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Interictal-ictal transition in the feline penicillin epileptogenic focus
Electroencephalography and Clinical Neurophysiology, 1978, 45:525--534 © Elsevier/North-Holland Scientific Publishers, Ltd.
525
INTERICTAL--ICTAL TRANSITION IN THE FELINE PENICILLIN EPILEPTOGENIC FOCUS IRA SHERWIN
1
Neurophysiology Unit, Research Service, Veterans Administration Hospital,Bedford, Mass. 01730, and Department of Neurology, Harvard Medical School, Boston, Mass. (U.S.A.) (Accepted for publication: March 21, 1978)
An epileptogenic focus in the interictal state has been likened to a smoldering ember which periodically bursts into flame, producing a seizure. In reflex epilepsy, the triggering mechanism underlying this interictal--ictal (II--I) transition may be obvious and highly specific (Servit 1963; Sherwin 1966). Generally, however, the mechanism involves complex determinants, possibly metabolic/neurohumoral in character (Sherwin 1963, 1967) and largely remains an enigma. That notwithstanding, the ability to accurately predict the timing of such transitions may be of major clinical importance. In some cases, such advance recognition of an impending ictus permits the employment of techniques which may abort the seizure (Efron 1956). Even when the II--I transition is too abrupt to employ such strategies, an early-warning device whose design basis we have previously described (Castillo and Sherwin 1971) may allow the patient to minimize his risk of physical injury. Previous studies of this problem have resulted in differing points of view. Earlier, King et al. (1953) had reported a decrease in the amplitude of the cortical discharges during the transition from interictal to ictal activity. Gastaut (1954) noted among other findings an increase in amplitude as the interictal activity gives way to ictal discharges.
Walker (1950), in a systematic investigation utilizing the penicillin focus, stressed a gradual increase in spike frequency preceding a seizure. Ralston (1958), using the same experimental model, reported increasing spike frequency occurred only in a minority of the II--I transitions he observed. He stressed the appearance of small, rapid (about 20--40/sec) rhythmical discharges superimposed on the slow component of the interictal spike and wave complex, as signaling an II--Itransition. He referred to this wave form as an 'afterdischarge' and noted it to be present in more than 90% of the seizures he encountered. Angeleri et ah (1972), however, found the type of 'after-discharge' described by Ralston to be an unreliable marker for II--I transitions in the feline penicillin focus. Moreover, in contrast to the findings of Walker (1950) they stress the appearance of an abrupt increase in spike frequency, in the 5 sec period just preceding a seizure. As pointed out above, the predictability of II--I transitions has potentially practical as well as theoretical implications. With this in mind, the present study was undertaken to re-examine this question, in general, and those previously reported conflicting findings, in the feline penicillin focus in particular. Methods and materials
I This work was supported by the Veterans Administration and by National Institutesof Health Grant No. NB06209.
The experiments described were conducted in adult mongrel cats weighing
526 between 2.5 and 3.5 kg. Surgery was performed under methoxyfiurane anesthesia with subsequent infiltration of the operative sites with a long lasting local anesthetic (Xyljectin). Following cannulation of the trachea and one femoral vein the animals were paralyzed with gallamine triethiodide (Flaxedil) and artificially ventilated. A metal bar fixed to the skull with dental acrylic was used to fasten the head to a stereotaxic frame. The skull was trephined to expose the midsuprasylvian gyrus of one hemisphere, and the exposed pial surface was covered with a pool of warm mineral oil to prevent drying. The brain potentials recorded with pial, silver-silver chloride ball electrodes were conventionally amplified, displayed on a multitrace oscilloscope and stored on analog magnetic tape for subsequent analysis. Throughout the experiment body temperature was maintained with a heating blanket and monitored by a rectal, electronic thermometer. End expiratory CO2 was intermittently sampled, using an infrared CO2 analyzer and kept between 3--4%. EEG spindles and pupillary meiosis indicative of sleep suggested the animals were not in pain.
Results
A principal aim of this study was to investigate the predictability of II--I transitions, therefore, we attempted to create foci whose activity was characterized by relatively long interictal periods, during which the focus could return to its 'steady state'. Based on previous experience (Sherwin 1973) in creating actively spiking foci with only infrequent seizures, the initial intracortical injections of penicillin were limited to 50 ~1 of a 50,000 U/ml solution of sodium penicillin in saline. Subsequent additional injections were carefully titrated in each preparation in an attempt to achieve the desired balance between ictal and interictal activity. Despite this, some foci produced long runs of semirhythmic (0.3--1/sec), interictal spikes with
I. SHERWIN only rare ictal episodes. Hyperventilation was employed in these preparations in an attempt to provoke electrical seizures. These data were considered for the assessment of unique wave forms, but were not included in the statistical treatments. By contrast, some foci produce~ frequent ictal episodes with only brief, interictal recovery periods. Data from these animals with 'quasi-status epilepticus' were similarly excluded from the statistical treatments but were compared with similar data from other studies for the occurrence of unique wave forms. To compare the present findings and those reported by others, we initially analyzed the interictal activity immediately preceding each fit for amplitude and frequency changes and for the appearance of unique wave forms. A variety of such features were encountered and some typical examples are shown in Fig. 1. In some II--I transitions (Fig. 1A), the semi-rhythmic spiking which was characteristic of the foci in this study might suddenly give rise to an organized electrical seizure without any obvious prior change in the interictal firing pattern. This was not a common finding and was seen in slightly less than 15% of the observed seizures. By contrast, some change in spike frequency preceded the remaining seizures. In some preparations, this consisted of a progressive increase in frequency gradually leading into the seizure (Fig. 1C). In the II--I transition shown in Fig. 1B, there is first an increase in spike frequency which then decreases immediately preceding the seizure. Complex changes, rather than simple increases or decreases, tended to characterize the observed spike frequency shifts. These frequency shifts were highly variable in different preparations and are described in detail below. A variety of amplitude changes were also observed which did not appear to follow any specific pattern. Occasionally, single spikes occurred that might be 50% larger than the background spikes. Neither these individual spikes nor any other changes in the background amplitude appeared to be consistently
INTERICTAL--ICTAL TRANSITION
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Fig. 1. Interictal--ictal transition patterns. A: abrupt transition without significant preceding change in the interictal spike frequency. B: increase followed by decrease of interictal spike frequency preceding the ictal discharge. C: gradually increasing interictal spike frequency leading to ictal discharge. D: interictal--ictal transition showing 'after-discharges'. Tic marks: intervals 1 sec; amplitude 20/~V.
