Introduction:
- Axial diameter of the eyeball: is the distance from tip of
cornea to the center of macula, which is normally 23-25 mm
(average 24 mm).
- The most important two refractive surfaces of the eyeball
are cornea and lens.
- The power of cornea and the lens depends on the curvature
of their surfaces, mostly the anterior surface of each one.
The cornea is a part of a sphere and normally it has a radius of
curvature for its anterior surface which is about 7.8 mm, while
the radius of curvature of anterior surface (capsule) of the lens
is 10 mm. so, the cornea is steeper. There is a reverse
relationship between the radius of curvature and steepness of
any sphere or a part of sphere structure e.g. the lens and
cornea respectively. If the radius of curvature of the cornea or
lens is decreased, there will be increase in the curvature (more
steepness). The increase in curvature of any of those 2 ocular
structures is associated with increase in refractive power of
them and vice versa.
- For any rays of light to come in parallel pattern to the eye,
their source must be at distance of six meters and more from
it, and if the source of light is at any distance less than 6
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ﺎﰊـنﳉا ﻞـﻣﰷ دﺎـﯾز
Refraction and
Refractive Errors
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meters, then the rays will come divergent. The more close
source to the eye, the more divergent rays coming from it.
- the total refractive power (converging power) of normal eye
(emmetropic eye) is 60D (Diopters) divided between the cornea
and the lens. Definitely, the cornea sharing in highest portion
(43D) of this refracting power while the lens having power of
residual 17 D only. So, the cornea is more important than the
lens regarding focusing of light on the retina for the following 2
reasons:
1- the anterior surface of the cornea is steeper than that of
the lens.
2- the light is transfer to the cornea passing through the air
which having the least refractive index (1.00) or (least density
of any other material) while the light transfer to lens passing
through the aqueous humor (water) which having refractive
index higher than that of the air (1.33). The refractive index
for the cornea is 1.37 while the lens is 1.38.
The deviation of light (refraction) is more when it is passing
between 2 media having more deference in their refractive
indices (density). So, the light is deviated more (more
convergence) when passing between air and cornea than when
passing between aqueous and the lens.
The difference in refractive index between cornea and air is:
1.37-1.0= 0.37 ,While between lens and aqueous is : 1.38-1.33=
0.05..0.05 is much less than 0.37.
Diopter: the reciprocal of the distance in meters from the
reference light source in air or vacuum, such that D=
1/(distance in meters).
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Emmetropic eye (eye with normal refraction):
It is an eye in which parallel rays of light tend to focus directly
on the retina when the eye is at rest{i.e. without accommodation
(the eye is using its normal power which is 60 D)}.
- Accommodation: contraction of Ciliary muscle in order to
increase curvature of lens (and so increase its refractive power
more than 17D) to visualize objects closer than 6 meters (near
objects).
- In order to see near objects, there will be contraction of
Ciliary muscles which lead to decrease the tone of Zonule and
their will be increase in the curvature of lens and increasing in
the refractive power of lens (>17D).
- Amplitude of accommodation: is the difference in the
converging power of the eye between maximum accommodation
and un accommodated eye (rest) , which depends on contraction
power of Ciliary muscles and elasticity of lens capsule, and both
of them decrease with advancing in age. The Amplitude of
accommodation is decrease with advancing age as the following:
* Early in life: it is 14D, so the child can focus an object
located 7 cm away from the eyes, i.e. the range of lens
refractive power can be increase from 17 D normally up to 31D.
* At age of 36y: due to atrophy of muscles and loss of lens
elasticity (sclerosis), the amplitude of accommodation will be
decrease to 6 D only, so the nearest object to the person which
can be focused is at a distance of about 15cm away from eye.
* At age of 45y: the amplitude of accommodation is 4D only,
and the nearest focus point is 25cm away from eye.
* At age of 60y: amplitude of accommodation is 1D only, and
the nearest focus point is 1meter.
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- Presbyopia: is a recession of near point with age (from 7 cm
early in life to 1 meter at 60 year) due to decreased amplitude
of accommodation (from 14 D early in life to 1 D at age of 60
year) making near vision uncomfortable , it is clinically
manifested after age of 40 and human start to wear
glasses(+ve) during near work e.g. reading.
