Kératocône

The kératocône (of the Greek kerato , horn, cornea and konos , cone) is a degenerative Maladie Eye which results in a loss of the sphericity of Cornea which then takes the form of a cone.

The kératocône can cause substancielles distortions vision, leading to multiple images, scratches and a over-sensitiveness with the light. The kératocône is most common of the Dystrophie S of the cornea. This disease assigns approximately a person on thousand and seems to touch all the categories of population in the world, although its prevalence is more marked in certain ethnicities.

The disease is typically diagnosed during the Adolescence of the patient and reached his most advanced stage towards 20-30 years.

The kératocône is a disease little known and whose causes are dubious. Its progression after the diagnosis remains unforeseeable. If it touches the two eyes, the deterioration of the vision can affect the capacity of the patient to drive a car or to read normally. In the majority of the cases, the corrective lenses are enough effective to make it possible to the patient to continue to lead legally and to find a normal activity. A more important progression of the Maladie can lead to a surgical operation including the corneal Clerc's Offices in particular. In spite of these uncertainties, the kératocône can be controlled successfully with a variety of medical or surgical techniques, often with little or not of weakening of the quality of life of the patient.

History

It is in a doctoral essay of 1748 that the German oculist Burchard Mauchard provides the oldest description of the kératocône which it calls staphyloma diaphanum . However, a clear description of this disease is given only in 1854 when the British John Nottingham makes the distinction of it among others Aneurism S of the cornea. Nottingham brings back the spherical cases of cornea which held its attention and describes several traditional characteristics of the disease such as the double vision (Diplopie), the weakness of the cornea, and the difficulty in preserving corrective lenses for the vision of the patient. In 1859, the British Chirurgie N William Bowman uses a Ophtalmoscope (invented by the German physicist Hermann von Helmholtz) to diagnose the kératocône, and described how to incline the mirror of the instrument so as to better distinguish the conical form from the cornea. Bowman also tries to restore the vision while drawing on the iris using a fine hook inserted through the cornea and by stretching the Pupil according to a vertical stenopeic slit (form which one can observe in the cat, for example). It reports that it obtained a success with this technique by restoring the vision of a 18 year old woman, until there unable to count on its fingers at a distance of 20 cm. In 1869, when the Swiss pionner of the Ophtalmologie Johann Horner writes a thesis entitled on the treatment of the kératocône , the disease already acquired its current denomination. The treatment at that time, approved by the principal German ophtalmologist Albrecht von Gräfe, is an attempt to physically reform the cornea by chemical cauterization via a solution of Silver nitrate and the application of an agent bringing to the contraction of the pupil (Myosis) by the application of a pressure. In 1888, the treatment of the kératocône becomes one of the first applications practical of the Contact lenses recently developped at the point by the French physicist Eugene Kalt which manufactures a shell of glass improving the vision by compressing the cornea in a more regular form.

Since the beginning of the twentieth century research on the keratocône improved comprehension of the disease and considerably increased the range of the options of treatment.

Causes and mechanisms

In spite of considerable research, the causes of the kératocône remain still a mystery. According to the United States National Keratoconus Foundation , it is probable that this attack can result from a combination multiples of factors: genetics, environmental or cellular, each one of those, being able to involve the release of the disease. Once contracted, the disease develops by the progressive dissolution of the layer of Bowman, the membrane being between the epithelium of the cornea and the Stroma, normally constituting the essence the thickness of this body. When these two last come into contact, the structural and cellular changes of the cornea compromise its integrity and lead to enflement characteristic of the disease. In any keratoconic cornea, one can detect areas of degenerative thinning which coexist with areas undergoing the curative wound.

