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. 2024 Nov 12;96(5):e334547. doi: 10.1136/jnnp-2024-334547

Prevalence of epilepsy: a population-based cohort study in Denmark with comparison to Global Burden of Disease (GBD) prevalence estimates

Jakob Christensen 1,2,✉, Betina B Trabjerg 3,4, Ryan G Wagner 5, Charles R Newton 6,7, Churl-Su Kwon 8, Kari Modalsli Aaberg 9, Eugen Trinka 10,11,12, Samuel Wiebe 13, Judith Helen Cross 14, Håkon Magne Vegrim 15, Theo Vos 16, Jaimie Steinmetz 16, Julie Werenberg Dreier 3,4
PMCID: PMC12015072  PMID: 39532519

Abstract

Background

The Global Burden of Disease Study (GBD) produces prevalence estimates for ‘idiopathic epilepsy’ (ie, of unknown aetiology) and ‘secondary epilepsy’ (ie, with known aetiology) but does not report prevalence by underlying aetiologies for ‘secondary epilepsy’.

Methods

We used nationwide, population-based register data from Denmark to identify underlying causes of epilepsy and their contribution to prevalence of ‘secondary epilepsy’ and compared with global prevalence data from GBD 2019. We identified all persons with a hospital-based epilepsy diagnosis and a filled prescription for antiseizure medication between 1 January 2009 and 31 December 2018. Epilepsy was categorised into ‘idiopathic’ or ‘secondary’ and ‘total epilepsy’ as the sum of the two epilepsy categories.

Results

On 31 December 2018, a total of 5 784 284 individuals (49.7% males) were living in Denmark including 40 336 with epilepsy (51.5% males). Perinatal conditions, traumatic brain injury, brain tumours and stroke were prominent underlying causes of ‘secondary epilepsy’. The prevalence of ‘total epilepsy’ in Denmark was 697 (95% CI 691 to 704) per 100 000 population (264 (95% CI 260 to 269) for ‘secondary epilepsy’ and 433 (95% CI 428 to 438) for ‘idiopathic epilepsy’). In the GBD 2019 Study, the prevalence of ‘total epilepsy’ in 2018 was 682 (95% uncertainty interval (UI) 586 to 784) per 100 000 population (359 (95% UI 324–397) for ‘secondary epilepsy’ and 324 (95% UI 249 to 404) for ‘idiopathic epilepsy’).

Conclusions

Prevalence estimates of ‘total epilepsy’, ‘idiopathic epilepsy’ and ‘secondary epilepsy’ in Denmark align with the GBD 2019 estimates. In future studies, it is suggested to explicitly include all types of epilepsy, including ‘secondary epilepsy’, which is currently estimated as sequelae (consequences) of underlying diseases.

Keywords: EPILEPSY, EPIDEMIOLOGY, STROKE, DEMENTIA

WHAT IS ALREADY KNOWN ON THIS TOPIC

  • The ‘total epilepsy’ prevalence is the sum of ‘idiopathic epilepsy’ (ie, in this context without known underlying cause) and ‘secondary epilepsy’ (ie, with known underlying cause).

  • Prevalence estimates of ‘total epilepsy’ and epilepsy subtypes vary with age, sex and underlying cause of epilepsy.

  • The contribution of specific underlying causes to the prevalence of ‘secondary epilepsy’ and their alignment with, and impact on, Global Burden of Disease (GBD) estimates of epilepsy are not known.

WHAT THIS STUDY ADDS

  • The study provides updated point prevalence estimates of ‘total epilepsy’ in Denmark; 697 (95% CI 691 to 704) per 100 000 population (264 (95% CI 260 to 269) for ‘secondary epilepsy’ and 433 (95% CI 428 to 438) for ‘idiopathic epilepsy’).

  • From the GBD 2019 Study, the study provides prevalence estimates of ‘total epilepsy’ in 2018; 682 (95% uncertainty interval (UI) 586 to 784) per 100 000 population (359 (95% UI 324 to 397) for ‘secondary epilepsy’ and 324 (95% UI 249 to 404) for ‘idiopathic epilepsy’).

  • Perinatal conditions, traumatic brain injury, brain tumours and stroke were prominent and highly age-dependent underlying causes of ‘secondary epilepsy’.

  • Prevalence estimates of ‘total epilepsy’, ‘idiopathic epilepsy’ and ‘secondary epilepsy’ in Denmark align with the GBD 2019 estimates.

HOW THIS STUDY MIGHT AFFECT RESEARCH, PRACTICE OR POLICY

  • In future studies of the burden of epilepsy, it would be advisable to explicitly include all types of epilepsy, including ‘secondary epilepsy’, currently considered a sequela (consequence) of underlying diseases.

  • The findings have significant implications for healthcare planning and resource allocation.

Introduction

Epilepsy is a disorder of the brain characterised by an enduring predisposition to generate epileptic seizures and by the neurobiological, cognitive, psychological and social consequences of this condition.1 Epilepsy may result from an underlying cause (eg, stroke or traumatic brain injury); in such cases, the epilepsy may be considered secondary to the underlying aetiology (‘secondary epilepsy‘. It is not always possible to identify an underlying aetiology of epilepsy in which case the epilepsy may be classified as ‘idiopathic epilepsy’. This terminology (ie, ‘secondary epilepsy’ and ‘idiopathic epilepsy’) was employed in the 2016 Global Burden of Diseases, Injuries and Risk Factors (GBD) Study of Epilepsy and in all subsequent rounds of the GBD through the present.2 We have chosen to maintain this dichotomy in the descriptions of epilepsy prevalence in this study, although, in the most recent International League Against Epilepsy (ILAE) classification of epilepsy,3 the term ‘idiopathic’ is reserved for the description of specific idiopathic generalised epilepsy syndromes. Thus, for clarity, in the present study of epilepsy prevalence, we have chosen to use the terms ‘secondary epilepsy’ for epilepsy where the first epilepsy diagnosis was preceded by an identified underlying cause of epilepsy and ‘idiopathic epilepsy’ for epilepsy with unknown underlying aetiology and ‘total epilepsy’ as the sum of the two types of epilepsy.

