The Epidemiology of Alopecia Areata

A Population-based Cohort Study in UK Primary Care

M. Harries; A.E. Macbeth; S. Holmes; W.S. Chiu; W.R. Gallardo; M. Nijher; S. de Lusignan; C. Tziotzios; A.G. Messenger

Disclosures

The British Journal of Dermatology. 2022;186(2):257-265. 

In This Article

Discussion

Our analysis of over 4 million people in primary care in England demonstrates that new-onset AA most commonly presents between 25 and 29 years of age. Females more commonly present with AA than males, although the overall difference was small and when age-stratified, a higher incidence in females was apparent only in children aged 4–18 years and adults aged 50+ years. Females also have a much broader peak than males in terms of age at AA onset. We show a threefold higher AA incidence in people of Asian origin, and a higher AA incidence in people from areas of higher deprivation and urban areas. One in four people with new-onset AA are referred to a dermatology specialist, with referral rates lowest in people from more deprived areas.

In comparison with other studies, to our knowledge, this is the largest population-based epidemiological study of AA to date, including 6765 people with AA compared with 530 people included in a recent US population-based study using data from the REP.[3] The IRs in both studies are similar: 0·26 in our study compared with 0·21 per 1000 person-years in the REP. The mean age at diagnosis was 33 years in the REP, similar to the incidence peak we observed. As expected, the incidence estimated in our primary care-based study is lower than that reported in secondary care cohorts (ranging from 0·7% to 3·8%),[5–9] where the denominator encompasses patients with skin conditions only.

Our population-based estimate of AA point prevalence (0·58%), which includes both active AA and historical AA diagnoses, is in line with the few previous epidemiological studies of AA prevalence. Historical estimates ranging from 0·2%[23] to 2·1%[3] have been observed in smaller US cohorts, while a more recent population-based study from France estimated an AA prevalence of 1·0%.[24]

We observed a higher rate of AA in those from nonwhite backgrounds and, in particular, a threefold higher rate in those of Asian origin. This is concordant with US data where a higher incidence of AA was observed in nurses of black compared with white ethnicity.[25] Interestingly, AA was more common in people of black ethnicity but less common among people of Asian ethnicity in the American National Alopecia Areata Registry.[26] Differences between our data and this study may relate to the lower prevalence of people from Asian backgrounds in the US cohorts, as well as reliance upon self-reporting in both US studies. While the exact pathogenesis of AA remains incompletely understood, it is generally considered to be an autoimmune phenomenon, occurring in those with a genetic predisposition in the presence of certain environmental triggers.[27] Genetic variations in the HLA region known to predispose to autoimmune conditions have been observed more frequently in those of nonwhite ethnicity compared with white individuals.[28,29] Rates of other autoimmune conditions including systemic lupus erythematous have also been reported to be higher in Black and Asian, compared with white, individuals.[30]

Ours is the first study to identify higher deprivation and urban environments as factors associated with AA. Urbanization and air pollution have previously been postulated as environmental triggers in the development of other autoimmune conditions such as rheumatoid arthritis, potentially via the induction of systemic inflammation by particulate matter.[31,32] Social deprivation could be linked to AA development indirectly due to higher rates of life stressors, depression and anxiety, although a causal link between these factors and AA onset has not been established.[33,34]

Considering the implications for primary care management of AA, 24% of patients with AA seen in primary care were referred to specialist dermatology in the first year after diagnosis, and referral rates increased from 19·4% in 2009 to 27·9% in 2017. This is comparable with secondary care referral rates for psoriasis in the UK (18·1% in 2009),[35] but is significantly higher than referral rates for atopic dermatitis (4·5% in 2018).[36] UK guidelines suggest dermatology referral for hair loss that does not respond to first-line treatment or if the diagnosis is uncertain.[12] Given that 80% of patients with limited AA may experience spontaneous remission within 1 year,[37] and hair regrowth with topical steroids has success rates of 18–60% compared with 0–27% success with placebo,[38–40] a specialist dermatology review rate of 24% seems concordant with guidance. Difference in primary care visit rates for people with AA were in line with known primary care consultation patterns, with females and those from more deprived backgrounds more likely to attend their GP.[41,42]

Despite higher rates of AA and primary care attendance, people from the most deprived areas were less likely to be referred to a dermatology specialist, a pattern previous reported in the UK for other conditions.[42] A potential explanation is that GP consultations in deprived areas are more often complicated by complex multi-morbid presentations, compared with patient-led, single-problem consultations in more affluent areas, where patient expectations of secondary care referral are greater.[43]

Potent TCS were the most frequently prescribed medication after AA diagnosis, in correspondence with national guidelines which recommend potent TCS as first-line treatment for limited AA.[10] The rise in the proportion of patients with AA prescribed potent TCS prescriptions over the study period may reflect a physician response to these 2012 guidelines. Low rates of systemic corticosteroid prescriptions are consistent with UK AA guidelines, which specifically state that the use of systemic steroids is not supported.[10]

The strengths of our study design include the use of a large, UK population-representative, primary care cohort.[14] Although we lacked a validated case definition for AA, our ontological approach to detecting incident AA has higher accuracy compared with the use of AA-specific codes alone.[44] Exclusions of recurrent AA episodes and other, potentially confounding, conditions (i.e. other causes of hair loss) are likely to have improved the accuracy of our estimates.

There are several study limitations. We are likely to have underestimated the true incidence and point prevalence of AA as some cases will have been coded using other terms including nonspecific alopecia. This study also does not capture mild cases of AA not presenting to primary care. We assume that all diagnoses of AA in primary care are correct and that AA is a simple diagnosis to make: diagnosis is usually easier in patch-type AA but it is possible that we underestimate true incidence based on the difficulty in diagnosing diffuse types of AA, although this will be a much smaller percentage of the overall incidence. Neither the reason for visits or specialist dermatology reviews, nor the indication of prescriptions, was available, which means we were unable to identify AA-specific events; our service utilization estimates therefore provide a measure of overall primary care use in people with AA. We were unable to assess secondary care prescribing in AA, as at present there is no mechanism in the UK for capturing secondary care prescriptions. As with all single-country epidemiological studies, our results should not be extrapolated to dissimilar populations.

In conclusion, by providing the first large-scale population-based estimates of the incidence and point prevalence of AA, our study helps to understand the burden of AA on the population across the lifespan. The marked variation in AA onset, especially across different ethnic groups, may give insight into the pathogenesis of AA and its management.

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