Someone once told
Bandolier
that 6% of us have a rare disease. Those rare diseases are many and varied, so many
and varied, in fact, that the majority of us will rarely or never have heard of more
than a few of them. So when
Bandolier
was asked to write about the epidemiology, diagnosis and treatment of Perthes'
disease the first question was 'What's Perthes' disease?'.
Perthes' disease is a developmental problem of the hip joint, usually unilateral,
affecting younger children, and recognised by history, examination and radiological
changes. Literature searches provided no systematic evidence about diagnosis or
treatment, but did show up some interesting population studies in the UK that can
help thinking about how often it can be expected and temporal trends.
Northern Ireland
Northern Ireland has a centralised orthopaedic service where four hospitals serve
children. Over a seven-year period 313 children were diagnosed with Perthes' disease.
Postcodes allowed spatial analysis by rural or urban area, and by deprivation index
for each postcode district.
The results showed that the overall annual incidence was 11.6 per 100,000 children
aged under 15 years. There were 256 boys and 57 girls (4.5 to 1), with mean age at
onset of 5.7 years and 16% of cases bilateral.
There was no relationship between settlement size and incidence, and no relationship
between population density and incidence. Incidence was highest in children living in
the most deprived areas (Figure 1). This was particularly noted for rural
deprivation, where the gradient was steepest (7.1 per 100,000 to 16.1 per 100,000 for
increasing deprivation), and for settlements up to 50,000 people. The relationship
was not seen in larger settlements.
Figure 1: Northern Ireland - Perthes' disease incidence by deprivation index
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Liverpool
Liverpool city was the most deprived of 310 English districts in 1998, with the
neighbouring districts of Knowsley ninth and Sefton 54
th
. There is a high incidence of Perthes' disease. Children from all three
districts are referred to the Alder Hey Hospital, which has maintained a Perthes'
disease register since 1978. The register has been reviewed up to 1999,
supplemented by computer searches of other activities in the hospital to ensure
no cases were missed.
Parents of affected children were interviewed to determine the district and ward
in which the children were born. Denominators for number of children under 15 in
wards and districts were determined from census figures. The average ward- and
district-specific rates of incidence were calculated for the periods 1976-1981,
1982-1989, and 1990-1995.
There were 122 children diagnosed with Perthes' disease in Liverpool, 60 in
Knowsley, and 38 in Sefton. Incidence rates declined in all three districts
(Figure 2). Lower incidence of Perthes' disease was seen in Sefton, the least
deprived district (Table 1). Multiple regression analysis was used to summarise
various measures of social deprivation and health on ward-specific rates.
Incidence increased where deprivation had increased, the prevalence of low
birthweight was highest, where free school meals were highest and where wards had
a low health index.
Figure 2: Liverpool - Perthes' disease incidence by time
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Table 1: Liverpool - Perthes' disease incidence by district and deprivation
index
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|
|
Liverpool |
Knowsley |
Sefton |
| Perthes' disease per 100,000 children
under 15 (1990-1995) |
8.7 |
11.3 |
4.4 |
| Deprivation index (higher values are
more deprived) |
10.5 |
9.4 |
4.4 |
Comment
Both studies make general points relating to rare diseases. Obvious is that both
ascertained all of the cases in their districts, and went to great lengths to do
this and ensure consistency in diagnosis. They went to great lengths to ascertain
the proper denominator for their incidence calculations. They went to great
lengths to determine accurately factors like deprivation and other indices of
deprivation. The result is that we can look on the results with some confidence,
despite the numbers of cases in total being under 600.
The message for Perthes' disease is that deprivation is the key. Not urban
deprivation, but rural deprivation also, making the results important for
Southern counties in the UK which are superficially affluent but where pockets of
rural deprivation occur.
Liverpool and Ireland have family links, and there may be genetic components
underlying these results. For instance in Liverpool 6% of the population is
formed by black or minority groups, but all 122 children diagnosed were white.
For a PCO of 100,000 people where about 20% may be under 15 years, high incidence
rates like those seen in Liverpool and Northern Ireland would result in two or
three children with Perthes' disease every year. Less deprived areas may see only
one.
References:
- WD Kealey et al. Deprivation, urbanisation and Perthes' disease in Northern
Ireland. Journal of Bone and Joint Surgery 2000 82-B: 167-171.
- BM Margetts et al. The incidence and distribution of
Legg-Calvé-Perthes' disease in Liverpool, 1982-95. Archives of Diseases
in Childhood 2001 84: 351-354.
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