related to the transition from interictal to ictal activity. Several examples of repetitive sharp waves superimposed on the slow wave c o m p o n e n t of the spike and slow wave complexes also were observed (Fig. 1D). As can be seen here and in Fig. 2 (arrows) these 'afterdischarges', described by Ralston, occurred at various times and were not consistently related to II--I transitions. Based on these data, it appeared that the most consistent and striking alteration heralding an II--I transition was some complex change in spike frequency. Modest increases and decreases in mean spike frequency often occurred but tended not to be significant. On the other hand, the individual spike-interval durations revealed prominent systematic variations as the ictus was approached. In 7 out of 13 preparations, we obtained several ictal episodes separated by relatively l o n g (at least 10 min, some more than 30 min) interictal periods. These II--I transitions were then subjected to more detailed analysis. The general plan for the quantitative analysis of the results was to divide the raw data into epochs of varying length starting with the last fit and working backwards to the pre-ictal activity preceding the first seizure in the sequence. In order to include the maxim u m a m o u n t of data in the analysis while
excluding 'post-ictal effects', a period equal to about one-half the duration of the shortest observed interictal interval initially defined the analysis epoch for that preparation. Consequently the analysis epoch varied from animal to animal. For each animal the analysis epoch was repeatedly redefined as some fraction of the original for each desired subsequent re-treatment of the data. Thus an optimal analysis epoch was sought for each preparation. Three successive II--I transitions from a typical example are shown in Fig. 2. For graphic clarity, only the last 80 sec of each interictal epoch is shown. The top tracing (A) represents 80 sec of pre-ictal activity occurring approximately 30 rain before the first ictal episode. The following traces (B--D) represent 80 sec of interictal activity immediately preceding each of the 3 subsequent seizures. The pre-ictal activity preceding the first seizure is characterized by semi-rhythmic spike firing at about 0.3--1/sec. This background pattern was similar for all the foci subjected to further analysis (see also Fig. 3A). Preceding each of these 3 seizures there is a complex change in the interictal spike firing frequency. Similar II--I transitions from another preparation are shown in more detail in Fig. 3. The trace pairs labeled A1, 2 through E1.2 each represent about 150 sec of activity
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Fig. 2. Pre-ictal activity 30 rain prior to a first seizure (A). Three successive interictal--ictal transitions characterized by complex changes in the interictal spike frequency (B--D). Arrows: mark the occurrence of 'afterdischarges'. Tic marks: intervals 1 sec; amplitude 20 #V.
taken from each of 5 sequential 5 min periods which preceded the first of 6 seizures. It can be seen that the complex changes in the spike firing pattern become progressively more striking as the ictus is approached. In Fig. 4, all of the interictal data related to all 6 seizures are depicted in the form of pooled, joint interval scattergrams. The 5 plots (E1-Es) correspond to 5epochs, as previously defined and the density of the dots is proportional to the frequency of occurrence of the various interval pairings. For the earliest inter-
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ictal epoch (El), it can be seen (upper right hand quadrant clustering) that the bulk of the intervals is followed by an interval of like length, i.e., semi-rhythmic spiking. There are a few instances in which short intervals are followed by longer intervals and vice versa. Relatively short--short interval pairings, however, are extremely rare. As the II--I t-ransition approaches, the previously observed modal pairings become less apparent and in the immediately pre-ictal epoch short--short pairings not seen earlier (cf., epochs 1 and 5)
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Fig. 3. Sequential changes in the spike activity preceding a seizure. Each trace pair (A1,2--E1,2) represents a 150 sec sample from the 5 sequential 5 min periods preceding the fit. Note, complex changes in the spike frequency become more prominent as the ictus is approached. Tic marks: intervals 2 sec; amplitude 20 #V.
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n o w become prominent. In addition, the modal pairings characteristic of the early interictal epochs are n o w greatly reduced in number and several preferred interval pairings emerge (E4 and Es). The minimum duration of the shortest intervals appeared to be constrained by some limiting factor. In this example and the other analyzed seizures, this minimum duration ranged between 4 0 0 and 500 msec. In order to quantify these changes, spikeinterval density histograms were then constructed from the data. As shown in Fig. 5, for each of the 6 consecutive seizures (rows), the transition from epoch 1 through epoch 5 ( E I , E s ) is characterized by a shift from a quasi-gaussian distribution o f the spike intervals to a multi-modal distribution with a series of distinct peaks. An analysis of
variance of these 30distributions revealed significant differences between 4 peaks located at about 700, 1700, 2 7 0 0 and 3 4 0 0 msec. Since a major goal of this study was to determine the reliability of predicting II--I transitions as far in advance o f a seizure as possible, the ordered mean peak values were then subjected to a Duncan Multiple Range Test for Correlated Means (Duncan 1957). In this example the first peak exceeds the critical value (significant at the 0.01 level) as early as epoch 3. By epoch 5 the critical values, at the 0.01 level, have been exceeded for all 4 peaks. Despite marked variations between preparations, the majority of the II--I transition patterns in each animal tended to be characteristic, although not completely consistent. For example, in Fig. 5, 4 of the
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Fig. 5. Interval histograms of the interictal spike activity preceding 6 consecutive seizures (rows). Interictal activity preceding each seizure is sampled in 5 successive epochs ( E l - - E s ). Abscissa scaled in sec, ordinate scaled as probability of spike occurrence. N o t e shift over time f r o m quasi-gaussian to m u l t i m o d a l distributions.