The visual deformation tested by the patient has two explanations, one being the irregular deformation of the surface of the cornea, the other the scars which occur on the most exposed areas. These factors bring to the formation of areas on the cornea which can form an image at various places on the Rétine causing the monocular symptom of polyopie. The effect can worsen under conditions of weak luminosity where the Pupille, adapted to the darkness, dilates to expose more an irregular large surface of the cornea. Marking seems to be an aspect of corneal degradation. However, a study suggests that abrasion by contact lenses can increase this probability of a factor two or more. A certain number of studies indicated that the keratoconic corneas show increased signs of metabolic activity via the Protéase S, a class of Enzyme S which degrade part of the Collagène present in the Stroma of the cornea, with simultaneously a reduction of the form of gene of the inhibiting enzyme of the protease. Other studies suggested that the reduced activity of the enzyme aldéhydedéshydrogénase can be responsible for a production of free radicals and species oxidizing in the cornea. It seems probable, in addition to the pathogenetic , that the damage caused in the cornea results in a reduction its thickness and its biomechanical force.

A genetic predisposition to the kératocône was observed, with, in certain families, a family transmission of the disease as well as agreements of case at identical twin. The probability of being reached when a member of its family is not yet clearly quantified, this number being between 6 and 19%. The responsible gene was not identified yet: two studies implying of the isolated and largely homogenetic communities gave contradictory results locating gene to the level of the Chromosome S 16q and 20q. The kératocône was associated with certain allergic diseases, including the Asthme and the Eczéma. A certain number of studies suggest that the vigorous friction of the eye can contribute to the progression of the kératocône, and that this practice should be disadvised among these patients.

Epidemiology

The National Eye Institute reports that the kératocône is most common of the corneal dystrophies to the United States, roughly assigning American over 2000 but certain reports/ratios mention a proportion of 1 per 500. This inconsistency can be due to the variations in the criteria of diagnosis, which in certain cases interpret a severe astigmatism like a kératocône, and vice versa . However its width is variable according to the consulted literature. An English study also suggested that the people of Asian origin would have 4,4 times more risk to suffer from the kératocône than people of the Caucasian type, and would be also earlier affected.

The kératocône is usually bilateral; the detection of a irregular Astigmatism suggesting a possibility of kératocône The serious cases can exceed the capacities of measurement of the instrument. Another indication can be provided by a retinoscopy (measurement of the state of refraction of the eye), during which a beam of light is concentrated on the Rétine of the patient. Reflected light by the eye of the patient causes reflexes observed by the expert. The kératocône is among the ophthalmologic conditions which show a reflex scissors ; the luminous reflection, instead of being distributed systematically, has a dark center which divides it into two branches. If the kératocône is suspecté, the Ophtalmologue or the optometrist will seek other features characteristic of the disease by means of an examination of the cornea. A case advance is usually completely obvious with the inspector, and this one can envisage a nonambiguous diagnosis before an examination more specialized. During a thorough examination, one can observe a ring of pigmentation of olive-green color to yellow-brown known, under the name of ring of Fleischer, around the eyes keratoconic. The ring of Fleischer, caused by the iron deposition on the epithelium of the cornea, is subtle and can not always not be detected in all the cases but it becomes more obvious when it is analyzed under a filter blue-cobalt. In a similar way, 50% of the patients present scratches of Vogt , i.e. fine lines on the cornea caused by its stretching and its thinning. This is why, although being a traditional sign of the disease, it tends not to be of first importance in the diagnostic step.

A manual keratoscope, also called disc of Placido , can allow a visualization simple and not-invasive surface of the cornea by projettant a series of concentric luminous circles on this one. A more final diagnosis can be obtained by the means of a corneal topography, during which an automatic instrument projects a luminous model on the cornea and determines its Topologie starting from the analysis of the digital image. The topographic chart indicates all the deformations of the cornea, the kératocône being marked by an increase characteristic of the curve which is usually below the central line of the eye. The technique can record instantaneous degree and extent of the deformation which can be used like locates in order to evaluate its rate of growth. It is of first importance because it makes it possible to detect the disease at its first stages whereas the other symptoms are not yet present.

Once the detected kératocône, its degree of importance can be defined by several measurements:

the slope of the most marked curve which can be: average (< 45 Dioptre S), advanced (up to 52 D) or severe (> 52 D).
the morphology of the cone: nipple (small: 5 mm and close to the center), oval (broad, below the center and often bending) or sphere (more than 75% of the affected cornea)
the smoothness of the cornea which can go from average (> 506 µm) to advanced (< 446 µm).

