Prevalence and incidence of epilepsy in the Nordic countries

Review article
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    Abstract
    BACKGROUND

    BACKGROUND

    Updated knowledge on the prevalence of epilepsy is valuable for planning of health services to this large and complex patient group. Comprehensive epidemiological research on epilepsy has been undertaken, but because of variations in methodology, the results are difficult to compare. The objective of this article is to present evidence-based estimates of the prevalence and incidence of epilepsy in the Nordic countries.

    METHOD

    METHOD

    The article is based on a search in PubMed with the search terms epilepsy and epidemiology, combined with each of the Nordic countries separately.

    RESULTS

    RESULTS

    Altogether 38 original articles reported incidence and/or prevalence rates of epilepsy in a Nordic country. Four studies had investigated the prevalence of active epilepsy in all age groups, with results ranging from 3.4 to 7.6 per 1 000 inhabitants. Only two studies had investigated the incidence of epilepsy in a prospective material that included all age groups. The reported incidence amounted to 33 and 34 per 100 000 person-years respectively. A prospective study that only included adults reported an incidence of 56 per 100 000 person-years.

    INTERPRETATION

    INTERPRETATION

    We estimate that approximately 0.6 % of the population of the Nordic countries have active epilepsy, i.e. approximately 30 000 persons in Norway. Epilepsy is thus one of the most common neurological disorders. The incidence data are more uncertain, but we may reasonably assume that 30 – 60 new cases occur per 100 000 person-years.

    Article

    Epilepsy is one of the most common neurological disorders and strikes people of all ages (1). An epilepsy diagnosis will often imply far more than recurring unprovoked epileptic seizures. The diagnosis may have an impact on the choice of education and profession, family, social contact and mental health (2, 3). As a result, this patient group has complex needs for health and social services. Updated knowledge on the prevalence of epilepsy is important for the planning of such programmes.

    It has been estimated that 30 000 – 40 000 people in Norway have this diagnosis (4, 5), but this estimate is based on epidemiological studies conducted outside the Nordic countries. However, geographic and socioeconomic conditions may influence the prevalence of epilepsy (6, 7). Thus, epidemiological data must be extrapolated with caution. The Nordic countries are not only in geographical proximity to each other, they are also fairly similar in terms of their cultures, living standards and economies. We have therefore chosen to review these countries as one entity. A good overview of Nordic epidemiological studies of epilepsy has so far been unavailable. The objective of this article is to report what we currently consider to be the best estimates of incidence and prevalence rates for epilepsy in the Nordic countries.

    Method

    Method

    We have undertaken a review of original articles based on searches in PubMed up to and including 1 January 2015. We searched for the terms epilepsy and epidemiology with the aid of the Boolean operator AND in combination with each of the Nordic countries separately. The search returned 141 hits for Norway, 262 for Sweden, 199 for Finland (including the Åland Islands, for which there were no hits), 213 for Denmark (including six for Greenland and one for the Faroe Islands) and 29 for Iceland.

    The search results were reviewed in light of the title and abstract. In total, we found 38 original articles in English, Norwegian or Danish in which prevalence and/or incidence rates were reported. We found no articles in Finnish, Icelandic or Swedish.

    An additional search with the terms «epilepsy AND (incidence OR prevalence)» for each of the Nordic countries returned 163 hits for Norway, 296 for Sweden, 247 for Finland (including the Åland Islands), 243 for Denmark (including six for Greenland and one for the Faroe islands) and 30 for Iceland. The additional search did not identify any further relevant original articles. The search results were cross-checked against reference lists in key English-language review articles and commentaries (6) – (9).

    Prevalence

    Prevalence

    Prevalence is defined as the proportion of individuals in a given population who have the diagnosis in question at a given point in time (the prevalence day). Prevalence is commonly reported as a percentage or thousandth part of the total population in the relevant area.

    Nordic studies that have investigated the prevalence of epilepsy are shown in Table 1 (10) – (38). Eight of these studies include all age groups (10) – (13, 26) – (29), but only four of them were restricted to active epilepsy (10) – (13). The procedure for identification of patients was identical in the four studies of active epilepsy, with hospital-based searches and retrospective review of patient records. Unfortunately, they used varying definitions of epilepsy.

