122 . Malouf R , Brust JC . Hypoglycemia: causes, neurological manifestations, and outcome . Ann Neurol 1985 ; 17 ( 5 ): 421 \200\223 30 . 123 . Dizon AM , Kowalyk S , Hoogwerf BJ . Neuroglycopenic and other symptoms in patients w ith insulinomas . Am J Med 1999 ; 106 ( 3 ): 307 \200\223 10 . 100 . Unterberger I , Trinka E . Diagnosis and treatment of paroxysmal dyskinesias revisited . Ther Adv Neurol Disord 2008 ; 1 ( 2 ): 4 \200\223 11 . 101 . Bruno MK , Hallett M , Gwinn-Hardy K , Sorensen B , Considine E , Tucker S , et al . Clinical ev aluation of idiopathic paroxysmal kinesigenic dyskinesia: new diagnostic criteria . Neurolo gy 2004 ; 63 ( 12 ): 2280 \200\223 7 . 124 . Martinovic Z . Fictitious epilepsy in Munchausen syndrome by proxy: family psychodynamics . Seizure 1995 ; 4 ( 2 ): 129 \200\223 34 . 125 . Meadow R . Fictiti ous epilepsy . Lancet 1984 ; 1 : 25 \200\223 8 . CHAPTER 9 The Electroencephalogram in the Investigation of Epilepsy Stephan U. Schuele , Adriana C. B ermeo , and Samden D. Lhatoo History of the electroencephalogram Typical for revolutionary innovations, the electroencep halogram (EEG) conquered the medical world within a decade. In 1929, Hans Berger reported o n the \201 rst recordings of the human EEG ( 1 ). His discovery became widely accepted th rough the corroborating studies of Adrian and Matthews published 5 years later ( 2 ). In 19 35, Foerster and Altenburger performed the \201 rst intraopera- tive electrocortigogram ( ECoG) from the human brain and by 1939, Pen \201 eld reported the \201 rst case of invas ive monitoring for epilepsy surgery ( 3 ). diameters, MRI compatible platinum electrodes, ampli \201 ers allow- ing 256 and more chan nel inputs, and sampling rates of 1 kHz and more are now widely available for the invasive recording of epilepsy patients. In the pre-digital era, simultanous recording and correlation of EEG and video was a major challenge. The \201 rst closed-circuit tele- vision (CCTV) for simultanous recording of E EG and video was reported in 1966 and similar analog recording systems were used for the ne xt three decades ( 9 ). Since the 1990s, digital recording equipment has almost completely replaced analog systems. Long-term monitoring is becoming increasingly available not only a s a tool for presurgical evaluation, but also as the diagnostic method of choice for patien ts with treatment resistant seizures ( 10 ). In the last few years, central servers and rem ote access within and from outside the hospital has permitted an increasing number of inten sive care units to offer continuous EEG monitoring for (subclinical) seizure detection and management, further expand- ing the role of EEG in neurological practice. The early years of clinical EEG were dominated by two schools, one led by Frederick A. Gibb s and his wife Erna L. Gibbs in Boston, US, the other by Herbert H. Jasper in Montreal, Can ada. Gibbs and Gibbs had the concept of epilepsy as a \200\230dysrhythmia \200\231 of the brain. They postulated that epilepsies could be classi \201 ed based on pathog- nomonic in terictal EEG \201 ndings. They deserve credit for describ- ing the classical 3Hz spike an d wave pattern seen with absence epilepsy but also erroneously assumed that rhythmic midtem poral theta known as \200\230psychomotor variant \200\231 is predictive of psychomotor epi lepsy ( 4 ). Gibbs and Gibbs emphasized the use of the ear refer- ence which is well suited to visualize the typical aspects of general- ized 3 Hz spike and wave discharges but also led them to mislocalize patients with temporal lobe epilepsy due to the proximity of the ab normal activity to the ear reference. Jasper and colleagues pre- ferred the use of a bipola r montage which is more sensitive for focal epileptiform discharges. The Montreal school pi oneered the localization of interictal and ictal activity for surgical evaluation in collab oration with Wilder Pen \201 eld ( 5 , 6 ). Physiological basis of the EEG The main generators of the scalp EEG are excitatory and inhi bitory postsynaptic potentials. EEG surface activity depends on the sum- mation of the loca l \201 eld potentials which is enhanced in perpen- dicular oriented apical dendrites of p yramidal neurons close to the surface. In addition to orientation and location, the size of the neuronal population necessary to show synchronized activity is estimated at 6 cm 2 or more to become visible on the surface EEG ( 11 , 12 ) (Fig. 9.1 ). The cellular correlate of interictal epileptiform activity is a par- oxysmal depolarization shift (PDS) which results in the initiation of a high-frequency burst of action potentials followed by a pro- longed hyperpolarization. The cardinal features of interictal epi- lept iform abnormalities visible on surface EEG mimic the cellular processes and consist of a pa roxysmal