|Year : 2013 | Volume
| Issue : 2 | Page : 48-54
Interictal electroencephalography in patients with epilepsy in northwestern Nigeria
Lukman F Owolabi1, Sale Shehu2, Shakirah D Owolabi2, Muhammad Umar2
1 Department of Medicine, Aminu Kano Teaching Hospital, Bayero University, Kano, Nigeria
2 Department of Psychiatry, Aminu Kano Teaching Hospital, Bayero University, Kano, Nigeria
|Date of Web Publication||23-May-2014|
Lukman F Owolabi
Department of Medicine, Aminu Kano Teaching Hospital, Bayero University, P.M.B 3452, Kano
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Background: Different studies have reported various frequencies of electroencephalographic (EEG) abnormalities in patients with epilepsy. There is, however, a paucity of data on EEG in Nigeria; hence, the need for this study.
Objective: The objective of the following study was to evaluate interictal EEG pattern in patients with epilepsy in Kano, Northwestern Nigeria.
Subjects and Methods: A cross-sectional study involving the analysis of EEGs of consecutive patients with clinical diagnosis of epilepsy over a 5 year period at two diagnostic centers in Kano, northwestern Nigeria. Information on socio-demographic and seizure characteristics was obtained. The recordings from patients were obtained using the standard methods and interpreted by two of the investigators. The International Federation of Societies for Electroencephalography and Clinical Neurophysiology definition of interictal epileptiform discharges (interictal epileptiform activity [IEA]) was adopted for the study.
Result: Out of 2219 patients referred for EEG at the two diagnostic centers during the study period, 2041 (92%) patients had a clinical diagnosis of epilepsy. Their age ranged between 0.04 and 75 years, with a mean age of 22.8 ± 14.9 years. Overall, EEG was abnormal in 1178 (57.7%) and 919 (45.1%) had an epileptiform pattern. A total of 1691 patients had hyperventilation (HV) and response to HV was unremarkable in 1286 (76%) of them. Out of 405 who had remarkable changes on HV; 302 had increase in epileptiform discharges, whereas 103 had abnormal discharges only on HV. Seventeen (89.5%) out of 19 patients with 3 Hz spike and wave complexes had activation by HV. The most common IEA were focal spike/sharp and wave and generalized spike/sharp and slow waves. More antiepileptic drug (AED) naive patients (678) than those that were on AED (500) had EEG abnormality and the difference was statistically significant, P < 0.001.
Conclusion: The study showed that the occurrence of interictal EEG abnormality in patients with epilepsy was about 58%. The proportion of interictal epileptiform discharges was 45% in routine first EEG studies. Among those with epileptiform activity, generalized sharp and wave complexes and focal sharp and slow wave complexes were the most common findings.
Keywords: Electroencephalographic, epilepsy, interictal
|How to cite this article:|
Owolabi LF, Shehu S, Owolabi SD, Umar M. Interictal electroencephalography in patients with epilepsy in northwestern Nigeria. Ann Nigerian Med 2013;7:48-54
|How to cite this URL:|
Owolabi LF, Shehu S, Owolabi SD, Umar M. Interictal electroencephalography in patients with epilepsy in northwestern Nigeria. Ann Nigerian Med [serial online] 2013 [cited 2020 Mar 31];7:48-54. Available from: http://www.anmjournal.com/text.asp?2013/7/2/48/133096
| Introduction|| |
Epilepsy is a major neurologic disease with considerable personal and social impact. It is one of the most common diseases of the nervous system that bring Africans to the hospital. ,, Apart from the patient's history and neurological examination, electroencephalography is still the most important tool in the evaluation of patients living with epilepsy. It helps to confirm the diagnosis of epilepsy and assists in classification.
