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Cornea and External Eye Disease
Special Focus
Dry eye or something more?
Howard Larkin
in London
IT'S not easy to make a definitive diagnosis of
dry eye with a simple test. Indeed, some
patients with normal tear production and
tear film break-up results, and no observable
corneal abrasions, complain of sandiness,
burning or even constant pain. Others with
impaired tears and significant cornea surface
staining voice no symptoms.
Even more challenging is predicting which
patients will develop dry eye after refractive
surgery.
“Dry eye is a frequent, if
not the most frequent,
complication of refractive
surgery. It is also a very
high factor for patient
dissatisfaction”
“Dry eye is a frequent, if not the most
frequent, complication of refractive surgery. It
is also a very high factor for patient
dissatisfaction,” Timo Tervo MD, of Helsinki
University Eye Hospital, Helsinki, Finland told
a symposium at the XXIV Congress of the
ESCRS.
Such difficulties diagnosing and anticipating
dry eye symptoms stem from the complexity
of the condition. Recent research has
revealed that the eye's ability to keep itself
moist and the cornea healthy depends on an
intricate and delicately balanced interplay of
lacrimal and meibomian glands, ocular surface
nerves, and the immune system that can
easily be disrupted by a variety of diseases
and environmental conditions.
While the dry eye process is still not fully
understood – and subjective elements such
as pain tolerance will always be a factor – a
number of objective tests and treatment
guidelines exist for evaluating the condition,
said Jesús Merayo-Lloves MD, of the
University of Valladolid, Spain. He
recommends a thorough evaluation of any
dry eye symptom before making a refractive
or cataract surgery decision.
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In some cases, the underlying causes of dry
eye symptoms greatly increase the risk of
major surgical complications, such as
postoperative infection or corneal incision
and surface recovery. Newer tests that can
detect chemical changes in the aqueous and
lipid elements of tears, and more reliably
measure tear film break-up and corneal
surface pathologies, often can distinguish
among the various infections and disorders
that can produce dry eye symptoms.
But the clinical tests don't show
everything, Dr Merayo-Lloves said. Listening
to the patient is just as important.
“If they have scratchy eyes now they will
have scratchy eyes later, so some people
won't do LASIK on someone with scratchy
eyes.”
Nerves: disease may underlie
symptoms
Dr Tervo believes that careful management of
any pre-operative symptoms of dry eye is
essential for a good refractive surgery
outcome. He recommends wetting agents,
cyclosporine, or punctal occlusion where
indicated.The benefits include more reliable
photoablation, less postoperative dry eye
symptoms, more accurate pre-op refraction
and wavefront analysis, and better
postoperative visual quality.
Measurable tear disruption and
hypersensitivity are common for seven to
nine days after surgery.These symptoms
likely result from interruption of nerve signals
from the corneal surface. However, in some
cases symptoms persist for months or even
years, Dr Tervo noted.
“For up to a year the symptoms may be
related to dry eye. If it goes on longer than
that there may be other conditions involved.”
One cause of long-term dry eye may be
corneal nerve damage, Dr Tervo said.The
cornea is richly supplied with nerve endings –
400 times as dense as the skin.These endings
are severed in PRK and LASIK procedures,
and they take a long time to regenerate, Dr
Tervo noted.
“You may see sub-basal nerve fibres
recover after five years and we continue to
see increases from year five to year 11.”
In addition, a significant portion of nerves
regenerate abnormally, particularly in patients
with Sjögren's syndrome, Dr Tervo said.
These abnormal nerves may be responsible
for dry eye symptoms that are not
accompanied by measurable tear
abnormalities.
The higher the correction, the more likely
it is that long-term dry eye symptoms will
result, he said. Other possible causes of
chronic pain may include loose epithelium
stretching the nerves as the eyes open and
close, and irregular epithelium.
Blepharitis is another chronic cause of dry
eye symptoms, noted James P McCulley MD,
FRCOphth, University of Texas Southwestern
Medical School, Dallas,Texas, US.When
blepharitis is present, it can be a source of
infecting organisms during surgery, and can
contribute to poor visual outcomes in
refractive or cataract procedures.
Dry eye also may indicate an underlying
inflammatory disease, such as Sjögren's, which
can greatly complicate recovery from
cataract surgery, said Thomas Neuhann MD,
of the Alz Augenklinik in Munich. He
recommended carefully considering the
potential complications of surface disease
before going ahead with cataract surgery.
“We should err on the careful side with
indication for surgery, intraoperatively and
with postoperative medication.”
Tests hint at dry eye causes
One must understand the underlying cause
of dry eye to assess its impact on potential
surgery, Dr Merayo-Lloves said. He
recommended several objective tests for
measuring both the quantity and quality of
tear production, as well as impact on corneal
tissues.That could be necessary in the cases
where clinical approach (clinical history and
exploration) are not enough to reach the
diagnosis.
He noted that staining with fluorescein,
rose Bengal and lissamine green could reveal
damage to corneal tissues due to disruption
of tear film. Lissamine green is less toxic than
rose Bengal, and are easy to obtain for
clinical use. Corneal staining could indicate a
break down in the mucin layer of the tear
film closest to the eye due to toxic
compounds produced by an infection or
inflammation.
He also suggested using the tear clearance
test to measures function.This correlates
well with corneal damage and gives
information of both production and
elimination.
Tear break-up time tests show the
integrity of the tear film.A time of eight
seconds or more may be a good refractive
surgery candidate; less than six seconds is a
contraindication. Between six and eight
seconds, a tear volume test may be indicated.
Short break-up time could be related to
improper lipid production resulting from
inflammation of the meibomian glands.
Dr Merayo-Lloves said that patients with
tear volume of 10mL or more may be good
candidates for refractive surgery.Those with
less should receive reflex test. Patients
without reflex tear production may not be
suitable refractive surgery candidates until
the problems have been identified and
treated.
Laboratory tests including tear lysozyme
and IGA concentrations, and conjunctival
impression cytology can assist to distinguish
between dry eye due to autoimmune disease,
evaporative problems possibly related to
infection, and allergic or toxic reactions.
Measurement of other tests such as tear
osmolarity will be helpful when they are
conducted at the office.
All of these tests provide objective data to
guide surgery and treatment decisions. But
once again, Dr Merayo-Lloves stressed the
importance of taking a careful history,
including specific questions that address
itching, burning, or foreign eye sensations, and
when they occurred.
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