6seizures revealed obvioasly homologous shifts of their spike interval distributions. Of the two seizures in which this is not obvious in the graphic display, the statistical treatment used did reveal a significantly similar shift in the spike interval distributions of one of them. In 73% of the analyzed seizures a significant firing pattern shift heralding the II--I transition was noted in the preceding 2 rain period, and in slightly less than a quarter of these, significant differences occurred as early as 5 rain prior to the II--I transition. The majority of the exceptions was of the type shown in Fig. 1A. By redefining their durations and thus the number of epochs, and by choosing lower protection levels, earlier although obviously less confident predictions could be made. It should be stressed that the selection of the analysis parameters was individually tailored to each preparation in order to optimize the II--I predictability.
Discussion
These data suggest that II--I transitions in the feline penicillin focus may occur without any obvious heralding EEG change. However, when changes in the EEG which signal an impending II--I transition do occur, they generally tend to be characteristic for an individual epileptogenic focus, but may differ markedly from one preparation to another. In the present study changes in spike frequency appeared to be the most significant predictor of such transitions. This usually was not a simple, gradual increase in frequency but rather, new patterns of discharge tended to occur. Although the mean spike frequency might not change significantly, a particular shift in the relative proportions of the different inter-spike intervals was often characteristic of an impending II--I transition. This finding is in keeping with the observation of Walker (1950) that, in the penicillin focus
IN TE R I C TAL- - I C TAL TRANSITION
'spikes appearing at first singly (rhythmic firing) become paired and then occur in runs'. Changes in the spike-interval distributions appeared to be the one feature most commonly related to all those seizures in which a reliable II--I prediction could be made. It should be emphasized, as previously noted, that the particular changes tended to be characteristic for each preparation. As with fingerprints, however, uniqueness did not make the individual markers less valuable. In his study of this problem, Ralston (1958) concluded that, in over 90% of the fits he observed II--I transitions were not characterized by an 'increase of frequency and fusion of spikes to form a seizure, but rather -- by the interpolation of a new and different form of electrical activity, the afterdischarge'. Although we found several examples of the type of 'after~lischarge' reported by Ralston, we did not find this to be a reliable marker for impending II--I transitions. On the one hand, we noted several instances in which this 'after~lischarge' pattern occurred without being followed by an organized electrical seizure. On the other hand, even when such an 'after
531
rence of this II--I transition marker (afterdischarge) is not peculiar to the preparation species nor to penicillin as an epileptogen. In the first of these studies (Chusid et al. 1953) several examples of 'after
532
increase in the spike frequency in the 5 sec period just preceding the seizure, when this is compared to the mean frequency for the immediately antecedent 55 sec. This latter observation was not a consistent finding in our preparations. A major difference between this study and the present investigation was seizure frequency. Based on their published recordings, some interictal periods appear to be as brief as 15 sec. As pointed out earlier, in the results section, we deliberately excluded from the present study preparations with very short interseizure recovery cycles. Perhaps if we had used more excitable preparations we too might have noted the changes reported to occur in the 5sec immediately preceding II--I transitions. We believe, however, that this limitation is more than offset by the advantage that our model may more closely simulate 'chronic' epilepsy. It is in that situation rather than in status epilepticus that we feel the predictability of II--I transitions is of particular clinical significance and interest. One additional feature of these experiments deserves comment. Prior to the actual onset of the high frequency seizure discharges, the emerging short inter-spike intervals tended to approach a minimum value of approximately 400--500msec. In previous experiments (Sherwin 1973) we noted that direct electrical stimulation of a penicillin focus at low frequencies could provoke long runs of one-for-one interictal spike driving. However, when driving was attempted at rates approaching 2/sec alternate stimuli might fail to provoke a spike. Stimulation at rates above 2/sec, however, consistently provoked organized seizure discharges. It is possible that our finding of an apparent constraint on the minimal duration (approximately 500msec) of the shortest interictal spike intervals may simply reflect the fact that in both sets of experiments we deliberately created relatively weak foci. Against this explanation, however, is the observation of Angeleri et al. that in their very active penicillin foci the minimum duration
I. S H E R W I N
of the shortest interictal spike intervals also was about 500 msec. Specifically, they noted that the duration of the relatively short (approximately 1.5 sec) spike intervals occurring early in an interictal period would be reduced to about 1/3 this value, i.e. 500 msec just prior to a seizure. Recently Scobey and Gabor (1977) proposed a statistical model to characterize the interictal excitability cycle of the penicillin focus. They noted that following a spontaneous interictal spike, a recovery period of about 400 msec was necessary before the probability of obtaining an evoked spike approached 100%. This finding is in keeping with the notion of an 'optimal' interictal spike frequency for the development of seizures (Elazar and Blum 1974). Scobey and Gabor attributed the temporal properties of this recovery function, indirectly, to the paroxysmal depolarizing shift characteristic of discharging neurons in the penicillin focus. Based on the present study and those cited, it is suggested that the emergence of repeated brief inter-spike intervals with durations of about 400-500 msec may indicate impending failure of those inhibitory mechanisms ordinarily operating in the interictal period and thus relate to an II--I transition. In sum, one aim of the present study was to specifically reexamine the reliability of previously reported markers of II--I transition in the feline penicillin focus. The present data strongly support and extend earlier conclusions (Walker 1950; Angeleri 1972) that in these foci certain changes occurring in interictal spike frequency are variable but important predictors of impending II--I transitions. A second aim of our work was to consider the general problem of seizure predictability based on an analysis of interictal EEG data. An obvious question that might be raised is whether the results derived from an acute model like the penicillin focus can be extrapolated to the chronic focus. Despite certain differences, the fundamental electrophysiologlcal properties of the penicillin focus are
INTERICTAL--ICTAL TRANSITION on balance mo r e like t h a t o f t he chronic focus th an d if f er ent f r o m it (Dichter and Spencer 1968; Sherwin 1977), and thus suggest t h a t similar t r e a t m e n t o f data f r o m chronic foci is at least reasonable. In the case o f chronic, h u m a n epilepsy cyclic variations in the spike activity occurring during b o t h seizure and interseizure periods have been r e p o r t e d (Stevens et al. 1972) which might require separate d a y / night analyses. It should be n o t e d t o o t h a t the specific tr eatmen ts o f the data e m p l o y e d in t h e present s tu d y represent only one approach and o t h e r statistical procedures m a y prove mo r e appropriate in particular instances. In any event, it is suggested t h a t an individualized, quantitative analysis of the stochastic properties o f the interictal E E G m a y be a potentially useful aid in the assessment and possible t r e a t m e n t of patients with frequent, p o o rl y controlled seizures.