However, there are pitfalls in the interpretation of electroencephalographic (EEGs). Most EEG patterns resembling that of epilepsy can also be caused by a wide variety of different neurologic diseases. Certain neurologic disorders may cause more than one type of EEG pattern. In individuals living with epilepsy, intermittent EEG changes including interictal epileptiform discharges can be infrequent and may not appear during the relatively brief period of routine EEG recording. Moreover, a large population based study reported a 0.5-4% incidence of EEG abnormality among normal individuals.  Other studies have also reported different frequencies of EEG abnormalities in patients with epilepsy. , However, there is paucity of data on the interictal EEG pattern of patients with epilepsy in Northwestern Nigeria, hence, the need for this study.
The study was aimed at evaluating interictal EEG patterns in patients with epilepsy in Kano, northwestern Nigeria.
| Subjects and Methods|| |
This was a cross-sectional study involving all patients with clinically diagnosed epilepsy. The patients were recruited consecutively over a 5 year period from two diagnostic centers, namely Abdullahi Bayero diagnostic center of the Aminu Kano Teaching Hospital and Fitrah diagnostic center, Kano. These two centers, which were the only facilities with EEG machines, attract referral from Kano as well as from the other states in northwestern Nigeria.
The patient's biodata and seizure characteristics including age at onset of epilepsy, classification of the epileptic syndrome, and medications were obtained from each patient using a standardized proforma.
Routine, scalp, awake, interictal EEG recording was performed on all the patients following a detailed explanation of the procedure to them. Cap electrodes were applied with collodion, according to the 10-20 system, with linked mandibular references. The data was obtained using 16-channel Grass and Medtronic EEG machines. Common average referential, longitudinal and transverse bipolar montages were used in all examinations. EEG recording was filtered with I Hz high pass, 35 Hz low pass and 60 Hz notch filters at paper speed of 30 mm/s. During the recordings, the subjects were instructed to remain relaxed but alert.
During eyes-open recordings, patients were told to fixate their gaze on a particular point on the wall and try to inhibit ocular movements. The behavioral state of the patient during the EEG recording was noted and spontaneous drowsiness and sleep encouraged. Standard activation procedure of hyperventilation (HV) was performed, except in cases where HV was contraindicated. Some of these contraindications included cardiac disease, respiratory disease, sickle disease, or situations where patient did not cooperate. Routine EEG recording was performed for 30-40 min and subsequently 5 min of HV. Only one of the EEGs had accessory for photic stimulation, thus, a few patients could have photic stimulation.
In most cases, two of the investigators independently interpreted the recordings from the patients and inter-rater agreement was determined. During interpretation, the following factors were documented; age at time of EEG, waking and sleeping background, interictal epileptiform activity (IEA) if any, location of focal IEA when identified, and the response of IEA to activation procedures. The International Federation of Societies for Electroencephalography and Clinical Neurophysiology definition of IEA was adopted in the study. 
Analysis was performed using Statistical Package for Social Sciences" (SPSS) program for Windows version 16.0 (SPSS Inc., Chicago, IL, USA). Mean and standard deviation were used to describe quantitative variables. Bivariate analysis was carried out using Pearson Chi-square or Fisher exact test for categorical variables. P < 0.05 was considered as statistically significant. The study was approved by the ethical committee of Aminu Kano Teaching Hospital, Kano.
| Results|| |
Out of 2219 patients referred for EEG at the two diagnostic centers during the study period, 2041 (92%) patients comprising 1249 males and 792 females were referred based on the clinical diagnosis of epilepsy. The other reasons for referral for EEG included headache, attention deficit hyperactive disorder, psychosomatic illness and mental retardation, accounting for the remaining 8%. Their age ranged between 0.04 and 75 years, with a mean age of 22.8 ± 14.9 years. Age and gender distribution of the study population are shown in [Table 1].
Prior to EEG, 833 (40.8%) were clinically classified to have generalized seizure type, whereas 210 (10.3%) were not clinically classified [Table 2]. 1237 (65%) patients were already on antiepileptic drugs (AEDs).
Inter-rater reliability between the interpreters (Cohen's kappa) was 0.74.
