J R Soc Med 2003;96:273-276
doi:10.1258/jrsm.96.6.273
© 2003 Royal Society of Medicine
Screening for diabetic retinopathy
D M Squirrell MRCOphth
J F Talbot FRCOphth
Department of Ophthalmology, Royal Hallamshire Hospital, Sheffield S10
2JF, UK
Correspondence to: J F Talbot E-mail:
john.talbot{at}sth.nhs.uk
 |
INTRODUCTION
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Throughout the industrialized world the complications of diabetic
retinopathy
remain the major cause of preventable visual loss in persons
of
working age. A reduction by one-third or more in new blindness
due to diabetes
has been adopted as one of the key 5-year targets
in the St Vincent
declaration,
1 and
the best way to achieve
this aim is a national strategy of screening for
diabetic retinopathy.
In this article we review the rationale and supporting
evidence
for a screening programme for diabetic retinopathy. We also
debate
the arguments for and against the screening modalities
that are currently used
in the UK.
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WHY SCREEN FOR DIABETIC RETINOPATHY AT ALL?
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One of the prime motivating factors behind the development of
a screening
programme for diabetic retinopathy is the efficacy
of laser photocoagulation
treatment in preventing visual loss.
The beneficial effect of laser treatment
was established by
two large randomized clinical trials—the Diabetic
Retinopathy
Study and the Early Treatment Diabetic Retinopathy Study (ETDRS).
The
essential findings of these trials were that, compared with
no treatment,
laser photocoagulation prevented visual loss in
patients with proliferative
diabetic retinopathy and macular
oedema by about
50%.
2,3
The ETDRS also served to identify points
in the natural history of diabetic
retinopathy at which laser
photocoagulation treatment should be applied. From
epidemiological
data we know that patients are usually symptom-free at these
threshold
levels of retinopathy: retinopathy may be well advanced before
visual
deterioration is noticed. That patients are generally symptom-free
when
they should receive preventive treatment is a strong argument
for establishing
a screening programme.
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WHAT EVIDENCE IS THERE THAT SCREENING CAN REDUCE BLINDNESS?
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In general, the progression of retinopathy is orderly and the
prevalence
and severity of retinopathy is related to duration
of
diabetes.
4,5,6
To date no randomized controlled trial has
been conducted to assess the
efficacy of screening for diabetic
retinopathy: the practical difficulties of
conducting such a
study would be enormous. The impact of a national screening
programme
in the UK has been estimated by use of mathematical models based
on
what is known about the disease's natural history, and these
models indicate
that an annual screening programme could yield
worthwhile health
gains.
7,8
These findings are supported by
the results of observational studies which
point to substantial
reductions in the incidence of new blindness due to
diabetic
retinopathy after the introduction of screening
programmes.
9,10
Mathematical models have also been used to examine the cost-effectiveness
of annual and semi-annual screening intervals in patients with diabetes. These
analyses come to broadly similar conclusions. Annual screening for diabetic
retinopathy in all patients with type 1 diabetes is cost-effective (provided
the screening modality is sufficiently sensitive), when the economic impact of
a person's blindness is balanced against the health costs incurred by
treatment and
screening.11,12,13,14
The economic argument for annual screening of all patients with type 2
diabetes is less convincing. The two analyses that have specifically
investigated this area concluded that only those patients with type 2 diabetes
who require insulin, or in whom retinopathy has been previously detected,
warrant annual
screening.12,15
Neither of these mathematical models, however, allows for the additional
administrative costs of running several different screening programmes for
patients with type 2 diabetes. The economic argument for the annual screening
of all patients with type 2 diabetes might have been more persuasive if these
costs had been taken into consideration.
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WHAT IS THE BEST SCREENING METHOD?
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For any screening programme to function effectively it must
fulfil certain
basic criteria. Firstly, the screening test must
have sufficiently high
sensitivity (true positive rate) to ensure
that substantial numbers of
patients with sight-threatening
retinopathy are not missed. Secondly, it must
have sufficiently
high specificity (true negative rate) to ensure that
ophthalmic
departments are not overwhelmed with unnecessary referrals.
