الفهرس 
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Abstract
Summary and Conclusion
Keratoconus is defined as a disorder characterised by progressive corneal steepening, most typically inferior to the center of the cornea, with eventual corneal thinning, induced myopia, and both regular and irregular astigmatism.
The prevalence of keratoconus in the general population appears to be relatively high and is one person per 2000.
The disease has no racial predilection and it is usually bilateral. The reports and studies showed variable sex distribution of the disease.
Onset of keratoconus occurs during the teenage years. The prognosis is unpredictable and progression is variable.
Most cases of keratoconus are sporadic. Hereditary transmission of keratoconus also had been reported. Dominant, recessive, and irregular transmissions all appeared to be implicated.
The proposed etiology of keratoconus includes biochemical and physical corneal tissue changes, but no one theory fully explains the clinical findings and associated ocular and non-ocular disorders.
Various biochemical abnormalities have been repoted in keratoconic corneas including decrease in hydroxylation of lysine and glycolysation of hydroxylysine, decreased total collagen and relative increase in structural glycoprotein.
Numbers of entities, both ocular (e.g., retinitis pigmentosa, floppy eyelid syndrome, fuch’s corneal dystrophy, cotact lens syndromes, keratectasia after LASIK) and systematic (e.g.,atopic disease, down syndrome, turner syndrem, systematic collagen diseas) are associated with keratoconus.
Although all layers of the cornea ultimately may show microscopic alterations, the earliest changes occur in the superficial layers of the cornea. The basal layer of epithelium involved in early stage and later the basal cells disappear, leaving only one or two layers of flattened superficial epithelial cells lying on an altered basement membrane. In advanced stage the Bowman’s layer is gradually destroyed, the stroma show abnormal thinning and anterior protrusion of the central portion of the cornea and the descemet’s membrane may be folded until becomes ruptured. Also the endothelial cells flattened and their nuclei lay father apart.
The shapes of cone may change according to the stage of keratoconus. There are nebulas, oval and globus shapes of cone.
Patients with keratoconus often complain of decrease in visual acuity, visual discomfort similar to a patient with uncorrected astigmatism. They will report, ”squinting” in order to see well. They are often more sensitive to light than the normal patient. They report glare or halos around lights.
The ocular manifestation of keratoconus is limited to the cornea. They include steepening of the cornea, especially inferiorly, alteration of red reflex, thinning of the corneal apex, clearing zones in the region of Bowman’s layer, prominent corneal nerves, scaring at the level of Bowman’ layer, and deep stromal stress lines that clear when pressure is applies to the globe (Vogt’s striae), iron deposition ring accumulates in the epithelium at the base of the cone, easily appreciated with oblique cobalt blue illumination (Fleischer’s ring). The steepening of the cornea leads to clinical signs, which include protrusion of the lower lid on down gaze (Munson’s sign), focusing of a light beam shown from across the cornea in an arrowhead pattern at the nasal limbus (Rizutti sign), reduced corneal sensation in the inferior cornea and appearance of corneal hyDROPs.
The keratometer aids in the diagnosis of keratoconus. The initial keratometric sign of keratoconus is absence of parallelism and inclination of the mires. The photokeratoscope or placido disc can provide an overview of the cornea and can show the relative steepness of any corneal area. The even separation of the rings in the spherical and the astigmatic cornea and the uneven spacing of the rings--especially inferiorly--in the keratoconic cornea should be noted. The central rings may show a tear-DROP configuration termed ”keratokyphosis”.
Ultra-sonic Pachymetry is a technique for measuring corneal thickness that aid in diagnosis and evaualte stages of keratoconus.
Keratoconus is more accurately distinguished from the normal population by video keratography than ulatrasonic pachymetry. Videokeratoscopy is used clinically to demonstrate pathology such as keratoconus, evaluate corneal contour before and after surgery and to diagnose contact lens –induced corneal distortion. Therefore, it has an important application before any anticipated refractive surgical procedures that based on normal, healthy cornea.
Many authors classifies the progression of keratoconus as mild, moderate and severe or advanced depend on the amount of corneal tissue affected that revealed by clinical sign and symptoms or by the corneal topographical changes.
Treatment of keratoconus depends on the severity of the condition and can be divided into two categories: surgical and non-surgical. There is no medical treatment currently available. The primary goal of all treatment is providing adequate vision.
The non-surgical management of keratoconus is essentially refractive. Early in the disease, spectacles are successful in restoring vision. As the condition advances and the cornea be become more distort, the spectacles are no longer offer acceptable correction then the contact lenses become more desirable option.
Rigid gas permeable contact lens is the main type of contact lens used in the treatment of keratoconus. Keratoconus is managed by many different contact lens designs and fitting methods according to the keratoconus type. In all cases, it should be consider offering the best visual acuity, comfort and corneal health.
Keratoconic patient who cannot tolerate contact lenses and have little or no corneal scarring may benefit from implantation of of intra stromal corneal ring or ring segments (INTACS) which is an ophthalmic device that induce overall corneal flattening, reduce the size of cone and migrate the cone centrally.
INTACS; intra srtomal corneal ring (ICR) and intrastromal corneal ring segments (ICRS) are relatively new devices that reinforce the cornea though the arc-shortening effect of the corneal lamellae that produces flattening of the central cornea. In addition, Intacs insertion is considered a minimally invasive approach that does not violate the visual axis of the patient and spares the central optical zone of the cornea.
Penetrating keratoplasty is the most treatment option used if the contact lens is no longer provides acceptable vision. The major complication of penetrating keratoplasty is a graft rejection which although its incidence is rare but it is major cause of surgical procedure failure.
It has been suggested that anterior lamellar keratoplasty used as alternative to penetrating keratoplasty for treatment of keratoconus as it eliminates the endothelial rejection despite of its difficult and time consuming technique. With the advances of deep anterior lamellar keratoplasty technique, the visual outcomes were improved and approaching the penetrating keratoplasty outcomes with the superior advantages of minimal rejection and complications.
The visual outcomes of Epikeratoplasty or onlay lamellar keratoplasty are worse than penetrating keratoplasty.
A keratoprosthesis is a device intended to restore vision to patients with severe bilateral corneal disease for which a corneal transplant is not an option where in some of the best outcomes, keratoprosthesis has been able to give patients 20/40 vision.
The patients of keratoconus with proud nebula at or near the apex of the cone usually suffer from contact lens intolerance. To overcome this problem, the phototherapeutic keratectomy should be considerable.