Arcus senilis is the name given to the circular white infiltrate seen around the margin of the cornea. The lens gradually loses its plasticity throughout life and this results in a progressive reduction in the focusing power of the eye. This loss of focusing ability is also contributed to by the progressive loss of ciliary muscle tone. A child might be able to observe details of an object held 5 cm from the eye, but as a result of hardening of the lens and weakening of the ciliary muscle, the nearest point at which an object can be kept in focus gradually recedes. This progressive degeneration tends to pass unnoticed until the eye is no longer able to focus to within the normal reading distance. This usually occurs at the age of 45 years if the eyes are otherwise normal, and the phenomenon is called presbyopia. Some degree of opacity of the lens fibres is common in old age and only when this becomes more extensive is the term "cataract" used. The pupil becomes smaller with age and does not show the wide range of adjustment to illumination seen in younger people. The vitreous shows an increase in small opacities visible to the subject as "vitreous floaters". A more dramatic degenerative change, which occurs in a high proportion of normal individuals in the 60-70-year age group, is detachment of the vitreous. The formed part of the vitreous separates from the retina, usually above at first, leaving a fluid-filled gap between the retina and posterior vitreous face. Movement of the vitreous face can cause troublesome symptoms, for example flashing lights and floaters, but often a vitreous detachment goes unnoticed and is an incidental finding on examination of the eye. The important association between sudden vitreous detachment and subsequent retinal detachment has already been discussed in Chapter 13. The appearance of the fundus also shows gradual changes; the retinal arterioles become straighter and narrower, as also do the venules. Colloid bodies or drusen are more commonly seen because of degenerative changes in Bruch's membrane and the pigment epithelium, and peripheral chorioretinal degeneration is more evident. The young retina is more shiny than the old retina and in the elderly the normal light reflex is less marked. The optic disc tends to become somewhat paler and a degree of optic atrophy is accepted by many clinicians as a senile change unrelated to disease.
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