Summary A study was u n d e r t a k e n to examine those E E G features which characterize interictal-ictal (II--I) transitions in the feline penicillin focus. The study describes certain c om pl ex changes in spike f r e q u e n c y and their statistical analysis, which appear t o have predictive value in signaling II--I transitions. T h e significance of these data with respect t o the conflicting results o f o t h e r studies and their possible application to clinical epilepsy are discussed. Rfisum~
Transition des pdriodes interictale et ictale dans les foyers dpileptogdnes d la pdnicilline du chat Une ~tude a ~t~ entreprise chez le chat p o u r examiner les donn~es E E G qui caractdrisent la transition des pdriodes interictale e t ictale (II--I) dons les f oye r s ~ la penicilline.
533 Cette ~tude d~crit certaines modifications complexes de la fr~quence des pointes et leur analyse statistique qui semble avoir une valeur predictive de signal de ces transitions II--I. J La signification de ces donndes par r a p p o r t aux r~sultats contradictoires d'autres ~tudes et leur application possible ~ l'~pilepsie clinique est discut~e. The author is pleased to acknowledge his indebtedness to Mr. David Goodman for invaluable assistance in the computer analysis of the data, and to Dr. Jeff Lieb for his critical review of the manuscript.
References Angeleri, F., Giaquinto, S. and Marchesi, G.F. Temporal distribution of interictal and ictal discharges from penicillin loci in cats. In: H. Petsche and M.A.B. Brazier (Eds.), Synchronization of EEG Activity in Epilepsies. Springer, New York, 1972: 221--234. Blum, B. and Posener, L.N. A stochastic analysis of inter-ictal epileptiform activity. Confin. neurol. (Basel), 1965, 26: 519--531. Castillo, H.T. and Sherwin, I. Encephalophone: an electronic stethoscope for the brain. J. Audit. Engng Soc., 1971, 19: 142--144. Chusid, J.G., Kopeloff, L.M. and Kopeloff, N. Experimental epilepsy in the monkey following multiple intracerebral injections of alumina cream. Bull. N.Y. Acad. Med., 1953, 29: 898--904. Dichter, M. and Spencer, W.A. Hippocampal 'spike' discharge: epileptic neurone or epileptic aggregate? Neurology (Minneap.), 1968, 18: 281--283. Duncan, D.B. Multiple range tests for correlated and heteroscedastic means. Biometrics, 1957, 13: 164--176. Efron, R. The effect of olfactory stimuli in arresting uncinate fits. Brain, 1956, 79: 267--281. Elazar, Z. and Blum, B. Interictal discharges in tungsten foci and EEG seizure activity. Epilepsia, 1974, 15: 599--610. Gastaut, H. The Epilepsies. Electroclinical Correlations. (English translation by M.A.B. Brazier.) Thomas, Springfield, Ill., 1954. Hanbery, J.W. and Ajmone Marsan, C. Intrathecal injection and cortical application of chloramphenicol -- an experimental study with a review of the local action of antibiotics on the central nervous system. J. Neuropath. exp. Neurol., 1954, 13: 297--317.
534 King, R.B., Schricker, J. and O'Leary, J.L. An experimental study of the transition from normal to convulsoid cortical activity. J. Neurophysiol., 1953, 16: 286--298. Ralston, B.L. The mechanism of transition of interictal spiking loci into ictal seizure discharges. Electroenceph. clin. Neurophysiol., 1958, 10: 217--232. Ralston, B.L. and Papatheodorou, C.A. The mechanism of transition of interictal spiking foci into ictal seizure discharges. Part II. Observations in man. Electroenceph. clin. Neurophysiol., 1960, 12: 297--304. Scobey, R.P. and Gabor, A.J. Properties of epileptogenic focus: activation field. J. Neurophysiol., 1977, 40: 1199--1213. Servit, Z. Reflex Mechanisms in the Genesis of Epilepsy. Elsevier, Amsterdam, 1963. Sherwin, I. Differential effects of hyperventilation
I. SHERWIN on intact and isolated cortex. Electroenceph. clin. Neurophysiol., 1963, 18: 559---607. Sherwin, I. Seizures precipitated by the use of language. Cortex, 1966, 2: 349--356. Sherwin, I. Alterations in the non-specific cortical afference during hyperventilation. Electroenceph. clin. Neurophysiol., 1967, 23: 532--538. Sherwin, I. Suppressant effects of diphenylhydantoin (Dilantin) on the cortical epileptogenic focus. Neurology (Minneap.), 1973, 23: 274--281. Sherwin, I. Stereotyped and structured bursts activity of single units in the penicillin epileptogenic focus in cats. Exp. Neurol., 1977, 55: 226--233. Stevens, J.R., Lonsbury, B.L. and Goel, S.L. Seizure occurrence and interspike interval: telemetered electroencephalogram studies. Arch. Neurol. (Chic.), 1972, 26: 409--419. Walker, A.E. Convulsive activity. Quart. Phi Beta Pi, 1950, 47: 108--115.