The use of these terms however tends to decline following the increasing use of the corneal topograhie.

The vision of the patient can fluctuate for one period of a few months, leading it to frequent changes in the regulations of lenses but the situation worsening, the Contact lenses become obligatory in the majority of the cases. The course of the disease can be however completely variable: it can even indefinitely remain stable among certain patients during years, whereas for others it can progress very quickly. Most generally, the kératocône progresses for one period from ten to twenty years Although invalidating for the patient, the signs are usually provisional and after one period from six to eight weeks the cornea returns to its transparency of origin. The re-establishment can be facilitated in a nonsurgical way by a binding with an osmotic saline solution. Although the hydrops cause usually the greatest marking of the cornea, from time to time they make it possible to the patient to profit from a flatter cone, facilitating the fitting of the contact lenses.

Some patients also find a good correction of vision and a comfort with a combination known as piggyback , in which rigid lenses are carried to the top of soft lenses, both providing a degree of correction of the vision. The lens piggyback makes use of a soft lens with a central zone milled to accept the rigid lens. The connection of a combination of lens piggyback requires a certain experiment on behalf of the assembler of the latter, and a tolerance on behalf of the patient keratoconic. Lenses sclérales are sometimes prescribed for cases of kératocônes advances or very irregular, these lenses covering a greater proportion of the surface of the eye and consequently being able to offer a better stability. The important size of the lenses can make them unpleasant or uncomfortable to certain patients. However their easier handling can make it possible to have the favors of patients having more reduced dexterity, the such elderly.

Surgical options

Transplantation of the cornea

Between 10% and 25% of the cases of kératocône evolve/move at a point where the correction of the vision by glasses or lenses is not satisfactory any more, the smoothness of the cornea becoming too excessive or a marking of the cornea because of the use of the contact lens which can itself pose problems. A transplantation of the cornea or kératoplastie transfixiante (or penetrating) is then necessary. The kératocône is one of the most common causes leading to a kératoplastie, generally representing a quarter of such procedures.

The surgeon trepans a duckweed of corneal fabric of the receiver positions on this level the graft of the donor, usually employing a combination of Suture S individual and current. The cornea does not need a direct supply blood, a compatibility between the blood groups or HLA is thus not required, the problem of rejection not being posed in a current way, contrary to other transplantations of bodies.

The period of re-establishment can take from four to six weeks and the full postoperative stabilization of the vision often takes a year or more but the majority of the Clerc's Offices are very stable on the long run. The national base of the kératocône announces that the penetrating kératoplastie has the most rate of success of all the procedures of Clerc's Office: in the precise case of the kératocône its success rate can be 95% or more.

The kératocône does not reappear normally in the transplanted cornea. certain cases were observed but are generally allotted to an incomplete excision of the cornea of origin or to an inadequate examination of fabric of the donor. The long-term prospects for the corneal Clerc's Offices realized in the case of kératocône are usually favorable once the made cicatrization and that several years were passed without problem.

Transplantation DALK

A manner of reducing the risk of rejection is the use of a recent technique called Deep Anterior Lamellar Keratoplasty , shortened by DALK . In this type of Clerc's Office, only the external epithelium and the principal part of the cornea, the stroma, are replaced, the endothéliale layer of the patient being preserved, giving an additional structural integrity to the post-grafted cornea. Since a rejection of Clerc's Office usually starts in the endothelium of the graft, the risk of rejection is some considerably reduced.

Moreover, it is possible to transplant a fabric which was freeze-dried, this process devitalizing this last thus, decreasing the risk of rejection notably.

Some surgeons prefer to remove the epithelium of the donor, others leave the cells of the donor in place; to remove can cause a light improvement in the comprehensive view, but this one goes from pair with an increase in the time of re-establishment of the vision.