interruption of the existing background activity, an initial sharp wave (80 \200 \223200 ms duration) or spike (40 \200\22380 ms) de \202 ection corresponding to a summati on of postsynaptic potentials. This is followed by a prominent slow wave, which re \202 ec ts the cellular hyperpolarization. The transition from single PDS generating interictal spi kes to a full electrographic In the pre-magnetic resonance imaging (MRI) era, epilepsy sur- gery almost always required an invasive evaluation which was ini- tially limited to intraoperative recordings using sub dural electrodes. In the late 1960s, the \201 rst implantable subdural strip electrodes w ere introduced in North America allowing seizure recording out- side the operating room ( 7 ). At the same time in France, Tailarach and Bancaud introduced a stereotactic method to a ccurately place intracerebral depth electrodes, which could be left in place for sev- eral days and allowed a systematic exploration of seizure onset and propagation ( 8 ). Over the subsequent half a century, these tech- niques have further evolved and thinner electrode pl ates and depth oxford textbook of epilepsy and epileptic seizures 96 \227\206 As con \201 rmatory testing for patients with a clinical exam apnea I + + test consistent with \200\230brain death \200\231. \210\222 \210\222 + + Inpatient continuous (video) EEG monitoring: + + II,III + + \227\206 Epilepsy monitoring unit (usually associated with medication \210\222 \210\222 tapering) \210\222 \210\222 \226 Diagnostic evaluation of patients with intractable seizures for IV more than 1 year. \210\222 \210\222 + + \226 Rapid treatment changes in patients with poorly controlled + + seizures. V + + + + \226 Presurgical evaluation of potential surgical candidates (non- invasive and invasive). VI Intensive care unit: \226 Detection of subclinical seizures in patient with altered mental A B status: Fig. 9.1 Generation of extracellular voltage \201 elds from graded synaptic activity. A) Excitatory postsynaptic potential (EPSP) at the apical dendrite is associated with a \202 ow of positive ions into the cell (an active current sink) and an extracellular negative \201 eld. A passive current source at the level of the cell body and basal dendrites is as sociated with an extracellular positive \201 eld. B) EPSP on the proximal apical dendrite at the level of cortical layer IV is associated with an active current sink and an extrace llular negative \201 eld. A passive current source at the distal apical dendrite in layer s II and III is associated with an extracellular positive \201 eld. With permission from Ebersole , 2003 ( 13 ). after convulsive status epilepticus. after an acute brain insult (trauma, hemorrhage, anoxic injury). \226 Monitoring of patients in burst suppression. \227\206 Procedures: \226 Carotid endarterectomy, aneurysm surgery, balloon occlusion. \226 Wada test. seizure is associated with the gradual loss of the afterhyperpolariza- tion and the progres sive appearance of an increasingly prolonged depolarization shift which leads to synchroniz ed recruitment of additional neuronal population ( 14 ). Scalp EEG Recording techniques Placement: EEG electrodes are placed according to the 10 \200\22320 system. This allows proportional coverage of individual head sizes using the 10t h and 20th percentile of a particular distance de \201 ned by anatomical landmarks (nasion to inion and pre-auricular to pre-auricular point) ( 15 ). Indication for EEG studies The indications for an EEG depend on the study type and clinical context: Routine outpatient EEG: Many centers use the new nomenclature: T7 (instead of T3) and P7 (instead of T5) for the le ft temporoparietal electrodes and T8 (instead of T4) and P8 (instead of T6) for the right t emporopari- etal electrodes ( 16 ). Paste is mostly used for 20 \200\22330 minute EEG recor dings. Collodium is paramount to assure persistent quality for multihour or -day recordings . \227\206 To determine the type of epilepsy (focal vs. generalized onset sei- zures, speci \201 c epilepsy syndrome). \227\206 To prognosticate risk of seizure recurrence (after a \201 rst unpro- voked seizure or prior to weaning medications). \227\206 For the differential diagnosis of paroxysmal events which could be epileptic or n on-epileptic (e.g. syncope, movement disorders, migraine, non-physiological events). Standard setting: the standard low frequency \201 lter for routine EEG is 1 Hz and the hi gh-frequency \201 lter 70 Hz. A notch \201 lter (60 Hz in the US and 50 Hz in Europe) c an be used to reduce elec- trical interference. Sensitivity is initially set at 7 V/mm. Electrode impedance should be at least 100 ohms and no more than 5 kohms. Ambulatory EEG: \227\206 To increase the yield to detect interictal epileptiform abnormali- ties in patients with normal routine