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STUDY OF INTERICTAL E.E.G IN EPILEPSYM. Usha Rani

1. Associate Professor. Department of Physiology, Andhra Medical College. Visakhapatnam, Andhra Pradesh.

CORRESPONDING AUTHOR:Dr. M. Usha Rani,Associate Professor,Dept. of Physiology,Andhra Medical College,Visakhapatnam, Andhra Pradesh.E-mail: [email protected]

ABSTRACT: The present study includes hundred consecutive patients with a clinical diagnosis of Epilepsy attending the department of Neurology. All these patients were evaluated clinically and data recorded in proforma. Electroencephalogram (E.E.G) was recorded with 16-channel Electroencephalograph and the standards of normality are established. The Aim of this study is to study the pattern of abnormalities in interictal EEG in Epilepsy. The Objectives of this study are 1. To confirm the diagnosis of epilepsy. 2. To study the EEG abnormalities in relation to different types of epilepsy. 3. To know the distribution of epilepsy in relation to age and sex. 4. To study the effect of antiepileptic drugs on EEG. 5. To study the effect of hyperventilation on EEG. 6. To study the effect of photic stimulation on EEG. 7. To study the effect of interval between last seizure and EEG recording on EEG abnormalities. Epilepsy is a very common neurological illness accounting for 10% of patients attending the Neurology Outpatient department. It most commonly affects people in the first 3 decades of life. Males seem to be slightly more frequently affected. The diagnosis of epilepsy is most often on clinical grounds; EEG is helpful in supporting the diagnosis of epilepsy in difficult situation and classification of the seizures. Single awake interictal recording is helpful only in approximately 40% of the patients. Provocative procedures like hyperventilation and photic stimulation increase the diagnostic yield, particularly primarily generalized epilepsies. Sleep records to be more informative should be recorded in light sleep. Generalized seizures either primary or secondary seem to be the commonest type. In patients with a clinical diagnosis of generalized seizures, EEG may demonstrate focal abnormalities revealing the true nature of the seizure. Partial seizures may not reveal focal onset in the EEG making classification of the seizures interacts (on such basis) unreliable. The study shows limitations of interictal EEG indicating that ictal records should be done for better information. KEYWORDS: Electroencephalograph, Electroencephalogram, interictal, Epilepsy.

INTRODUCTION: Like other cells, nerve cells also show changes of electrical potential during activity which can be detected, amplified and recorded in the form of waves with an instrument called electroencephalograph. The EEG waves are due to current flow in the fluctuating dipoles formed on the dendrites of the cortical cells and cell bodies. Cortical dendrites are the forest of densely units placed in the superficial layers of the cerebral cortex. Dendrites are the sites of non-propagated hypopolarising and hyperpolarizing local potential changes in the excitatory and inhibitory axo-dendritic synapses. Dendrites are not the processes for conduction and do not propagate action potentials. Action potentials are propagated through the axonic terminals. When the excitatory axo-dendritic synapses are activated, current flow into and out in between

Journal of Evolution of Medical and Dental Sciences/ Volume 2/ Issue 16/ April 22, 2013 Page-2732

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the cell body and axo-dendritic endings, causing a wave-like potential fluctuation in the volume conductor. Thus the EEG is the potential fluctuation in volume conductor, but not the action potential and is conducted through the axon only. The present study includes the different interictal EEG patterns in epilepsy.

Electro encephalogram is the recordings from the surface of the brain and from the outer surface of the head demonstrate continuous electrical activity in the brain. Both intensity and the pattern of this electrical activity are determined to a great extent by the level of excitation of the brain resulting from sleep, wakefulness, and brain diseases such as epilepsy and even psychoses. The undulations in the recorded electrical potentials are called brain waves and the entire record is called an EEG. The main generates of EEG are thought to be post synaptic potentials, with the largest contribution arising from pyramidal cells in cortical layer 3. Dr.Hansberger an Austrian psychiatrist was the first to record electroencephalographs from humans.

