RETINITIS
DISORDERSOFTHE RETINA
Despite its small size, the retina is
subject to a wide variety of disorders,
many of which seriously affect vision
and, in some cases, result in blindness.
Congenital and genetic disorders
Colour blindness (see
c o lo u r visio n
d e ficie n cy
),
an abnormality of retinal
cones (colour receptors in the retina),
usually has a genetic basis. Hereditary
degenerative disorders of the macula
may appear at any age, leading to
serious impairment of central vision.
Other degenerative disorders of the
retina with a genetic basis include
Tay-
S a c h s d is e a se
and
re tin itis p ig m e n to s a
.
R etrolen tal fib ro p la s ia
may result from
exposure of a premature baby to
excessive oxygen concentration (needed
to treat lung immaturity), which causes
abnormalities in the retinal vessels.
Infection
T o x o p la sm o sis
is an infection of the
retina that is acquired before birth and
recurs later in life, causing progressive
damage to the retina.
T
oxocara canis
is a parasitic worm
whose larvae may lodge in the retina
and cause severe retinal destruction,
producing a white mass resembling a
tumour (see
to x o ca ria sis
).
O n c h o c e rc ia sis
,
an infestation by a tropical worm, may
cause severe retinal damage.
Bacterial and fungal infections
elsewhere in the body can be carried
in the blood to the retina. People
whose immune systems are impaired
are more susceptible to infections
that can involve the retina.
Tumours
R etin o b la sto m a
is a cancerous tumour
that usually appears in the first three
years of life. There may be visual loss in
the affected eye and a visible whiteness
in the pupil;
s q u in t
often develops.The
tendency to this cancer can be inherited.
Secondary cancerous tumours,
spreading to the eye from primary
tumours elsewhere in the body, can
also occur. Malignant melanoma (see
m e la n o m a , m a lig n a n t
) can arise from the
choroid (the layer beneath the retina).
A variety of noncancerous tumours
may also occur in the retina.
Injury
The retina may be torn or detached
due to severe penetrating or blunt (non-
penetrating) injury (see
retin a l d e ta ch m en t
;
retin a l tear
). Permanent damage may be
caused by a retinal burn, which is
sometimes caused by looking directly
at an eclipse of the sun.
Metabolic disorders
D ia b e te s m e llitu s
may cause
re tin o p a th y
,
with fluid leakage and haemorrhage into
the retina (see
retin a l h a e m o rrh a g e
)
and
with the growth of new, fragile blood
vessels on the retinal surface, which bleed
readily.Vitreous haemorrhage (bleeding
into the vitreous humour, the gel-like
substance that fills the rear compartment
of the eye) may occur from blood vessels,
and fibrous tissue can grow forwards
on to the vitreous humour in cases of
“proliferative’’ retinopathy.This is a major
cause of permanent loss of vision.
Impaired blood supply
R etin a l vein o c c lu sio n
(or
retin a l a rtery
o c c lu s io n
), a common cause of blindness,
results from blockage of the central vein
(or artery) of the retina. Hypertensive
retinopathy is damage to the retina
caused by high blood pressure, which
leads to narrowing and
a th e ro s c le ro s is
(fat deposits on the artery walls) of the
retinal arteries, both of which may lead
to retinal damage.
Poisons
A combination of heavy tobacco-
smoking, heavy alcohol intake, and
poor nutrition may lead to visual loss.
Vitamin deficiency in combination
with lead poisoning may cause visual
loss.
M eth a n o l
causes widespread and
permanent destruction of certain retinal
tissues, leading to blindness.
Drugs
Many drugs can damage the retina.
Examples are
c h lo ro q u in e
,
used in large
doses over a long period for the
treatment of conditions such as
rheumatoid arthritis.
Other disorders
Age-related
m a c u la r d e g e n e ra tio n
,
causing progressive loss of vision, is
a common condition in older people.
Detachment of the retina often occurs
in the absence of any injury, and it
may be more common in individuals
who suffer from severe
m yo p ia
(shortsightedness).
INVESTIGATION
Retinal disorders are investigated
by checking the visual acuity and
the visual fields (see
vision tests;
retinoscopy).
After dilating the pupils
with drops, the retinas are inspected
by means o f a direct or indirect
ophthalmoscope (an instrument used
to examine the eye). Fluorescein can
be injected into an arm, where it is
carried by the blood to outline the
retinal vessels and to detect leaks.
Electrophysiological tests can also
be carried out to study certain ocular
diseases. Ultrasound scanning can
be used to study tumours in or under
the retina.
R
by degeneration. A retinal tear is more
likely to occur in people who have
severe
m y o p ia
(shortsightedness) but the
condition may also be caused by a
severe eye injury.
R e tin a l d e ta c h m e n t
usu-
ally follows a retinal tear.
If a retinal tear is diagnosed at a
stage before detachment has occurred,
the hole may be sealed by means of
la s e r tre a tm e n t
or cryopexy (the appli-
cation of extreme cold).
re tin a l v e in o c c lu s io n
Blockage of a vein carrying blood from
the
re tin a
(the light-sensitive inner layer at
the back of the eye). It usually results
from
th ro m b o s is
(abnormal blood clot for-
mation) in the affected vein and is more
common in people who have
g la u c o m a
(a
condition in which the pressure of fluid
in the eye is abnormally high). Retinal
vein occlusion may affect either the main
retinal vein or one of its branches.
Retinal vein occlusion may cause visual
disturbances, glaucoma, and can result
in blindness,
depending on the vein
affected and how quickly the condition
can be treated. (See also
re t in a l a rte ry
o c c lu s io n
.)
re tin itis
Inflammation affecting the
re tin a
(the
light-sensitive inner layer at the back of
the eye). (See also
re t in o p a th y .)
661
previous page 660 BMA A Z Family Medical Encyclopedia   2004 read online next page 662 BMA A Z Family Medical Encyclopedia   2004 read online Home Toggle text on/off