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Dictionary

AMBLYOPIA ('lazy eye') - is a functional impairment manifested by lowering of visual acuity that originates in childhood mostly as a result of squint, high refractive error, anisometropia (i.e. high refractive difference between eyes) or other eye disorder such as congenital cataract and so on. Visual function of a child develops until 6 to 8 years of age and any obstacle in normal eye function in early childhood may alter the visual acuity and permanently weaken the eye. In practice it means that amblyopic ('lazy') eye is not able to achieve visual acuity such as of the healthy eye even with the best dioptric correction (glasses or contact lenses). That also limits the possible result of refractive eye surgery, which can fix only refractive error, but unfortunately not amblyopia.

ASTIGMATISM - Astigmatism is a refractive error caused by irregular curvature of the cornea or (less frequently) of the lens. The curvature and thus also the refractive power of the cornea or lens differs in different meridians (axes) and the resulting retinal image is again blurry and deformed. The error develops in childhood and is frequently combined with the basic errors - short-sightedness or far-sightedness. Astigmatism impairs vision both for distant and near objects and is corrected with cylindrical glasses or toric contact lenses. Slight degrees of astigmatism are physiological (the cornea is never perfectly regularly curved) and do not require optical correction.

 


ASTIGMATIC KERATOTOMY ("incisions of the cornea") - is a microsurgical method appropriate for the treatment (reduction) of higher astigmatism. The method consists in placing two arcuate incisions at the border of the cornea using a special diamond knife (blade). Varying the length, depth and location of the incisions the surgeon can achieve the required change of the corneal curvature and correct thereby the astigmatism. For higher degrees of astigmatism and combined refractive errors, a combination of this procedure with LASIK can be considered. As a first stage, the incisions are used to reduce corneal astigmatism, and the residual error is than approached - after the first incisions have healed and refraction has stabilised (about 6 months after the incisions) - with a laser procedure.

CATARACT - is mostly acquired, less frequently congenital opacification of human lens. It is found usually in people 50-60 years old and older as a result of metabolic changes in human lens, but it can also appear as a complication of certain other ocular or general health disorders such as diabetes, the use of some drugs or ocular trauma. The correction of cataract is uniquely surgical. Cataract is extracted from the eye and replaced by an artificial lens, which is implanted into the remaining lens capsule.

CLE (clear lens extraction) - represents another possibility of refractive error correction - especially in patients with higher hyperopia and over 40 years of age. The procedure consists of clear human lens extraction and its substitution by artificial intraocular lens in order to correct refractive error of the eye. The operation is performed similarly like cataract extraction. The most modern operation is the clear lens extraction associated with multifocal intraocular lens implantation; multifocal lens is artificial lens that can focus-up close and far at the same time.

EPI-LASIK – The surface method of laser refractive surgery similar to the LASEK and PRK methods. The surface covering layer of the cornea is cut off using a special “epikeratome”. The laser surgery itself then removes a small layer of corneal stroma. At the end of the surgery, the epithelial flap is returned to its original place. Over the next few days, the epithelium will recover and regenerate. At the end of the operation, similar to the PRK method, it is necessary to cover the eye with a special contact lens to protect the eye during the first days of recovery and alleviate pain. The lens usually remains in the eye for four days. Most people feel pain associated with the healing of the eye surface during the first few days. Eyesight recovery is also longer as compared to the LASIK method. The sight will usually stabilize within several months.

EXCIMER LASER - An excimer laser is a computerised system employing an ultraviolet light beam with a wave length of 193 nanometers. This radiation is used to model the surface of the cornea according to the refractive error present, and the new optical system of the eye can focus light beams closer to the surface retinal layers. Each single pulse of the excimer laser is capable of removing corneal tissue with an accuracy of 0.25 micrometers! Just for comparison, human hairs have a width of 50 micrometers. The precision of the excimer laser and the predictability of changes made in the surface curvature, as well as its safety, made the use of excimer laser procedures the leading method in refractive surgery.

EYE-TRACKER - Eye-tracker is a control operating system, which optically fixes the patient's eye throughout the operation and thus prevents the undesired deviation of laser radiation. During the surgery, the position of the eye is permanently monitored and evaluated by automatic system, which then adjusts the direction of laser pulses according to eye movement evaluation and ensures the correct performance of the procedure. The modern generation of lasers uses highly effective system of eye-tracking, which enables very accurate removal of refractive error even in less compliant patients.