METHODS AND MATERIALSEEG MACHINE

The above figure is a 16-channel EEG machine (Recorders and Medicare) which was used for our study.


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EEG ELECTRODES: These are gold plated discs (or cups) to be placed on the scalp with the help of EEG paste. These can be cleaned with normal saline or warm water or sodium hydroxide solution; alternatively, they can be kept overnight in any one of them. 10-20 ELECTRODES PLACEMENT SCHEME:

The standard set of electrodes for adults consists of 22 electrodes including one ground electrode. The international 10-20 system of electrode placement uses the distances between 3 bony landmarks of the skull-nasion(bridge nose),inion(occipital protuberance on the back of the head), and pre-auricular point-to generate a system of lines which run along and across the head and intersect at intervals of10% or 20% of their total length. The electrodes are named with a letter and a subscript. The letter denotes the underlying region—frontopolar(FP),frontal(F),Central(C),parietal(P),occipital(O),and auricular(A).The subscript is either the letter Z indicating zero or midline placement or a number indicating lateral placement, odd numbers on the left and even numbers on the right side of the head. Thus, Cz is placed at 50% of the nasion-inion distance in the midsagittal plane, while C3&C4 are 20% of this distance to the left and right of Cz.


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Different types of Normal E.E.G:


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RECORDING AN EEG:EEG recording is done in the following sequence:a) With open eyes.b) With closed eyes. c) hyperventilation for 3 minutes.d) Photic stimulation at frequencies 1, 3, 5, 6, 7, 10, 15, 20, 25 and 30/Sec.e) Sleep record.

OBSERVATIONSTable -1 Showing the Age Distribution :

AGE GROUP (IN YEARS) PATIENT PERCENTAGES1-10 38%11-20 41%21-30 14%31-40 3%41-50 2%51-60 2%

It is obvious that a large majority (91%) belong to first three decades of life.


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Table -2 Showing the Gender Distribution : TOTAL 100Males 60Females 40

60% of the patients in the study group are Males.

Table -3 Showing the different seizure types as per the ILEA Classification : SEIZURE TYPES NUMBER OF PERSONSGeneralized tonic/ clonic seizures

65

Absence seizures 4Myoclonic seizures 6Simple partial seizures 14Complex partial seizures 11TOTAL 100

Great majority (65%) are generalized tonic/ clonic (GTCS) in type followed by partial seizures (25%). Absences were recorded in 4% and myoclonic seizures in 6%.

Table -4 Showing the details of Antiepileptic Medication used : DRUG NUMBER OF PERSONSPhenytoin 27 (52.%)Carbamazepine 16 (30.8%)Sodium valproate 6 (11.5%)Clobazam 3 (5.8%)TOTAL 52

Of the 100 patients, 52 were already on antiepileptic drugs before they were enrolled in the study. The commonest antiepileptic drug used was Phenytoin (52%) followed by Carbamazepine (30.8%). Sodium valproate and clobazam were used by others.

Table -5 Showing the percentage of Abnormal EEGs : TOTAL No. of E.E.G.’s 100Normal 58%Abnormal 42%

Of the interictal records done in 100 patients, 42% disclosed abnormality, while the rest were normal.

Table -6 Showing the records with activation procedures : ACTIVATION PROCEDURE

NUMBER OF PERSONS UNDERWENT ACTIVATION PROCEDURE

NUMBER OF EEG +VE CASES

1. Sleep (Total) 16Drug induced sleep 9 5 (55.6%)Natural sleep 7 2 (28.6%)2. Hyperventilation 84 3 (35%)4. Photic stimulation 84 2 (2.4%)


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Abnormal EEG patterns seen only on hyperventilation 1%Abnormal EEG patterns seen only on photic stimulation 2%

84 Patients underwent activation procedures both hyperventilation and photic stimulation. In 3%, hyperventilation procedures precipitated abnormality in the form of spike and wave 3HZ/sec. Photic stimulation precipitated abnormality in 2% of patients in the form of generalized spike and wave discharge.

