Classification of myopia
The different forms of myopia: classifications
We can distinguish several types of myopia, according to the mechanism involved, the type of evolution, or the existence of associated pathologies. The degree of myopia is usually quantified based on the power of corrective lenses required to correct: the degree of short-sightedness, or myopia power is expressed as a number of vergence (negative convention). example:-3 D. The calculation of the inverse of the vergence (ex: 1/3D = 0.33 m = 33 cm) provides the distance at which the nearsighted eye sees net without correction: this distance is by definition one of the punctum point (furthest point which is seen net). Beyond the point punctum, the vision is blurred (it's even more blurred as you move away from the point punctum)
Myopia is related to a power (posterior focal length) excessive retractives structures of the eye: rays from distant sources are focused by the cornea and the crystalline lens in front of the retina (see the blurred vision of the myopic). The power of glass needed for the correction of myopia correction indicates the severity of it (a myopia of 5 d is more severe than a myopia of 1 d), but not its cause, at least for low and medium myopia.
Severity of myopia and degree of correction
Can be arbitrarily classified depending on its importance, established myopia from the degree of correction needed (diopters) is it necessary to wear (in glasses):
-low myopia < 3D
-myopia average between 3 and 6 d
-high myopia > 6 d
This classification does not take into account associated pathologies that can participate in the severity of myopia (retinal pathologies, in particular). It is of some interest in refractive surgery; If the PKR is performing as well as the LASIK for the low and medium myopia, there (increased accuracy of the correction and less scarred regression) superiority of LASIK for the high myopia (greater than 6 D).
High myopia may also be defined biometric: it concerns in general eyes whose axial length is greater than 26 mm. The deformation of the posterior pole of the eyeball in high myopia case is responsible for the existence of retinal complications such as the posterior staphylome, or myopic choroidose;
Classification of myopia based on its mechanism
Recall that the focal power of the eye depends on the refractive elements of the eye: horny and crystalline. (Inversely proportional to focal length) focal power of these elements is so dependent on the power of the cornea, the distance between the cornea and the lens, and the power of the lens.
Depending on the degree of eye elongation, and the existence of other anomalies, there are mainly the so-called myopia axiles so-called myopia refractive. Axiles myopia are associated with a "significant" increase in the axial length of the eyeball. The refractive myopia are related to a mismatch between the focal power (cornea/lens) and axial length (this axial length is relatively normal, but the cornea and the lens focus too incident light. This occurs when the cornea is particularly and snap, which increases sound Optical power - vergence).
Excessive axial length (the axial length is the distance between the top of the cornea, and the fovea which is a retinal fine vision) is an obvious cause of myopia. However, some short-sighted eyes present an identical to that of normalsighted eyes axial length, see sometimes lower!
There are large variations between human eyes for a parameter as the corneal power (between 40 and 47 d for healthy corneas), as well as the depth of anterior Chamber, (and probably the lens power).
As a result, it is difficult to blame systematically isolated excessive axial length to explain the occurrence of myopia; We speak of refractive myopia when it is rather related to a sort of "disharmony" between axial length and focal power of the refractive elements of the eye. The length of the eye is not excessive in itself (close to 24 mm) with respect to a reference population composed of no myopic eye. but it is excessive with respect to the Optical power of the eye. Generally, this type of short-sightedness does not exceed 3 or 4 diopters; beyond that, the probability of axile myopia is strong.
Corneal myopia is a special type of refractive myopia. The increase in power corneal trained an increase in focal power of the eye: from light rays from a distant source are too refracted by the cornea (they converge in front of the retina), while the axial length is not statistically higher than the average. This mechanism is at least partially offending in myopia associated to the Keratoconus, because this condition causes a central corneal curve, and therefore a marked increase in the corneal vergence. The risk factors for Keratoconus are genetic and environmental. According to the author of this site, the repeated rubbing of the eyes which are the direct and necessary of Keratoconus cause.
Here is an example of corneal myopia, for which the measure of the axial length in optical biometry is normal, while the keratometry is high.
It's a myopia that the main mechanism is the elongation of the eyeball. The average axial length of the human eye "Emmetropic" is close to 23 mm (2.3 cm).
A study found that the axial length of farsighted eyes averaged 22.62 ± 0.76 mm), and was therefore logically shorter than short-sighted eyes, who was measured at 25.16 ± 1.23 mm (axile and refractive myopia combined). These figures show that axial length values differ on average, but in terms of the standard deviation of the respective distributions, they overlap significantly between the two groups (farsighted vs myopic). In this study, the myopic eyes had a slightly more powerful than the farsighted eyes cornea, but the measured difference was not significant)Llorente et al. Myopic versus hyperopic eyes: axial length, corneal shape and optical aberrations. Journal of Vision, 2004; 4, 288-298).
It is difficult to distinguish some axiles 'refractive' myopia myopia, because the variability of the axial length in the general population is important. However, most of the myopia greater than 4 D are axiles, and most of the eyes with axial length greater than 25.5 mm are achieved axial myopia (Hendicott and Lam. Myopic crescent, refractive error and axial length in Chinese eyes. Blink Exp Optom, 1991; 74:168 - 174).