The many forms of epilepsy regardless of aetiology have major socioeconomic consequences for patients, families and society and are associated with reduction in health metrics such as life years lost due to premature mortality and years lived with disability often summarised as disability-adjusted life-years (DALYs).2 4 In an analysis of loss of DALYs, epilepsy ranked fifth among neurological disorders,5 and recently, the 73rd World Health Assembly of the WHO recognised that epilepsy and other neurological disorders are the leading cause of DALYs lost and the second-leading cause of death worldwide.6 Epilepsy is associated with significant health loss and because estimates of the burden of epilepsy are determined by the prevalence, precise estimates of epilepsy prevalence are essential for healthcare planning and resource allocation.7 8 A major contribution to our understanding of the burden of various disorders comes from the GBD Studies, which have reported detailed disease-specific estimates of prevalence, incidence, deaths and loss of DALYs by age and sex.9 10 The GBD also provides more detailed estimates of burden of individual disorders, that is, the 2016 GBD Epilepsy Study measured burden of epilepsy as deaths, prevalence and health loss (summarised as loss of DALYs), by age, sex, year, location and sociodemographic index.2 10 While the comprehensive GBD studies9 10 provide estimates of years lived with disability for ‘total epilepsy’ including ‘secondary epilepsy’, the 2016 GBD Epilepsy Study primarily focused on ‘idiopathic epilepsy’ and its burden.2 The burden of ‘secondary epilepsy’ (eg, secondary to stroke and traumatic brain injuries) was instead quantified as sequelae, or consequences, of the underlying causes of ‘secondary epilepsy’.2 Thus, the underlying epilepsy prevalence estimate that contributes to the 2016 GBD Epilepsy Study estimation for Years Lived with Disability for epilepsy is generated from the prevalence of ‘idiopathic epilepsy’ (329.3 per 100 000 population (95% uncertainty interval (UI) 280.3 to 381.2)), and not the ‘total epilepsy’ prevalence (ie, the sum of ‘idiopathic epilepsy’ and ‘secondary epilepsy’ prevalence; 621.5 per 100 000 population; (95% UI 540.1 to 737.0)).2

To assess the contribution of ‘secondary epilepsy’ and ‘idiopathic epilepsy’ to overall estimates of ‘total epilepsy’ prevalence and to assess the contribution of different underlying causes of ‘secondary epilepsy’, we estimated population-based prevalence of ‘total epilepsy’, ‘secondary epilepsy’ and ‘idiopathic epilepsy’ in a nationwide study in Denmark and compared estimates with updated GBD 2019 epilepsy prevalence estimates.

Materials and methods

Study design and participants

In this population-based cohort study, we included all individuals identified from the Danish Civil Registration System11 who were alive and living in Denmark on 31 December 2018. The Danish Data Protection Agency, Statistics Denmark and the Danish Health Data Authority approved this study. According to Danish law, informed consent was not required for register-based studies. All data were deidentified and not recognisable at an individual level.

Procedures

The Danish Civil Registration System11 holds information on all Danish residents, including sex, date of birth, date of death and a unique personal identification number that can be used to link information from various national registers. We obtained information on epilepsy from the Danish National Patient Registry,12 which contains information on all patients discharged from Danish hospitals since 1 January 1977 and on inpatients, emergency department and outpatient specialty clinic contacts since 1995. The diagnostic system used in these registers was the Danish modification of the International Classification of Diseases, 8th Revision (ICD-8) until 31 December 1993, and the ICD-10, from 1 January 1994. The use of antiseizure medication (ASM) was identified using the Danish National Prescription Registy,13 which holds information on filled prescriptions from 1995. Persons were classified as having prevalent epilepsy if they had received an epilepsy diagnosis (ICD-8: 345 (excluding 345.29) or ICD-10: G40) and had filled an ASM prescription (ATC N03A (excluding N03AX12 N03AX16) or N05BA09) between 1 January 2009 and 31 December 2018. Since the Danish National Patient Registry contains data from 1 January 1977, we restricted those having their first epilepsy diagnosis after 1 January 1979 to have at least 2 years of data before epilepsy onset, to capture underlying causes of epilepsy. The positive predictive value of an epilepsy diagnosis in the Danish National Hospital Register was 81% (95% CI 75% to 87%),14 but the completeness is not known, and persons with epilepsy only followed by general practitioners, by privately practising neurologists/paediatricians or patients who did not receive a diagnosis may have been missed. Combining information from hospital contacts with information from prescriptions filled for ASM has been shown to increase the positive predictive value where the best model correctly classified 90% of the epilepsy cases.15 Thus, the current algorithm using a combination of epilepsy diagnoses and ASM use provides estimates with a moderate to high positive predictive value and completeness.7

However, as we may not capture patients with epilepsy who were diagnosed in the hospital setting before 1 January 2009, and patients diagnosed outside the hospital, we added the following in sensitivity analyses: (1) To capture prevalent patients, who were diagnosed in a hospital setting in Denmark before 1 January 2009, and afterwards had been seen by general practitioners or privately practising neurologists only, we classified persons as having prevalent epilepsy if they had received an epilepsy diagnosis (ICD-8: 345 (excluding 345.29) or ICD-10: G40) between 1 January 1979 and 31 December 2008 and had filled an ASM prescription (ATC N03A (excluding N03AX12 N03AX16) or N05BA09) between 1 January 2009 and 31 December 2018 and (2) To capture prevalent persons who were diagnosed with epilepsy outside the hospital and had not received an epilepsy diagnosis in the hospital setting, we classified persons as having prevalent epilepsy if they had filled two prescriptions for an ASM (ATC N03A (excluding N03AX12 N03AX16) or N05BA09) with the indication ‘epilepsy’ between 1 January 2009 and 31 December 2018 and were not captured by any of the two other definitions of epilepsy.

