emmetropia
refractive state of a normal eye in which all the rays of light from a distantly fixated object are imaged sharply on the retina without the necessity of any accommodative effort.
What are the three basic abnormalities in the refractive state of the eye?
hyperopia or hypermetropia, myopia, and astigmatism
hyperopia or hypermetropia
farsighted
rays of light from a distant object come to a focus at a point behind the retina with respect to the unaccommodated eye
the chief cause is a shortening of the anteroposterior axis of the eye.
axial hyperopia vs. curvature hyperopia
a shortening of the anteroposterior axis of the eye vs. the front surface of the eye (the cornea or lens) has less curvature than normal
Define latent, manifest, and absolute hyperopia
- completely corrected by the eye’s own accommodation
- can be corrected either by convex lenses or by the patient’s own accommodation
- refractive error that is not compensated for by accommodation
Why is cylopegic refraction not appropriate for correction of hyperopia?
Since cycloplegic drops paralyze accommodation, cyclopegic examination indicates the magnitude of the refractive error. Noncycloplegic examination reveals the acceptability of a particular correction.
Why is hyperopia often asymptomatic in the young?
Young patient have a greater power of accomodation: hyperopia is often not symptomatic until patients are older.
When does hyperopia need to be treated in the young?
accommodative strabismus: part or all of the strabismus may be corrected by the use of convex lenses, decreasing the need for accommodation and the associated excessive convergence.
myopia
parallel rays of light come to focus at a point just in front of the retina with respect to the unaccommodated eye: there is a fixed far point in space depending on the power.
the eye has too much plus power for its size: near vision always good
What is index myopia?
a change in the index of refraction of the lens, usually from diabetes or cataract
water loss in anterior chamber from elevated sugar vs. hardening of lens
axial myopia vs. curvature myopia
The eyeball is too long for the normal refractive power of the lens and the cornea vs. the eye is of normal size but the curvatures of the cornea and lens are increased.
Describe the usual progression of myopia
Rare at birth, typical onset around age 4, progresses with growth often rapidly during puberty, typically stable between ages 20-40
Characteristics of progressive myopia
Children with -6.00 diopters or more that can increase rapidly: potential complications include thinning of sclera, vitreous degeneration, retinal tears or detachments, macular hemorrhage
all myopia more than 10.00 diopters in magnitude is axial
astigmatism
Rays of light are not refracted equally in all directions, so that a point focus on the retina is not attained.
regular astigmatism
Axes of the principal meridians of the astigmatism are at right angles to each other.
List the three types of regular astigmatism
simple, compound, and mixed
simple astigmatism
One of the focal lines always falls on the retina (emmtropic), the other meridian may have its focus behind the retina (hyperopic) or in front of it (myopic).
simple hyperopic as