    Table 1

    Original articles on prevalence of epilepsy in the Nordic countries. The studies are sorted by the age groups included and by whether the studies were restricted to patients suffering from active epilepsy

    First author (reference)

    Year

    Country

    Age (years)

    Prevalence per 1 000

    Only active epilepsy

    Number of cases

    Gudmundsson (10)

    1966

    Iceland

    All

    3.4

    Yes

    635

    Joensen (11)

    1986

    Faroes

    All

    7.6

    Yes

    333

    Olafsson (12)

    1999

    Iceland

    All

    4.8

    Yes

    428

    Syvertsen (13)

    2015

    Norway

    All

    6.5

    Yes

    1 771

    Breivik (14)

    2008

    Norway

    0 – 14

    3.8

    Yes

    114

    Sillanpää (15)

    1973

    Finland

    0 – 15

    3.2

    Yes

    348

    Eriksson (16)

    1997

    Finland

    0 – 15

    3.9

    Yes

    329

    Sidenvall (17)

    1996

    Sweden

    0 – 16

    4.2

    Yes

    155

    Larsson (18)

    2006

    Sweden

    0 – 16

    3.4

    Yes

    205

    Brorson (19)

    1967

    Sweden

    0 – 19

    3.5

    Yes

    195

    Becker-Christensen (20)

    1998

    Greenland

    0 – 19

    4.1

    Yes

    35

    Waaler (21)

    2000

    Norway

    6 – 12

    5.1

    Yes

    198

    Keränen (22)

    1989

    Finland

    > 15

    6.3

    Yes

    1 233

    Forsgren (23)

    1992

    Sweden

    > 17

    5.5

    Yes

    713

    Svendsen (24)

    2007

    Norway

    31, 41, 46, 61, 76

    8.2

    Yes

    90

    Brodtkorb (25)

    2008

    Norway

    18 – 65

    6.7

    Yes

    12

    de Graaf (26)

    1974

    Norway

    All

    3.5

    No

    749

    Juul-Jensen (27)

    1975

    Denmark

    All

    6.9

    No

    1 675

    Christensen (28)

    2007

    Denmark

    All

    5.7

    No

    28 303¹

    Bolin (29)

    2014

    Sweden

    All

    8.8

    No

    81 606¹

    Surén (30)

    2013

    Norway

    0 – 12

    6.6

    No

    5 269¹

    Olesen (31)

    1996

    Greenland

    0 – 15

    5.9

    No

    15

    Blichfeldt (32)

    2004

    Greenland

    0 – 15

    3.4

    No

    43

    Li (33)

    2014

    Sweden

    2 – 17

    9.0

    No

    9 309¹

    Sillanpää (34)

    1992

    Finland

    4 – 15

    6.8

    No

    104

    Baldin (35)

    2012

    Iceland

    7 – 15

    7.7

    No

    75

    Wagner (36)

    1983

    Denmark

    16 – 66

    4.3

    No

    1 054

    Bakken (37)

    2014

    Norway

    18 – 82

    9.0

    No

    33 571¹

    Löfgren (38)

    2009

    Finland

    39

    19

    No

    222

    [i]

    [i] ¹   Registry-based

    One of these studies had been conducted in Iceland more than 50 years ago. Unsurprisingly, it found a significantly lower prevalence rate (3.4/1 000) than the three others (10). This finding is in line with an older study from Northern Norway (prevalence 3.5/1 000), which included all age groups, but was not restricted to active epilepsy (26).

    The three remaining studies reported prevalence rates of 4.8/1 000 (12), 6.5/1 000 (13) and 7.6/1 000 (11). The study with the lowest prevalence rate stemmed from Iceland. Here, untreated patients who had been seizure-free for more than one year were excluded (12). The highest prevalence rate was found in the Faroe Islands, despite exclusion of all patients who had been seizure-free for more than five years, irrespective of treatment. As an explanation, the author noted that the patient records had frequently been written by personnel who had no particular interest in epilepsy, and that this may have resulted in overdiagnosis. The final study stemmed from Norway. In it, active epilepsy was defined as ongoing treatment and/or at least one seizure over the last five years (13). To date, this remains the only study that has used the most recent guidelines for epidemiological epilepsy research from the International League Against Epilepsy (ILAE) and their comprehensive proposals for amendment of the terminology from 2010 (39, 40).

    Eight studies investigated the prevalence of active epilepsy in children (14) – (21). The prevalence rates range from 3.2 to 5.1 per 1 000 inhabitants. When considering that the prevalence of epilepsy increases with age (12), it is unsurprising that the study reporting the highest prevalence rates did not include the youngest children (the age group 0 – 5 years) (21).

    Prevalence figures for active epilepsy in adults have been reported in four Nordic studies (22) – (25), two of which stem from Norway (24, 25). We believe there is reason to assume that both of these have a high sensitivity and specificity with regard to the identification of patients, since they used population-based surveys combined with individual interviews and hospital searches/retrospective record reviews respectively. The cohorts investigated, however, were small. Their prevalence rates of active epilepsy amounted to 6.7 (24) and 8.2 per 1 000 (25) respectively. The two other Nordic studies were based on larger populations, with prevalence rates found to be 5.5 (23) and 6.3 per 1 000 (22) respectively. In the latter study, all included patients were clinically examined by the authors, and we may thus assume that it has high specificity.