We considered a person to have ‘secondary epilepsy’ if the first epilepsy diagnosis was preceded by any of the following recognised causes of epilepsy: central nervous system (CNS) infections and other specific infections, perinatal conditions associated with epilepsy, selected malformations related to the brain, brain neoplasm, traumatic brain injury, stroke and dementia (online supplemental table 1), and a person to have ‘idiopathic epilepsy’ if the first epilepsy diagnosis was not preceded by any of these conditions.

Persons diagnosed with monogenic/chromosomal disorders (eg, tuberous sclerosis and Down syndrome) were classified into ‘secondary epilepsy’ and persons with a presumed genetic aetiology of epilepsy (eg, juvenile myoclonic epilepsy) with complex genetic inheritance were classified with ‘idiopathic epilepsy’. From the Danish National Patient Register,12 we included diagnoses of causes given before or on the date of the first epilepsy diagnosis (ie, from 1977 to the date of epilepsy diagnosis). We did not include diagnoses of infections, traumatic brain injury and stroke if they were given in the 14 days leading up to the first epilepsy diagnosis (ie, to exclude early postinjury seizures).16 In cases where multiple potential causes of ‘secondary epilepsy’ were identified, the one occurring closest to the first epilepsy diagnosis was chosen.

Updated GBD 2019 estimates of epilepsy prevalence in the year 2018

The Institute of Health Metrics, Washington, USA regularly publish refined GBD data that are publicly available, and based on the most up-to-date iteration of this dataset, GBD 2019, the estimates of prevalence of epilepsy in the year 2018 via the Global Health Data Exchange database to collect age-specific prevalence (and the corresponding 95% UIs) for epilepsy were collected.17 The updated GBD 2019 estimates for ‘total epilepsy’ prevalence and prevalence of ‘secondary epilepsy’ and ‘idiopathic epilepsy’ from the Global Burden of Disease Collaborative Network17 were collected for the year 2018 to enable comparison with the data derived from Danish registers for the same year. Estimates of ‘total epilepsy’ prevalence by year, age, sex and country were produced using prevalence and incidence data from population-based studies as inputs to a mixed effects meta-regression modelling tool, DisMod-MR.9 ‘Total epilepsy’ prevalence was then split between ‘idiopathic epilepsy’ and ‘secondary epilepsy’ using proportion data collected from 89 studies that categorised epilepsy cases as either ‘idiopathic’ or ‘secondary’ to a known cause. These data were entered into a linear mixed-effects model to produce proportional estimates by year and location. Uncertainty was estimated by creating 1000 values for each prevalence estimate and performing aggregations across causes and locations at the level of each of the 1000 values for all intermediate steps in the calculation.2 GBD uses a standard population calculated for all age-standardised estimates, as the non-weighted average across all countries of the percentage of the population in each 5-year age group from the United Nations Population Division’s World Population Prospects (2012 revision).2

The 2016 GBD Epilepsy Study estimated global, regional and country-specific prevalence from 1990 to 2016 based on 317 studies on the prevalence of epilepsy, and additional studies of incidence, mortality and on the severity of the epilepsy were selected for the calculation of disability weights.2 The reference definition for epilepsy was based on the ILAE Guidelines for Epidemiologic Studies on Epilepsy,18 which defined an epilepsy case as someone with an active, recurrent condition of epileptic seizures (two or more) unprovoked by an immediate cause and who has had at least one epileptic seizure in the past 5 years regardless of ASM treatment. Thus, the definition used in both the 2016 GBD Epilepsy Study2 and the GBD 2019 study deviates slightly from the definition use in the analyses of the Danish data in this study (ie, persons who had received a hospital-based epilepsy diagnosis and had filled a prescription for ASM in the past 10 years).

Statistical analyses

In the Danish data, we calculated the point prevalence of epilepsy as the number of persons who were alive and diagnosed with epilepsy divided by the total number of persons alive in Denmark on 31 December 2018. We calculated the prevalence of ‘secondary epilepsy’ by conditions (ie, infections, traumatic brain injury, stroke, brain neoplasm, perinatal complications, dementia or congenital malformations) and the prevalence of ‘idiopathic epilepsy’ as the number of persons alive with these diagnoses divided by the total number of persons alive in Denmark on 31 December 2018. In the Danish population, we estimated 95% CIs for the point prevalence estimates. The population-based prevalence based on Danish data was directly compared with the UI from the GBD study

Patient and public involvement

The UK Epilepsy Priority Setting Partnership used the James Lind Alliance rigorous methodology to identify and prioritise unanswered questions that are most important to those affected by and working in epilepsy and identified top 10 research priorities for epilepsy.19 On this top 10 list, the question ‘What underlying mechanisms cause epilepsy in children and in adults?’ was prioritised as number two indicating that studies of underlying causes of epilepsy is highly prioritised by epilepsy charities, healthcare professionals and the epilepsy community. We will work with patient organisation associations on plans for dissemination of the study results to participants and wider relevant communities as we believe that the findings have significant implications for healthcare planning and resource allocation.

Role of the funding source

The funders of the study had no role in study design, data collection, data analysis, data interpretation or writing of the report.

