七年制医学课件眼科角膜病karatitis.ppt

CORNEA DISEASE Physiology The cornea functions as a protective membrane and a “window” through which light rays pass to the retina. Its transparency is due to its uniform structure , avascularity ,and deturgescence. Deturgescence, or the state of relative dehydration of the corneal tissue, is maintained by the active bicarbonate “pump” of the endothelium and the barrier function of the epithelium and endothelium. Corneal resistance to infection The epithelium is an efficient barrier to the entrance of microorganisms into the cornea. Once the epithelium is traumatized, however, the avascular stroma and Bowman’s layer become susceptible to infection with a variety of organisms, including bacteria ,amebas, and fungi. Physiology of symptoms Since the cornea has many pain fibers, most corneal lesions, superficial or deep (corneal foreign body, corneal abrasion, interstitial keratitis), cause pain and photophobia . Since the cornea serves as the window of the eye and refracts light rays, corneal lesions usually blur vision somewhat, especially if centrally located. Photophobia in the corneal disease is the result of painful contraction of an inflamed iris. Symptoms The first symptoms are usually irritation, photophobia, and tearing. When the central cornea is affected, there is also some reduction in vision. Since corneal anesthesia usually occurs early in the course of the infection, the symptoms may be minimal and the patient may not seek medical advice. lesions The most characteristic lesions is the dendritic ulcer. It occurs in the corneal epithelium, has a typical branching, linear pattern with feathery edges, and has terminal bulbs at its ends. Geographic ulceration is a form of chronic dendritic disease in with the delicate dendritic lesion takes a broader form. Corneal sensation, as with dendritic disease, is diminished. lesions Subepithelial opacities can be caused by HSV infection. Disciform keratitis is the most form of stromal disease in HSV in