The
British Diabetic Association proposed that any screening
programme for
diabetic retinopathy should have at least 80%
sensitivity and specificity, and
it is against these figures
that any screening modality for diabetic
retinopathy must be
judged. A survey conducted across England and Wales during
1996
revealed that the provision of diabetic retinopathy screening
services
was uneven, with different screening modalities being
employed.
16 Though there is
now general acknowledgment that
we need a national strategy for diabetic
retinopathy screening,
debate continues on how this screening should be
performed.
Direct ophthalmoscopy alone has no role in a screening programme
since
the method consistently fails to meet the 80% sensitivity and
specificity
targets.
17,18,19,20
There are two principal candidates—retinal
photography (in one of its
many guises), and screening by optometrists
using the indirect ophthalmoscope
or the slit lamp biomicroscope.
As yet no randomized trial has been conducted
to compare these
options. One study did use a mathematical model to analyse
the
effectiveness of various screening strategies but did not include
optometrist
screeners.
12 To date, only
one large systematic review has
addressed this issue.
21 The authors
concluded that the most
effective screening modality for diabetic retinopathy
was retinal
photography through dilated pupils. Unfortunately this comparison
is
flawed, because they used data from studies in which most optometrists
used
the direct ophthalmoscope. This is important since the
authors conceded that
indirect ophthalmoscopy was an effective
screening strategy in trained hands.
Subsequently, further evidence
has emerged on the effectiveness of optometrist
screeners using
slit lamp biomicroscopy.
22 The widely
assumed superiority of
photographic screening over optometrists using
appropriate equipment
therefore remains unproven.
The effectiveness of an individual screening modality to deliver the
desired sensitivity and specificity targets for detecting diabetic retinopathy
is not, in itself, sufficient justification for adopting that modality. It
must in addition be acceptable and convenient for patients, be sensitive to
local needs and have inbuilt quality control mechanisms. We will now briefly
review each of the screening modalities currently in use, outlining their
merits and disadvantages. The principal advantages and disadvantages of each
technique are summarized in Table
1.
Retinal photography
Fundus photography, without mydriasis, utilizing 45° Polaroid colour
prints was the first retinal photographic technique to be applied to diabetic
retinopathy screening. Whilst Polaroid photography offered an instant
hard-copy image of the retina, concerns were soon raised about the adequacy of
the technique to detect sight-threatening retinopathy in the peripheral
retina, particularly when the pupils were small.
23 These concerns
were borne out by a large comparative study which revealed sensitivities as
low as 35% with this technique.
19 In contrast,
retinal photography through dilated pupils using 35 mm transparencies has
proved highly effective, achieving sensitivities and specificities of 89% and
86%, respectively.
18 But this method
of retinal photography likewise has limitations. Lenticular and corneal
opacities, a poor tear film and patient movement can all render the acquired
image useless for grading purposes. The technique thus has an associated
technical failure rate of about 8%. In addition, the processing and storage of
large numbers of transparencies can be costly in resources. The use of digital
imaging systems may be part of the answer to these difficulties. The instant
image acquisition afforded by a digital system has the potential to reduce the
technical failure rate, and the electronic image facilitates easy storage and
cataloguing. 24
Controlled studies evaluating the latest digital systems in this role have
been very promising, with reported sensitivities and specificities of around
90%.
25,26
Retinal photography through dilated pupils using 35 mm transparencies, or
digital imaging, is therefore an effective technique for diabetic retinopathy
screening. But whatever the imaging system employed, retinal photography has
several inherent weaknesses as a screening tool. First, it requires special
equipment and a pool of trained personnel and equipment, which mean high
capital set-up costs. Second, concern has been expressed that, as in other
screening programmes, there might be difficulties in maintaining the
motivation of screening staff.