Results

On 31 December 2018, a total of 5 784 284 individuals (2 876 754 (49.7%) males and 2 907 530 (50.3%) females) were alive and living in Denmark, including 40 336 individuals with epilepsy (20 763 (51.5%) males and 19 573 (48.5%) females). Thus, the point prevalence on this date of ‘total epilepsy’ was 697 (95% CI 691 to 704) per 100 000 population (722 (95% CI 712 to 732) per 100 000 population for males and 673 (95% CI 664 to 683) per 100 000 population for females). The point prevalence of ‘secondary epilepsy’ was 264 (95% CI 260 to 269) per 100 000 population (297 (95% CI 291 to 303) per 100 000 population for males and 232 (95% CI 227 to 238) per 100 000 population for females), and the point prevalence of ‘idiopathic epilepsy’ was 433 (95% CI 428 to 438) per 100 000 population (425 (95% CI 417 to 432) per 100 000 population for males and 441 (95% CI 433 to 449) per 100 000 population for females). In the expanded definition of prevalent epilepsy adding persons with a hospital-based diagnosis of epilepsy before 1 January 2009, who filled an ASM prescription between 1 January 2009 and 31 December 2018 to the main definition, we identified 48 952 persons with epilepsy alive on 31 December 2018 giving an overall point prevalence of ‘total epilepsy’ of 846 (95% CI 839 to 854) pr 100 000 population (online supplemental figure 1). In the further expanded definition of epilepsy adding persons who filled two prescriptions for ASM with the indication ‘epilepsy’ between 1 January 2009 and 31 December 2018 to the previous definitions, we identified 70 451 persons with epilepsy alive on 31 December 2018, giving an overall point prevalence of ‘total epilepsy’ of 1218 (95 % CI 1209 to 1227) pr 100 000 population (online supplemental figure 1).The age-specific prevalence of ‘total epilepsy’, ‘secondary epilepsy’ and ‘idiopathic epilepsy’ in Denmark is shown in figure 1. In general, the prevalence of ‘total epilepsy’ increased slightly with age, with a plateau between 15 and 50 years of age before increasing until the end of life. The prevalence of ‘idiopathic epilepsy’ increased in the first 10 years of life and then was stable throughout life. In contrast, the prevalence of ‘secondary epilepsy’ increased with age into late adult life.

Figure 1. Age-specific point prevalence of ‘secondary epilepsy’ (with known aetiology) and ‘idiopathic epilepsy’ (with unknown aetiology) in Denmark on 31 December 2018. Prevalence estimates are estimated for the age groups <10 years, 10–14, 15–19, …, 80–84, 85–89 and 90+ years (see tables1 2 for more detailed age-specific and sex-specific prevalence estimates).

Figure 1

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For ‘total epilepsy’, the prevalence was similar in the two sexes up to 50 years of age, after which the prevalence of epilepsy was higher in males than in females (table 1 and online supplemental figure 2).

Table 1. Point prevalence of ‘total epilepsy’ by sex and age in Denmark on 31 December 2018.

Age groups Males and females Males Females
N Prevalence (95% CI) per 100 000 N Prevalence (95% CI) per 100 000 N Prevalence (95% CI) per 100 000
0–1 99 80 (64 to 96) 56 88 (65 to 111) 43 72 (50 to 93)
2–3 227 186 (162 to 210) 122 194 (160 to 229) 105 177 (143 to 211)
4–5 345 295 (264 to 326) 200 333 (287 to 379) 145 254 (213 to 295)
6–7 418 342 (309 to 374) 212 338 (293 to 384) 206 345 (298 to 392)
8–9 607 459 (422 to 495) 331 487 (435 to 539) 276 428 (378-479)
10–11 761 560 (520-599) 405 579 (523-635) 356 539 (483-595)
12–13 791 583 (543-624) 416 600 (542-657) 375 566 (509-623)
14–15 883 654 (611-697) 449 650 (590-710) 434 659 (597-720)
16–17 871 644 (601-687) 425 613 (555-671) 446 676 (614-739)
18–19 1010 716 (672-759) 501 693 (633-754) 509 739 (675-803)
20–21 1019 692 (650-734) 539 714 (654-774) 480 669 (609-729)
22–23 1055 676 (635-717) 543 680 (623-737) 512 671 (613-729)
24–25 1063 664 (624-704) 532 653 (598-709) 531 675 (618-732)
26–27 1042 662 (622-702) 540 673 (616-729) 502 650 (594-707)
28–29 986 645 (605-685) 494 634 (578-690) 492 657 (599-715)
30–34 2405 698 (671-726) 1148 654 (616-691) 1257 745 (704-786)
35–39 2206 667 (640-695) 1089 654 (615-693) 1117 681 (641-721)
40–44 2352 633 (608-659) 1106 595 (560-630) 1246 672 (635-709)
45–49 2572 661 (635-686) 1285 658 (622-694) 1287 663 (627-699)
50–54 2921 695 (670 to 720) 1515 715 (679 to 751) 1406 675 (640-710)
55–59 2794 742 (715 to 769) 1473 782 (742 to 822) 1321 702 (664 to 740)
60–64 2720 794 (765 to 824) 1479 873 (828 to 917) 1241 718 (678 to 757)
65–69 2703 844 (813 to 876) 1470 941 (893 to 989) 1233 752 (710 to 794)
70–74 3058 932 (899 to 965) 1676 1058 (1007 to 1108) 1382 814 (772 to 857)
75–79 2395 1070 (1028 to 1113) 1280 1232 (1165 to 1299) 1115 930 (876 to 984)
80–84 1685 1186 (1130 to 1242) 873 1415 (1322 to 1508) 812 1010 (941 to 1080)
85–89 890 1157 (1082 to 1233) 429 1455 (1318 to 1592) 461 972 (884 to 1061)
90–94 372 1074 (965 to 1182) 147 1392 (1169 to 1616) 225 934 (812 to 1055)
95+ 86 853 (673 to 1032) 28 1349 (853 to 1846) 58 724 (539 to 910)
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For ‘secondary epilepsy’, the prevalence of epilepsy was higher in males than in females throughout life, but for ‘idiopathic epilepsy’, the prevalence was higher in females than in males up to about 60 years of age, after which the prevalence of ‘idiopathic epilepsy’ was higher in males than in females (table 2 and online supplemental figure 3).