    Incidence

    Incidence

    Incidence is defined as the proportion of individuals in a certain population who are diagnosed with the condition is question within a given period of time. Incidence is commonly reported as the number of new cases per 100 000 persons per year. Nordic studies that have investigated the incidence of epilepsy are shown in Table 2 (10, 11, 14, 15, 18, 22, 26) – (28, 41) – (49). Because such studies are often difficult to undertake in practice, there are fewer studies of incidence than of prevalence. To ensure the highest possible sensitivity and specificity, incidence studies ought to be prospective in nature (9). Moreover, it should be noted whether the study also includes single, unprovoked seizures or whether it is restricted to the ILAE definition of epilepsy (40, 50). This is especially relevant for incidence studies, since many of them register all those who are recorded with a first-time epileptic seizure and report this figure, without waiting to see whether the patient suffers a second seizure. The definitions of epilepsy used in the studies we are referring to in this article are summarised in Table 3 (10) – (38, 41) – (49).

    Table 2

    Original articles on incidence of epilepsy in the Nordic countries. The articles are sorted by design and age of the included patients

    First author (reference)

    Year

    Country

    Age (years)

    Incidence per 100 000 person-years

    Design

    Olafsson (41)

    2005

    Iceland

    All

    33

    Prospective

    Adelöw (42)

    2009

    Sweden

    All

    34

    Prospective 1

    Sidenvall (43)

    1993

    Sweden

    0 – 15

    73

    Prospective 1

    Braathen (44)

    1995

    Sweden

    0 – 16

    53

    Prospective

    Forsgren (45)

    1996

    Sweden

    > 17

    56

    Prospective 1

    Gudmundsson (10)

    1966

    Iceland

    All

    26

    Retrospective

    de Graaf (26)

    1974

    Norway

    All

    33

    Retrospective

    Juul-Jensen (27)

    1975

    Denmark

    All

    30

    Retrospective 1

    Joensen (11)

    1986

    Faroes

    All

    43

    Retrospective

    Olafsson (46)

    1996

    Iceland

    All

    47

    Retrospective

    Sillanpää (47)

    2006

    Finland

    All

    53

    Retrospective

    Christensen (28)

    2007

    Denmark

    All

    69

    Retrospective

    Breivik (14)

    2008

    Norway

    0 – 14

    47

    Retrospective

    Sillanpää (15)

    1973

    Finland

    0 – 15

    25

    Retrospective

    Blom (48)

    1978

    Sweden

    0 – 15

    82

    Retrospective

    Larsson (18)

    2006

    Sweden

    0 – 16

    40

    Retrospective

    Brorson (49)

    1987

    Sweden

    0 – 19

    50

    Retrospective

    Keränen (22)

    1989

    Finland

    > 15

    24

    Retrospective

    [i]

    [i] ¹   Including single unprovoked seizures

    Table 3

    Definitions of epilepsy and active epilepsy used in the included original studies of prevalence and/or incidence

    Definition of epilepsy

    Reference

    > 1 unprovoked epileptic seizure

    27, 42, 43, 45

    > 2 unprovoked epileptic seizures

    19, 36, 49

    > 2 unprovoked epileptic seizures over a period of > 24 hours

    11, 13, 14, 17, 18, 21 – 23, 25, 41, 46

    > 3 unprovoked epileptic seizures over a period of > 1 week

    15

    Chronic organic brain disorder with recurring epileptic seizures

    26, 34

    Recurring epileptic seizures over the last three years

    48

    Recurring unprovoked epileptic seizures

    12, 16, 24

    Recurring unprovoked seizures of cerebral origin

    44

    Paroxysmal and transitory disturbance of the brain function that develops suddenly, stops spontaneously and tends to recur

    10

    > 2 unprovoked epileptic seizures/1 seizure and finding of epileptic activity by EEG/approved reimbursement of costs for antiepileptic drugs

    38

    Registered with an ICD-10 code for epilepsy

    28 – 30, 33, 37

    Approved reimbursement of costs for antiepileptic drugs

    47

    Affirmative answer to a question about epilepsy in a questionnaire

    35

    Not stated

    20, 31, 32

    Definition of active epilepsy

    Reference

    > 1 seizure over the last year and/or antiepileptic medication

    12

    > 1 seizure over the last 2 years and/or antiepileptic medication

    20

    > 1 seizure over the last 3 years, irrespective of medication

    19

    > 1 seizure over the last 4 years, irrespective of medication

    15, 21

    > 1 seizure over the last 5 years, irrespective of medication

    11, 14, 18, 25

    > 1 seizure over the last 5 years and/or antiepileptic medication

    10, 13, 16, 17, 22 – 24

    Only five prospective incidence studies of epilepsy (41) – (45) have been undertaken in the Nordic countries, of which only two include all age groups. The study with the largest population basis, that included single, unprovoked seizures (42), found an annual incidence of 34/100 000. A prospective study from Iceland found an incidence of 33/100 000. Here, single unprovoked seizures had been excluded (41).

