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In a 1748 doctorial dissertation, a German oculist named Burchard Manchart provided an early description of a case keratoconus, which he called staphyloma diaphanum (Greer, 1989). However, it was not until 1854 that British John Nottingham clearly described keratoconus and distingushed it from other ectasis of the cornea. Nottingham reported the cases of "conical cornea" that had come to his attention and described several classic features of the disease including polyopia, weakness of the cornea and difficulty matching corrective lenses to the patient vision. Keratoconus is a degenerative non-inflammatory disorder of the eye in which structural change within the cornea (trasparent front part of the eye that covers the iris, pupil and the anterior chamber) cause it to thin and change to a more conical shape.
Keratoconus can cause substantial distortion of vision with multiple images reported by the patient .According to (Feder, 2005) keratoconus is the most common dystrophy of the cornea affecting around one person in a thousand, and it is seems to occur in all ethnic groups worldwide .It is typically diagnosed in the patient's adolescent years and attains its most severe state in the twenties and thirties. The visual distortion experienced by the patient comes from two sources, one being the irregular deformation of the surface of the cornea, and the other being scarring that occurs on its exposed high points. These factors act to form regions on the cornea that map an image to different locations on the retina and give rise to the symptoms of monocular polyopia.The effect can worsen where there is low light as the dark adapted pupil dilates to expose more of the irregular surface of the cornea (Yanoff, 1996)
It is likely that keratoconus can arise from a number of different factors, genetic, environmental or cellular, any of which may form the trigger for the onset of the disease. Once initiated, the disease normally develops by the progressive dissolution of Bowman's layer, located between the front epithelium and the stroma in the cornea. As the two come into contact, cellular and structural changes in the cornea adversely affect its integrity and lead to the buldging and scarring that are characteristic of the disorder. Within any individual keratonic cornea, there may be found regions undergoing wound healing. People with an early keratoconus typically notice a minor blurring of their vision, at an early stage, the symptoms of keratoconus may be no different from those of any other refractive defect of the eye. As the disease progresses, vision deterorates sometimes rapidly. Visual acuity becomes impaired at all distances and night vision is often quite poor. Some individual have vision in one eye that is worse than that in the other eye. Some develop photophobia (sensitivity to bright light), eye strain from squinting in order to read or itching in the eye; usually there is little or no pain. The classic symptom of keratoconus is the perception of multiple images, known as monocular polyopia.
The diagnosis of keratoconus frequently begins with an assessment of the patient medical history, particularly visual symptoms, the presence of any history of ocular disease or injury which might affect vision and the presence of any family history of ocular disease. If keratoconus is suspected, the ophthalmologist or an optometrist will search for other characteristic finding of the disease by means of slit lamp examination of the cornea. A diagnosis of keratoconus easily can explain patient's symptoms or greatly alter a plan topography useful as a screening device for keratoconus, for documentation of coneal status, it also should be included in the patient's examination. Indeed most cases of keratoconus will be demonstrated by automated topography (figure 2), however, the surgeon must resist notion that automated topography replaces manual keratometry. The two modalities should be additive to provide increased information about patients cornea and should not be considered in competition with each other, as implied by some.
The important information obtained from automated topography is the determination of the optical quality of the cornea. Refractive surgery is designed to correct a refractive error; it is not a topographical map. Those topography units that afford the observer the most accurate information about corneal irregular astigmatism are valuable. Because irregular astigmatism is linked directly to keratoconus, the more sensitive an automated topography is in detecting irregular astigmatism, the better will it detect the subtle levels of keratoconus (Trevor, 1981). Corneal topography plays an important role in laser in situ keratomileusis (LASIK). Preoperative screening permits the detection of karatoconus and other corneal shape anomalies that may be contradiction for refractive surgery. Monitoring the progress of the surgical outcome with topography is helpful to detect postoperative complications such as decentration and clinically significant irregular astigmatism. LASIK compares favorably with PRK for the treatment of low moderate myopia in terms of early achievement of refractive stability, but induction of modest amount of irregular astigmatism can be a concern.
In early stages of keratoconus, spectacles can suffice to correct for mild astigmatism. As the condition progresses, spectacles may no longer provide the patient with a satisfactory degree of visual acuity, and most clinical practitioners will move to managing the condition with contact lenses.
In keratoconic patients, contact lenses improve vision by means of tear fluid filling the gap between the irregular corneal surface and smooth regular inner surface of the lens, thereby creating the effect of a smoother cornea. Many specialized types of contact lenses have been developed for keratoconus, and affected people may seek out both doctors and specialized in conditions of the cornea, and contact lens fitters who have experience managing patients with keratoconus. The irregular cone present a challenge and the fitter will endeavor to produce a lens with optimal contact stability and steepness. Traditionally, contact lenses for keratoconus have been the hard or rigid gas permeable variety although manufactures have also produced specialized soft or hydrophilic lenses. A soft lens has a tendency to conform to the conical shape of the cornea, thus diminishing its effect (Zadnik, 1999)
Keratoconus will progress to a point where vision correction is no longer possible; thinning of the cornea becomes excessive, or scarring as a result of contact lens wear causes problems of its own, and a corneal transplantation or penetrating keratoplasty becomes required. Keratoconus is the most common grounds for conducting a penetrating keratoplasty, generally accounting for around a quarter of such procedures. The corneal transplant surgeon trephines a lenticule of corneal tissue and then grafts the donor cornea to the existing tissue (citation) usually using a combination of running and individual sutures. The cornea does not have direct supply (avascular) and so donor tissue is not required to be blood type matched. Eye banks (retrieve and store eyes for cornea transplant and research) check the donor corneas for any disease or cellular irregularities. The acute recovery period can take four to six weeks and full post operative vision stabilization often takes a year or more but most transplants are very stable in the long term.
One way of reducing the risk of rejection is to use a newer technique called a Deep Anterior Lamellar Keratoplasty, DALK. According to (citation) in DALK graft, only the outermost epithelium and the main bulk of the cornea, the stroma is replaced. The patient's rearmost endothelium layer is retained, giving some additional structural integrity to the post graft cornea. Because a graft rejection usually begins in the endothelium, the chance of a rejection episode is greatly reduced.
Rarely, a non penetrating keratoplasty known as epikeratophakia (epikeratoplasty) may be performed in case of keratoconus. The corneal epithelium and lenticule of donor cornea grafted on top of it. The procedure requires a greater level of skill on the part of the surgeon, and less frequently performed than penetrating keratoplasy as the outcome is generally less favorable. It may however be seen as an option in a number of cases, particularly for young patients (Rabonitz, 2004). Other treatments of keratoconus are corneal ring segments insert, radial keratotomy, a refractive surgery procedure where by the surgeon makes a spot -like pattern of incisions into the cornea to modify its shape.
Despite its uncertainties, keratoconus can be successfully managed with a variety of clinical and surgical techniques and often with little or no impairment to the patient's quality of life.
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- Greer, C, H (1989) Ocular pathology: Keratoconus, 4th edition, Blackwell Scientific, Oxford, pp 284-298
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- Rabonnitz, Y, (2004) Ectactic disorder of the cornea, 40: 284-302
- Trevor-Roper, P, D (1981) The cornea in health and disease, journal of ophthalmology: 21: 901-920
- Yanoff, M, (1996) Ocular pathology, diagnosis of keratoconus, 4th edition,Mosby-Wolfe, London, pp 420-434
- Zadnik, K, Barr J (1999) Diagnoses, contact lenses prescribing and care of the keratoconus patient, 29: 284-304