27 Finally, there
is the problem of how to deliver the service to those patients who need to be
screened. One solution is to mount camera systems in mobile vans. Whilst this
option does have the advantage of flexibility, a large administrative team is
required to coordinate the programme and it also demands purchase and
maintenance of a fleet of vehicles. An alternative would be to locate several
fixed camera systems within the target community. If careful consideration was
given to the location and access to these facilities, such a system might work
well in urban areas. It would be less suitable for rural populations, and even
in urban areas the number required to ensure high attendance rates might be
prohibitively high.
Optometrist screening
Use of an optometrist practice based scheme to screen for diabetic
retinopathy has several potential advantages. The practices have long flexible
opening hours and their proximity to the populace facilitates easy access.
Optometrists who offer home visits can even screen housebound patients. In
Sheffield, a system has proved popular with patients, with over 90% of our
6500 target patients presenting for screening annually. This contrasts sharply
with the high non-attendance rates of our hospital-based diabetic eye and
general diabetic clinics. Furthermore, the screening itself is performed by
personnel who can offer a holistic package of care, since they are in a
position to screen for non-diabetic eye disease as well. The ability of an
optometrist to detect sight-threatening retinopathy obviously depends on which
instrument is used to examine the retina. In the early studies with
optometrist screeners, all used the direct ophthalmoscope. Although the
results were encouraging,
28,29,30
we now know that, even when done by an experienced optometrist, screening by
this method is unacceptable.
19 The results with
the slit lamp biomicroscope have been much more impressive, yielding
sensitivities and specificities as high as 80% and 95%, respectively.
22,31
With an associated technical failure rate of less than 1%, these results
compare favourably with retinal photography. The one crucial weakness of a
screening programme with optometrist screeners is the need for an elaborate
quality control mechanism if it is to be audited effectively.
32 This means
either re-examination of a substantial number of patients or secondary
photography with the ETDRS gold standard 7 field stereo images. In our
experience the secondary wave of screening for quality assurance is poorly
attended, with non-attendance rates of over 50% (unpublished). This failure to
attend seriously undermines the audit process. Furthermore, since the expected
prevalence of sight-threatening eye disease in the screened population is low,
a substantial proportion of the screened population must be audited to obtain
meaningful data. Add this to the high non-attendance rate, and we calculate
that one-fifth of our original screening population would need to be
secondarily screened. This is obviously not practicable and is the principal
obstacle to use of optometrists in screening for diabetic retinopathy.
Combined modalities
The remaining option for diabetic retinopathy screening is to combine
screening modalities and site camera systems within optometrist practices.
Such a system might offer both the desired accessibility and a holistic
package of eye care amenable to audit.
33 Combination of
screening modalities is not a new idea. Previous studies have shown that
sensitivities of around 90% can be achieved by optometrists using
ophthalmoscopy and dilated fundus photography,
34,35
and these figures are all the more impressive when one considers that they
were achieved with the direct ophthalmoscope. Optometrists also represent an
available, well trained and motivated work force. With minimal training they
could perform many of the tests that, in a purely photographic screening
programme, would have to be delegated to a team of photographers and graders.
Recruitment of this workforce might overcome the potential difficulties of
training and retaining personnel to grade retinal photographs. One
disadvantage of a combined modality programme is that the capital set-up costs
may be of the same magnitude as those of a purely photographic system.
Furthermore, if these costs are not to be prohibitive the camera systems might
have to rotate around optometrist practices, thus nullifying the accessibility
that is one of the strengths of an optometrist-based system.
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CONCLUSION
|
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No single modality satisfies all the requirements for a screening
programme.
Currently the preferred method for screening is a retinal
photographic
service based on digital systems. In any one region, the
screening
programme that is adopted is likely to be a compromise between
efficacy
of the method, the existing infrastructure and local expertise.
|
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INTRODUCTION
WHY SCREEN FOR DIABETIC...