525
INTERICTAL--ICTAL TRANSITION IN THE FELINE PENICILLIN EPILEPTOGENIC FOCUS IRA SHERWIN
1
Neurophysiology Unit, Research Service, Veterans Administration Hospital,Bedford, Mass. 01730, and Department of Neurology, Harvard Medical School, Boston, Mass. (U.S.A.) (Accepted for publication: March 21, 1978)
An epileptogenic focus in the interictal state has been likened to a smoldering ember which periodically bursts into flame, producing a seizure. In reflex epilepsy, the triggering mechanism underlying this interictal--ictal (II--I) transition may be obvious and highly specific (Servit 1963; Sherwin 1966). Generally, however, the mechanism involves complex determinants, possibly metabolic/neurohumoral in character (Sherwin 1963, 1967) and largely remains an enigma. That notwithstanding, the ability to accurately predict the timing of such transitions may be of major clinical importance. In some cases, such advance recognition of an impending ictus permits the employment of techniques which may abort the seizure (Efron 1956). Even when the II--I transition is too abrupt to employ such strategies, an early-warning device whose design basis we have previously described (Castillo and Sherwin 1971) may allow the patient to minimize his risk of physical injury. Previous studies of this problem have resulted in differing points of view. Earlier, King et al. (1953) had reported a decrease in the amplitude of the cortical discharges during the transition from interictal to ictal activity. Gastaut (1954) noted among other findings an increase in amplitude as the interictal activity gives way to ictal discharges.
Walker (1950), in a systematic investigation utilizing the penicillin focus, stressed a gradual increase in spike frequency preceding a seizure. Ralston (1958), using the same experimental model, reported increasing spike frequency occurred only in a minority of the II--I transitions he observed. He stressed the appearance of small, rapid (about 20--40/sec) rhythmical discharges superimposed on the slow component of the interictal spike and wave complex, as signaling an II--Itransition. He referred to this wave form as an 'afterdischarge' and noted it to be present in more than 90% of the seizures he encountered. Angeleri et ah (1972), however, found the type of 'after-discharge' described by Ralston to be an unreliable marker for II--I transitions in the feline penicillin focus. Moreover, in contrast to the findings of Walker (1950) they stress the appearance of an abrupt increase in spike frequency, in the 5 sec period just preceding a seizure. As pointed out above, the predictability of II--I transitions has potentially practical as well as theoretical implications. With this in mind, the present study was undertaken to re-examine this question, in general, and those previously reported conflicting findings, in the feline penicillin focus in particular. Methods and materials
I This work was supported by the Veterans Administration and by National Institutesof Health Grant No. NB06209.
The experiments described were conducted in adult mongrel cats weighing
526 between 2.5 and 3.5 kg. Surgery was performed under methoxyfiurane anesthesia with subsequent infiltration of the operative sites with a long lasting local anesthetic (Xyljectin). Following cannulation of the trachea and one femoral vein the animals were paralyzed with gallamine triethiodide (Flaxedil) and artificially ventilated. A metal bar fixed to the skull with dental acrylic was used to fasten the head to a stereotaxic frame. The skull was trephined to expose the midsuprasylvian gyrus of one hemisphere, and the exposed pial surface was covered with a pool of warm mineral oil to prevent drying. The brain potentials recorded with pial, silver-silver chloride ball electrodes were conventionally amplified, displayed on a multitrace oscilloscope and stored on analog magnetic tape for subsequent analysis. Throughout the experiment body temperature was maintained with a heating blanket and monitored by a rectal, electronic thermometer. End expiratory CO2 was intermittently sampled, using an infrared CO2 analyzer and kept between 3--4%. EEG spindles and pupillary meiosis indicative of sleep suggested the animals were not in pain.
