Download Ocular refractive system

Refraction
Department of Ophthalmology
Tianjin Medical University Hospital
Ocular refractive system

Cornea
 Aqeous humor
 Lens
 Vitreous
Refractive condition

Refractive power
 Length of ocular axis
 The unit of refractive power : diopter (D)
Concepts

Emmetropia
Under nonaccommodated condition, parallel rays
(from beyond 5m) refracted by ocular refractive
system accurately focus on the retina
 Accommodation
In order to see near object clearly, the eye
increases lens curvature to strengthen ocular
refractive power to make near object form clear
image on the retina, the function to change ocular
refractive power called accommodation
The mechanism of ocular accommodation

Contraction of ciliary muscle
 Lens zonule relax---lens becomes thicker
 Increase total refractive power
 The unit of accommodation is diopter (D)
 The accommodative power is larger in
juvenile, and less in old
Concepts

Far point
Under nonaccommodated condition, the
farthest point the eye can see clearly is called
far point
 Near point
Under the largest accommodation, the point
the eye can see clearly at the nearest distance
is called near point
Non-emmetropia or ametropia

The refractive power and the length of the
globe are not correlated so that parallel light
rays refracted by ocular refractive system
do not come to focus on the fovea
 Hyperopia, myopia, astigmatism
Myopia

Under nonaccommodated condition,
parallel light rays refracted by ocular
refractive system come to focus in front of
the retina
 Mild, moderate, high myopia
Myopia

Axial myopia: The axis of the eye is quite
long, but the refractive power is normal
 Refractive myopia: The axis is normal but
the refractive power increases.
 The curvature of the cornea is large and
increases
 The curvature of the lens increases
 Spasm of accommodation
Myopia---clinical findings





Visual acuity: the distance vision of myopia is bad, the
near one may be in normal
Asthenopia: without wearing glasses, long-term near work,
overusing of convergence, not or less use of
accommodation
Exotropia: hypofunction of convergence is induced to
bring about exotropia
Ocular fundus: a grey crescent in temporal side of the disc,
degeneration of Bruch membrane, choroidal atrophy,
posterior scleral staphyloma, macular hemorrhage, Fuch’s
spot
Vitreous liquefaction and opacity
Myopia

Correction: concave lens, glasses, contact
lens
 Radial keratotomy
 PRK
 LASIK
 LASEK
Hyperopia

Concept: under nonaccommodated
condition, parallel light rays refracted by
ocular refractive system focus behind the
retina
 Mild, moderate, high hyperopia
Hyperopia

Axial hyperopia: the axis is short, refractive
power is normal
 Refractive hyperopia: the axis is normal,
refractive power is weak
Hyperopia---clinical findings





Visual acuity: near vision is much worse than far
one, long-term and excessive accommodation is
easy to arouse accommodative spasm
Asthenopia: long-term near work, excessive
accommodation often may induce asthenopia
Esotropia
Ocular fundus: the disc is smaller than normal
with blurred margin, a little elevated
Correction: convex lens, glasses, contact lens
Astigmatism








Concept: Due to the difference of refractive power
in every meridian of eye, so outside light rays
can’t focus on the retina to form clear image
Regular astigmatism
Simple hyperopic astigmatism
Simple myopic astigmatism
Compound hyperopic astigmatism
Compound myopic astigmatism
Mixed astigmatism
Irregular astigmatism
Astigmatism---clinical findings

Visual acuity: blurred both in seeing far or
near, double image
 Asthenopia, distending ophthalmalgia
 Ocular fundus: the disk is vertically
elongated with blurred margin
 Treatment: cylindrical lens, contact lens
Review

What is myopia ?
 What is hyperopia ?
 What is astigmatism ?
 Ocular refractive system includes ?
 What is ametropia ?
 Ametropia includes ?
 Regular astigmatism includes ？