EEG. Montages: digital recording offers the ability to change the mon- tage setting during revie w. Most laboratories have a set of standard montages which should at minimum include a long itudinal bipo- lar (double banana), referential, and transverse bipolar montage. \227\206 For the diagnostic evaluation of patients with events which are frequent enough and do not require medication tapering. \227\206 To determine seizure frequency in patients who are unable to report their events (subclinical events, patients unaware of unwit- nessed clinical events) to guide treatment and lifestyle restrictions. Localization EEG channels have two inputs, the second one being either a refer- ence (refer ential montage) or the next electrode in line (bipolar montage). Inpatient EEG: \227\206 For evaluation of patients with unexplained altered mental status. Rules of localization: Fig. 9.2 (two channel bipolar montage) depicts that a de \202 ectio n can be the result of a single generator over \227\206 For prognostication for patients in coma. CHAPTER 9 the electroencephalogram in the investigation of epilepsy 97 \210\222 + cortex which is not covered by an electrode (e.g. mesial and basal cortex). Given the minim um amount of cortex required to be detectable on surface EEG, one can argue that the descri ption \200\230focal \200\231 is not appropriate and \200\230regional \200\231 is a better term for surface abnormalities. Even the absence of EEG changes during a seizure does not exclude the diagnosis of epilepsy: only 20 \200\22330 % of focal seizures without loss of c onsciousness have an EEG correlate on scalp EEG ( 17 ). On the other hand, in seizures with altered respon- siveness ictal EEG changes are seen in 85 \200\22395 % of cases and the la ck thereof is highly suspicious of a non-epileptic phenomenon ( 18 ). Sampling: a routine E EG captures only 20 \200\22330 minutes and spo- radic or state dependent interictal epilept iform abnormalities may easily be missed. In serial EEG recordings, 50 % of patients with e pilepsy were found to have epileptiform abnormalities on the \201 rst EEG, 84 % by the th ird EEG, and in 92 % by the fourth EEG ( 19 ). A similar sampling phenomenon can be seen du ring continuous EEG monitoring in the intensive care unit during which seizures in patients with non-convulsive status are mostly captured within the \201 rst 1 \200\2232 hours of recording. However, 12 % of patients with non- convulsive seizures will not be detected unt il the second monitor- ing day ( 20 ). A A I-II A-B + \210\222 B B II-III B-C \210\222 + C C Fig. 9.2 Polarity convention: left column \200\224 the negative (upward) de \202 ection i n channel 1 (input A minus input B) is caused either by a relative negativity over input A or a relative positivity over input B. Right column \200\224 the positive (downward) de \202 ection in channel 1 is caused either by a relative positivity over input A or a relati ve negativity over input B. Phase reversal channel 1 and 2: left column \200\224 the posit ive phase reversal over B is either due to a positive generator over B or two negative gene rators over A and C. Right column \200\224 the negative phase reversal over B is either du e to a negative generator over B or two positive generators over A and C. In clinical pract ice, we are usually dealing with a single and mostly negative generator, i.e. the scenario indicated by column 2 with a negative generator over B. Methods to increase the yield of a routine EEG \227\206 Sleep deprivation: the major bene \201 t of sleep deprivation is prob- ably that it ensures that the patient falls asleep during the record- ing the next morning although the sleep deprivation by itself may have some additional effect. Recording of sleep enhance s the yield of the EEG to capture epileptiform abnormalities par- ticularly in young patien ts and patients with generalized epilepsy syndromes ( 21 ). the phase reversal or two generators of opposite polarity at the end of the 3-electrode cha in. Biologically most plausible is a single, usually negative generator (Fig. 9.2 example o n the right). The presence or absence of a phase reversal determines the max- imum of the electrical acti vity not only in bipolar but also referen- tial montage con \201 gurations (Table 9.1 ). \227\206 Hyperventilation: the physiological response to hyperventil ation is a rhythmic p olymorphic delta slowing (not to be overinterpreted as evidence of a generalized abnormalit y!). Hyperventilation can bring out epileptiform activity in 10 % of generalized epilepsies and rarely in focal epilepsies. It may induce absence seizures in 50 % of childhood absenc e epilepsies. Choice of reference: ideally, the chosen reference is not involved in the activity to be ma pped. In that case, all channels will point in one direction (upward