Results
A principal aim of this study was to investigate the predictability of II--I transitions, therefore, we attempted to create foci whose activity was characterized by relatively long interictal periods, during which the focus could return to its 'steady state'. Based on previous experience (Sherwin 1973) in creating actively spiking foci with only infrequent seizures, the initial intracortical injections of penicillin were limited to 50 ~1 of a 50,000 U/ml solution of sodium penicillin in saline. Subsequent additional injections were carefully titrated in each preparation in an attempt to achieve the desired balance between ictal and interictal activity. Despite this, some foci produced long runs of semirhythmic (0.3--1/sec), interictal spikes with
I. SHERWIN only rare ictal episodes. Hyperventilation was employed in these preparations in an attempt to provoke electrical seizures. These data were considered for the assessment of unique wave forms, but were not included in the statistical treatments. By contrast, some foci produce~ frequent ictal episodes with only brief, interictal recovery periods. Data from these animals with 'quasi-status epilepticus' were similarly excluded from the statistical treatments but were compared with similar data from other studies for the occurrence of unique wave forms. To compare the present findings and those reported by others, we initially analyzed the interictal activity immediately preceding each fit for amplitude and frequency changes and for the appearance of unique wave forms. A variety of such features were encountered and some typical examples are shown in Fig. 1. In some II--I transitions (Fig. 1A), the semi-rhythmic spiking which was characteristic of the foci in this study might suddenly give rise to an organized electrical seizure without any obvious prior change in the interictal firing pattern. This was not a common finding and was seen in slightly less than 15% of the observed seizures. By contrast, some change in spike frequency preceded the remaining seizures. In some preparations, this consisted of a progressive increase in frequency gradually leading into the seizure (Fig. 1C). In the II--I transition shown in Fig. 1B, there is first an increase in spike frequency which then decreases immediately preceding the seizure. Complex changes, rather than simple increases or decreases, tended to characterize the observed spike frequency shifts. These frequency shifts were highly variable in different preparations and are described in detail below. A variety of amplitude changes were also observed which did not appear to follow any specific pattern. Occasionally, single spikes occurred that might be 50% larger than the background spikes. Neither these individual spikes nor any other changes in the background amplitude appeared to be consistently
INTERICTAL--ICTAL TRANSITION
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related to the transition from interictal to ictal activity. Several examples of repetitive sharp waves superimposed on the slow wave c o m p o n e n t of the spike and slow wave complexes also were observed (Fig. 1D). As can be seen here and in Fig. 2 (arrows) these 'afterdischarges', described by Ralston, occurred at various times and were not consistently related to II--I transitions. Based on these data, it appeared that the most consistent and striking alteration heralding an II--I transition was some complex change in spike frequency. Modest increases and decreases in mean spike frequency often occurred but tended not to be significant. On the other hand, the individual spike-interval durations revealed prominent systematic variations as the ictus was approached. In 7 out of 13 preparations, we obtained several ictal episodes separated by relatively l o n g (at least 10 min, some more than 30 min) interictal periods. These II--I transitions were then subjected to more detailed analysis. The general plan for the quantitative analysis of the results was to divide the raw data into epochs of varying length starting with the last fit and working backwards to the pre-ictal activity preceding the first seizure in the sequence. In order to include the maxim u m a m o u n t of data in the analysis while
excluding 'post-ictal effects', a period equal to about one-half the duration of the shortest observed interictal interval initially defined the analysis epoch for that preparation. Consequently the analysis epoch varied from animal to animal. For each animal the analysis epoch was repeatedly redefined as some fraction of the original for each desired subsequent re-treatment of the data. Thus an optimal analysis epoch was sought for each preparation. Three successive II--I transitions from a typical example are shown in Fig. 2. For graphic clarity, only the last 80 sec of each interictal epoch is shown. The top tracing (A) represents 80 sec of pre-ictal activity occurring approximately 30 rain before the first ictal episode. The following traces (B--D) represent 80 sec of interictal activity immediately preceding each of the 3 subsequent seizures. The pre-ictal activity preceding the first seizure is characterized by semi-rhythmic spike firing at about 0.3--1/sec. This background pattern was similar for all the foci subjected to further analysis (see also Fig. 3A). Preceding each of these 3 seizures there is a complex change in the interictal spike firing frequency. Similar II--I transitions from another preparation are shown in more detail in Fig. 3. The trace pairs labeled A1, 2 through E1.2 each represent about 150 sec of activity
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taken from each of 5 sequential 5 min periods which preceded the first of 6 seizures. It can be seen that the complex changes in the spike firing pattern become progressively more striking as the ictus is approached. In Fig. 4, all of the interictal data related to all 6 seizures are depicted in the form of pooled, joint interval scattergrams. The 5 plots (E1-Es) correspond to 5epochs, as previously defined and the density of the dots is proportional to the frequency of occurrence of the various interval pairings. For the earliest inter-
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ictal epoch (El), it can be seen (upper right hand quadrant clustering) that the bulk of the intervals is followed by an interval of like length, i.e., semi-rhythmic spiking. There are a few instances in which short intervals are followed by longer intervals and vice versa. Relatively short--short interval pairings, however, are extremely rare. As the II--I t-ransition approaches, the previously observed modal pairings become less apparent and in the immediately pre-ictal epoch short--short pairings not seen earlier (cf., epochs 1 and 5)
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Fig. 3. Sequential changes in the spike activity preceding a seizure. Each trace pair (A1,2--E1,2) represents a 150 sec sample from the 5 sequential 5 min periods preceding the fit. Note, complex changes in the spike frequency become more prominent as the ictus is approached. Tic marks: intervals 2 sec; amplitude 20 #V.
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n o w become prominent. In addition, the modal pairings characteristic of the early interictal epochs are n o w greatly reduced in number and several preferred interval pairings emerge (E4 and Es). The minimum duration of the shortest intervals appeared to be constrained by some limiting factor. In this example and the other analyzed seizures, this minimum duration ranged between 4 0 0 and 500 msec. In order to quantify these changes, spikeinterval density histograms were then constructed from the data. As shown in Fig. 5, for each of the 6 consecutive seizures (rows), the transition from epoch 1 through epoch 5 ( E I , E s ) is characterized by a shift from a quasi-gaussian distribution o f the spike intervals to a multi-modal distribution with a series of distinct peaks. An analysis of
variance of these 30distributions revealed significant differences between 4 peaks located at about 700, 1700, 2 7 0 0 and 3 4 0 0 msec. Since a major goal of this study was to determine the reliability of predicting II--I transitions as far in advance o f a seizure as possible, the ordered mean peak values were then subjected to a Duncan Multiple Range Test for Correlated Means (Duncan 1957). In this example the first peak exceeds the critical value (significant at the 0.01 level) as early as epoch 3. By epoch 5 the critical values, at the 0.01 level, have been exceeded for all 4 peaks. Despite marked variations between preparations, the majority of the II--I transition patterns in each animal tended to be characteristic, although not completely consistent. For example, in Fig. 5, 4 of the
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Fig. 5. Interval histograms of the interictal spike activity preceding 6 consecutive seizures (rows). Interictal activity preceding each seizure is sampled in 5 successive epochs ( E l - - E s ). Abscissa scaled in sec, ordinate scaled as probability of spike occurrence. N o t e shift over time f r o m quasi-gaussian to m u l t i m o d a l distributions.