Table -7 Showing the effect of interval between last seizure and EEG recording on EEG abnormalities :

DURATION OF INTERVAL

TOTAL NUMBER OF EEG +VE CASES

One week duration 63 30 (47.6%)Two weeks duration 23 8 (34.8%)Beyond two weeks 14 4 (28.6%)

It is obvious that patients in whom the interval between the last seizures and EEG recording was short, showed more frequent abnormalities.

Table -8 Showing the types of Abnormalities in EEG : TYPES OF ABNORMALITY NUMBERSPECIFIC:

Spike and Wave 31 (73.8%)Spike 3 (7.1%)Sharp wave 1 (2.4%Sharp wave & slow wave 1 (2.4%

NON SPECIFIC:Slow wave 6 (14.3%)

TOTAL 42

Out of the total number of 100 records, 42 demonstrated an abnormality in the EEG. Of the 42 abnormal EEG records, 31 demonstrated an abnormality in the form of spike and wave discharges (73.8%), followed by spikes in 3 (7.1%) and 1 showed sharp wave (2.4%). Non-specific abnormality in the form of generalized slow waves were seen in 6 (14.3%).

Table -9 Showing distribution of Focal and Generalized Seizures:SEIZURE TYPE CLINICAL DIAGNOSIS EEG ABNORMALITIESGeneralized seizures

75 34 (45.33%)

Focal seizures 25 8 (32%)TOTAL 100 42

Of the 75 E.E.Gs recorded from patients with a clinical diagnosis of generalized seizures, 34 demonstrated abnormalities consistent with diagnosis of epilepsy. Only one out of 34 records showed focal EEG abnormalities over right hemisphere along with showing of back ground activity indicating an underlying structural lesion.


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Among the generalized EEG abnormalities (33 records), majority (87.9%) showed a generalized spike and wave discharge which were bilateral symmetrical and synchronous. Generalized sharp waves (3.03%) and sharp wave & slow wave (3.03%) were seen in a small percentage. In two of them the abnormality was only in the form of generalized slowing of background activity.

Among 8 abnormal records taken from 25 patients with partial seizures (both CPS & SPS) all showed only generalized abnormalities. 2 of the records of patients with a clinical diagnosis of SPS(simple partial seizures) showed generalized spikes in 2 patients and generalized slow waves in 2. Of the 4 patients with a clinical diagnosis of CPS(complex partial seizures) the EEG abnormalities were generalized in the form of slow waves in 2, and spike, spike & wave in one each.

DISCUSSION: Epilepsy is a condition which affects all ages but with highest incidence in extremes of life. Age specific incidence rates are consistently high in the younger age groups with highest rates occurring during first few months of life. The rates are lowest during adult years and again show an increasing incidence in elderly .

The present study also reflects a similar pattern of distribution with highest number in the first two decades of life. The absence of higher incidence in the later decades of life may be due to the sample bias, which included less number of patients of that age group (Table-1).

In all the studies reporting gender specific incidence rates of epilepsy, the incidence is higher in males compared to females. Most studies report male to female ratio varying between 1.1 and 1.7. The present study also shows higher occurrence of epilepsy in males (Table-2).

Most of the studies which follow the classification devised by ILEA(International League Against Epilepsy) taking both clinical features and EEG as their criteria, report a higher incidence of partial seizures (with or without secondary generalization. In the present study, where the initial diagnosis was only on clinical grounds, there was an over representation of generalized seizures which included both primary generalized and secondary generalized seizures. There is also a higher representation of simple-partial seizures in the present study compared to Western studies (Sander et.al.) because neurocysticercosis which is commonest cause of seizures in this area presents as simple partial focal motor seizures (Table-3).