de \202 ection when r ecording a negative gen- erator). If the reference is involved and close to the maximum (e. g. temporal sharp wave in an ipsilateral ear reference), some de \202 ec- tions will go up and other down in relation to the reference and it can be dif \201 cult to visualize the maximum and the associated volt- age \201 eld. \227\206 Photic stimulation: \226 Photic driving is composed of rhythmic activity elicited over the posterior regions often at a stimulation frequency close to normal posterior background. Photic driving is a non-speci \201 c \201 nding without clinical relevance. Phase reversal: it is important to note that phase reversals are not equivalent with abnorm alities. They are simply indicators of the \201 eld maximum seen with normal and abnormal electrical activity. \226 Photoparoxysmal response: spike-and-waves or polyspike-and- waves triggered by photi c stimulation. A type I response \200\224 non-sustained and limited to posterior head regi on \200\224 is in more than half of patients not associated with epilepsy. A type II \200 \224 outlasting the stimulus and generalized in distribution \200\224 is Limitations of scalp EEG Resolution: scalp recordings cannot detect electrical synchroniza- tion of less than 6 cm 2 of cortex, and have often poor resolution of Table 9.1 Rules of localization Montage type Phase reversal Conclusion Common clinical situation, assuming a single generator No Bipolar Maximum activity is located at the end of the chain Generalized slowing, maximum (negativity) frontal Yes Maximum activity is located over the phase reversal Posterior alpha activity, maximum P7/P8 (Fig. 9.3 A) Spike/sharp wave, maximum over the pha se reversal No Referential Reference electrode is either maximum or minimum Maximum is indicated by the largest upward de \202 ection Yes Reference electrode is neither maximum nor minimum Maximum is indicated by the largest upward de \202 ection oxford textbook of epilepsy and epileptic seizures 98 A B C Fig. 9.3 Normal EEG activity. A) Mu rhythm: 7 \200\22312 Hz, comb-like activity, attenuated by thought or movements. Seen in 20 % of normal individuals. B) Lambda waves: bi-or tripha sic (check mark appearance) elicited by looking at a patterned design in a well-illuminated room in this case a computer screen. C) Sleep stage II: sleep spindles, K complex and post erior sharp transients of sleep (POSTS) are seen. CHAPTER 9 the electroencephalogram in the investigation of epilepsy 99 Interictal abnormalities Non-epileptiform abnormalities abnormal and a typical \201 nding for genetic epilepsies in par- ticular juvenile myoclon ic epilepsy (JME). \226 Photomyoclonic response: brief repetitive spikes generated by contraction of muscles of the face or scalp that are driven by the photic stimulation; seen in normal and epilept ic patients. Slowing \227\206 Generalized slowing indicates a diffuse dysfunction (Fig. 9.5 A) which correlates clinically with an encephalopathy. Normal EEG Normal activity and a number of physiological variants can be confused with abno rmal activity and may lead to overinterpreta- tion of the EEG (22; Table 9.2 ). \226 Intermittent generalized slowing is typical for a mild encepha- lopathy, continuous slowing (usually with loss of posterior back- ground activity) indicates a moderate or seve re encephalopathy. \226 Other patterns seen in comatose patients include background suppression (amplitude < 10 V), burst suppression and alpha, theta or delta coma consistent of diffuse, uniform, u nreactive activity in that particular frequency range. Table 9.2 Normal EEG activity and benign variants Normal activity \227\206 Focal slowing indicates focal dysfunction. Awake \226 Intermittent focal slowing, particularly over the temporal region can be normal in d rowsiness, young individuals or patients over 40 years. Alpha rhythm 8 \200\22313 Hz, typically 15 \200\22345 V in adults Beta rhythm > 13 Hz and < 40 Hz, typically < 20 V Mu rhythm (Fig. 9.3 A) 20 % of normal individuals. 7 \200\22312 Hz, comb-like, attenuated by thought or movements \226 Continuous focal slowing is a reliable indicator for a focal structural abnormality (Fig. 9.5 B). Theta > 4 Hz and < 8 Hz. More pronounced in drowsiness Normal intermittent temporal slowing (2 \200\2236 waves, theta range) over age 40 years \226 Intermittent rhythmic focal slowing may represent an electro- graphic seizure equiva lent particularly if associated with a clinical correlate. Posterior slow waves of youth Most common in children and adolescents. Seen in 15 % of young adults Asymmetries Alpha rhythm asymmetries up to 50 % occur normally with the right side often hi gher than the left. Lambda waves (Fig. 9.3 B) Bi-or triphasic (check mark appearance) elicited by looking at a patterned design in a well -illuminated room \227\206 Increase: increased amplitude is seen after burr holes or craniot- Sleep (Fig. 9.3 AC) omies known as breach rhythm (Fig. 9.5 B). Vertex waves (stage I) Diphasic, sharp transients with negative polarity \227\206 Decrease: reduction in amplitude may occur because of cortex injury or \202 uid collection between the cortex surface and the elec- trodes (e.g. subdural hemorrhage). Positive occipital sharp transient of sleep (POSTS) Sleep spindles (stage II) Sigmoid activity, 0.5 \200\2232 s in adults, 10 \200\22314 Hz, 20 \200\22340 V \227\206 Frequency asymmetry: hemispheric background frequencies should be within 1 Hz; fr equency asymmetry is a non speci \201 c \201 nding. K complex Diphasic Benign variants ( 22 ) Generalized periodic discharges This includes a heterogeneous group of abnormalities: Subclinical rhythmic electrographic discharge in adults (SREDA) Rare. Rhythmic sharply contoured 5 \200\2237 Hz theta waves, widespread, bilaterally synchr onous, posterior head regions \227\206 Generalized (epileptiform) discharges after an anoxic injury Midline theta Rhythmic sinusoidal, archiform theta over FZ/CZ often associated with myoclonic seizures. Frontal arousal rhythm Trains of monophasic 7 \200\22310 Hz activity bifrontal \227\206 Triphasic waves typical for a metabolic encephalopathy (Fig. 14 and 6 positive bursts \200\230Ctenoids \200\231 \200\224 comb (Greek). 0.5 \200\2231 s duration of 14 and 6 Hz positive spikes 9.5 A). \227\206 Periodic discharges recurring every 1 \200\2232 seconds typical for Small sharp spikes (SSS, Fig. 9.4 A) Low amplitude (50 V) and duration (<50 ms), anterior to midtemporal patients with Creutzfeldt \200\223Jakob disease. 6 Hz spike and wave \200\230Phantom spike and wave \200\231-spike <25 V compared to slow wave Epileptiform abnormalities \227\206 Focal spikes and sharp waves are the hallmark of epilepsies with Wicket spikes (Fig. 9.4 B) Monophasic, arciform 6 \200\22311 Hz, isolated or in trains, anterior to mid temporal. 1 \200\2233 % of normal individuals focal seizures (Fig. 9.6 A). \226 Epileptiform abnormalities are seen in 80 \200\22390 % of epileptic patients with fo cal seizures and are detected in less than 1 \200\2232 % of the normal population. Rhythmic midtemporal discharges (RMTD) \200\230Psychomotor variant \200\231. Drowsiness. Midtemporal train of rhythmic theta acti vity, lasting up to 10 s, variable morphology, often with a notched appearance. 0.5 % \226 Generalized or multifocal spikes or sharp waves, generalized spike and wave complexe s and polyspikes are reliable indica- tors of epilepsies with generalized seizures (Fig. 9. 6 B). Physiological artifacts: ocular, cardiac, myogenic, glossokinetic, and others. oxford textbook of epilepsy and epileptic seizures 100 A Fig. 9.4 Benign variants. A) Small sharp spikes: low amplitude (<50 V) and duration (<50 ms), anterior to midtemporal. B) Wicket spikes: monophasic, arciform 6 \200\22311 Hz, isola ted or in trains, anterior to mid temporal. Seen in 1 \200\2233 % of normal individuals. B Ictal patterns EEG seizures can be classi \201 ed based on onset and early evolution into focal (or regional), lateralized, generalized, or non-localizable seizure patterns. Focal s eizures Focal seizure patterns are characterized by rhythmic, often sinusoi- dal activity i n the beta, alpha, or theta range or by repetitive discharges with evolve in frequency, amp litude, or distribution (Fig. 9.7 A). They often start with a generalized desynchronization of the background and end with irregular slow activity, in second- arily generalized seizu re often more pronounced over the site of onset. The ictal pattern is commonly not detectab le during the aura but emerges with alteration of consciousness indicating a nec- essary sp read outside the onset zone for the ictal discharge to become visible on scalp recordings. Hence, interictal epileptiform \226 In combination with a normal awake and sleep background, generalized 3 \200\2234 Hz spike and wave complexes and polyspikes are typical for a group of genetic epilepsy syndrom es: child- hood absence epilepsy (CAE), juvenile absence epilepsy (JAE), juvenile myoclonic epilepsy (JME). \226 Generalized epileptiform discharges are less speci \201 c than focal epileptiform d ischarges and 10 \200\22320 % of patients with general- ized epileptiform discharges, recor ded either spontaneously or triggered with photic stimulation or hyperventilation, have no history of seizures. \227\206 Periodic lateralized epileptiform discharges (Fig. 9.6 C): lateralized (and somet imes focal) epileptiform discharges that occur repetitively and semiperiodic every 1 \200 \2235 seconds; they are usually seen in the context of acute or subacute focal cortical inj ury. CHAPTER 9 the electroencephalogram in the investigation of epilepsy 101 A Fig. 9.5 Slowing. A) Generalized