0521779407-18 CUNY1086/Karliner 0 521 77940 7 June 13, 2007 8:1
1274 Refractive Disorders (Ametropias)■Causes – whereas regular astigmatism is generally a naturally occur-
ring condition, irregular astigmatism is generally caused by disorders
that may cause irregularity of the corneal surface or tear film (e.g.,
dry eye; corneal scarring or irregularity due to infection, trauma, or
surgery; or ectatic conditions such as keratoconus).
■Correction – regular astigmatism can be corrected with spectacles
or toric soft contact lenses. Rigid gas-permeable contact lenses can
be used to correct both regular astigmatism and irregular corneal
astigmatism.Presbyopia
■Prevalence – Presbyopia is a universal, progressive, age-related loss
of accommodative power, with symptomatic onset typically between
ages 42–46. Onset occurs earlier in hyperopes since a portion of the
accommodative reserve must be used to correct the hyperopia.
■Definition – In accommodation, the ring-like ciliary muscle con-
tracts, relaxing the zonules attached to the lens, thereby increasing
convexity and refractive power, allowing focus on near images.
➣Causes – Presbyopia is caused by age-related oxidative and other
changes to the lens proteins (crystallins), resulting in reduced
deformability in response to ciliary muscle contraction.
➣Correction – presbyopia is commonly treated with bifocal spec-
tacles, reading glasses, or bifocal contact lenses.tests
Ametropias (refractive errors) can be diagnosed using subjective or
objective means.
Subjective refraction is the process of determining the underlying
refractive error by presenting lenses of varying powers until the patient
reports that a small target image is in sharpest focus.
Objective refraction is the process of measuring the underlying
refractive error using retinoscopy (manual or automated) or aberrom-
etry.
Objective refraction is invaluable in measuring ametropias in chil-
dren and in uncooperative patients.differential diagnosis
n/amanagement
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