Validation of epilepsy diagnoses in the Danish National Hospital Register
Introduction
Population-based health registries have provided valuable insight into the epidemiology of epilepsy (Hauser et al., 1993, Jallon et al., 1999, Annegers et al., 1984), although population-based registries are rarely developed or designed for research purposes (Christensen et al., 2005a, Kwan and Brodie, 2000). The use of these registries in research often requires information about the clinical criteria used for registering people with epilepsy (Sorensen et al., 1996).
In 1993, the International League Against Epilepsy (ILAE) proposed epidemiologic guidelines in which an epileptic seizure was defined as “a clinical manifestation presumed to result from an abnormal and excessive discharge of a set of neurons in the brain. The clinical manifestation consists of sudden and transitory abnormal phenomena, which may include alteration of consciousness, motor, sensory, autonomic or psychic events, perceived by the patient or an observer” (Commission, ILAE, 1993). For epidemiological purposes it was recommended that an effort should be made to classify seizure type based predominantly on clinical criteria (Commission, ILAE, 1993).
An epilepsy diagnosis was defined as “A condition characterized by recurrent (two or more) epileptic seizures, unprovoked by any immediate identified cause. Multiple seizures occurring in a 24-h period are considered a single event. An episode of status epilepticus is considered a single event. Individuals who have had only febrile seizures or only neonatal seizures as herein defined are excluded from this category” (Commission, ILAE, 1993). The use of the ILAE Classification of epilepsy syndrome was encouraged, however, it was acknowledged that in many settings a proper classification of epileptic syndromes could not always be obtained (Commission, ILAE, 1993).
The ILAE criteria for seizures (Commission, ILAE, 1981), and epilepsies and epileptic syndromes (Commission, ILAE, 1989) have been used in epidemiological studies on epilepsy (Christensen et al., 2005a). However, the ILAE terminology and classification have not been unequivocally used in population-based registries, such as the ones based on the WHO's ICD. Limited evidence indicates that epilepsy classification based on ICD varies greatly compared with ILAE criteria, and that correspondence between the two classifications depends on factors such as number of hospital admissions and use of antiepileptic drugs (Ehrenstein et al., 2006, Holden et al., 2005a, Holden et al., 2005b, Tomson and Forsgren, 2003).
Our aim was to validate epilepsy diagnoses according to ILAE criteria from hospital admissions and outpatient contacts in the Danish National Hospital Register based on WHO's ICD, taking epilepsy syndrome into account.
Section snippets
Danish Civil Registration System
We used data from the Danish Civil Registration System (Pedersen et al., 2006) to identify all persons born in Denmark and alive and resident in Denmark from 1 January 1977 to 31 December 2002 (N = 6,543,341 persons). All live-born children with a permanent address in Denmark are assigned a unique personal identification number (CPR number), which is stored in the Danish Civil Registration System together with information on vital status, emigration, and place of living. The CPR number is used as
Validation process
Medical records were collected from 57 departments at 41 hospitals. Medical records were missing for 12 (6%) patients. In the remaining 188 medical records, we found information on neuroimaging in 90 (48%) patients (MRI in 25 (28%) of patients and CT in 65 (72%) of patients) and EEG in 150 (80%) patients.
Epilepsy diagnosis
Overall, the epilepsy diagnosis was confirmed in 153 patients (definite epilepsy, n = 131, probable epilepsy, n = 22), corresponding to a positive predictive value of 81% (95% CI: 75–87%) (the
Validity of the epilepsy diagnosis
We found that 81% of persons registered with epilepsy in the Danish National Hospital Register fulfilled the ILAE criteria for an epilepsy diagnosis. Although care should be taken when extending these finding to other populations, the results are quite similar to what was found in a hospital-based register in Stockholm, Sweden covering the years 1980–1989 (Tomson, 2000). Among 171 patients admitted for inpatient care with a diagnosis of epilepsy, the epilepsy diagnosis was found to be correct
Acknowledgements
We are indebted to Birgitte Jensen Hanner for enduring the hard work of collecting and systematizing information from the medical records. The study could not have been performed without help from the hospital secretarial staff at the involved departments. We would like to thank Carsten Bødker Pedersen and Marianne Gørtz Pedersen for help with the retrieval of epilepsy cases from the Danish National Hospital Register and with the calculation of proportion patients registered with the individual
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