pronounced form axile myopia can be defined by his power expressed in diopters (ex: myopia superior to-8 d), or by measurement of ocular axial length (the length of the eye: superior to 25 mm). The LASIK is the technique of choice for low and medium myopia, but it is rivaled by other surgical techniques in the field of high myopia. These techniques are to insert an implant in the anterior segment of the eye (keeping the lens in young patients, or after its withdrawal in beginner cataract case): they need to make an incision and inject a corrective lens (implant) in the eye.
It is a frequent demand for surgical correction of high myopia. High myopia is very debilitating and is accompanied by a total dependence on the additive corrections means traditional (glasses, contact lenses). Corrective lenses of glasses for the high myopia are particularly unsightly (thick edges, concentric narrowing of the face from them through print, reflections) and sometimes uncomfortable because of their weight. Finally, they decrease the size of the retinal image and therefore the image perceived.
High myopia is often experienced by those who suffer as a genuine infirmity refractive surgery could "erase" permanently. The word 'disability' is often in the mouths of patients when they designate their Visual function.
Contact lenses is a seductive optical solution for correcting high myopia, as long as it is comfortable and the patient accepts the constraints (maintenance and manipulation). The appearance of intolerance to contact lenses is a very common reason for consultation for refractive surgery.
Benefits of LASIK for correction of high myopia
One of the major benefits of LASIK on implementation techniques is that this operation doesn't involve penetration surgical endoculaire, because no implant is placed. She looks less invasive than techniques that require the installation of an intraocular implant (implant phakic), and less mutilating only when the placement of an implant is associated with excision of the lens (surgery of the clear lens with an pseudophake implant).
Back and observed long-term security enjoyed by LASIK (20 years for the first case) also represent a substantial profit. However, it is crucial for the surgeon to know the technical characteristics and the limits of LASIK for the correction of the strong myopiques ametropia. The preoperative assessment is particularly important, in order to determine the operability of the patient and provide a realistic prognosis on the expected result.
Moreover, certain situations can lead to indicate in the myopic LASIK technique particularly hard. In case of gap correction important between the two eyes (difference of degree between the eyes, called "Anisometropia" high myopia), the Equalization of refraction between right and left eye represents an interesting benefit to patients. Indeed, these can not tolerate a total correction in glasses because of the difference in size of image between the two eyes (aneisoconie) that causes the difference in power of corrective lenses.
High myopia and presbyopia
Become presbyopic, strong myopic patients are often surgery to midlife seeker, so they don't have to juggle different type of lens correction (lenses and glasses for presbyopia, progressive lenses, etc.). For these patients, myopia is not really a help to see close without glasses: for example, the punctum point (the point where NET vision without effort) of a short-sighted patient-8 dioptres is located only 1/8 = 0.125 m = 12.5 cm from his face, which is not very convenient reading distance... In these patients, however, it discuss the possibility (or insufficiently thick cornea case, the need) a under correction, aiming for a close residual refraction-1.50 to-2D which will allow them to read at a comfortable distance after surgery, while greatly reducing the blur vision from afar.
A short-sighted including the ametropia goes-10 d to-1 d perceived a significant improvement in its capacity of Visual discrimination. Even if it remains insufficient for driving or shows, she allows him to walk comfortably in a familiar environment (home, office) without necessarily having to wear corrective lenses-1 d, it can however keep on hand in case of specific need.
In total correction (emmetropisation) case, the joy of finally being issued their myopia and the restoration of good uncorrected distance Visual acuity compensates largely correction close dependence in presbyopic patients motivated to see from a distance without glasses or lenses.
When the conditions for a functional outcome in line with the expectations of the patient are collected, LASIK is a technique well suited to surgical correction of high myopia.
High myopia and limits of LASIK
If low and medium myopia certainly represent the best indications of LASIK, the correction of high myopia is more discussed. It is difficult to set a specific upper limit for the correction of myopia by LASIK. The excimer laser can be programmed pourcorriger 15 diopters of myopia: but the number of operations in LASIK for high myopia to-12D remains very low.
For each case, the feasibility of surgery will be determined through the confrontation of the evidence gathered during the preoperative assessment and the expectations of the patient. In particular, the initial corneal thickness is a crucial point, since most nearsightedness is strong, more the laser must 'dig deeper' deep. The operability of a strong myopic is mainly conditioned by the thickness of the cornea. It shall also submit a good regularity and the lack of topographic signs of Keratoconus fruste (see the) page dedicated to the corneal thickness and LASIK).