slowing and triphasic waves. Referential montage. Generali zed 2 \200\2233 Hz slowing, maximum frontal. In second 7, a triphasic wave is seen. B) Foca l slowing and asymmetry. Continuous irregular slowing is seen over the right temporoparieta l region. Increased beta activity is noted over the right frontocentral area. Patient statu s post right-sided craniotomy for a right anterior temporal resection. B activity often provides a more accurate localization of the epilep- togenic zone. generalized distribution, a phenomenon called secondary bilateral hypersynchrony. The localizing value of the ictal EEG depends on the region of onset. Most temporal lobe ep ilepsies have localized EEG \201 ndings, seen in 90 % of mesial temporal lobe epilepsies and 75 % of neocor- tical temporal lobe seizures which may present with a lateralized rathe r than focal/regional onset ( 23 ). A rhythmic theta pattern at onset is highly specific fo r temporal and exceedingly rare in extratemporal lobe epilepsies. Parietal and occipital lobe epilepsies show a localized onset pat- tern in up to half of th e cases. However, similar to mesial frontal lobe epilepsies, occipital lobe seizures can be falsely localized to the contralateral site due to the orientation of the ictal vector. In addi- tion, parieto-occipital lobe epilepsies have a tendency to spread rapidly into eithe r the ipsilateral or contralateral frontal or tempo- ral lobe or may even present with a ge neralized seizure pattern. Frontal lobe epilepsies, particularly from the mesial structures tend to present with a dif fuse suppression or generalized EEG onset and only around a third of cases have a localized EEG seizure pat- tern. This is further complicated by the fact that interictal epilepti- f orm discharges in frontal lobe epilepsies can present in a Generalized seizures The EEG in generalized seizures often correlates with the seizure type . In the subgroup of genetic epilepsies, i.e. CAE, JAE, JME, the interictal EEG \201 ndin gs usually mimic the ictal EEG pattern. oxford textbook of epilepsy and epileptic seizures 102 A B C Fig. 9.6 Interictal epileptiform abnormalities. A) Left anterior temporal sharp wave (bipol ar montage without and with sphenoidal \200\224 upper panel; referential montage to Vertex and Ear reference \200\224 lower panel). B) Generalized 2 \200\2233 Hz (poly)spike and wa ve complex in a patient with genetic generalized epilepsy. Referential montage. C) Periodic lateralized epileptiform discharges (PLEDS). Trains of 2 Hz lateralized semiperiodic epile ptiform discharges over the right hemisphere, maximum T8 (phase reversal). can outlast the initial myoclonic activity and be associated with staring and unresponsiven ess. Seizures are triggered by sleep deprivation and often occur after awakening. \227\206 The EEG during typical absence seizures in CAE and JAE shows the same 2.5 \200 \2233.5 Hz spike and wave pattern which is seen interic- tally. Ictal burst are usually lon ger than asymptomatic interictal activity. However, even short bursts of 1 \200\2232 second s duration can lead to symptoms and vise versa, patients with prolonged runs of 3 Hz spike and wave activity lasting 30 seconds and more can remain symptom free despite careful testi ng. \227\206 Generalized tonic \200\223clonic seizures associated with CAE and JME are often p receded by an absence or myoclonic seizure with cor- responding EEG changes as previously o utlined (Fig. 9.7 B). The tonic phase is mostly obscured by a dense electromyography (EMG) artifact but if not, may show a brief attenuation and low voltage fast followed typically b y a sharply contoured, rhythmic \227\206 The EEG during myoclonic seizures in JME typically shows a burst of polyspike and wave discharge in the 3 \200\2234 Hz range. The burst CHAPTER 9 the electroencephalogram in the investigation of epilepsy 103 A Fig. 9.7 Ictal pattern. A) Focal seizure. 20 s page. Five seconds after clinical onset, abr upt appearance of sharply contoured, rhythmic theta activity over the right temporal region spreading 8 s later into the left temporal area. B) Generalized seizure. At second 4, sudd en appearance of diffuse polyspikes and low voltage fast activity, progressively obscured b y irregular 2 \200\2233 Hz EMG correlated with myoclonic jerks followed by dense EMG artifa ct indicating tonic muscle activity. B treated for intractable seizures for many years by their neurologists or primary care physi cian have non-epileptic events. A referral for patients who continue to have seizures after 1 year of medical treat- ment to an epilepsy center is therefore recommended to avoid long -term morbidity and mismanagement ( 24 ). activity in the alpha range coinciding with the tonic phase of the seizure. The rhythmic ac tivity progressively slows down and at 4 \200\2235 Hz, a