- Farsightedness (Hyperopia) means that objects seen far away are clearer than those closer. In most cases, this condition is because the eyeball is shorter than normal from front to back. The rays of light coming into the eye are not focused as sharply at the time they reach the retina, with the point of focus falling behind the retina. The problem can also be caused by either a cornea or lens that does not have a steep enough curvature. Usually present from birth, farsightedness tends to run in families. Most young people are unaware of the problem because their lens is flexible and usually able to accommodate enough to focus the light sharply onto the retina. But the constant overworking of the muscles, makes the lens assume a more tightly curved shape which can leave aching or burning eyes, headaches, fatigue, or blurred vision after much reading or close-up work. As one ages, the lens becomes less elastic and unable to make the necessary shape adjustment. Most farsighted people need corrective lenses by middle age. Correcting the condition is accomplished with a convex lens, which is thicker in the center than at the edges. This moves the point of focus forward, onto the surface of the retina. Surgery is another option, but the procedure is complicated and, therefore, not performed as often as for the correction of nearsightedness.
Fluorescein angiography is a diagnostic test commonly used for evaluating diseases of the retina and the choroid. The procedure, also called fundus photography, allows a more detailed examination than the ophthalmoscope. Fluorescein, a dye also used in checking corneal abnormalities, is injected in an arm vein. As the dye circulates through the eye, the blood vessels in the retina and choroid stand out as bright yellow. A specially filtered camera takes flash pictures every few seconds for several minutes. Analysis of the resulting angiogram can determine the presence of tumors or clogged or leaking blood vessels, diabetic retinopathy, or macular edema. Fluorescein angiography may be used after photocoagulation (sealing of blood vessels with laser treatments) to determine the results of the procedure.
Fluorescein eye staining is a procedure performed to examine the cornea. The test allows the examiner to look for suspected scratches, irritations, or infections. The examiner applies fluorescein dye into the eye by touching the lower eyelid with a fluroescein-laden paper strip. The dye covers the cornea and settles into any cuts or scratches of the surface. The excess dye is washed from the cornea with the natural tearing process. Observation of the eye takes place under ultraviolet light as the fluorescein dye glows a bright green, illuminating such irregularities as corneal abrasions, ulcers, burns, or overexposure to light.
Fovea is a small hollow or indentation in the macular section of the retina. The macula is packed with cones, the cells that distinguish detail and color. The fovea is in the center of the macula and contains the highest concentration of cones. Light is focused directly on the fovea, making it the site of greatest perception. (See more under macular degeneration.).
Fuch’s endothelial dystrophy is a hereditary disease in which the corneal endothelium is destroyed. It is the leading cause of corneal transplantation in the United States. The endothelium is the protective inner lining of the cornea made up of a single layer of endothelial cells. These cells prevent the aqueous fluid of the anterior chamber from penetrating the cornea. A reduction of endothelial cells allows seepage into the cornea, resulting in swelling, loss of transparency and vision. The disease usually becomes apparent after the age of forty and is more common in women than men. It is diagnosed from a slit-lamp examination by dystrophic spots in the endothelium. As the condition progresses, corneal edema or fluid retention and swelling develops, the cornea thickens, and vision is lost. In later stages, blisters form on the epithelium, resulting in painful ruptures. Early treatment of Fuch’s endothelium dystrophy may include corneal transplantation (keratoplasty), or replacement of a disc of healthy cornea. Keratoplasty is not effective for later-stage dystrophy. Severe, painful forms of this disease are treated with cryotherapy in which a cryoprobe is applied to the sclera of the ciliary body to alleviate the pain.
- Fundus is a term for the back section of the inner eye. It contains the retina, the choroid, and the optic nerve. The fundus can be viewed with an ophthalmoscope, a handheld instrument that lights and magnifies the fundus for examination. Examination of the fundus, called funduscopy or ophthalmoscopy, allows the examiner to detect disorders within the eye and to determine such other diseases of the body as hypertension, diabetes, and arteriosclerosis.