Table 2. Point prevalence of ‘secondary epilepsy’ (with known underlying aetiology) and the point prevalence of ‘idiopathic epilepsy’ (with unknown underlying aetiology) per 100 000 persons by sex and age in Denmark on 31 December 2018.

Age groups ‘Secondary epilepsy’ (with known underlying aetiology) ‘Idiopathic epilepsy’ (with unknown underlying aetiology)
Males and females Males Females Males and females Males Females
N Prevalence (95% CI) N Prevalence (95% CI) N Prevalence (95% CI) N Prevalence (95% CI) N Prevalence (95% CI) N Prevalence (95% CI)
0–1 29 23 (15 to 32) 19 30 (16 to 43) 10 17 (6 to 27) 70 57 (43 to 70) 37 58 (40 to 77) 33 55 (36 to 74)
2–3 84 69 (54 to 84) 43 69 (48 to 89) 41 69 (48 to 90) 143 117 (98 to 136) 79 126 (98 to 154) 64 108 (81 to 134)
4–5 113 97 (79 to 114) 63 105 (79 to 131) 50 88 (63 to 112) 232 198 (173 to 224) 137 228 (190 to 267) 95 166 (133 to 200)
6–7 120 98 (81 to 116) 56 89 (66 to 113) 64 107 (81 to 133) 298 244 (216 to 271) 156 249 (210 to 288) 142 238 (199 to 277)
8–9 175 132 (113 to 152) 98 144 (116 to 173) 77 120 (93 to 146) 432 326 (296 to 357) 233 343 (299 to 387) 199 309 (266 to 352)
10–11 221 163 (141 to 184) 128 183 (151 to 215) 93 141 (112 to 169) 540 397 (364 to 431) 277 396 (350 to 443) 263 398 (350 to 446)
12–13 234 172 (150 to 195) 137 197 (164 to 230) 97 146 (117 to 175) 557 411 (377 to 445) 279 402 (355 to 449) 278 420 (370 to 469)
14–15 259 192 (169 to 215) 131 190 (157 to 222) 128 194 (161 to 228) 624 462 (426 to 499) 318 461 (410 to 511) 306 464 (413 to 516)
16–17 243 180 (157 to 202) 125 180 (149 to 212) 118 179 (147 to 211) 628 464 (428 to 500) 300 433 (384 to 482) 328 497 (444 to 551)
18–19 299 212 (188 to 236) 155 214 (181 to 248) 144 209 (175 to 243) 711 504 (467 to 541) 346 479 (428 to 529) 365 530 (476 to 584)
20–21 287 195 (172 to 217) 162 215 (182 to 247) 125 174 (144 to 205) 732 497 (461 to 533) 377 499 (449 to 549) 355 495 (443 to 546)
22–23 335 215 (192 to 238) 193 242 (208 to 276) 142 186 (156 to 217) 720 461 (428 to 495) 350 439 (393 to 484) 370 485 (436 to 534)
24–25 361 225 (202 to 249) 203 249 (215 to 283) 158 201 (170 to 232) 702 438 (406 to 471) 329 404 (360 to 448) 373 474 (426 to 522)
26–27 333 212 (189 to 234) 188 234 (201 to 268) 145 188 (157 to 218) 709 450 (417 to 483) 352 439 (393 to 484) 357 463 (415 to 510)
28–29 341 223 (199 to 247) 194 249 (214 to 284) 147 196 (165 to 228) 645 422 (390 to 455) 300 385 (342 to 429) 345 461 (412 to 509)
30–34 840 244 (227 to 260) 451 257 (233 to 281) 389 231 (208 to 253) 1565 455 (432 to 477) 697 397 (368 to 426) 868 514 (480 to 549)
35–39 872 264 (246 to 281) 502 301 (275 to 328) 370 226 (203 to 249) 1334 404 (382 to 425) 587 353 (324 to 381) 747 455 (423 to 488)
40–44 798 215 (200 to 230) 423 228 (206 to 249) 375 202 (182 to 223) 1554 419 (398 to 439) 683 368 (340 to 395) 871 470 (438 to 501)
45–49 806 207 (193 to 221) 468 240 (218 to 261) 338 174 (156 to 193) 1766 454 (433 to 475) 817 418 (390 to 447) 949 489 (458 to 520)
50–54 957 228 (213 to 242) 552 261 (239 to 282) 405 194 (175 to 213) 1964 467 (447 to 488) 963 455 (426 to 483) 1001 480 (451 to 510)
55–59 1054 280 (263 to 297) 621 330 (304 to 356) 433 230 (208 to 252) 1740 462 (440 to 484) 852 452 (422 to 483) 888 472 (441 to 503)
60–64 1125 329 (309 to 348) 653 385 (356 to 415) 472 273 (248 to 298) 1595 466 (443 to 489) 826 487 (454 to 520) 769 445 (413 to 476)
65–69 1236 386 (365 to 408) 722 462 (429 to 496) 514 314 (286 to 341) 1467 458 (435 to 482) 748 479 (445 to 513) 719 439 (407 to 471)
70–74 1498 456 (433 to 479) 870 549 (513 to 585) 628 370 (341 to 399) 1560 475 (452 to 499) 806 509 (474 to 544) 754 444 (413 to 476)
75–79 1167 522 (492 to 551) 645 621 (573 to 669) 522 435 (398 to 473) 1228 549 (518 to 579) 635 611 (564 to 659) 593 495 (455 to 534)
80–84 819 577 (537 to 616) 451 731 (664 to 798) 368 458 (411 to 505) 866 610 (569 to 650) 422 684 (619 to 749) 444 552 (501 to 604)
85–89 457 594 (540 to 649) 205 695 (600 to 790) 252 532 (466 to 597) 433 563 (510 to 616) 224 760 (661 to 859) 209 441 (381 to 500)
90–94 190 548 (471 to 626) 76 720 (559 to 881) 114 473 (386 to 560) 182 525 (449 to 601) 71 672 (517 to 828) 111 461 (375 to 546)
95+ 41 407 (282 to 531) 10 482 (184 to 780) 31 387 (251 to 523) 45 446 (316 to 576) 18 867 (468 to 1266) 27 337 (210 to 464)
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Figure 2 shows the age-specific prevalence (figure 2A) and proportion (figure 2B) of epilepsy by underlying aetiology (unknown and known by underlying cause). The causes of ‘secondary epilepsy’ varied considerably with age; up to 40–50 years of age, CNS malformations and perinatal conditions were the most common causes of ‘secondary epilepsy’, after 50 years, brain tumours and stroke became significant causes of ‘secondary epilepsy’, and in end-of-life, dementia contributed to the prevalence of ‘secondary epilepsy’. Traumatic brain injury and infections contributed to the prevalence of ‘secondary epilepsy’ throughout life.