polyspike and wave pattern emerges accompanied by clonic jerking which becomes progressively more irregular. In about one-hal f of the patients the seizures end with a diffuse sup- pression, followed by a period of ir regular generalized slowing. For most patients admitted to an epilepsy monitoring unit, tapering of medications is neces sary to record seizures in a reason- able amount of time. This requires continuous nursing supervision and technical support to assure patient safety and optimized data utilization. Placement of additional electrodes over the area of interest (anterior temporal electrodes, sphenoidal electrodes) can be helpful to increase the yield of interictal abnormalities an d to facilitate source localization. The goal is usually to record 2 \200\2233 typi- cal ev ents. Standardized testing during the event is mandatory to demonstrate the subjective symp toms, ictal and postictal lateraliz- ing signs (ictal speech and postictal aphasia, Todd pa ralysis), to examine the level of consciousness, and to correlate clinical \201 nd- ings with the EEG. It is important to determine if the patient is aware of the events to assess the reliability of the patient \200\231s seizure account. For many patients, seeing their o wn seizure can be helpful to understand the impact of a seizure on their functioning and to facilitate their decision-making for surgery and adherence to medi- cal management. \227\206 Atonic and tonic seizures are typical for Lennox \200\223Gastaut syn- drome and a variety of EEG seizure pattern have been described. Most typically, the seizures are assoc iated with a voltage attenua- tion or electrodecremental response with or without visible s uperimposed low voltage fast. A generalized (poly)spike and wave complex can precede the ev ent and a brief period of gener- alized slowing is usually seen after the event. Video EEG monitoring In the past, patients were usually referred to an epilepsy center with a video EEG monitoring unit for a presurgical evaluation assuming that they are potential surgical candidates. However, selection cri- teria who constitutes a good surgical candidat e (e.g. suspected epi- leptogenic zone based on semiology and interictal EEG, lesion on epi lepsy protocol MRI), depend on a high level of expertise and technical infrastructure. It h as become evident that 25 % of patients oxford textbook of epilepsy and epileptic seizures 104 to surgery and the ability to include the patient in the informed consent process after all data are obtained further adds to the ben- e \201 t of extraoperative recording (Fig. 9.8 ). The typical length of stay is around 3 \200\2235 days which allows the recording of seizure s in approximately 70 % of patients ( 25 ). Complications are rare and related to falls, st atus epilepticus, pos- tictal psychosis, and, in rare events, SUDEP ( 26 ). Advanced EEG techniques EEG source localization With conventional visual analysis of focal EEG activity, where the observer attempts to de \201 ne the source of this activity, certa in assumptions are made. It is assumed that ( 1 ) the source lies close to the electrode th at has maximum negativity or minimum positivity; ( 2 ) that focal sources produce focal epi leptiform abnormalities; and ( 3 ) that broad potential \201 elds are indicative of diffu se or multi- focal generators ( 27 ) However, this is not always true and clear exceptions to these assumptions are frequently found in clinical practice when depth electrodes are us ed to localize EEG activity because generators of EEG activity are dipolar with a \201 el d potential of one polarity at the active synaptic site (current sink) and a \201 eld of the opposite polarity at a distant site (current source) ( 28 ). What is seen on the surfac e therefore, may arise from a distant and/or deep source. An example of this is the mesial temporal EEG spike of hippocampal sclerosis. These spikes can be demonstrated to arise from the hippocampal \200\223amygdalar complex with depth elec- trodes. Voltage distribution ma ps of simultanous scalp recordings of these same spikes with a standard surface EEG montage will show these to arise from the anterior temporal or antero-basal temporal lobe (electro des F7, T3, FT9). EEG source localization (EEG source imaging, ESI) is therefore aimed at l ocating the true generator of electrical activity as a direct or indirect indicator of the individual \200\231s epileptogenic zone. This is important in the charac- terization of a f ocal epilepsy although ESI techniques are also capa- ble of source localizing eloquent cort ex through analysis of evoked potentials. See Figs 9.9 and 9.10. Invasive EEG Intraoperative electrocorticography Intraoperative ECoG with subdural and, les s commonly, stereotactic depth electrodes attempts to capture interictal abnormalities to f