Overall, EEG was abnormal in 1178 (57.7%) and 919 (45.1%) had epileptiform pattern. When EEG findings in the elderly (age ≥60 years) were compared with those less than 60 years of age, 40 out of 54 (74.1%) of the former and 1138 out of 1987 (57.3%) of the latter had EEG abnormalities and the difference was statistically significant (P = 0.01) [Table 3]. Similarly, greater frequency of EEG abnormality was recorded in patients that were AED naïve who had EEG abnormality compared with those that were already on AED (678 (84.3%) and 500 (40.4%) respectively, and the difference was statistically significant, P lt; 0.0001 [Table 3].
Out of the 1691 patients that had HV, response to HV was unremarkable in 1286 (76%) patients. Out of 405 patients who had remarkable changes, 302 had increase in epileptiform discharges while 102 had abnormal discharges only on HV. Physiologic slowing occurred in 348 (20.6%) patients. Seventeen (89.5%) out of 19 patients with 3 Hz spike and wave complexes had activation by HV. A total of 18 patients had photic stimulation, but none of them had abnormal discharges in response to photic stimulation.
The distribution of IEA across age category of the patients and cerebral origin of the IEA are shown in [Table 4] and [Table 5], respectively. [Figure 1], [Figure 2], [Figure 3] show three of the EEGs.
|Figure 1: Bilateral temporal and parietal discharges in a patient with complex partial seizure|
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|Figure 2: Paroxysms of generalized spike and slow wave complexes with burst suppression in a child with generalized epilepsy|
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|Figure 3: 3 Hz spike and wave complexes in a patient with absence seizure|
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| Discussion|| |
In this study, over 90% of the patients were referred due to clinical suspicion of epilepsy. Other reasons for referral included headache,  psychosomatic illness, dementia, brain tumor and encephalopathy among others. This referral pattern is in conformity with reports elsewhere.  However, it is pertinent to note that the number of patients seen during the study period is a small fraction of those with clinically diagnosed epilepsy in the study area.
Diagnosis of epilepsy is based mainly on clinical history and examination. EEG constitutes the single most valuable laboratory test in the evaluation of patients with epilepsy. It is a safe, non-invasive procedure for evaluation of electrophysiological state of patients with epilepsy in the ictal or interictal period; but for reasons of cost of the investigation and scarcity of the facility, only a few of the patients would have the opportunity of EEG assessment.
Male preponderance in this study agreed with the earlier studies. , This finding may be a reflection of predominance of male patients with epilepsy over their female counterpart in the region. 
In the current study, about 58% of the EEG recordings were abnormal and IEA was found in 45%. This figure was similar to that reported in the previous studies. , Epileptiform activity is specific, but not sensitive, for diagnosis of epilepsy.  EEG has relatively low sensitivity in epilepsy, ranging between 25 and 56% respectively. Specificity is better, but again variable at 78-98%.  These wide ranges can be explained partly by diverse methods of case selection and differences in clinical requirements for diagnosis of epilepsy in population studies used to determine the specificity and sensitivity of EEG. 
The finding of a normal interictal EEG in a patient who has unquestioned seizures frequently baffles clinicians. In general, normal EEG does not exclude epilepsy, which ultimately may have to be diagnosed on clinical grounds.  A routine EEG examination is essentially a sampling procedure, thus, interictal abnormality may occur infrequently and the sampling time may be so short that no abnormality appears during EEG recording. Apart from this sampling effect, age of the patient, type of seizure, frequency of seizure, and medication effects were some of the factors advanced for variation in positive record in EEG. 
Our study found a high rate of interictal epileptic activity in elderly patients with epilepsy. This finding contrasted the report of Drury and Bedoun, in which a low rate of IEA detection was reported in the elderly compared with younger patients with epilepsy.  The number of elderly patients enrolled in our study was small, this may account for the discrepancy between our study and the previous study.