Epikératoplastie

Seldom, a not-penetrating kératoplastie, called épikératoplastie can be carried out in the case of a kératocône. The corneal epithelium of the patient is removed and a duckweed of the cornea of the donor is grafted with the top. The process is more complex and is less frequently carried out than a kératoplastie transfixiante because the results are generally less favorable. The épikératoplastie can however be perceived like option in a certain number of cases, in particular for young patients.

The use of the intra-corneal rings

A recent alternative to the transplantation of the cornea is the insertion of intra-corneal rings. A small incision is carried out with the periphery of the cornea and two fine arcs of Polymethylmethacrylate are slipped between the layers of the corneal stroma on the two sides of the pupil before the end of the incision. The segments exert a pressure against the curve of the cornea, applatissant the peak of the cone and bringing back the cornea to a more natural form. The method, carried out under local anesthesia, offers the benefit of the reversibility because it does not imply any final lesion of fabric of the eye.

The two principal types of rings intra-stroma available are known under commercial names INTACs and rings Ferrara . INTACs flatter and are less centrally placed than the prismatic rings Ferarra. INTACs were approved for the first time into 1999 in the United States by the Food and Drug Administration (FDA) in the case of the Myopie. Their utisitaion was then extended to the treatment of the kératocône in July 2004. The Ferrara rings await the approval of FDA for the kératocône. Conceptually, the technique rests on the injection of a transparent synthetic gel in a channel reamed by the stroma. When freezing polymerizes, it stiffens and takes the properties similar to those of the pre formed rings.

The clinical studies on the effectiveness of the rings intrastromals for the treatment of the kératocône are with their first developments, and the results are rather encouraging up to now, although they must still obtain the agreement of all the surgeons ophtalmologists. The rings offer a good luck of improvement of the vision, but the results are not guaranteed and in some cases the state can even worsen.

In addition to one a kératoplastie transfixiante, the use of absorbent contact lenses or glasses can remain necessary for a certain correction of vision.

The potential complications of intra-corneal rings include the accidental penetration through the former room of the eye, the post-operative infections of the cornea, and the migration or the extrusion of the rings. One discovered more recently than of better results could be obtained for the cones located with the periphery of the cornea by using a simple arc INTACs. What leads to the preferential flatness of the cone, but also to increase higher the punt part above the cornea.

Radial Keratotomy

The radial keratotomy is a procedure surgical, known as refractive, where the surgeon carries out an incision in the cornea in order to modify the form of it. This old surgical option for the Myopie is largely replaced by the methods LASIK (by use of a laser beam) and other similar procedures. Procedure LASIK in itself is contra-indicated in the case of the kératocônes and of corneal thinnings because the displacement of the corneal fabric stromal will damage an already thin and fragile cornea in the long term.

For similar reasons, the radial keratotomy was generally not used on patients keratoconic Cependant, an Italian private clinic brought back certain successes with a prodécure of asymmetrical radial keratotomy during which the incisions are confined with a sector of the eye. The smoothness of the cornea esr initially measured via a Pachymètre, the surgeon then carrying out an incision a depth of 70 80% measured thickness. The patient initially undergoes tests of Photophobie and fluctuation of the vision, commune with other forms of refractive surgery. Riboflavin, when it is activated under ultraviolet light during approximately 30 minutes, increases the reticulation of collagen with the stroma thus modifying part of the constraints of tension of the cornea. The form C3-R, development with the University of technology of Dresden, showed a deceleration or a stop in the progression of the kératocône, and certain cases a reduction, in particular when the technique is combined with the use of rings intracornéens. A recent study shows that one obtains an improvement twice higher when the C3-R is combined with of INTACs, compared to the use of INTACs only. The technique thus seems promising because it makes it possible to be able to treat the disease as of these primary symptoms.

Corrective lenses can still be necessary after the treatment but it is hoped that this one will be able to more limit the deterioration of the vision of the patient and to reduce the use of the Clerc's Offices of corneas.

See too

Cornea
ocular List of the diseases and disorders
Ophthalmology

References

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