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

Patients and Methods

Study Design

The study protocol was prespecified as part of an AA observational study series (Harries et al., paper submitted), registered with ClinicalTrials.gov (NCT04239521), and conducted following REporting of studies Conducted using Observational Routinely collected Data (RECORD) guidelines.[13]

Population-based primary care data were extracted from the Oxford-Royal College of General Practitioners (RCGP) Research and Surveillance Centre (RSC) network database. The RCGP RSC cohort is drawn from a large network of GP practices across England, representing a broadly representative sample of the UK population.[14] The RCGP RSC database contains information about demographics, clinical diagnoses, clinical measurements, laboratory test results and prescriptions. Studies using RCGP RSC have been published across many common diseases, including obesity, atrial fibrillation, diabetes and rheumatoid arthritis.[15–18]

Study Population

All adults and children registered between 1 January 2009 and 31 December 2018 were eligible for inclusion. People who opted out of record sharing were excluded (approximately 1·8% of adults).

Alopecia Areata Cases. We identified people diagnosed with AA (AA cases) using diagnostic Read codes specific to the condition (Harries et al., paper submitted). AA cases required an AA-specific Read code and, in the subsequent 365 days, no code for an alternative diagnosis (scarring alopecia,[19] traction alopecia, congenital alopecia, androgenetic alopecia, telogen effluvium, tinea capitis, trichotillomania or secondary syphilis of the scalp).

Sociodemographic Factors. Age was categorized into 5-year age groups. Ethnicity was grouped into standard major UK ethnic groups: white, black, Asian, mixed and other.[20] Socioeconomic status was defined using the Index of Multiple Deprivation (IMD),[21] calculated at the point of data extraction using patient postcode, and stratified by national deprivation quintile.

Statistical Analyses

Incidence of Alopecia Areata. Incident cases were defined as people with a new-onset AA during the study period. People with a diagnosis of AA prior to the study period were excluded. To increase certainty that an AA diagnosis was incident, those diagnosed within 6 months of registering with a practice were excluded, unless under 1 year of age. We calculated IRs, stratified by age category and sex, by dividing the number of incident people by the sum of person-years of follow-up for the total eligible population. Multivariable IR ratios (IRRs) adjusted for sex, ethnicity, IMD quintile, geographical region and geographic area (urban/rural), were estimated using Poisson regression.

Point Prevalence of Alopecia Areata. We estimated point prevalence of AA for adults, overall and age/sex stratified, on 31 December 2018, the last date of follow-up. Point prevalence represents people actively registered with their GP practice on that date, who have either active AA or who have had AA in the past. Point prevalence was calculated by dividing the total number of actively registered people with an AA diagnostic code (at any prior timepoint) by the total number of actively registered people. All people were included regardless of the date of AA diagnosis in relation to the date of registration with a practice. People with < 365 days of follow-up from date of registration were excluded from the analysis (unless under 1 year of age).

Service Utilization and Prescribing Patterns. In people newly diagnosed with AA, we examined primary care visit rates, specialist dermatology review rates and prescribing of medications indicated for AA, in the first year after AA diagnosis.

We used a matched-cohort design to compare rates of primary care visits in people newly diagnosed with AA (AA cases) and people without AA (matched controls). Matched controls were identified by matching each AA case with four controls never diagnosed with AA at the date of diagnosis of the AA case. We used nearest-neighbour matching, by age, sex and time since practice registration, at the GP practice level.[22] Matched controls required at least 1 year of follow-up when matched to minimize the risk of a nonrecorded existing diagnosis of AA. Follow-up for each control began at the start of follow-up of their corresponding AA case.

Annual rates of primary care visits were calculated by dividing the total number of visits each year by the sum of person-years of follow-up each year. The proportion of AA cases referred for specialist dermatology review and the proportion prescribed medications of interest were calculated using the Kaplan–Meier estimator, as not all people had an entire year of follow-up after diagnosis available. Visit and referral rates were stratified by age, sex, ethnicity, IMD quintile, geographical region and rural/urban geography. AA medication classes of interest were categorized as mild, moderate, potent or very potent topical corticosteroids (TCS), systemic steroids, minoxidil and dithranol.[10] In sensitivity analyses, we excluded people with a diagnosis of atopic dermatitis or psoriasis prior to either their AA diagnosis or first medication prescription.

Sensitivity Analysis. To check the validity of the AA case definition and identify possible changes in coding practice, annual incident trends and point prevalence of alopecia coded as 'nonspecific' and 'other specified' (either scarring, congenital, traction or traumatic, or androgenetic alopecia) (Harries et al., paper submitted) were estimated using the same approach as for the primary cohort.

Changes to the Study Protocol. The matched cohort was added to provide a comparator for rates of primary care visits. This provides important context as the reason underlying a visit cannot be ascertained from primary care records, meaning we could not identify whether visits related to AA itself. No other changes were made; all a priori defined objectives were reported. R v3.4.1 was used for analysis.

Ethics Approval

Study approval was granted by the RCGP RSC Research Committee. Using the NHS Health Research Authority research decision tool (https://www.hra-decisiontools.org.uk/research/) showed that formal ethics board review was not required for this study.

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