Screening for diabetic retinopathy
- Sensitivity: The proportion of individuals with diabetic retinopathy who are correctly identified by the screening test.
- Specificity: The proportion of individuals who do not have diabetic retinopathy who are correctly identified by the screening test.
Diabetes remains the commonest cause of blindness in the working age group in most Western societies, although the level may be diminishing with effective early treatment.1, 2Multicentre studies have demonstrated that the incidence of blindness from diabetic retinopathy (DR) can be significantly reduced by early intervention with laser treatment.3,4 This applies to both proliferative retinopathy and maculopathy, apart from ischaemic maculopathy which is untreatable. However, once DR is symptomatic, with significant visual loss, the chance of recovering vision is greatly reduced. Laser treatment may not be possible, and even vitrectomy may not lead to visual gain.
Effective treatment, therefore, has to be initiated at an early stage before the patient is aware of any symptoms, and patients in this category can only be identified by systematic screening.
DR fulfills all the criteria for screening programmes,3and several have been in existence in the UK for 20 years. Different districts have developed individual methods of screening depending on the equipment and personnel available, and no single method has emerged as clearly superior. Screening should consist of measurement of visual acuity and a retinal examination.6
Requirements of a diabetic retinopathy screening programme
- Identification of the population at risk.
- An efficient register and recall system.
- An effective instrument for viewing the retina.
- An experienced interpreter of the findings.
- A screening protocol.
- A system for managing the cases identified.
- Feedback to the screening personnel.
- Quality control.
Identification of the population at risk
The identification of the population at risk requires a diabetic register. In the UK these are now in place in almost all districts, held by diabetologists, Local Diabetes Advisory Groups or Health Authorities. These may run screening programmes themselves or delegate to others, subject to controls on confidentiality. If a register does not already exist, it may be compiled from GP disease registers, hospital diabetic clinic records, pharmacy prescriptions for diabetic preparations and by direct advertisement to the public.
A diabetic register can only contain those with diagnosed diabetes. It has been estimated that there may be one undiagnosed diabetic in the community for every one that is known. Up to 30% of non insulin-dependent diabetics (NIDDM) have retinopathy at diagnosis. Population screening for diabetes is a separate issue, but optometrists may not infrequently diagnose diabetes, either because of an unexpected change in refraction or by finding retinopathy. In our district with a diabetic population of 3,500, about 5 patients a year are diagnosed by optometrists.
Once the population is identified, a system must be developed for running the screening programme. It is essential that adequate funding is provided. A programme coordinator is required to run the service and at least 10 hours a week of secretarial time are necessary for a programme for 3,500 diabetics. A computerised recall system is preferable but manual systems are effective although labour-intensive. The register must be constantly updated to ensure that patients are not lost to follow-up.
Screening instruments and methods
The ideal screening instrument for DR does not exist. It would be an instrument with a large field and thus a high sensitivity, which could be used with undilated pupils, and would be easy to use and not too expensive. (In general, the sensitivity of screening for DR resides with the instrument and the specificity with the screener.)
The screening instruments currently in use are listed below (see Table).
A combination of a non-mydriatic camera taking two views (macular and nasal) through dilated pupils, together with fundoscopy using an ordinary ophthalmoscope, improves the screening sensitivity to over 90%.7,8However, this increases the cost and it would be preferable to have a reliable single modality.
Table. Screening instruments currently in use: advantages and disadvantages
|Ophthalmoscope||Inexpensive; widely available; easy to use||Dilated pupils; small field; low sensitivity|
|Indirect ophthalmoscope||Large field; relatively inexpensive||Dilated pupils; difficult to use|
|Slit lamp + wide angle lens||Large field||Dilated pupils; difficult to use|
|Non-mydriatic camera||Large field; easy to use; undilated pupils; can be transported to the community in mobile units; new models can be linked to computers for electronic data storage||Poor quality films if media opacities; specialist review of films needed; more than one view required for adequate coverage, which increases cost; Polaroid films not as good as 35 mm.|
|Non-mydriatic camera used with mydriasis||As above; but dilated pupils for better quality photos||As above; dilated pupils|
|Conventional fundus camera||Large field; easy to use; a good viewing instrument||Dilated pupils; expensive; not widely available|
|Scanning laser ophthalmoscope (SLO)||Undilated pupils; high sensitivity||Very expensive|
|Fluorescein angiography with fundus camera||High sensitivity||Expensive; side effects; impractical on a large scale|
|Stereo-ophthalmoscope||Large field; easy to use||Now out of production|
Screeners should have a sufficient case load to acquire expertise and should be permanent staff providing continuity of care.