Figure 2. Age-specific point prevalence (top) (A) and proportions (bottom) of ‘idiopathic epilepsy’ (with unknown aetiology) and ‘secondary epilepsy’ (with known aetiology) (B) by underlying cause of epilepsy in Denmark on 31 December 2018. Prevalence estimates and proportions are estimated for the age groups <10 years, 10–14, 15–19, …, 80–84, 85–89 and 90+ years. Only diagnoses given before the first epilepsy diagnosis are considered for preceding causes (not counting infections, traumatic brain injuries and strokes if occurring within the 14 days prior to first epilepsy diagnosis). In case of multiple diagnoses before onset of epilepsy, the one occurring closest to the first epilepsy diagnosis was chosen. ‘Idiopathic’ indicates epilepsy with unknown aetiology. ‘Malformations’ are grouped with ‘idiopathic’ for age groups 80 years and above due to small numbers. ‘Dementia’ is grouped with ‘stroke’ for age groups below 60 years due to small numbers. The Danish Hospital Register was established in 1977 and is not able to identify birth related and perinatal conditions as causes of epilepsy in persons over 41 years of age because these persons were born before the register was established. Numbers are given in online supplemental table 2.

Figure 2

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The GBD 2019 estimate of global prevalence of ‘total epilepsy’ for the year 2018 was 682 (95% CI 586 to 784) per 100 000 population, the estimated point prevalence of ‘secondary epilepsy’ was 359 (95% CI 324 to 397) per 100 000 population, and the estimated point prevalence of ‘idiopathic epilepsy’ was 324 (95% CI 249 to 404) per 100 000 population (figure 3).

Figure 3. Age-specific point prevalence of ‘secondary epilepsy’ (with known aetiology) and ‘idiopathic epilepsy’ (with unknown aetiology) in the 2019 Global Burden of Epilepsy Study for the year 2018. Prevalence estimates are estimated for the age groups <10 years, 10–14, 15–19, …, 80–84, 85–89 and 90+ years. Global epilepsy estimates are obtained from the Global Burden of Disease (Global Burden of Disease Collaborative Network. Global Burden of Disease Study 2019 (GBD 2019) Results. Seattle, United States: Institute for Health Metrics and Evaluation (IHME), 2020. Available from https://vizhub.healthdata.org/gbd-results/.

Figure 3

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The age-specific prevalence of ‘total epilepsy’ in Denmark was very similar to the age-specific, global, ‘total epilepsy’ prevalence estimates of epilepsy from GBD 2019. However, the Danish population prevalence of ‘idiopathic epilepsy’ was higher than the GBD 2019 global prevalence (p<0.0001), and the GBD 2019 global prevalence of ‘secondary epilepsy’ was higher than the prevalence in the Danish population (p<0.0001) (figures1 3).

Discussion

In this population-based study of more than five million people living in Denmark, we found prevalence estimates of ‘total epilepsy’ similar to the global prevalence estimates reported by the 2016 GBD Epilepsy Study2 and in updated GBD 2019 data for the year 2018.17 A little more than half of the ‘total epilepsy’ prevalence is accounted for by ‘secondary epilepsy’. The 2016 GBD Epilepsy Study estimates the burden of epilepsy but focuses on the burden of ‘idiopathic epilepsy’,2 and accordingly, the burden from ‘secondary epilepsy’ does not contribute to this estimate. The significant contribution of ‘secondary epilepsy’ to ‘total epilepsy’ prevalence shows why incorporating cases with ‘secondary epilepsy’ is important to capture the full burden in all persons with epilepsy. However, the heterogeneity of the underlying causes contributing to ‘secondary epilepsy’ varies substantially across the lifespan as shown in this study, and this variation in underlying conditions and overlap with other disorders is challenging to the methods used to estimate the burden of epilepsy and when comparing to other conditions that may themselves be causes of epilepsy, for example, stroke20 and traumatic brain injury.2 21 The GBD 2019 study of disease burden does not explicitly quantify the contribution from stroke, traumatic brain injury and CNS tumours to ‘secondary epilepsy’, even though these are important contributors, but does include years lost due to disability following neonatal conditions and meningitis, encephalitis and neurocysticercosis.22

