A man who is color-blind with 20/300 vision has LASIC surgery that connects his vision to 20/20 in each eye. Years later, however, he experiences difficulty reading and must get reading glasses. A few years later he develops cloudy patches in his right eye that the physician says will require surgery to replace the lens. A few years after his vision is restored by that surgery, the clarity of his central vision deteriorates and it is dangerous for him to drive at night.
-Explain the most likely cause of his color blindness, the visual defect indicated by 20/300 vision, and how the LASIK surgery changed his vision to 20/20.
-Explain why he needed reading glasses years after his LASIK surgery.
-Identify the most likely cause of the cloudy patches in his vision that was corrected by surgical replacement of his lens, and the likely cause of the later deterioration of his central vision.
The lens is normally completely clear, due to its unique structure. It is composed of about one thousand layers of cells aligned in parallel and tightly joined together, so that gaps don’t form as the shape of the lens is changed. The lens is transparent because (1) it is avascular (2) its cell organelles have been destroyed in a controlled process that halts before the cells die; and (3) the cell cytoplasm is filled with proteins called crystallin. Because of this structure, every region of the lens normally has the same refractive index. However, damage from ultraviolet light, dehydration, or oxidation may cause the crystallin proteins to change shape and aggregate to produce the cloudy patches in a person’s visual field known as cataracts. Cataracts interfere with vision in more than half of people over the age of 65. This is generally treated by surgically replacing the lens with an artificial lens.
Many people with refractive problems choose to have a surgical procedure known as LASIK (laser-assisted in situ keratomileusis). A computer-guided laser reduces the curve of the cornea for myopia so that the focus is moved back to the retina. It makes the cornea more steeply curved for hyperopia, and more spherical to correct for astigmatism. This does not correct for presbyopia, so reading glasses may be required. Alternatively, if the person has myopia, one eye can be deliberately undercorrected for reading while the other eye is corrected for distance vision. This is called monovision, and is tolerated better by some people than others.
Glaucoma occurs when there is loss of retinal ganglion cell axons along with blood vessels and glia, in the optic nerve that produces characteristic changes in the appearance of the retina through an ophthalmoscope. Glaucoma is generally caused by a problem in teh flow of aqueous humor and can be classified as closed-angle or open-angle. Closed-angle glaucoma occurs when the drainage pathway for aqueous humor is blocked (by a tumor or inflammation); open-angle glaucoma occurs when the angle formed by the iris and cornea is unobstructed bu the drainage (through the canal of Schlemm) is still inadequate. Primary open-angle glaucoma is the second leading cause of blindness in the United States, and can occur when the aqueous humor pressure is either normal or elevated. Elevated intraocular pressure due to inadequate drainage of aqueous humor is the most important risk factor, and the only risk factor that can be treated to prevent this disease or slow its progression.
Clinical examination of the fundus (ophthalmoscopy) can aid the diagnosis of a number of ocular and systemic (body) diseases. The features noted in these examinations include the condition of the blood vessels; the color and shape of the disc; the presence of particles, exudates, or hemorrhage; the presence of edema and inflammation of the optic nerve (papilledema); and myopia and hyperopia.
The only part of the visual field actually seen clearly is that tiny part (about 1%) that falls on the fovea centralis. We are unaware of this because rapid eye movements shift different parts of the visual field onto the fovea. A common visual impairment, particularly in older people, is mascular degeneration–degeneration of the macula lutea and its central fovea. People with macular degeneration lose the clarity of vision provided by the fovea in the central region of the visual field. In most cases, the damage is believed to be related to the loss of retinal pigment epithelium in this region. In some cases, the damage is made worse by growth of new blood vessels (neovascularization) from the choroid into the retina.
The eye is a receptor, transducing light into electrical nerve impulses. We actually see with our brain. Impulses from the retina pass, via the lateral geniculate bodies, to the visual cortex of the occipital lobe where the patterns of impulses are integrated to produce an image. The importance of the visual cortex in vision is illustrated by strabismus, a condition in which weak extrinsic eye muscles prevent the two eyes from converging on an object and fusing the images. To avoid confusion the cortical cells eventually stop responding to information from one eye, making that eye functionally blind. Visual information is integrated with input from the other senses in the cortex of the inferior temporal lobe. If this area is damaged (the Kluver-Bucy syndrome), visual recognition is impaired. Although the image is seen, it lacks meaning and emotional content.
The gene for the S cone pigment is located on autosomal chromosome number 7, whereas the genes for the M and L cones are located on the X chromosome. Color blindness is caused by an inherited lack of one or more type of cones, usually the M (green) or L (red) cones. These people have only two functioning types of cones, and their vision is thus dichromatic. People who have only one cone in the middle to long wavelength region (M or L) have difficulty distinguishing reds from greens. Because the photopsins of the M and L cone pigments are coded on the X chromosome, and because men have only one X chromosome (and so cannot carry the trait in a recessive state), such red-green color blindness is far more common in men than in women.