Visual impairment from cataract in England and Wales: the NHS surgical burden  
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Correspondence:
Darwin C Minassian
Institute of Ophthalmology
UCL, and Moorfields Eye Hospital
London
EC1V 2PD

Darwin C Minassian


Angela Reidy

Institute of Ophthalmology
UCL, and Moorfields Eye Hospital
 

Abstract

The age specific prevalence and incidence of cataract are combined demographic data to provide an estimate of the cataract pool within the UK population. Cataract surgery rates and age specific mortality rates are then combined with the pool data in a holistic ‘system dynamics’ model to explore how the cataract pool might change over a 5 year period, given a series of options in the levels and pattern of cataract surgery. Finally, the cost to the NHS of the various options are considered.

Keywords

Cataract surgery, epidemiology, cataract surgery rates, cataract cost.

Introduction

Historically, for policy and planning purposes, estimation of the magnitude of the NHS cataract surgical burden has been derived from utilisation data, such as the number of cataract operations and the size of the waiting lists. Even the Department of Health Needs Assessment For Cataract Surgery document (1994) found it difficult to estimate need in any broader context than the numbers of patient/GP consultations about cataract. None of these figures are based upon population estimates of prevalence and incidence, the two fundamental epidemiological measures of disease magnitude in a defined population. These two measures in isolation, however, are also inadequate, since they do not fully reflect the dynamic nature of the cataract problem. For an appreciation of the potential burden a holistic approach is required, whereby the main factors and their interactions which influence the population cataract pool are taken into account. To meet this requirement, a dynamic model may be constructed to illustrate the magnitude of the problem, not only at one point in time, but also how it might change in a defined time period, under a variety of service level scenarios. Such models may be constructed for the whole NHS as well as for particular districts and regions. Estimates from these models will provide an indication of the magnitude of the potential burden; i.e. the population need for intervention to restore good vision, based on clinical measures of visual impairment due to cataract.

In this article, we present first the data on a number of indices of the NHS cataract burden in England and Wales. We then bring these indices together in a holistic model to explore how a number of proposed changes in service provision might affect the population cataract pool over a period of time.

Data inputs

Prevalence and Incidence

In our context, prevalence is defined as the proportion of cases (impaired vision <6/12 due to cataract) in a defined population. This proportion may also be expressed as the number of cases in the population. Prevalence reflects the population status at a given fixed time. Prevalence figures presented here were derived from the North London Eye Study (NLES),1 and were age-specific figures for people aged 65 and older.

Incidence, for our purpose, is defined as the number of new cases that occur in the defined population within a given period of time. This definition reflects ‘cumulative incidence’, which may be expressed as a proportion, e.g. the number of new cases in 5 years, divided by the size of the population. Age-specific prevalence data from the NLES have been used here to derive 5-year cumulative incidence estimates using the method of Podgor & Leske.2

Generally, direct measures of these figures for the whole population are not available, but they may be estimated from the findings in unbiased random samples drawn from the population of interest, as in the case of the NLES.

Demographic and vital statistics

Data for England and Wales were obtained from the Office for National Statistics, giving the estimated resident population by age in 1996. The relevant mortality rates were derived from life tables in the 1998 Annual Abstract of Statistics.

Our estimates of age-specific prevalence and incidence were then applied to the demographic profile in order to obtain the prevalence and incidence figures for the population of England and Wales.

Population prevalence estimates

The findings on population prevalence indicate that 2.4 million people aged 65 and older have cataract causing visual impairment (i.e. <6/12) in one or both eyes. This figure is an estimate of the ‘current’ pool of cataract cases in England and Wales, i.e. a snapshot at one point in time. The age-specific prevalence (numbers) are shown in Figure 1.

Additions to the pool over time

Our incidence estimates suggest that 1.1 million new cases of cataract are expected over a 5-year period. This figure assumes that the underlying age-specific incidence rates remain constant over the 5-year period. These data allow a first glimpse of the dynamics of the cataract pool in the population: during a 5-year period, 1.1 million are added to the 2.4 million already in the pool, but we know that substantial numbers leave the pool through having cataract surgery and/or through natural mortality. At this stage the more 'hands on' measures of activity can be incorporated from utilisation data and surveys of surgical practice.

Cataract surgery rates

Age-specific cataract surgery data were obtained from the Department of Health in England and from the Welsh Office. The data source was Hospital Episode Statistics, coded C71-C72 (i.e. Finished Consultant Episodes). Data were also extracted from the recent ‘Reference Costs 2000’ document from the Minister of State for Health.

The proportion of operations performed for visual acuity levels of <6/12 and for <6/18 in each age group was obtained from the National Cataract Surgery Survey 1997-8,3,4 as was the proportion of second eye cataract operations.