The GBD Epilepsy Study defined an epilepsy case as someone with two or more seizures that were unprovoked by an immediate cause and who has had at least one epileptic seizure in the past 5 years regardless of ASM treatment.2 18 In contrast, the identification of a person with epilepsy in the Danish data was based on persons who had received a hospital-based epilepsy diagnosis and had filled a prescription for ASM in the past 10 years. In a previous study addressing various definitions of prevalent epilepsy using Danish Register data,7 increasing the ‘look back’ period from 5 to 10 years was associated with higher point prevalence estimates. Thus, it is reasonable to assume that the requirement of a seizure within the past 5 years (as in the GBD study) would provide lower prevalence estimates compared with data based on hospital diagnoses over a 10 year period in the Danish data. However, the Danish epilepsy definition required a prescription for ASM, which on the contrary would lead to lower prevalence estimates compared with the GBD study that did also include persons with epilepsy not using ASM.

The causes contributing to ‘secondary epilepsy’ do not only vary by sex and age, as shown in this study but may also vary by region and socioeconomic level.2 9 Differing age distribution of populations, together with differences in causes, might act together and influence the healthcare needs of a population. Estimates of the underlying aetiology of epilepsy might be of great importance for planning healthcare, including both potential preventive measures and treatment needs. Possible preventive measures will differ by cause and age group; immunisation for the prevention of infections, improved perinatal care for reducing perinatal causes and measures to prevent brain trauma in the younger and stroke in the elderly population.20 21

Estimates of prevalence according to aetiology are also important as the causes are associated with the prognosis of epilepsy; for example, ‘secondary epilepsy’ has a higher risk of developing into drug-resistant epilepsy compared with ‘idiopathic epilepsy’.23 Drug-resistant epilepsy has been found to have higher burden and cost of illness.8 Drug-resistant epilepsy, affecting up to 3 in 10 patients with epilepsy,24 often requires treatments in addition to preventive ASM, like epilepsy surgery. Prevalence estimates including ‘secondary epilepsy’ are therefore crucial for planning such specialised healthcare needs, for example, access to CT scans and MRI that will allow identification of underlying causes of ‘secondary epilepsy’.25

We used the terminology used by the 2016 GBD Epilepsy Study2 and the GBD 2019 Study17 to describe epilepsy (ie, ‘secondary’ and ‘idiopathic’), although these descriptions do not fully comply with the terminology suggested by the ILAE.3 The issue is further complicated by the terminology used in previous classifications proposed by the ILAE from 198926 that differs from the terminology used in the current ILAE classification.3 In the ILAE 1989 classification, epilepsy was defined as ‘symptomatic’ (or secondary) when the epilepsy was considered to be the consequence of a known or suspected disorder of the CNS, ‘idiopathic’ when there was no underlying cause other than a possible hereditary predisposition and ‘cryptogenic’ when the epilepsy was presumed to be symptomatic, but where the aetiology was not known.26 However, with the most recent ILAE classification,3 underlying aetiology of epilepsy is described separately, the term ‘idiopathic’ is reserved for the description of specific idiopathic generalised epilepsy syndromes (‘idiopathic generalised epilepsies’, ie, childhood absence epilepsy, juvenile absence epilepsy, juvenile myoclonic epilepsy and epilepsy with generalised tonic-clonic seizures alone), and the term ‘cryptogenic’ has been omitted.3 27 Thus, the terminology has changed over time along with the increased understanding of epilepsy, and ‘idiopathic’ and ‘secondary’ have different meanings in different settings. However, the distinction between epilepsy with and without underlying aetiology is helpful in epidemiological studies18 that aim to provide population-based estimates of the prevalence and burden of epilepsy.5 7

Limitations of the study

In this study, the identification of epilepsy patients was based on data from healthcare registers with limited clinical information, but the combination of epilepsy diagnoses and prescriptions for ASM has a high sensitivity and specificity for epilepsy.15 28 Persons who were not seen in a hospital setting and persons with epilepsy who were not prescribed ASM may have been missed, leading to underestimation of the epilepsy prevalence.7 29 The use of register-based information to identify persons with epilepsy comes with limitations and we previously estimated that the positive predictive value of an epilepsy diagnosis in the Danish National Hospital Register is about 80%.14 30 Thus, not all persons registered with epilepsy in the hospital register meet the diagnostic criteria for epilepsy. Misdiagnosis of epilepsy includes coding errors and inclusion of persons with a condition misdiagnosed as epilepsy. Including persons erroneously diagnosed with epilepsy will result in inflated estimates of epilepsy prevalence. We tried to address this potential bias of the prevalence estimate by combing the register diagnoses with prescription fill for ASM.7 15 The prevalence estimate from the Danish Register data was based on a conservative algorithm with specificity of 94%.15 28 Expanding the definition of epilepsy to include also cases diagnosed prior to 2009, who filled prescriptions for ASM within that last 10 years leading up to 31st December 2018 provided much higher prevalence estimates of epilepsy than when using the initial algorithm (online supplemental figure 1). Even higher prevalence estimates of epilepsy were found when further expanding the definition of epilepsy to include cases who in the last 10 years have filled at least two prescriptions for ASM with the indication "epilepsy". However, even though these prevalence estimates, based on expanded definitions of epilepsy, are higher than when using the initial algorith, these prevalence estimates of epilepsy are still within the variation of prevalence estimates reported in the meta-analysis,31 and even the highest prevalence estimate is similar to the most recent national data from the 2021 National Health Interview Survey in the USA.32