FAR-SIGHTEDNESS - In far-sightedness (hypermetropia) on the other hand, the eye is too short, or the refractive power of cornea and lens is tow low. As a result of this disproportion, the beams of light meet "behind" the retina and the ensuing image is again blurry. The optical apparatus of the eye is capable of correcting this error, to a certain degree, by means of accommodation (through changes in the convexity of the lens). This possibility, however, is limited, and diminishes with increasing age. More severe refractive errors of this kind can manifest in childhood already, while a less severe error does not have to cause complaints till adulthood - the vision of a patient is then impaired for near objects, but can worsen also for distant objects as well later on. A plus (convex) glass is used in corrective glasses or contact lenses to correct this error. 

 


GLARE (starbusting, halo) - and halos around lights in dim lightning and at night (blurring and fogging of vision or scattering of light sources) are to some extent natural effects caused by pupil dilation in dim lightning and different refraction of light rays in central and peripheral portion of the cornea. This phenomenon is known by majority of patients that are using glasses or contact lenses and sometimes it may appear also following laser refractive procedures. The intensity of this phenomenon is related to the number of dioptries removed, optical zone width and pupil size. Combination of high refractive error, thin cornea and large pupils increases the risk of this phenomenon in postoperative phase - that is why every planned laser procedure has to be well evaluated.

HAZE - is the possible complication of healing of the cornea following the PRK method. It involves the opacification of the cornea resulting from scarring due to impaired corneal epithelial basal membrane. Mild degree of haze does not usually affect the visual acuity, but moderate to severe degree of opacification may have negative impact on visual function. Therapy includes long-term application of steroid eye drops. Haze does not practically appear after the LASIK method.

HYPERMETROPIA - viz Far-sightedness

KERATOCONUS - is a disorder of corneal connective tissue; it is usually bilateral. The cornea of keratoconus patient is gradually bulging and thinning, so the dioptric power is decreasing. The disorder is first manifested as an increasing myopia, often in combination with higher and irregular astigmatism. Keratoconus can be diagnosed using the ORBOSCAN equipment that provides specialized examination of the cornea and reveals early stages of the disease. Keratoconus can be cured by rigid contact lenses; advanced stages must be treated by corneal transplant. Laser refractive procedures are contraindicated in keratoconus.

 

LASEK – The surface method of laser refractive surgery which is, in fact, a variant of the older PRK method. The covering surface layer of the cornea, i.e. epithelium, is not fully removed but only prepared as a thin flap after the application of a special solution. The laser surgery itself then removes a small layer of corneal stroma. At the end of the surgery, the epithelial flap is returned to its original place. Over the next few days, the epithelium will recover and regenerate. At the end of the operation, similar to the PRK method, it is necessary to cover the eye with a special contact lens to protect the eye during the first few days of recovery and alleviate pain. The lens usually remains in the eye for four days. Most people feel pain associated with the healing of the eye surface during the first few days. Eyesight recovery is also longer as compared to the LASIK method. The sight will usually stabilize within several months.

LASIK (laser-assisted in situ keratomileusis) - is a more recent and advanced method that is currently preferred at all leading centres where it is used to treat short-sightedness, mild and moderate far-sightedness, and astigmatism. The principle of the method consists in cutting away a narrow surface lamella of the cornea (most commonly 160 micrometers thick) and treatment of the tissue with laser after the lamella has been lifted off. After the deeper layers have been treated the lamella is returned back to its original site. It takes just a few minutes for the lamella to reattach and there is no need for sutures or covering the surface of the eye with a contact lens. The healing after the LASIK procedure is very rapid and practically without pain, the rehabilitation of vision lasts several days making an early surgery on the other eye possible, while the limitation of patient activities in the postoperative course is minimum. LASIK is thus a method that is more demanding in terms of its conduction, and demands also better technical equipment of the centre as well as higher degree of experience of the surgeon, but the high precision of the method, the good postoperative stability and painlessness of both the surgery and postoperative course explain why the method is preferred to PRK.

MONOVISION – It is a technique used to correct dioptric defects in near-sighted patients in a presbyopic age group (i.e. patients over 45 who have different dioptres for distance and near vision). One eye retains a minor defect for distance vision (ca -1.5 dioptre) and will therefore see better to near distances while the other eye is fully corrected for distance vision. The resultant condition enables efficient vision to both long and near distances. However, as it may affect spatial vision, the simulation of the condition using contact lenses is recommended.

MULTIFOCAL INTRAOCULAR LENS – an artificial intraocular lens which – unlike the conventional monofocal artificial lens – enables distance vision through the unique arrangement of optical zones of distance and near viewing, similar to multifocal glasses or contact lenses. It is used to correct dioptric defects particularly in patients in a presbyopic age group (over 45) when the natural human lens loses its elasticity and thus the ability to refocus to long and near distances (PRELEX,CLE).