The standard practice is that if the recording performed in the basal waking state disclosed no abnormalities or only non-specific abnormalities, an attempt must be made to activate epileptiform discharge. In our study, about 24% of the patients had HV-activated IEA. This finding, which conformed to another report elsewhere,  was much more pronounced in patients with absence seizure in the current study. HV most consistently activates 3 Hz spike and slow wave complexes and often produces a clinical absence. In a study conducted by Dalby, activation was reported in 50% of patients with absence seizure with or without convulsions. 
Only few patients had photic stimulation and none of them had activation of IEA. Even though the number of patients that had this activation procedure was so small that no valid conclusion could be drawn, the finding further corroborates the report of Danesi and Oni. 
Of the abnormal tracings in the study, epileptiform activities in form of focal spike/sharp and wave as well as generalized spike/sharp and slow waves were recorded in the majority of the patients. This finding is consistent with many other reports from similar studies. ,,, Focal epileptiform discharges represent fast, synchronous potentials in a large number of neurons over a discrete area of the brain. They can occur as interictal activity and represent an area of cortical irritability that may be predisposed to producing epileptic seizures. However, interictal discharges are not wholly reliable for definitive diagnosis of epilepsy or origin of seizure.  Conversely, generalized epileptiform discharges occur synchronously throughout the entire brain. They are strongly suggestive of generalized epilepsy. 
Low frequency of periodic lateralizing epileptiform discharges (PLEDS) in our study is in keeping with reports from other studies.  Majority of the patients with PLEDS in the current study had focal neurologic deficit. PLEDS usually overlie an area of acute structural brain lesion such as herpes encephalitis and cerebral infarct. Regarding nonspecific paroxysmal discharges, they often occur in form of bursts of rhythmic bisynchronous slow wave (delta or theta activity), which can occur normally during drowsiness in infants and young children.
In conformity with reports from other studies, more of the IEA were recorded in the left hemisphere.  The reason for this differential "epileptogenicity" of the left hemisphere is not well known. 
It is, however, worthy of note that the findings reported in the current study were findings on first EEGs conducted on the patients. In a study conducted on 308 patients by Marsan and Zivin in which serial EEGs were carried out on patients with epilepsy, an abnormal record was obtained on first EEG examination in 56% of patients. However, in subsequent recordings, 26% had abnormal records and only 18% of the patients had consistently abnormal recordings.  Thus, a repeat of EEG in our patients with normal EEG could have further yielded abnormality in some patients.
Our study found significant difference in the frequency of abnormality among AED naïve patients when compared with their counterparts on AED. This finding, which is in agreement with previous studies, further lend support to the long-held view that AED suppresses both ictal and IEA. 
It is noteworthy that our study had some limitations; lack of more sensitive EEG machines with more channels, and lack of photic stimulation accessories as well as inability to use other provocation tests such as sleep, sleep deprivation and drugs were some of the limitations of this study.
In spite of these shortcomings, selection of patients from the various states in northwestern Nigeria, fairly large sample size and the fact that the EEGs were independently interpreted by 2 electroencephalographers with fairly high inter-rater reliability, which is the degree of agreement between the two investigators in respect of their reports, are some of the strengths of this study.
Diagnosis of epilepsy is often not straightforward and misdiagnosis is not uncommon.  A detailed and reliable account of the event by an eyewitness is the most important part of the diagnostic evaluation, but this may not necessarily be available. EEG can provide support for the diagnosis of epilepsy and also assists in classifying the underlying epileptic syndrome. Therefore, in order to optimize the use of EEG in the evaluation of patients with suspected epilepsy, the clinician must understand the strengths and weaknesses of EEG, specifically as they relate to the diagnosis of seizures and epilepsy.
| Conclusion|| |
Based on the above study it can be concluded that the occurrence of interictal EEG abnormality was about 57%. The proportion of epileptiform pattern was 45% on routine first EEG studies among patients with epilepsy in northwestern Nigeria. Among those with epileptiform EEG discharges, generalized sharp and wave complexes and focal sharp and wave complexes were the most common findings.
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[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]