The main middle watery layer is what we may think of as 'tears'. The
watery fluid comes from the lacrimal glands. There is a lacrimal gland
just above, and to the outer side, of each eye. The lacrimal glands
constantly make a small amount of watery fluid which drains onto the
upper part of the eyes. When you blink the eyelid spreads the tears
over the front of the eye.
Tiny
glands in the eyelids (meibomian glands) make a small amount of lipid
(oily) liquid which covers the outer layer of the tear film. This layer
helps to keep the tear surface smooth and to reduce evaporation of the
watery tears.
Cells of the conjunctiva at the front of the eye
and inner part of the eyelids also make a small amount of mucus-like
fluid. This allows the watery tears to spread evenly over the surface
of the eye.
The tears then drain down small channels
(canaliculi) on the inner side of the eye into a tear 'sac'. From here
they flow down a channel called the tear duct (also called the
naso-lacrimal duct) into the nose.
Who gets dry eyes and what causes it?
Dry
eyes can affect anyone, but it becomes more common with increasing age.
Dry eyes affects about 7 in 100 people in their 50s, and about 15 in
100 people in their 70s. Women are affected more often than men.
The causes include:
-
Ageing. You tend to make less tears as you get older. In particular, some women notice dry eyes developing after the menopause.
-
Medication. Some drugs sometimes have a side-effect of
causing dry eyes, or make dry eyes worse. These include: diuretics
('water tablets'); some antidepressants; antihistamines; some
treatments for anxiety and other psychological problems; the
contraceptive pill; beta-blockers such as propranolol, atenolol, etc;
some eye drops used to treat other eye conditions. This is not an
exhaustive list. Tell you doctor if you suspect that a drug is causing
your dry eyes.
-
Illness. Some people develop dry eyes as a symptom of a more
general disease. For example, dry eyes may occur with rheumatoid
arthritis, SLE (systemic lupus erythematosus), and Sjogren's syndrome.
In these situations you would normally have other symptoms in other
parts of the body. For example, joint pains.
-
Increased evaporation of tears. This may be due to:
- Low humidity. For example, from central heating or air conditioning.
- Low blink rate, often combined with opening your eyes wider than
normal. For example, spending a long time looking at a computer, TV or
microscope.
- Windy conditions when you are outside.
- Inability to completely cover the eyes when closing the eyelids.
For example, due to eye problems that some people have related to
thyroid disease. Also, some people sleep with their eyes partly open.
-
Damage to the outer part of the eyes, eyelids, etc, from disease, injury or surgery.
-
Blepharitis (inflammation of the eyelids) is often associated with dry eyes.
-
Unknown. Some younger people have no apparent cause.They simply produce less than the normal amount of tears.
What are the symptoms of dry eyes?
Both eyes are usually affected. The eyes may not actually feel 'dry'. Symptoms include:
- Irritation
in the eyes. The eyes may feel gritty or burning. However, the eyes do
not go red. If they do, another eye problem or a complication is
usually present.
- Slight blurring of vision from time to time. However, dry eyes do
not affect the seeing part of the eye, and dry eyes do not usually
cause permanent damage to vision.
- You may not like bright lights.
- If you wear contact lenses, you may find they become uncomfortable.
What are the possible complications of dry eyes?
Complications
are uncommon. Inflammation of the conjunctiva (conjunctivitis) or the
cornea at the front of the eye (keratitis) sometimes occurs. In severe
cases, small ulcers may develop on the cornea. Rarely, the corneal may
perforate (puncture).
See a doctor if the eye goes red or if
vision becomes affected (more than slight temporary blurring). Also if
eye pain develops other than the grittiness or irritation that goes
with dry eyes. These are not normally symptoms of dry eyes and may
indicate another eye condition or a complication of dry eyes.
How are dry eyes diagnosed?
A doctor can
usually diagnose dry eyes from the symptoms. However, as dry eyes can
be a 'symptom' of an underlying disease (such as Sjögren's syndrome),
don't be surprised if your doctor asks about all sorts of other
symptoms.
Sometimes a test is done to confirm the diagnosis of
dry eyes. This is called the Schirmer Tear Test. This measures the
amount of tears that you form. Special filter paper is placed under the
lower lid of your eye and left for five minutes. The amount of tears
that you make can be assessed by how wet the filter paper becomes after
five minutes.
What is the treatment for dry eyes?
Artificial tears
These come as eye drops and
gels, and are usually good at relieving symptoms. You can buy them at
pharmacies or get them on prescription. At first, you may need to use
them every hour or more to improve symptoms. Once symptoms improve, you
may then only need to use them three or four times a day. You may need
to use them regularly to keep symptoms away.
There are several
types of artificial tear drops and gels with different ingredients.
Occasionally, some people find one type may irritate. A change to a
different preparation may help if the first does not suit.
Note:
some types of artificial tears contain preservatives such as
benzalkonium hexachloride. If you use drops that contain benzalkonium
hexachloride for long periods, they may damage the front of the eye
(the cornea). Therefore, if you use artificial tears more than four
times per day long-term, it is best to use a 'preservative-free' brand
which does not contain benzalkonium chloride.
Eye ointment
It may also help to use a
soothing and lubricating ointment at bedtime for overnight. You can buy
this too at pharmacies, or get it on prescription. You should not use
ointment during the day as it may make the artificial tear drops less
effective, and can blur vision. (Also, do not use eye ointment if you
use other eye drops for other conditions such as glaucoma. The other
eye drops may not work so well on top of an ointment.)
Other treatments
Artificial tears and
soothing ointments work well in most cases. Other treatments may be
advised by a specialist in severe cases not helped by the above. For
example, drugs are sometimes used to boost tear production by the tear
gland and surgery is sometimes done to block the tears from draining
away. Also, some people may need other treatments if they have an
underlying cause for their dry eyes.
Contact lens wearers
You should not wear
contact lenses whilst using many types of eye drops. Check with your
doctor or pharmacist. It is often the preservative in the drops that
may cause problems. Some types of drops are available without
preservative which are suitable for contact lens wearers.
You should not wear contact lenses whilst using eye ointment.
Disclaimer: This article is for information only and should not
be used for the diagnosis or treatment of medical conditions. EMIS and
PiP have used all reasonable care in compiling the information but make
no warranty as to its accuracy. Consult a doctor or other health care
professional for diagnosis and treatment of medical conditions.
© EMIS and PiP 2008 Reviewed: 23 Jul 2008
