Presbyopia Flashcards
Why does the amplitude of accommodation decline with age
This is due mainly to sclerosis of the fibres of the crystalline lens and changes in its capsule which reduce the spontaneous steepening of its surfaces when the ciliary muscle contracts
Also it may be that the ciliary muscle itself becomes less efficient with advancing age (after 40 years)
How much accommodation is the eye capable of in infancy
14 Dioptres but by the age of 45 years this has fallen to about 4 D. After the age of 60 years only 1 D or less remains
How much accommodation does the eye keep in reserve
for comfortable near vision one-third of the available accommodation must be kept in reserve.
Therefore, the patient will begin to experience difficulty or discomfort for near vision at 25 cm when his accommodation has decayed to 6 D
Why is the onset of presbyopia earlier in uncorrected hypermetropia than emmetropia
the patient with hypermetropia must accommodate more to achieve near vision. For example, a patient with 3 D of hypermetropia needs to exert 3D of accommodation to see clearly at infinity. Therefore, to see clearly at 25 cm 7D of accommodation are needed (3D + 4D)
Which portion of a bifocal lens is larger
distance portion is usually the larger of the two.
The near portion usually occupies the lower part of the lens.
What is the distance and near visual points
The distance (DVP) and near (NVP) visual points are positions on a lens through which it is assumed the visual axis is directed while the spectacles are in use
The NVP is 2 mm nasal to and 8 mm below the DVP
What is the distance and near visual points
The distance (DVP) and near (NVP) visual points are positions on a lens through which it is assumed the visual axis is directed while the spectacles are in use
The NVP is 2 mm nasal to and 8 mm below the DVP