6seizures revealed obvioasly homologous shifts of their spike interval distributions. Of the two seizures in which this is not obvious in the graphic display, the statistical treatment used did reveal a significantly similar shift in the spike interval distributions of one of them. In 73% of the analyzed seizures a significant firing pattern shift heralding the II--I transition was noted in the preceding 2 rain period, and in slightly less than a quarter of these, significant differences occurred as early as 5 rain prior to the II--I transition. The majority of the exceptions was of the type shown in Fig. 1A. By redefining their durations and thus the number of epochs, and by choosing lower protection levels, earlier although obviously less confident predictions could be made. It should be stressed that the selection of the analysis parameters was individually tailored to each preparation in order to optimize the II--I predictability.
Discussion
These data suggest that II--I transitions in the feline penicillin focus may occur without any obvious heralding EEG change. However, when changes in the EEG which signal an impending II--I transition do occur, they generally tend to be characteristic for an individual epileptogenic focus, but may differ markedly from one preparation to another. In the present study changes in spike frequency appeared to be the most significant predictor of such transitions. This usually was not a simple, gradual increase in frequency but rather, new patterns of discharge tended to occur. Although the mean spike frequency might not change significantly, a particular shift in the relative proportions of the different inter-spike intervals was often characteristic of an impending II--I transition. This finding is in keeping with the observation of Walker (1950) that, in the penicillin focus
IN TE R I C TAL- - I C TAL TRANSITION
'spikes appearing at first singly (rhythmic firing) become paired and then occur in runs'. Changes in the spike-interval distributions appeared to be the one feature most commonly related to all those seizures in which a reliable II--I prediction could be made. It should be emphasized, as previously noted, that the particular changes tended to be characteristic for each preparation. As with fingerprints, however, uniqueness did not make the individual markers less valuable. In his study of this problem, Ralston (1958) concluded that, in over 90% of the fits he observed II--I transitions were not characterized by an 'increase of frequency and fusion of spikes to form a seizure, but rather -- by the interpolation of a new and different form of electrical activity, the afterdischarge'. Although we found several examples of the type of 'after~lischarge' reported by Ralston, we did not find this to be a reliable marker for impending II--I transitions. On the one hand, we noted several instances in which this 'after~lischarge' pattern occurred without being followed by an organized electrical seizure. On the other hand, even when such an 'after
531
rence of this II--I transition marker (afterdischarge) is not peculiar to the preparation species nor to penicillin as an epileptogen. In the first of these studies (Chusid et al. 1953) several examples of 'after
532
increase in the spike frequency in the 5 sec period just preceding the seizure, when this is compared to the mean frequency for the immediately antecedent 55 sec. This latter observation was not a consistent finding in our preparations. A major difference between this study and the present investigation was seizure frequency. Based on their published recordings, some interictal periods appear to be as brief as 15 sec. As pointed out earlier, in the results section, we deliberately excluded from the present study preparations with very short interseizure recovery cycles. Perhaps if we had used more excitable preparations we too might have noted the changes reported to occur in the 5sec immediately preceding II--I transitions. We believe, however, that this limitation is more than offset by the advantage that our model may more closely simulate 'chronic' epilepsy. It is in that situation rather than in status epilepticus that we feel the predictability of II--I transitions is of particular clinical significance and interest. One additional feature of these experiments deserves comment. Prior to the actual onset of the high frequency seizure discharges, the emerging short inter-spike intervals tended to approach a minimum value of approximately 400--500msec. In previous experiments (Sherwin 1973) we noted that direct electrical stimulation of a penicillin focus at low frequencies could provoke long runs of one-for-one interictal spike driving. However, when driving was attempted at rates approaching 2/sec alternate stimuli might fail to provoke a spike. Stimulation at rates above 2/sec, however, consistently provoked organized seizure discharges. It is possible that our finding of an apparent constraint on the minimal duration (approximately 500msec) of the shortest interictal spike intervals may simply reflect the fact that in both sets of experiments we deliberately created relatively weak foci. Against this explanation, however, is the observation of Angeleri et al. that in their very active penicillin foci the minimum duration
I. S H E R W I N
of the shortest interictal spike intervals also was about 500 msec. Specifically, they noted that the duration of the relatively short (approximately 1.5 sec) spike intervals occurring early in an interictal period would be reduced to about 1/3 this value, i.e. 500 msec just prior to a seizure. Recently Scobey and Gabor (1977) proposed a statistical model to characterize the interictal excitability cycle of the penicillin focus. They noted that following a spontaneous interictal spike, a recovery period of about 400 msec was necessary before the probability of obtaining an evoked spike approached 100%. This finding is in keeping with the notion of an 'optimal' interictal spike frequency for the development of seizures (Elazar and Blum 1974). Scobey and Gabor attributed the temporal properties of this recovery function, indirectly, to the paroxysmal depolarizing shift characteristic of discharging neurons in the penicillin focus. Based on the present study and those cited, it is suggested that the emergence of repeated brief inter-spike intervals with durations of about 400-500 msec may indicate impending failure of those inhibitory mechanisms ordinarily operating in the interictal period and thus relate to an II--I transition. In sum, one aim of the present study was to specifically reexamine the reliability of previously reported markers of II--I transition in the feline penicillin focus. The present data strongly support and extend earlier conclusions (Walker 1950; Angeleri 1972) that in these foci certain changes occurring in interictal spike frequency are variable but important predictors of impending II--I transitions. A second aim of our work was to consider the general problem of seizure predictability based on an analysis of interictal EEG data. An obvious question that might be raised is whether the results derived from an acute model like the penicillin focus can be extrapolated to the chronic focus. Despite certain differences, the fundamental electrophysiologlcal properties of the penicillin focus are
INTERICTAL--ICTAL TRANSITION on balance mo r e like t h a t o f t he chronic focus th an d if f er ent f r o m it (Dichter and Spencer 1968; Sherwin 1977), and thus suggest t h a t similar t r e a t m e n t o f data f r o m chronic foci is at least reasonable. In the case o f chronic, h u m a n epilepsy cyclic variations in the spike activity occurring during b o t h seizure and interseizure periods have been r e p o r t e d (Stevens et al. 1972) which might require separate d a y / night analyses. It should be n o t e d t o o t h a t the specific tr eatmen ts o f the data e m p l o y e d in t h e present s tu d y represent only one approach and o t h e r statistical procedures m a y prove mo r e appropriate in particular instances. In any event, it is suggested t h a t an individualized, quantitative analysis of the stochastic properties o f the interictal E E G m a y be a potentially useful aid in the assessment and possible t r e a t m e n t of patients with frequent, p o o rl y controlled seizures.