The high myopia stabilizes later than low myopia or average; It is preferable to not operate on patients 'too young' (ex: before 25 years), unless the stability of myopia is well-documented, and warn them about the risk of renewed progressive myopia after correction.
The pre-surgery LASIK for high myopia
As always in refractive surgery, it is important to eliminate the contraindications.
At the strong myopic LASIK contraindications
High myopia is rarely isolated ocular abnormality. The preoperative assessment of a short-sighted patient strong applicant for refractive surgery is not limited to collecting information in optical sight, but must also eliminate a contraindication for LASIK:
-Keratoconus: strong myopia is common in case of advanced Keratoconus, because the increase of the Central corneal curvature causes an increase in the refractive power of the cornea. There is also a strong astigmatism myopic, often myopic or reverse direction. Corneal topography, as well as Tomography to detect Keratoconus in its more moderate, or infra clinical forms (cf screening of Keratoconus fruste). These forms of expression variables may not be that the translation of a corneal deformation induced then accentuated by the eye frottemens (caused by a related to a field of allergic pruritus).
-Beginner cataract (nuclear shape in particular): in this case, excision of the lens and the replacement with an implant is more appropriate (cataract surgery of the strong myopic:) example)
-glaucoma: critical eye pressure shall be readjusted based on the corneal thinning after LASIK. However, management and modern monitoring of glaucoma is based on factors other than the eye pressure. It is also possible to apply that reduction of the corneal "rigidity" would be beneficial to the stress exerted by hypertension on the emergence at the level of the ganglionic cells nerve fibers riddled blade. However, the simple section of the Bowman membrane by the realization of the flap changes the viscoelasticity of the cornea (9).
-the existence of a retinal disease of the macula must reconsider the surgical indication in myopes
Additional tests before LASIK for high myopia
The examinations are designed to assess the feasibility of the technique, i.e. ensure the adequacy between the degree of myopia to correct and (as well as some topographic features) thickness of the cornea.
Them topographers of elevation allow the measurement of corneal thickness, which can also be obtained by optical product or ultrasonic. Extended contact lens wear may be responsible for changes in corneal thickness. It is higher within hours of waking up. In case of extended wear rigid lenses, it is important to meet a deadline of at least one month of removal of lens, or more in case of deformation induced by these rigid lenses (corneal warpage).
Whatever the significance of the correction to be made, the diagnosis of infra clinical Keratoconus (Keratoconus frustrates) must be eliminated, and it will be raised through a review of corneal topography, revealing one or several signs following (nonexhaustive list): astigmatism irregular, low thickness in lower temporal eccentric position, low corneal thickness, and reduction of the enantiomorphism (the symmetry between the eyes). The occurrence of corneal ectasia after a LASIK done on eyes with a residual posterior wall thickness 250 microns is suspected the existence of forms not detected in subclinical (Keratoconus fruste) Keratoconus. We have recently demonstrated that these early forms of infra clinical Keratoconus screening, posterior topography and tomography (product optical point-by-point gained from the topographers of elevation as the Orbscan) offered a clear benefit in terms of sensitivity and specificity on the use of the previous specular topography of Placido. In some case,. the measurement of the corneal viscoelasticity by the Ocular Response Analyzer® (ORA) can be useful to support a suspicion of cornea 'at risk ectatique' (reduction of the hysteresis, aspect of the signal unfortunately).
It is accepted that the risk of luminous halos depends on the importance of output correction, as well as the dimensions of the zone of ablation: the influence of the diameter pupillary scotopic is controversial. In addition to the corneal topography, pupillary diameter can be measured at the average of conventional methods (scale, type pupillometer Colvard) or newer (measure then the) collection of topo aberrometrique review). Pupillary diameter measures provided by the analysis module of the anterior segment of the topographers as the Orbscan or the Pentacam are difficult to interpret because of the lighting related to lighting of the targets projected in the review.
Constraints of thickness for LASIK in high myopes
In LASIK, you should always calculate the predicted the posterior residual wall thickness which is equal to the initial thickness of the cornea, less the thickness of the flap and the photoablation. Predicted the posterior residual wall thickness should in no case be less than 250 microns: this rule is enacted in an empirical manner, and represents a minimum threshold beyond which the risk of corneal ectasia is greatly increased.
Thus, in order to prevent a secondary corneal ectasia, the respect of a posterior stromal wall thickness greater than or equal to 250 microns is consensus adopted by the majority of refractive surgeons. The physiological minimum thickness of the cornea is located in the Central para-apicale region. A meta-analysis found an average central corneal thickness equal to 534 microns (standard deviation close to 30 microns). There is however a significant dispersion around this average, thicker corneas so offering the possibility of a larger photoablation.
