7: Refractive States of the Eye Flashcards
in an unaccommodated myopic eye, light from a distant object is image:
anterior to the fovea
in an unaccommodated hyperopic eye, light from a distant object is imaged:
posterior to the fovea
in myopia, the eye’s power is too ____ for its axial length
great
in hyperopia, the eye’s power is too ____ for its axial length
low
define the far point of the eye (R)
it is the object point conjugate with the fovea (R’) of the unaccommodated eye
define the far point distance (k)
is that from the first principal point (P) of the eye to the far point (R)
where is the far point of the myopia eye located?
is located in real space somewhere closer than infinity
are the far point distances (k) negative or positive for myopia and hyperopia
myopic eye: k is negative (-)
hyperopia eye: k is positive (+)
where is the far point of the hyperopic eye located?
the far point is a virtual object point located somewhere behind the corneal vertex
what is the ocular refraction (K)?
it is the reciprocal of the far point distance k
ocular refraction depends on a balance between the:
eyeball length and the eye power
total hyperopia is
the amount of hyperopia determined with ‘adequate’ cycloplegia
manifest hyperopia is
the portion of the total that can be obtained without cycloplegia
latent hyperopia is
the portion of the total that remains hidden (or latent) without cycloplegia
(determined by calculation)
facultative component of hyperopia
can be overcome with accommodation
this includes the latent component
absolute component of hyperopia
cannot be overcome with accommodation
in young patients, the amplitude of accommodation far exceeds total hyperopia,
total= ? and absolute=?
total=facultative
absolute=zero
regular astigmatism is when
the eye power is maximal and minimal at two meridians separated by 90 degrees
irregular astigmatism can occur when
- the meridians of maximal and minimal power are not separated by 90 degrees
- when the eye cannot be corrected by a regular spherocylinder lens
example of irregular astigmatism and how it occurs
keratoconic eyes develop markedly irregular astigmatism, due to a highly aberrated corneal shape
with the rule astigmatism
- the meridian of highest positive ocular power is closer to vertical
- the eye is corrected with a negative cylinder device that has its axis in the range of 180
against the rule astigmatism
- the meridian of highest positive ocular power is closer to horizontal
- the eye is corrected with a negative cylinder device that has its axis in the range of 90
oblique astigmatism
the axis of the correcting cylinder is neither with-the-rule nor against-the-rule
anisometropia
the mean spherical equivalent ocular refraction (M) is different in the two eyes
what occurs in an individual with anisometropia
they are unable to obtain a clear focus in both eyes simultaneously
what is ametropia and how can it occur
when one eye is hyperopia and the other is myopic, occurs if the eye is too short or too long for its dioptric power
the process of emmetropization occurs in which
- the distribution mean moves toward emmetropia or low hyperopia
- the standard deviation of the distribution decreases
with time, the distribution of ocular refraction becomes
leptokurtotic and skewed towards myopia
ocular diffraction has a ______ distribution in neonates, but this distribution changes with development
Gaussian
leptokurtotic distribution is one that has
relatively lower proportions in the shoulders and relatively higher proportions near the peak and in the tails
skewed distribution is one in which
the mean and mode do not coincide
in the case of ocular refraction, the mean is more myopic than the mode
many individuals are classified as having correlation ametropia. this is when
their components are within the usual range for emmetropes, but those components do not balance correctly
what is component ametropia?
when one (or more) components fall outside the usual range for emmetropes -there are two types
describe the two types of component ametropia?
axial ametropia occurs when the axial length is abnormal, and refractive ametropia occurs when one or more of the refractive components are abnormal
Javal’s rule proposed what relationship
between corneal astigmatism (Cc), presumed lenticular astigmatism, and total astigmatism (Ct)
Ct= (1.25)Cc - 0.5
Javal’s rule sign conventions are
+ indicates with the rule astigmatism
- indicates against the rule astigmatism