This study compares worldwide estimates of the prevalence of epilepsy from the GBD 2019 Study with epilepsy prevalence estimates from Denmark, that is, a resource-rich country in Northern Europe. Although overall prevalence estimates of ‘idiopathic epilepsy’ and ‘secondary epilepsy’ in Denmark were similar to the GBD 2019 Study estimates, differences in age and sex distribution of the populations and underlying risk factors of ‘secondary epilepsy’ (eg, CNS infections and traumatic brain injury) in other countries and parts of the world suggest that the underlying aspects of the epilepsy prevalence estimate including age and sex distribution as well as aetiology of ‘secondary epilepsy’ may not be generalisable to other regions with different demographics and access to healthcare.33 The analyses of the Danish health registry data may limit the generalisability of the prevalence estimates to other populations. Although the overall prevalence estimated from the Danish data was similar to the pooled point prevalence of active epilepsy from a systematic international meta-analysis of 63 studies (638 per 100 000 persons (95% CI 557 to 730)), this meta-analysis also identified significant heterogeneity between estimates.31 In addition to variation by age and sex, the prevalence also varied by country, income and underlying epilepsy aetiology,29 31 suggesting that the estimates derived from the Danish register data may not apply to other countries and regions with different healthcare systems and income levels.

We included infections, perinatal conditions, selected malformations, brain neoplasms, traumatic brain injury, stroke and dementia as potential underlying causes of ‘secondary epilepsy’ (online supplemental table 1). However, there are other underlying causes of epilepsy that we did not address specifically, for example, those with metabolic and immune aetiology.3 Inborn errors of metabolism in the young and metabolic insults in the elderly may be associated with epilepsy, but the proportion of ‘secondary epilepsy’ due to these causes is probably low.34 35 There are no population-based studies providing the incidence and prevalence of autoimmune epilepsy.36 The condition is only recently recognised and the condition may be underdiagnosed.37

Precise estimates of epilepsy prevalence, including identification of possible underlying aetiology identified in this study, are essential for healthcare planning and resource allocation as prevalence is a key parameter when estimating the burden associated with epilepsy.5 7 Current GBD 2019 methodology estimates total ‘secondary epilepsy’ but only explicitly calculates the proportion of secondary epilepsy attributable to neonatal conditions and infections (eg, malaria, encephalitis, meningitis and cysticercosis).22 Other major contributions to the prevalence of ‘secondary epilepsy’ including stroke, traumatic brain injury and brain tumours are not explicitly estimated. In future studies of the consequences of epilepsy, it would be advisable to explicitly include all types of epilepsy, including the underlying causes of ‘secondary epilepsy’ that are estimated as sequelae (consequences) of underlying diseases. Studies such as the one presented here based on Danish register data have limitations including the ability to identify underlying cases of epilepsy. Although the Danish Hospital Register was established in 1977 and captures a wide range of potential causes of epilepsy, the register is not able to identify birth related and perinatal conditions as causes of epilepsy in persons over 41 years of age because these persons were born before the register was established. This will lead to underestimation of ‘secondary epilepsy’, and accordingly, we found that the prevalence estimate of ‘secondary epilepsy’ in the Danish cohort was lower than the prevalence estimates of ‘secondary epilepsy in the GBD study’.

Conclusion

This nationwide, population-based register study and the GBD study provide the age-specific and sex-specific prevalence of epilepsy of similar overall magnitude and underscore that ‘secondary epilepsy’ constitutes a major proportion of persons living with epilepsy. Underlying causes of epilepsy such as perinatal conditions, infections, traumatic brain injury, brain tumours and stroke are potential targets for prevention and thus could potentially reduce the global burden of epilepsy.

Supplementary material

online supplemental file 1
jnnp-96-5-s001.docx (448KB, docx)
DOI: 10.1136/jnnp-2024-334547

The funders had no role in the design of the study, the data collection and analysis, the preparation of the manuscript and the decision to publish. The lead author (JC) affirms that the manuscript is an honest, accurate and transparent account of the study being reported; that no important aspects of the study have been omitted; and that any discrepancies from the study as originally planned have been explained.

Footnotes

Funding: This work was funded by the Novo Nordisk Foundation (NNF16OC0019126 and NNF22OC0075033), the Lundbeck Foundation (R400-2022-1205), the Central Denmark Region and the Danish Epilepsy Association. KMA received funding from the South-Eastern Norway Regional Health Authority.

Provenance and peer review: Not commissioned; externally peer reviewed.

Patient consent for publication: Not applicable.

Ethics approval: Register research projects based on pseudoanonymous data from Danish registers are exempted from ethics approval.

Data availability free text: Due to data protection laws, researchers need to apply to the Danish Health Data Authority and Statistics Denmark to have access to the underlying person-level data. All programming code and summary outcomes are available by contacting the corresponding author.

Author note: Transparency statement: The lead author (JC) affirms that the manuscript is an honest, accurate and transparent account of the study being reported; that no important aspects of the study have been omitted; and that any discrepancies from the study as originally planned have been explained.

Correction notice: Since this paper first published, the middle initial of the author Ryan Wagner has been updated.

Data availability statement

Data may be obtained from a third party and are not publicly available.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

online supplemental file 1
jnnp-96-5-s001.docx (448KB, docx)
DOI: 10.1136/jnnp-2024-334547

Data Availability Statement

Data may be obtained from a third party and are not publicly available.

Articles from Journal of Neurology, Neurosurgery, and Psychiatry are provided here courtesy of BMJ Publishing Group

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