MYOPIA - viz. Short-sightedness

PACHYMETRY (corneal thickness assessment) - is essential part of examination before the refractive laser procedure. Corneal thickness plays an important role in deciding how many dioptries can be safely removed in each patient. The measurement is most often performed by ultrasound following the previous topical drop anaesthesia. More modern and accurate method is the optical method using the ORBOSCAN equipment. This equipment allows the measurement of thickness of the cornea in all its area including the localization of the thinnest point. The assessment is non-contact, so there is no need for topical anaesthesia.

PHAKIC LENS - Presbyopia is not a refractive error in the true sense of the word, but rather a manifestation of the physiologic process of ageing of the eye associated with a loss of its ability to accommodate - focus to near objects. This accommodative mechanism becomes gradually weaker starting already in childhood leading to the so-called near point becoming more and more distant (the near point is the point nearest to the eye at which an object is still accurately focused on the retina). Around mid-40s the distance of the near point shifts to a distance of about 35- 40 cm in front of the eye and the patient starts having difficulty reading or working with near objects. The error is corrected with plus "reading" glasses the dioptre value of which gradually increases with advancing age. The problem of presbyopia (i.e. the need to wear glasses for reading or work at a short distance after mid-40s) cannot yet be solved reliably with the current laser refractive procedures.

PRELEX (Presbyopic Lens Exchange) – The replacement of a human lens for an artificial lens in order to correct dioptric defects in patients in a presbyopic age group (over 45) when the natural human lens loses its elasticity and thus the ability to re-focus to long and near distances. This operation is particularly suitable in patients who have different dioptres for vision to long and near distances. The surgery is identical to cataract surgery. The human lens is removed using an ultrasonic phacoemulsifying apparatus and an artificial lens is then inserted in the lens capsule. The optical power of the artificial lens is chosen individually according to the dimensions of the patient’s eye and the size of his/her dioptric defect to correct the defect in distance vision, or in near vision by using a multifocal artificial lens.

PRK (photorefractive keratectomy) - is a method consisting in a modification of the curvature of the corneal surface after removing its surface cellular level - epithelium. The surface of the cornea is first anaesthetised using a topical anaesthetic administered in the form of drops and the epithelium is then removed; this is followed by the laser procedure carried out in the surface layer of corneal stroma. As the epithelium layer forms a protective cover without which the eye becomes rather sensitive and even painful, the cornea must be covered with a contact lens after the procedure. Regeneration of the covering epithelium layer occurs within several days but even so the first days of the healing process of the eye are accompanied by pain and fuzzy (blurred) vision. Definitive healing of the cornea and stabilisation of vision vary to a large degree and can be expected to take several weeks to months. At present, PRK is an alternative to LASIK and is used to treat mild and moderate forms of short-sightedness.

PTK method (Photo-therapeutic keratectomy) – is a laser therapeutic procedure to treat corneal surface diseases, for example recurrent erosions, corneal scars, etc. The course of the procedure and postoperative recovery are similar to those for the PRK technique. The procedure is based on the laser ablation of corneal surface layers after the covering epithelium has been removed. The procedure is performed under local anaesthesia using anaesthetic eye drops, and takes about 5 minutes. Following the procedure, a therapeutic contact lens will be applied on the eye and it remains there for 3-4 days until the cornea has healed completely. The patient has to apply antibiotic drops over this period of time. The eye may hurt. After the removal of the therapeutic lens, the patient will continue to apply anti-inflammatory eye drops.

SHORT-SIGHTEDNESS - In short-sightedness (myopia), the eye is too long, or the combined refractive power of the cornea and lens is to high. This disproportion causes the light beam to converge (meet) in front of the retina, and the ensuing image is blurry. The error most frequently develops at school age and progresses throughout growth and adolescence. The patient with short-sightedness has difficulty to see distant objects and has usually no problems with reading. This refractive error is usually corrected with a minus (concave) corrective glases. 

 


STRABISMUS - or squint is condition in which the parallel position of eye globes is impaired. Mostly, it is a result of congenital or acquired functional defect of oculomotor muscles. Each patient suffering from this disorder should undergo specialized evaluation of strabismus. There is a certain connection between the position of eye globes and refractive errors. In some cases, refractive surgery may cause accentuation of squint along with the development of uncomfortable diplopia. The laser refractive procedure is not recommended in patients that appear to be at risk.

 

WAVEFRONT – It is a diagnostic system based on the principle of a wave-front which enables us to perform individualized measurement of the eye’s optic properties and evaluate higher-order aberrations. In laser surgery, they help to make a complete and exact picture of the qualitative and quantitative condition of the eye’s optical apparatus, assess the quality of vision in the dark, and quantify the occurrence of visual side phenomena.



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