The antiepileptic drugs used for control of seizures in different parts of the world very not only on the availability but also the affordability. For the same reason Phenytoin and carbamazepine are the most commonly prescribed antiepileptic drugs. The less frequent use of sodium valproate reflects not only the cost factor but also the lower incidence of primary generalized epilepsies in the present series for which it is often reserved (Table-4).

It is an established fact that only 30-40% of the patients with epilepsy show epileptiform discharges on a single wake record, although, of course the sensitivity will vary according to the population(Azmane, Marson, Znn). In the present study also, only 42% of interictal records showed some abnormality suggestive of a seizure disorder (Table-5).

When routine EEG record is normal, a sleep EEG is of great help. Epileptiform discharges are seen in a sleep EEG IN 70-80% of patients with clinical epilepsy. About 50% of patients with unhelpful wake records show definite interictal epileptiform activity during the sleep record(Gastaut et al). Most the additional yield occurs during light sleep or drowsiness rather than deep sleep. In the present study, while 35 out of 84 (i.e., 41.7%) wake records showed abnormalities, 7 out of 16 (43.8%) sleep records registered diagnostic abnormalities. The apparent lack of additional benefit of sleep study may be due to relatively a smaller sample size


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or the fact that they are deeply sedated often with a medication because the child was uncooperative (Table-6).

Hyperventilation is a very useful provocative procedure in children with absence seizures which establishes a diagnosis in 90% of the people. In this study, all the 3 children who had hyperventilation documented 3Hz spike and wave activity confirming the diagnosis (Table-6).

Photosensitivity is age dependent. In the peak age group of 7-19 years, 10% of patients with newly presenting seizures show unequivocal photosensitivity on EEG with female: male ratio of 3:2, while it rarely presents at other ages. Photosensitivity is usually seen in idiopathic generalized epilepsy. In the present study, of the patients who had a photic stimulation, only 2 demonstrated a diagnostic response (Table-6), the low yield is partly because of fewer number of primary generalized epilepsies who underwent EEGs in the second decade.

Out of 63 patients who underwent EEG recording within one week of least seizure, 47.6% showed diagnostic abnormalities. In those whose EEGs were recorded within two weeks of the last seizures. 34.8% showed some abnormality. Beyond 2 weeks, the yield was much less (28.6%). The Table-7 suggests, the shorter the interval between the last seizure and EEG recording, the higher are the changes of documenting an abnormality.

It is a well known fact patients with generalized seizures on clinical grounds show focal abnormalities on EEG proving that in fact they are secondary generalized seizures. In our study, only one out of 33 records showed such abnormality in a patient in whom focal onset was not recognized clinically.

If a focal lesion is deeply placed like in medial temporal lobe or sub frontal area which are inaccessible to surface electrodes, it is possible that EEG may not document focal abnormalities. It is likely that the same explanation holds good in our study in which all the 8 records done in patients with partial seizures recorded generalized abnormalities.

CONCLUSION:1. Epilepsy is a very common neurological illness accounting for 10% of patients attending

the Neurology outpatient department, King George Hospital. It most commonly affects people in the first 3 decades of life. Males seem to be slightly more frequently affected.

2. The diagnosis of epilepsy is most often on clinical grounds. EEG is helpful in supporting the diagnosis of epilepsy in difficult situation and classification of the seizures.

3. Single awake interictal recording is helpful only in approximately 40% of the patients. Provocative procedures like hyperventilation and photic stimulation increase the diagnostic yield, particularly primarily generalized epilepsies.

4. Sleep records to be more informative should be recorded in light sleep.5. Generalized seizures either primary or secondary seem to be the commonest type. 6. In patients with a clinical diagnosis of generalized seizures. EEG may demonstrate focal

abnormalities revealing the true nature of the seizure. 7. Partial seizures may not reveal focal onset in the EEG making classification of the

seizures interacts (on such basis) unreliable.8. The study shows limitations of interictal EEG indicating that ictal records should be

done for better information.

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