    A smaller prospective study among children found an annual incidence of epilepsy of 53/100 000 (44). A prospective study of adult patients found an incidence of 56/100 000. Here, single unprovoked seizures had been included (45).

    Discussion

    Discussion

    In 2011, ILAE published new guidelines for epidemiological research on epilepsy (39), because substantially differing methodologies had made comparisons of studies difficult (6, 7). Epidemiological studies commonly define epilepsy as a minimum of two unprovoked epileptic seizures during a period of more than 24 hours (39, 51). This remained ILAE’s definition until 2014, when it was expanded to also include a single unprovoked seizure if it is part of an epilepsy syndrome or if the risk of recurring seizures is estimated to exceed 60 % (50).

    Studies that include single unprovoked and/or acute symptomatic seizures will naturally result in a higher prevalence of epilepsy than studies that have deemed such seizures as not qualifying for the diagnosis.

    According to the ILAE definition, active epilepsy is a condition involving ongoing treatment with antiepileptic medication and/or at least one epileptic seizure over a specified period of time, commonly the last 2 – 5 years. It is recommended that epidemiological studies restrict their reporting to active epilepsy (39, 51). However, there is still room for some discrepancy – the shorter the period that has elapsed since the last seizure, the more patients will be excluded from the group with active epilepsy. For those who have been seizure-free for more than ten years and have gone without antiepileptic medication for more than five years, the ILAE has introduced the concept of «resolved epilepsy» (50). This means that their epilepsy is in remission, not that the disease has been cured. This concept has not yet been applied in epidemiological studies.

    Another cause of substantially different findings is the method used for identification of patients. No method is totally watertight, and recommending a single specific procedure is therefore difficult. Studies that use a population-based questionnaire may have limitations caused by low response rates and selection bias. Moreover, such studies may for practical reasons be restricted to smaller populations or delimited age groups, as are door-to-door surveys.

    Studies based on registered diagnostic codes may provide for larger study populations, although there is a risk of overestimating the prevalence of the disease. In addition, it is difficult to restrict such studies to active epilepsy. A study from Denmark showed that approximately 20 % of all patients who were registered with an epilepsy code did not fulfil ILAE’s definition of epilepsy (52). In our recent study from Buskerud county we made similar findings (13). A retrospective review of patient records to validate the diagnosis would naturally increase the specificity, but this is conditional on the correctness of the information in the records and that the registrations have been made by competent personnel.

    Age variation in the study population is another problem. Only a handful of studies from the Nordic countries include all age groups. Studies that are limited to a specific age group can only with difficulty be extrapolated to the entire population, since it has previously been proven that there is an elevated prevalence of epilepsy in the oldest section of the population (12). This trend is also evident in the material we have collected for this article. In studies that have investigated the prevalence of active epilepsy in children and adolescents (0 – 19 years), the prevalence rate varies from 3.2 to 4.1 per 1 000 (14) – (20), while the prevalence of active epilepsy among adults is higher (5.5 – 8.2/1 000) (22) – (25). In the Nordic countries, no isolated studies of prevalence among the elderly have been made, nor has this been investigated in the adolescent group.

    As regards incidence, there are too few studies in our material and their results are too divergent to permit any conclusions regarding the age distribution. It is known, however, that the incidence curve for epilepsy is double-humped, with a peak early in life and a new increase in the oldest age groups (53).

    Unfortunately, some stigma remains attached to the diagnosis of epilepsy (54). A wish to keep this diagnosis secret on the part of some people in certain communities or age groups may cause underreporting. This source of error may perhaps be greater in older studies, and perhaps also in studies undertaken in rural areas.

    Conclusion

    Conclusion

    Epidemiological research on epilepsy is not an exact science. All the methods in use include sources of error. An adequate description of the methodology and its limitations, as well as a shared definition of epilepsy, is essential in all such research efforts. Studies that are based on updated guidelines and classifications of epilepsy remain scarce – not only in the Nordic countries, but also globally.

    On the basis of this literature review, it is reasonable to assume that the real prevalence of active epilepsy in the Nordic countries amounts to approximately 6 per 1 000, i.e. that approximately 30 000 individuals in Norway suffer from active epilepsy. In light of the few prospective studies that have been undertaken, the annual incidence can be estimated to 30 – 60 new cases per 100 000 inhabitants.

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