Ammetropia:
is either → Hypermetropia ,Or → Myopia, Or → Astigmatism.
Hypermetropia (hyperopia):
Is a type of refractive errors in which parallel rays of light are
brought to a focus some distance behind the retina when the
eye is at rest.
Etiological classification:
1- Axial Hypermetropia: shorter antero-posterior axial length,
i.e. the eye has normal converging power (60D) but its axial
length is less than 24 mm.
2- Curvature Hypermetropia: due to decreased curvature
(flattening) of the cornea congenitally or as a result of trauma
or disease e.g. corneal ulcer, microbial keratitis.
3- Index Hypermetropia: decrease in effective refractivity of
the lens.
* the power of the lens depends on the difference between
refractive indices of the nucleus and cortex, so the more the
difference the more converging power and vise versa. In index
Hypermetropia, there is decrement of the difference.
Clinical classification:
1- Facultative hypermetropia: hypermetropia corrected by
accommodation (depends on age and degree of refractive error).
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2- Absolute hypermetropia: hypermetropia out of amplitude of
accommodation and corrected by glasses.
3- Manifest hypermetropia: maximum hypermetropia that can
be corrected with a convex lens with accommodation active, i.e.
= Facultative + Absolute.
4- Latent hypermetropia: hypermetropia hidden behind ciliary
body tone which equal to the difference between total and
manifest hypermetropia. The refractive power of the ciliary
body is about 1-1.5D (usually it considered as part of the
refractive power of lens, i.e. 17D of lens = 16D + 1D (of the
ciliary body tone), it also decreases with age.
5- Total hypermetropia: amount of hypermetropia present
with all accommodation suspended (with cycloplegic drugs (e.g.
Atropine, Cyclopentolate and Homotropine) , to exclude
accommodation and tone of ciliary body) Manifest + Latent.
* With advancing age, the facultative will decrease and the
manifest will be represented by absolute only, and at the same
time, the latent will also decrease and the total will be
represented by manifest only, so in old people the total is made
of the absolute only.
Symptoms:
1- Blurred vision: for near work (as it needs more power) and
even far vision if the degree of hypermetropia is high and
beyond the amplitude of accommodation.
2- Eye strain: headache due to excessive accommodation and
dissociation between Accommodative Convergence and
Accommodation "AC/A.
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* To see the near objects, the eye will show the near reflex,
which consists of accommodation, convergence and miosis. When
one of these processes occur, it will stimulate the other two.
Normally, for each 1D of accommodation, there will be
associating 4 prism D convergence (i.e. AC/A=4 prism D/1D).
A normal eye to see an object at distance of 1/2 meter needs
2D and this in turn will produce 8 prism D convergence, while an
eye with 7D hypermetropia has at rest 28 prism D convergence,
so to see an object at 1/2 meter distance the 2D needed will
produce 8 prism D more, so totally there will be convergence 36
prism diopters for the same distance which is 1/2 meter.
3- General symptoms like nausea and fatigue.
4- early presbyopia clinically manifested at late 30 or early 40.
Treatment:
- Convex lens in spectacles (+ve lenses to increase refractive
power).
- Contact lenses.
- Excimer laser photorefractive keratectomy: reshaping of the
cornea to increase the refractive power, each 1mm decrease in
radius of curvature increases the refractive power about 6D.
e.g. Laser in situ keratomileusis (Lasik)
- Non-contact laser thermal keratoplasty (Holmium laser
spots): to change the coneal curvature.
- Phakic intraocular lens (IOL).
Myopia or short-sightedness:
That form of a refractive error where parallel rays of light
come to a focus in front of the retina when the eye is at rest.
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- As there is increase in the refractive power of the eye, the
near objects (closer than 6m) will be seen normally, while far
objects (whom rays come parallel) will be focused in front of
retina.
Aetiological classification:
1- Axial myopia: anteroposterior length is longer than normal.
2- Curvature myopia: increased curvature of cornea, or one or
both surfaces of lens.
3- Index myopia: increased refractivity of lens, e.g. nuclear
sclerosis (stage before nuclear cataract), due increase the
difference between refractive indices of nucleus and cortex.
Clinical classification:
1- Simple (stationary): <6D, start after the age of 4y,
stationary (stops before 20y), normal physiological variety and
the VA can be corrected to 6/6.
