title: The Correlation between Electroencephalography Amplitude and Interictal Abnormalities: Audit study.
abstract: OBJECTIVE
  The aim of this study was to establish the relationship between background amplitude and interictal abnormalities in routine electroencephalography (EEG).
  
  
  METHODS
  This retrospective audit was conducted between July 2006 and December 2009 at the Department of Clinical Physiology at Sultan Qaboos University Hospital (SQUH) in Muscat, Oman. A total of 1,718 electroencephalograms (EEGs) were reviewed. All EEGs were from patients who had been referred due to epilepsy, syncope or headaches. EEGs were divided into four groups based on their amplitude: group one ≤20 μV; group two 21-35 μV; group three 36-50 μV, and group four >50 μV. Interictal abnormalities were defined as epileptiform discharges with or without associated slow waves. Abnormalities were identified during periods of resting, hyperventilation and photic stimulation in each group.
  
  
  RESULTS
  The mean age ± standard deviation of the patients was 27 ± 12.5 years. Of the 1,718 EEGs, 542 (31.5%) were abnormal. Interictal abnormalities increased with amplitude in all four categories and demonstrated a significant association (P <0.05). A total of 56 EEGs (3.3%) had amplitudes that were ≤20 μV and none of these showed interictal epileptiform abnormalities.
  
  
  CONCLUSIONS
  EEG amplitude is an important factor in determining the presence of interictal epileptiform abnormalities in routine EEGs. This should be taken into account when investigating patients for epilepsy. A strong argument is made for considering long-term EEG monitoring in order to identify unexplained seizures which may be secondary to epilepsy. It is recommended that all tertiary institutions provide EEG telemetry services.
authors.affiliation: Department of Clinical Physiology, Sultan Qaboos University Hospital;
authors.forename: Sami F
authors.initials: SF
authors.lastname: Al-Rawas
authors.affiliation: Department of Clinical Physiology, Sultan Qaboos University Hospital;
authors.forename: Rajesh P
authors.initials: RP
authors.lastname: Poothrikovil
authors.affiliation: Department of Mathematics & Statistics, College of Science, Sultan Qaboos University, Muscat, Oman;
authors.forename: Khidir M
authors.initials: KM
authors.lastname: Abdelbasit
authors.affiliation: Department of Neurology, Institute of Epileptology, King's College Hospital, London, UK.
authors.forename: Robert S
authors.initials: RS
authors.lastname: Delamont
keywords: Electroencephalography, abnormalities; Epilepsy