The realization of a flap end to the femtosecond laser by technique LASIK helps preserve a larger posterior residual wall thickness. Greater predictability offered by the femtosecond laser with the mechanical microkeratome for cutting stromal thickness control makes an undeniable asset. However, the thin flaps are more unstable and prone to neck folds, especially when they are repositioned on a deeply revamped geometry after residual wall. In our experience, it is best to not realize cutting stromal of less than 110 microns thickness if you want to escape the risk of micro pleats.
As to the depth of ablation of a conventional refractive treatment P, it is proportional to the treated correction (in diopters: D) and to the square of the diameter of the optical zone programmed (in mm, S):
P = 1/3 x D x S^2 This formula - called Munnerlyn formula simplified - is an approximate value
It is not possible for physico-mathematiques reasons to reduce the depth of ablation compared with this estimate. Some laser manufacturers sometimes advertise lower depths over large areas of ablation, but these also include the transition (to non optical sight) area, and the reduction in the depth of ablation is done so at the expense of the real optical zone diameter. Insist on the fact that the laws of physics are a must and we teach on an optical zone of 6 mm, it is not possible to correct a diopter of myopia without digging less than 12 microns year Center (in reality this revenue, the simplified formula of Munnerlyn, is rather under estimator: in practice, we can count 14 microns per diopter on this same diameter).
The depth of ablation increases with the square of the diameter of the optical zone. This diameter reduction "saves" thickness, but may expose to the greater risk of halos and myopic regression.
Using some programs of photoablation says 'tissue reduction' ('tissue saving") Although legitimate, represents an option that shouldn't have to be, and in fact points out that some platforms were originally designed to deliver the most expensive photoablation in corneal tissue than impose the laws of optics! At the time of the design of these lasers, tissue economy was not necessarily a priority. These photoablation programs could be abandoned, because there is no interest to remove more corneal tissue that it is necessary. Some excimer lasers have been immediately with programs in line with the laws of optics, and in fact equivalent to the so-called modes of 'tissue reduction '. This has paradoxically deprived their manufacturers (or even the user surgeon) an asset of communication marketing...!
Thus, the correction of high myopia by LASIK meets anatomical boundaries and functional specific to each patient, and it is difficult to have a dogmatic attitude in this area. In practice, the maximum power of myopia treated are between-10 and-12 diopters, for an optical zone diameter between 5.5 and 6 mm. In a patient whose cornea is healthy but thin - ex: corneal thickness of 510 microns - the maximum depth of ablation shall not exceed 150 microns (based on flap of 110 Micron thick).
Reduction of the diameter of the optical zone helps reduce the depth of ablation Central (this effect is even more pronounced as the relationship between these two parameters is exponential).
The realization of a zone of 5.5 mm Optics (diameter that we consider minimum if it does not induce excessive degradation of the quality of vision) will not treat over 12 dioptres; but that figure is in itself significant and the great majority of refractive surgery consultants present with a much lesser myopia.
Technical peculiarities of LASIK for correction of high myopia
The orbital and eye of the strong myopic conformation usually favors a good exposure of the eyeball for LASIK. Apart from a higher than normal axial length and depth of anterior Chamber, there is no strong myopia-specific biometric features. The size of the flap is determined by the dimensions of the zone of ablation; It will have to be slightly higher in case of astigmatism associated with myopia, because necessary enlargement of the transition next to the initially less arched Meridian area often induces an elliptical edge to the area where photoablation will be issued. The pupil is often slightly shifted in nasal at the strong myopic, and a slight refocusing of the flap on it allows to optimize the dimensions of the stromal bed available for the photoablation.
Obtaining an end flap may be a deliberate goal to reduce the risk of ectasia secondary: the creation of the flap using the femtosecond laser seems preferable to a mechanical cutting in the microkeratome, because there are significant variations in the thickness of the flap with mechanical systems no laser cutting. Because of the greater risk travel and folds of the flap; epithelial marking should be systematic.
The duration of treatment is proportional to the degree of correction and the use of an eye-tracker (tracking of the movements of the eye system) performance is required for the excimer laser. The lasers beam full (Visx) or scanning by slot (Nidek) are well suited to the treatment of high myopia, as well as the lasers to scan by spots on condition that the firing frequency is high (Wavelight EC 500).
The following video concerns the correction of a 10-D myopia:
LASIK has a place in the arsenal of techniques able to correct high myopia, and can be indicated until - 12D about for the most favorable case. Respect for the contraindications and the patient information on the peculiarities of the achievement and the results of LASIK in this context to balancing safety and postoperative satisfaction for the patient and the surgeon.