2- Pathological (progressive): >6D, start before the age of 4y,
progress even after age of 20y, real pathological process and VA
cannot be corrected to 6/6 and ocular pathological changes
involving vitreous, retina and optic nerve which are:
-
general chorioretinal atrophy.
-
myopic crescent at the optic disc.
-
posterior staphylloma.
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vitreous degeneration.
-
macular degeneration.
Symptoms:
1- Distance object "Blurred", near objects clear.
2- Headache due to sustained contraction of occipito-frontalis
muscle in order to make slit of palpebral fissure and increase
vision and eye strain due to dissociation of AC/A.
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* A normal eye to see an object at distance of 1/2 meter needs
2D and this in turn will produce 8 prism D convergence, while a
eye with 2D myopia will not need any accommodation to see this
object, but there will convergence 8 prism D to see this near
object. Thus is in turn will produce 2 D accommodation which is
not needed by myopic patient. So any myopic should wear
glasses for far and near to create a normal relation between
accommodative convergence and accommodation.
3- general symptoms like fatigue and nausea.
Treatment:
- Concave lenses in spectacles.
- Contact lens.
- Radial keratotomy:
it is done by making several radial corneal
incisions at periphery of cornea to create scarring that lead to
changes in corneal curvature(flattening)(It is old method not
done now a days).
- Excimer laser photorefractive surgery:
either P.R.K.
(photorefractive keratectomy) it is done in simple type or
LASIK (laser in situ keratomileusis) it is done in high degree.
Both types done by excimer laser, in first method (P.R.K.) we
just remove corneal epithelium and excimer laser done on bare
surface while second method (LASIK) we made corneal flap and
then applying laser on corneal tissue and at end of surgery we
return the flap again to its position.
- Phakic IOL.
- Normal (clear) Lens extraction.
Astigmatism:
Astigmatism is that condition of refraction in which a point of
focus of light cannot be formed upon the retina. The optical
condition is that instead of a single focal point, there are two or
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more focal lines (depending on the type of astigmatism),
separated from each other by a focal interval.
Etiological classification:
1- Curvature astigmatism: Most commonly, it is corneal, less
commonly it is lenticular (induced by lens). The cornea or the
lens having unequal radii of curvature.
2- Decentering astigmatism: subluxation of lens.
3- Index astigmatism: due to cataract in one meridian more
than other.
Clinical classification:
1- Regular: two principle meridians at right angle are involved
(i.e. there is difference in the refractive power of two
meridians 90° in between):
a- Simple astigmatism: myopic or hypermetropic; one of the
foci falls on retina, other in front or behind it.
b- Compound astigmatism: neither of the two foci lies upon
retina, but are both placed in front or behind retina (myopic or
hypermetropic respectively).
c- Mixed astigmatism: one focus in front and other behind
retina.
2- Irregular astigmatism: refraction in different meridians is
quite irregular. Found in pathological condition of cornea;
irregular healing after trauma or inflammations or keratoconus.
Treatment:
- Cylindrical lenses in spectacles: only for simple astigmatism.
-Contact lenses: used for compound and mixed astigmatism,
where we correct one of the meridians by them, and then
correct the other meridian by cylindrical lenses (i.e. we switch
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it to simple and correct it accordingly). Either soft or hard
Contact Lenses.
- Photorefractive Excimer laser surgery: to correct one
meridian.
-LASIK: like photorefractive Excimer laser surgery, but used
for higher refractive error.
- Phakic Toric IOL.
- Keratoplasty (corneal graft): for more than two meridians
and central corneal opacity.
Aphakia:
Definition: absence of lens from the eye or its absence from
pupillary area (luxated).
- Aphakic eye is usually strongly hypermetropic in which parallel
rays of light are brought to focus behind the retina
- All accommodation is abolished
Treatment:
1- high power convex lens in spectacles
*Disadvantages:
-high magnification (30%) of normal size.
-limitation of visual field.
-heavy weight.
2- Contact lens (1% magnification)
3- Intra ocular lens implantation: best correction as there is
no magnification at all.
ﺎﰊـنﳉا ﻞﻣﺎـﻛ دﺎـﯾز .د
F.A.B.Ophth, F.I.C.O, V.R.S, M.B.ch.B.
۰۱