Further Findings

The findings are summarised in Table 1. According to the Minister of State for Health, some 189,000 cataract operations were performed in the UK in the year 1999-2000. We estimate that about 87% (164,000) of these operations are performed on cases aged 65 years and older.

Surgery
Number of procedures
Average cost £
Total cost £
With Phaco and IOL:
Day cases
Inpatients
137,229
31,869
568
877
77,946,072
27,949,113
Other cataract extractions:
Day cases
Inpatients
All Cataract extractions
13,093
6,367
188,558
577
826
630
7,554,661
5,259,142
118,708,988
Table 1. Cataract operations in the UK and the cost to the NHS for the year 1999-2000. Summarised from: the NHS Executive, Minister of State for Health, ‘Reference Costs 2000’ document. The total costs are calculated from the given average costs, except for the grand total, which is the column total and is used to calculate the overall average cost.

 

Data from the National Cataract Surgery Survey indicate that about 73% of all operations are directed at cases with visual acuity <6/12. This proportion amounts to a total of about 120,000 operations on people with visually impairing cataract aged 65 and older in England and Wales. These operations should effectively remove cases from the population cataract pool during the year. The situation, however, is more complex than this, because some of these operations are on the second eye of patients who have already had one eye operated on earlier in the year, so that the 2 operations contribute to the removal of only one case from the population cataract pool. In the National Cataract Surgery Survey, 35% of the operations were on the second eye.

Mortality

The effect of mortality on the pool of the elderly cataract cases in the population is substantial. Our calculations estimate that in the population pool of 2.4 million cataract cases, 233,000 deaths are expected annually, amounting to a death rate of 9.9%. The 5-year cumulative mortality in the cohort of 2.4 million cases is estimated at about 854,000 deaths.

A model of the population dynamics

In bringing together all the above factors into a holistic ‘system dynamics’ model5, we explore how the population cataract pool might change over a 5-year period, given a number of options in the levels and pattern of cataract surgery, as listed below.

(a) If the current practice in cataract surgery provision remains unchanged (with only an expected 10,000 or so additional operations per year), the population pool of cases will not be controlled rather it will grow by more than 200,000 cases in 5 years, an increase of 8.5%.

(b) Directing all surgery towards the age group of 65 and older with vision impairing cataract (<6/12) may improves the situation, but the backlog will still increase by 3.4%.

(c) We estimate that if about 95,000 additional operations per year are allocated to cases aged 65 yr. and older, the population cataract pool should remain virtually unchanged over the 5-year period.

(d) The same control described in (c) could be achieved by directing all operations in the elderly group to eyes with impaired vision, and allocating 27,000 additional operations per year, similarly directed.

Costs to the NHS

Conceptually, the simplest of the above options to prevent the backlog from worsening would be (c), a proposed provision of 477,000 additional operations over a 5-year period. The most cost-effective, however, might be (d) or a variant of it, whereby the operations are directed at the elderly and/or at eyes with vision below the level of 6/12. Determining costs for the proposed changes to the level and pattern of operations can be a complex issue, because the proposed additional operations may require additional theatre/hospital capacity over and above any ‘spare’ capacity that may exist now, and could be very costly. We are currently researching these issues. For background information, however, the reader may be interested in our crude estimate of the cost of the present NHS service for the UK. Using the ‘Reference Costs 2000’ figures on average costs for day-case and inpatient cataract surgery with and without phakoemulsification, we estimate that the minimum annual cost to the NHS for cataract surgery is currently at £119 million (Table 1). The proportion spent on cases aged 65 and older is £103 million

 
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1 Reidy A, Minassian DC, Vafidis G, Joseph J, Farrow S, Wu J, Desai P, Connolly A. Prevalence of serious eye disease and visual impairment in a north London population: population based, cross sectional study. BMJ 1998;316:1643-1646

2 Podgor MJ, Leske MC. Estimating incidence from age-specific prevalence for irreversible disease with differential mortality. Statistics in Medicine 1986;5:573-578.

3 Desai P, Minassian DC, Reidy A. National cataract surgery survey 1997-8: a report of results of the clinical outcomes. Br J Ophthalmol 1999;83:1336-1340

4 Desai P, Reidy A, Minassian DC. Profile of patients presenting for cataract surgery in the UK: national data collection. Br J Ophthalmol 1999;83:893-896

5 Minassian DC, Reidy A, Desai P, Farrow S, Vafidis G, Minassian A. The deficit in cataract surgery in England and Wales and the escalating problem of visual impairment: epidemiological modelling of the population dynamics of cataract. Br J Ophthalmol 2000;84:4-8