11. Presbyopia Flashcards
what happens to the amplitude of accomodation as we age
steadily declines
why does accomodation decline as we age
- sclerosis of the fibres of the cystalline lens
- changes to the capulse which reduce the spontaneous steepining of its srugfaces
- ciliary muscle becomes less efficient with advancing age (after 40years)
how much accommodation is the infant eye capable of
14D
how much accommodation AT 45 YEARS
4d
how much accommodation at 60 yeas
1D
how much accomdation is required at reading distance (25cm)
emmetropic eye 4D
how much accomadative reserve is required for comfortable near vision
1/3
when would you start getting discomfot
Decayed to 6D
usually between 40 to 45 years of age
why can’t presbyopia be defined in amplitude of accommodative reserve
onset of symptoms varies for patients
how to work out presbyopic correction
Need to know:
1. remaining amplitude at near point
2. desired working distance
Presbyopic correction example:
emmetropic patient with remaining accomodation of 3D at 25cm
3D at 25cm
comfort = use 2D out of 3D (1/3 remaining)
to see well at 25cm need 4D
Thf presbyopic correction would require 2D
presbyopia in hypermetropia
Occurs earlier in uncorrected hypermetropia than emmtropia
as hypermetropia must accommodate more to achieve near vision
presbyopic correction
hypermetropia with 3D
Hypermetrope needs to exert 3D of accommodation to see clearly at infinity
therefore to see clearly at 25cm 7D of acommodation is needed
3D + 4D
Myopia and presbyopia
presbyopia in uncorrected myopia is delayed
presbyopia and working distance
correction must be adjusted for working distance
how to check for over-correction
ensure patient can read N5 at reading
and N8 at arms lenght
what are bifocal lenses
sinlge pair of spectacles provudes separate distance and near prescriptions for each eye
Split franklin bifocals
- flat bottom of two lenses that abutt together
Cemented wafer bifocals lens
- have near portion constrecuted by attaching a supplementary lens to the surface of a distance lens of the same refractive infex
- UV cured epoxy is used
Fused bifocals
- near portion is made by heat-fusing a button of flint glass to a corresponding depression in a crown glass main lens which has a lower refractive index
- different types of flint glass allow variation in the refrective index and power of the segment
segment shapes for right eye fused bifocal lenses
solid bifocals
- single piece construction
- near is produced by a different curvature of either the front or back surfacep
plastic bifocals are always
solid / exectutive bifocals
DVP and NVP
Distance visual point
Near visual point
DVP and NVP are assumed to be
the visual axis is directed while the spectacles are in use
NVP is relation to DVP
2mm nasal to and 8mm below the DVP