Summary A study was u n d e r t a k e n to examine those E E G features which characterize interictal-ictal (II--I) transitions in the feline penicillin focus. The study describes certain c om pl ex changes in spike f r e q u e n c y and their statistical analysis, which appear t o have predictive value in signaling II--I transitions. T h e significance of these data with respect t o the conflicting results o f o t h e r studies and their possible application to clinical epilepsy are discussed. Rfisum~
Transition des pdriodes interictale et ictale dans les foyers dpileptogdnes d la pdnicilline du chat Une ~tude a ~t~ entreprise chez le chat p o u r examiner les donn~es E E G qui caractdrisent la transition des pdriodes interictale e t ictale (II--I) dons les f oye r s ~ la penicilline.
533 Cette ~tude d~crit certaines modifications complexes de la fr~quence des pointes et leur analyse statistique qui semble avoir une valeur predictive de signal de ces transitions II--I. J La signification de ces donndes par r a p p o r t aux r~sultats contradictoires d'autres ~tudes et leur application possible ~ l'~pilepsie clinique est discut~e. The author is pleased to acknowledge his indebtedness to Mr. David Goodman for invaluable assistance in the computer analysis of the data, and to Dr. Jeff Lieb for his critical review of the manuscript.
References Angeleri, F., Giaquinto, S. and Marchesi, G.F. Temporal distribution of interictal and ictal discharges from penicillin loci in cats. In: H. Petsche and M.A.B. Brazier (Eds.), Synchronization of EEG Activity in Epilepsies. Springer, New York, 1972: 221--234. Blum, B. and Posener, L.N. A stochastic analysis of inter-ictal epileptiform activity. Confin. neurol. (Basel), 1965, 26: 519--531. Castillo, H.T. and Sherwin, I. Encephalophone: an electronic stethoscope for the brain. J. Audit. Engng Soc., 1971, 19: 142--144. Chusid, J.G., Kopeloff, L.M. and Kopeloff, N. Experimental epilepsy in the monkey following multiple intracerebral injections of alumina cream. Bull. N.Y. Acad. Med., 1953, 29: 898--904. Dichter, M. and Spencer, W.A. Hippocampal 'spike' discharge: epileptic neurone or epileptic aggregate? Neurology (Minneap.), 1968, 18: 281--283. Duncan, D.B. Multiple range tests for correlated and heteroscedastic means. Biometrics, 1957, 13: 164--176. Efron, R. The effect of olfactory stimuli in arresting uncinate fits. Brain, 1956, 79: 267--281. Elazar, Z. and Blum, B. Interictal discharges in tungsten foci and EEG seizure activity. Epilepsia, 1974, 15: 599--610. Gastaut, H. The Epilepsies. Electroclinical Correlations. (English translation by M.A.B. Brazier.) Thomas, Springfield, Ill., 1954. Hanbery, J.W. and Ajmone Marsan, C. Intrathecal injection and cortical application of chloramphenicol -- an experimental study with a review of the local action of antibiotics on the central nervous system. J. Neuropath. exp. Neurol., 1954, 13: 297--317.
534 King, R.B., Schricker, J. and O'Leary, J.L. An experimental study of the transition from normal to convulsoid cortical activity. J. Neurophysiol., 1953, 16: 286--298. Ralston, B.L. The mechanism of transition of interictal spiking loci into ictal seizure discharges. Electroenceph. clin. Neurophysiol., 1958, 10: 217--232. Ralston, B.L. and Papatheodorou, C.A. The mechanism of transition of interictal spiking foci into ictal seizure discharges. Part II. Observations in man. Electroenceph. clin. Neurophysiol., 1960, 12: 297--304. Scobey, R.P. and Gabor, A.J. Properties of epileptogenic focus: activation field. J. Neurophysiol., 1977, 40: 1199--1213. Servit, Z. Reflex Mechanisms in the Genesis of Epilepsy. Elsevier, Amsterdam, 1963. Sherwin, I. Differential effects of hyperventilation
I. SHERWIN on intact and isolated cortex. Electroenceph. clin. Neurophysiol., 1963, 18: 559---607. Sherwin, I. Seizures precipitated by the use of language. Cortex, 1966, 2: 349--356. Sherwin, I. Alterations in the non-specific cortical afference during hyperventilation. Electroenceph. clin. Neurophysiol., 1967, 23: 532--538. Sherwin, I. Suppressant effects of diphenylhydantoin (Dilantin) on the cortical epileptogenic focus. Neurology (Minneap.), 1973, 23: 274--281. Sherwin, I. Stereotyped and structured bursts activity of single units in the penicillin epileptogenic focus in cats. Exp. Neurol., 1977, 55: 226--233. Stevens, J.R., Lonsbury, B.L. and Goel, S.L. Seizure occurrence and interspike interval: telemetered electroencephalogram studies. Arch. Neurol. (Chic.), 1972, 26: 409--419. Walker, A.E. Convulsive activity. Quart. Phi Beta Pi, 1950, 47: 108--115.