VISION TESTS
t h e e y e ) ,
m y o p ia
( s h o r t s i g h t e d n e s s ) , a n d
h y p e rm e t ro p ia
( l o n g s i g h t e d n e s s ) , w h i c h
c a n a l m o s t a l w a y s b e c o r r e c t e d b y
g la s -
s e s
o r
c o n ta c t le n s e s .
O t h e r d i s o r d e r s o f
v i s i o n i n c l u d e
a m b ly o p ia
( l a z y e y e ) ;
d o u -
b le v is io n ;
a n d d i s o r d e r s o f t h e
e y e
o r
o p t ic n e rv e ,
o f t h e n e r v e p a t h w a y s c o n -
n e c t i n g
t h e
o p t i c
n e r v e s
t o
t h e
b ra in ,
a n d o f t h e b r a i n i t s e l f .
CAUSES IN THE EYE
T h e
e y e
m a y
l o s e
i t s
t r a n s p a r e n c y
t h r o u g h
c o r n e a l
o p a c i t i e s ,
c a ta ra c t,
o r
v itre o u s h a e m o rrh a g e
( b l e e d i n g i n t o t h e
g e l o f t h e e y e b e h i n d t h e l e n s ) . D e f e c t s
n e a r t h e c e n t r e o f t h e r e t i n a c a u s e l o s s
o f t h e c o r r e s p o n d i n g p a r t s o f t h e
v is u a l
f ie ld
( s e e
m a c u la r d e g e n e r a t io n ). F lo a te rs ,
w h i c h a r e u s u a l l y i n s i g n i f i c a n t , m a y i n -
d i c a t e a
re t in a l t e a r
o r h a e m o r r h a g e , o r
t h e y
m a y
h e r a l d
a
re t in a l d e ta c h m e n t
.
O p tic n e u r it is
( i n f l a m m a t i o n o f t h e o p t i c
n e r v e )
c a n
c a u s e
a
b l i n d
s p o t
i n
t h e
c e n t r e o f t h e v i s u a l f i e l d .
CAUSES IN THE BRAIN
D a m a g e t o t h e b r a i n ( f o r e x a m p l e , f r o m
a
s tro k e
)
m a y c a u s e v i s u a l i m p a i r m e n t
s u c h
a s
h e m ia n o p ia
( l o s s
o f h a l f t h e
v i s u a l f i e l d ) ,
a g n o s ia
( f a i l u r e t o r e c o g -
n i z e
o b j e c t s ) ,
v i s u a l
p e r s e v e r a t i o n
( i n
w h i c h a s c e n e c o n t i n u e s t o b e p e r c e i v e d
a f t e r t h e d i r e c t i o n o f g a z e h a s s h i f t e d ) ,
a n d v i s u a l h a l l u c i n a t i o n s .
v is io n , lo s s o f
T h e i n a b i l i t y t o s e e . L o s s o f v i s i o n m a y
d e v e l o p s l o w l y o r s u d d e n l y , a n d i t m a y
b e t e m p o r a r y o r p e r m a n e n t , d e p e n d i n g
o n t h e c a u s e . L o s s o f v i s i o n m a y a f f e c t
o n e e y e o r b o t h o f t h e e y e s . I t c a n c a u s e
c o m p l e t e
b lin d n e s s
o r
m a y a f f e c t o n l y
p e r i p h e r a l o r c e n t r a l v i s i o n .
SLOW VISION LOSS
P r o g r e s s i v e l o s s o f v i s u a l c l a r i t y i s c o m m o n
w i t h a g e a n d m a y b e d u e t o a n u m b e r o f
d i s o r d e r s ( s e e
visio n , d is o rd e rs o f).
SUDDEN VISION LOSS
S u d d e n l o s s o f v i s i o n m a y b e c a u s e d b y
d i s o r d e r s
s u c h
a s
h y p h a e m a
( b l e e d i n g
i n t o t h e a q u e o u s h u m
o u r ) , s e v e r e
u v e i-
tis
( i n f l a m m a t i o n
o f
t h e
i r i s
o r
t h e
c h o r o i d ) ,
v it r e o u s h a e m o rrh a g e
( b l e e d -
i n g
i n t o
t h e
g e l
w i t h i n
t h e
e y e ) ,
o r
re t in a l h a e m o rrh a g e .
O p tic n e u r it is
( i n -
f l a m m a t i o n
o f
t h e
o p t i c
n e r v e )
c a n
r e d u c e v i s i o n i n o n e e y e .
D a m a g e
t o
t h e
n e r v e
c o n n e c t i o n s
b e t w e e n t h e e y e s a n d b r a i n , o r t o t h e
v i s u a l a r e a o f t h e b r a i n , c a n c a u s e l o s s
o f p e r i p h e r a l v i s i o n . T h i s n e r v e d a m a g e
m a y
b e
c a u s e d
b y
a
n u m b e r
o f d i s -
o r d e r s s u c h a s a n
e m b o lis m
,
is c h a e m ia
,
t u m o u r , i n f l a m m
a t i o n , o r i n j u r y .
v is io n te s ts
T h e p a r t o f a n
ey e e x a m in a tio n
t h a t d e -
t e r m i n e s r e d u c t i o n i n t h e a b i l i t y t o s e e .
VISUAL ACUITY TESTS
T e s t s o f
v is u a l a c u ity
( s h a r p n e s s o f c e n t r a l
v i s i o n ) i n c l u d e a
S n e lle n c h a rt,
w h i c h i s
u s e d t o t e s t a p a t i e n t ’s d i s t a n c e v i s i o n ,
a n d c l o s e - r e a d i n g t e s t c a r d s , w h i c h m a y
b e u s e d t o a s s e s s h i s o r h e r n e a r v i s i o n .
REFRACTION TESTS
R e f r a c t i o n t e s t s a s s e s s t h e a b i l i t y o f t h e
e y e t o f o c u s l i g h t c l e a r l y o n t h e r e t i n a
v i a t h e
c o r n e a a n d
l e n s . T h e y
c a n
b e
u s e d t o d e t e c t
h y p e rm e t ro p ia
( l o n g s i g h t -
e d n e s s ) ,
m y o p ia
( s h o r t s i g h t e d n e s s ) , a n d
a s tig m a t is m
( a b n o r m a l c u r v a t u r e o f t h e
f r o n t
o f
t h e
e y e ) .
B y
o b s e r v i n g
t h e
e f f e c t o f d i f f e r e n t l e n s e s o n m o v e m e n t
TYPES OF VISION TESTS
These tests are performed to measure a number ofvariables: the acuity ofa
patient’s distance vision and the power ofthe lenses he or she may need (visual
acuity and refraction test), the extent of peripheral vision (visual field test), and
the ability to focus on near objects (accommodation tests).
Visual acuity test
These tests use the familiar Snellen chart. Visual
acuity is measured according to how far down
the chartthe patient can read accurately.
One eye is covered and the other is fixed on a
central target inside the test bowl. A response
button is pressed when the patient sees lights
flashing in different parts ofthe visual field.
Lenses in the phoropter are changed until the
letters near the bottom ofthe Snellen chart can
be read. This allows the optician to make a
prescription for appropriate lenses.
Accommodation test
After any distance-focusing ability has been
corrected with glasses, the abilityto read small
print close-up is measured to test accommodation.
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