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Incidence of Perthes' disease

Northern Ireland
Liverpool
Comment

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



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




Table 1: Liverpool - Perthes' disease incidence by district and deprivation index

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:

  1. WD Kealey et al. Deprivation, urbanisation and Perthes' disease in Northern Ireland. Journal of Bone and Joint Surgery 2000 82-B: 167-171.
  2. 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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