Axiles myopia, include the scalable high myopia, characterised by a progressive distension of the posterior pole and the appearance of associated pathologies related to it. These misuse are often referred to as pathological.
The most easily recognizable myopia is definitely pathological, also called myopia "evolutionary" myopia, progressive myopia, degenerative myopia. It is characterized by an appearance more early (early childhood, before adolescence) and progresses quickly. It is characterized by an excessive distension of the posterior pole of the eye, with important extension of the length of the eye, and is accompanied by degenerative retinal chorio. This degeneration is related to the fact that the chorio-retinal, little stretch fabric, is undergoing a significant increase of the inner surface of the eye (linked to sclerale expansion). This tissue thins, rips and atrophy according to the degree of myopia. Fortunately, the prevalence of this type of myopia seems in regression. Myopia may exceed 10 D (up to 30 D into serious, sometimes called shapes "") malignant myopia »). Axial length is reached, see exceeds 30 mm.
The optical correction of these myopias appealed to rigid contact lenses. Glasses, the thickness of the concave lenses (even at high factor) is such that sometimes, special equipment must be designed.
These misuse are also associated with a risk of loss of best corrected Visual acuity. In other words, retinal lesions observed in patients with severe myopia can result in a functional loss not correctable by glasses or lenses. Refractive surgery of this type of short-sightedness, when it is possible, is generally based on the placement of an implant (implant phakic) eye intra or excision of the lens and its replacement with an implant whose power is calculated to reduce myopia.
An interesting phenomenon is often noted during surgery of the lens on eyes with high myopia. by the focus on the cornea, then moving away slightly the microscope of a few centimetres from the eye, it is possible to observe the details of the eye (retina) the myopic background hard. This is the to the fact that the retina of the strong myopic is combined with the map of the punctum point, which is located at a distance (in meters) equal to the inverse of the myopic correction (expressed in diopters). For example, the plan of the punctum point is located about 7 cm for a myopic to-15 D (1/15 = 0.07 about).
Myopias: rarer forms
There are far less frequent than the refractive myopia myopia forms and or axiles.
It is associated with diabetes, some pathological States and pregnancy, and based on likely changes the refractive media (mood acqueuse) chemicals, responsible for an increase in their refractive index, more or less associated with accommodatifs spames.
The myopia of index:
Index myopia is a form of refractive myopia: she is related to nuclear cataract: the nucleus of the lens is bomb and its index increases. The myopia of index may appear at an Emmetropic, at a former farsighted, or aggravate a pre-existing myopia (see the page dedicated to) the study aberrometrique of the myopia of index)
Accommodative myopia (or by "accommodative spasm" ""):
It's a myopia caused by a prolonged contraction of the ciliary muscle secondary to sustained efforts in vision (fine craftwork, school work or computer very extended, etc.). The instillation of eyedrops "cycloplegics" (paralyzing the ciliary muscle), allows to correct the myopia.
Myopia associated with a subluxation of the lens
This exceptional shape surprises with the 'normal' dimensions of the eyeball with respect to the degree of myopia, which is linked to a loosening of the suspensory ligament of the lens (zonule). The lens spontaneously adopts a more rounded form, increasing its refractive power and causes excessive convergence of the refracted rays (see the) description of a case of myopia by subluxation of the lens)
Type of myopia and refractive surgery
(PKR, LASIK) excimer laser refractive surgery concerns mainly the refractive myopia, and part of the axiles myopias (keeping in mind that the border between these types of myopia is fuzzy). Axiles myopia are the myopias characterized by a generally more important (more than 6 D) correction and are rather corrected by LASIK, as long as the thickness and corneal regularity are compatible with this technique. Axiles myopia have a longer evolution: they start earlier in childhood, and change later than «refractive» say myopia Low myopia of refractive type (ex:-2.50 D) is generally stable around the age of 20 to 22 years, a high myopia of refractive type (ex:-8 d) tends to stabilize later (25-30 years), and keeps an 'evolutionary potential. "
The myopia corneal can receive a LASIK or a PKR, to the exclusion of the related to Keratoconus, corneal myopia who are against formal indication to LASIK.
The pathologic myopia (scalable high myopia, etc.) are by definition of 'fragile' eyes: refractive surgery is conceived as a functional surgery. The appearance of cataract (more frequent and earlier on this type of eyes) is an occaseion to reduce myopia through the removal of the lens, and its replacement by an intra ocular lens whose power is calculated to compensate for the excessive lengthening of the posterior segment of the eye. The lens usually has an optical power of 22D (in the plane of the lens). Emmetropic patient therefore, wishing to stay that boasts an implant of power close to 22D. On the other hand, a myopic-16 d surgery of cataract and wishing a total reduction of near-sightedness in theory benefit from the implementation of an implant whose optical power (vergence) will be close to 6 D.
Finally, the myopia of index is an indication of the cataract surgery.