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
Cataract surgery, epidemiology, cataract surgery rates, cataract
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.
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
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.
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.
Phaco and IOL:
| Day cases
All Cataract extractions
|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.
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