ur- ther delineate the irritative cortex (as indicated by spikes) or the area of dysfunctio n (represented by slowing or attenuation of normal activ- ity). The additional procedural r isk of intraoperative ECoG is compa- rably small and usually extends the time of surgery by not more than 30 \200\22360 minutes. In many lesional (mesial temporal sclerosis (MTS), ca vernous malformations, low grade tumors) and non-lesional (non-dominant) temporal lobe case s, intraoperative ECoG can be a useful adjustment to delineate the extent of the surgical r esection without the risks associated with extraoperative invasive recordings. For these patients, functional mapping can be often accom- plished preoperatively through W ada testing or functional MRI (fMRI). If necessary, the relation to eloquent cortex can be further defined through central sulcus mapping with somatosensory evoked potentials or elec trical cortical stimulation under anaesthe- sia (recording motor response from EMG electrod es) or in the awake patient (for language mapping). Extraoperative invasive recording Extraoperative invasive monitoring either through subdura l elec- trode placement or a depth electrode exploration with \200\230stereo \200\231-EEG is the preferred approach to invasive epilepsy surgery in many adult centers ( 7 , 8 ). Par ticularly non-lesional extratemporal, dominant temporal, or bitemporal lobe cases and the g roup of patients with suspected dual pathology, post-traumatic epilepsy, or focal cortical dysplasia bene \201 t from the invasive de \201 nition of the ictal onset area. Sampling of interictal epileptiform activity to delineate the irrita- tive cortex is often more reli able outside the operation room with all its pitfalls in terms of time constraint and the e ffect of anaesthe- sia. A more systematic and reproducible functional mapping prior Sophisticated mathematical modeling techniques and algo- rithms are used to determine ESI a nd several commercially availa- ble software programs incorporate these for clinical and re search usage. With both ESI and source imaging with magnetoencepha- lography, two important concepts require consideration \200\224 the for- ward and inverse problems; the \200\230 solutions \200\231 to which add complexity Electrocorticography Location Extratemporal Temporal Dominant Non-dominant Extraoperative Intraoperative Noninvasive Fig. 9.8 Intra- vs. extraoperative electrocorticography. The decision between non-invasive and intraoperative or extraoperative invasive recording for epilepsy surgery depends on loc ation and etiology. The suspected etiology of a lesion determines the overlap between irrit ative cortex, ictal onset area, and lesion and the location impacts the proximity to eloque nt cortex and the need for mapping. Post-traumatic FCD Non-lesional Epileptogenic zone Cavemous angioma Low-grade tumor MTS Etiology Irritative zone Two or more lesions e.g. dual pathology, TS Technical Mesial or basal coverage Depths Ictal onset zone Epileptogenic lesion CHAPTER 9 the electroencephalogram in the investigation of epilepsy 105 Fig. 9.9 A patient with a left temporal lobe epilepsy due to a mesial temporal cavernoma (t op right \200\224 white arrow). The EEG shows a left temporal spike with a surface voltage distribution as seen in the top left \201 gure. to the modeling techniques and algorithms used. The forward problem refers to the problem o f accurate localization of a \201 eld potential at the recording scalp electrodes for a g iven dipole source. This is made dif \201 cult by the fact that there are intervening phys io- logical planes with varying tissue anisotropy (brain, cerebrospinal \202 uid, meninge s, skull) and additionally, head shapes and sizes vary from individual to individual. Conve rsely, the inverse problem refers to the problem of working backwards from the \201 eld p oten- tial recorded at the scalp electrodes to identifying its source. The source can be a \201 xed, moving, or rotating dipole source, multiple dipoles or a distributed source and different algorithms are used for different situations. Clinical validation for these tech niques is slowly building evidence for the utility of ESI ( 29 ), particularly in source lo calizing the interictal spike. Source localization of the ictal discharge is less convincin gly studied. demonstrating localization in relation to surrounding brain struc- tures. To lend con \201 dence to ESI accuracy, it is essential that two important criteria are ful \201 lled. Fir stly that the electrode positions incorporated into analysis are the subject \200\231s rath er than the soft- ware \200\231s default positions. Secondly, the MRI images should be the patient \200\231s own images rather than the default software images. Non- ful \201 lment of either of these caveats can result in inaccurate results.