- Ophthalmologists, although the most competent to do it, have insufficient time to screen for as well as treat retinopathy.
- Diabetologists, although usually competent, can only screen those patients attending hospital clinics.
- GPs, although well placed to screen their own patients, rarely have a large enough case load to become competent and are limited by using ophthalmosopes which have a low sensitivity.
- Clinical assistants, specifically trained and using a good instrument, are ideal.
- Junior medical staff are not suitable as they do not acquire sufficient experience in their training period.
- Optometrists are ideally placed to provide screening in the community, but need extra training and should be part of an organised programme with a direct referral system and specific funding arrangements.
- Technicians may operate photographic or SLO systems (see Table) but the films need to be read by an ophthalmologist, diabetologist or clinical assistant.
Programmes should have a written protocol, covering both the procedure to be followed for the screening method used and the action to be taken on the findings.
The findings may be divided into 3 categories:
- Patients requiring urgent referral, i.e., all patients with new vessels.
- Patients requiring routine referral: those with potentially sight-threatening retinopathy i.e., any retinopathy within 2 disc diameters of the fovea; pre-proliferative retinopathy (venous beading, loops, multiple cotton-wool spots, intraretinal microvascular abnormalities).
- Patients to be retained in the screening programme – the vast majority (Fig. 1)
Programmes vary on the extent of retinopathy which requires referral; some require all retinopathy to be referred while others will expect screeners to monitor background retinopathy, depending on the experience and training of the screeners and the available resources.
For patients remaining in the screening programme, the standard re-screening interval is one year. Patients in a low risk category – those over the age of 70 at diagnosis, with no retinopathy at first screening and no other risk factors; and children under the age of 12 years – may be screened every 2 years. Decisions to extend the interval beyond one year should be taken by a doctor. Patients in high risk categories (pregnancy or after episodes of ketoacidosis) should be screened at 3 month intervals.
All NIDDM patients should be screened for the first time soon after diagnosis, because the duration of the disease is unknown, but young insulin-dependent patients may safely wait 2 years.
Some protocols may allow treated patients to be returned to routine screening but this depends on the experience of the screeners.
When setting up a screening programme it is very important that adequate arrangements are made for treatment of the cases identified. Initially there will be a backlog of untreated cases before a steady state is reached (Fig. 2). Our population of 3,500 diabetics, with an established screening programme of 18 years, yields an average of 12 new cases of proliferative retinopathy and 42 new cases of maculopathy to be treated each year. The workload increases every year in the assessment and treatment clinic, as well as the screening clinic, as new patients enter the programme (Fig. 3). There is a net gain of over 200 patients into the programme each year and 7% of patients screened are transferred to the assessment clinics.
There should be feedback to the screeners on the outcome of their referrals. This is educational and should form part of ongoing training. It also helps to maintain interest and motivation, as screening in isolation is unrewarding.
Ideally, quality control should be built into the system. In research programmes a sample of patients may be re-screened by an assessor. In practice this is difficult as patients are unwilling to be screened twice and it may undermine their perception of the screener’s competence.
Audit of the referrals is more practicable. This will show:
- The number of appropriate referrals
- The number of false positives: those who have spontaneously regressed, and those referred inappropriately (e.g., macular degeneration, drusen) – to be transferred back to screening
- The number with advanced disease – these are a test of the sensitivity of the screening method.
Screening for diabetic retinopathy is well-established and of proven efficacy. However, the variety of screening methods indicates the lack of a single reliable system. When new equipment becomes available the sensitivity may improve.
1 DHSS. Blindness and Partial Sight in England 1969-76 [HMSO]. Reports on Public Health and Medical Subjects No.129.
2 Evans JE, Rooney C, Ashwood F, Dattani N, Wormald R. Blindness and Partial Sight in England and Wales: April 1990-March 1991. Health Trends 1996; 28 (1): 5-12.
3 British Multicentre Study Group. Photocoagulation for proliferative diabetic retinopathy: a randomised controlled clinical trial using the xenon-arc. Diabetologia 1984; 26: 109-15.
4 The Early Treatment Diabetic Retinopathy Study Research Group. Photocoagulation for diabetic macular oedema. Early Treatment Diabetic Retinopathy Study Report No.1. Arch Ophthalmol 1985; 103: 1796-806.
5 Wilson JMJ, Jungner G. Principles and practice of screening for disease. WHO Public Health Paper 1968: 34.
6 Retinopathy Working Party. A protocol for screening for diabetic retinopathy in Europe. Diabetic Med 1991; 8 Symposium: S4-S10.