epidemiology of sight loss in the uk astrid fletcher london school of hygiene & tropical...

Epidemiology of sight loss in the UK

Astrid Fletcher

London School of Hygiene & Tropical Medicine

Overview

• What do we know about the prevalence of sight loss in the UK

• What are the major conditions leading to sight loss?

• Do we need more research?• What are the gaps in knowledge? • What are the main research questions

arising from the data on prevalence and causes?

Use of prevalence data

• Prevalence defined as proportion of people with sight loss at a specific time point

• Describes the relative importance of a health problem in the population

• Usually reported for different age groups• Prevalence rates applied to age specific

population data provide estimates of number of people affected

• Knowledge of prevalence and numbers by causes of sight loss is important for planning services and identifying unmet need

Prevalence estimates of sight loss are only the first step

Largely uninformative without data on the underlying conditions leading to sight loss

Approaches to measurement and categorisation of sight loss

• Vision difficultiesSelf report of difficulties with vision related functions ranging from single item questions to disability scales

• Vision related quality of life scales Describe the impact of vision problems on

everyday functioning and well-being • Clinical measures

“Objective” measures eg Distance and near acuity, visual fields etc

Definition of visual impairment

• WHO cut-points are based on best eye and after full refraction– Visual impairment <6/18 – Low vision <6/18 to 3/60 – Blindness <3/60

• Definitions used in UK studies – <6/12 (approximates to UK driving requirement)– <6/18 & <3/60– Presenting or pinhole corrected or after refraction

Prevalence of best-corrected visual acuity <6/12 in population-based studies

Congdon et al Arch Ophthalmol 1998

Surveys of adult population in the UK using visual acuity measurements

Survey

Setting

Age Number

Response

Lavery 1988 Melton

Mowbray GP

76+ 529

78%

Wormald 1992

Inner London GP

65+ 207 72%

Reidy 1998 N. London

17 GP practices

65+ 1547 84%

National Diet & Nutrition Study 2000

Postcode sampling

& Sample of nursing

homes

65+ 1,362

75% private households

94% nursing homes

MRC Assessment trial 2002

53 GPs across GB

75+ 14,600 69%

Prevalence of VI and blindness

0

5

10

15

20

25

30

35

40

45

Age

Pre

vale

nce VI M

VI F

Blind M

Blind F

MRC Assessment Trial Prevalence of binocular visual impairment

(<6/18) and blindness (<3/60)

74-79 80-84 85-89 90+

Study Age V.I. N %<6/18

%<6/12

Lavery 76+ Refraction 474 26.2 -

Wormald 75+ Refraction 106 14.2 21.8

North London 65+ Presentingbilateral

1547 - 30.2

NDNS 75+ Pinhole 1362 15.2 32.4

MRC 75+ Presentingbinocular

14600 12.410.8-13.9

20.117.8-22.0

UK studies: prevalence visual acuity

Study Age V.I. N %<6/18

NDNS 75+ Pinhole 1362 15.2

MRC 75+ Presentingbinocular

14600 12.410.8-13.9

Rotterdam 75+ Refraction 1806 4.7

Baltimore 70+ Refraction 836 4.8

Beaver Dam 75+ Refraction 795 6.0

Blue Mountains

70+ Refraction 783 5.0

Melbourne 70+ Refraction 605 6.2

Comparison with other non UK studies: VA <6/18 in 75+

Variation in estimates between studies

• Definitions• Measurement quality • Age structure

– especially in oldest age groups where prevalence is highest

• Sampling error – Small numbers in older age groups

May be “true” differences between populations

What is the significance of differences in prevalence between

populations?

• Variations in prevalence reflect variation in the prevalence of underlying conditions– Availability and use of eye care services– Aetiology of specific eye problems in

different populations

Comparison between UK and non UK studies

• Most non UK studies use only best corrected visual acuity

• Exclude data on vision impairment due to refractive error

• Presenting vision is the most appropriate measure of a person’s everyday vision

• Recommended by WHO in 2003 that presenting VA <6/18 be used as the main definition of visual impairment

MRC TrialCauses of visual impairment (VA

<6/18) aged 75+

31.6

32.6

20.46.4

2.1

2.6

3.8

Refractive error

AMD

Cataract

Glaucoma

Diabetic eyedisease

Myopic deg

OtherPrevalence of VA <6/18 excluding RE = 8%

Comparison with other studies – blindness (VA <3/60)

0%

20%

40%

60%

80%

100%

Other

Diabetes

Glaucoma

Cataract

AMD

Comparison with other studies - low vision (VA <6/18-3/60)

0%

20%

40%

60%

80%

100%

Other

Diabetes

Glaucoma

Cataract

AMD

Visual impairment in older people

50% to 70% of visual impairment in the older age group is due to “remediable” causes and could be improved by:

specs/ new specs

cataract surgery

Visual impairment in older people

Often not known to health services

Of people aged >65 (Reidy et al 1999):• only 12% of people with cataract were in touch with eye services• only one third of those with uncorrected refractive error had seen an optician in the past 12 months

MRC assessment trial

• Of people eligible for referral to an ophthalmologist around a half were referred by the GP

• Among those referred, 88% attended• Over 80% of people advised to see an

optician did so• New lenses were obtained by 45%• The main reasons given for not obtaining

glasses were ‘not needed’ and cost

Should new evidence on prevalence and causes of vision

impairment be a research priority?

• Probably not for the older age group. Evidence is reasonably consistent with other developed countries

• Lack information on ethnic minorities in whom prevalence of VI, underlying causes and eye care use may be different from the majority population

• Evaluation of strategies to reduce the high proportion of untreated remediable conditions should be priority for action