Transcortical reflexes and focal motor epilepsy

Ictal haemodynamic changes in a patient affected by "subtle" Epilepsia Partialis Continua

We report on a 64 year-old woman presenting with Epilepsia Partialis Continua (EPC) affecting the left hand since the age of 24 without neurological deficit. Structural MRI showed a region of focal cortical dysplasia (FCD) over the right central gyrus and lesions in the mesial frontal and occipital cortex secondary to perinatal hypoxic injury. Ictal spike haemodynamic mapping using simultaneous EEG-fMRI revealed significant BOLD signal changes prominent in the region of FCD (larger cluster), occipital cortex (global statistical maximum), prefrontal cortex and cerebellum. The cluster over FCD was in good agreement with the result of EEG source analysis. Our findings provide an interesting illustration of the ability of EEG-fMRI to reveal epileptogenic networks confirming the intrinsic epileptogenic properties of dysplastic neurons.

The role of subcortical structures in human epilepsy

Like normal cerebral function, epileptic seizures involve widespread network interactions between cortical and subcortical structures. Although the cortex is often emphasized as the site of seizure origin, accumulating evidence points to a crucial role for subcortical structures in behavioral manifestations, propagation, and, in some cases, initiation of epileptic seizures. Extensive previous studies have shown the importance of subcortical structures in animal seizure models, but corresponding human studies have been relatively few. We review the existing evidence supporting the importance of the thalamus, basal ganglia, hypothalamus, cerebellum, and brain stem in human epilepsy. We also propose a "network inhibition hypothesis" through which focal cortical seizures disrupt function in subcortical structures (such as the medial diencephalon and pontomesencephalic reticular formation), leading secondarily to widespread inhibition of nonseizing cortical regions, which may in turn be responsible for behavioral manifestations such as loss of consciousness during complex partial seizures.

Involvement of proprioceptive feedback in brainstem-triggered convulsions

PURPOSE

In rodents, specific motor components of generalized convulsive seizures depend on two distinct anatomic substrates: (a) forebrain networks are responsible for facial and forelimb clonus with or without rearing and falling; and (b) brainstem networks are responsible for running-bouncing fits and tonic convulsions. To investigate the requirement of proprioceptive inputs in the generation of these two different types of seizures, we compared the effects of neuromuscular blockade by D-tubocurarine on the EEG expression of brainstem- and forebrain-triggered seizures.

METHODS

Unilateral electrical stimulations were applied for 50 consecutive days in freely moving male adult rats through a bipolar electrode aimed at the dorsal hippocampus (n = 5), the occipital cortex (n = 4), the inferior colliculus (n = 6), or the midbrain reticular formation (n = 6). Two days after the last stimulation, rats were paralyzed with d-tubocurarine and stimulated in the same way.

RESULTS

In brainstem structures, the first electrical stimulation induced tonic seizures concomitant with low-voltage cortical activity; repetition of daily stimulations progressively induced tonic-clonic seizures associated with high-amplitude cortical spike-wave discharges. After immobilization by d-tubocurarine, brainstem stimulations failed to induce any EEG paroxysm. In forebrain structures, repeated electrical stimulations produced a classic kindling with progressive occurrence of clonic seizures associated with large cortical discharges; d-tubocurarine left unchanged the EEG pattern of these latter seizures.

CONCLUSIONS

These data suggest that proprioceptive reafferentation resulting from movement is necessary for the generation of self-sustained brainstem seizures but is not implicated in the elaboration of forebrain seizures.

Suppression of motor seizures after specific thalamotomy in chronic epileptic monkeys

In the monkey with a chronic motor epileptogenic focus, thalamic lesions restricted to the anterior part of the ventro-postero-lateral nucleus produced a long-lasting improvement, leading in most cases to almost complete seizure suppression. This improvement was attributed to the interruption of a long-loop recurrent excitatory mechanism, involving muscular afferents which are known to relay in the thalamic region where lesions were produced. The specific nature of this effect was suggested by the preliminary results of lesions performed in thalamic regions with non-specific cortical projections. Compared with the pre-lesional situation in which focal activity resulted in permanent motor troubles, the post-lesional state did not show any additional motor deficit. It is concluded that such lesions could be a new approach to controlling intractable motor epilepsies in man.

Kojewnikow's Epilepsia Partialis Continua: two cases associated with striatal necrosis

The authors report two cases of children suffering from Epilepsia Partialis Continua (EPC). The first case concerned a boy primarily affected by abdominal neuroblastoma and secondarily by bilateral EPC; "pallidal posture" was the prominent clinical feature. An acute measles encephalitis was diagnosed and the CT scan showed necrosis of the putamina. The second case concerned a girl suffering from increased intracranial pressure due to suprasellar craniopharyngioma. Seven days after intervention, Diabetes Insipidus and EPC appeared. Enlargement of rolandic and sylvian spaces and lacunar necrosis of the putamen on the left side were also evident on the CT scan. The authors emphasize the significance of occasional metabolic disturbances, especially natremia, in the development of EPC.

Distinction between the myoclonus-related potential and the epileptic spike in epilepsia partialis continua

Neurophysiological analysis of spontaneous and reflex-triggered myoclonus in a case of epilepsia partialis continua is reported. A central positive wave (P1), part of the high voltage evoked potential and of the spontaneous spike, is held to be responsible for the myoclonic jerk. It is demonstrated that P1 is a distinct phenomenon from both the evoked potential and the epileptic spike, and so should be identified as a myoclonus-related potential (MRP).

Evoked potentials from passive elbow movements. II. Modification by motor intent

Subjects were instructed to remain passive or to react to a forearm perturbation by opposing the imposed movement. Evoked potentials (EPs) were recorded at 8 scalp sites in both conditions. In the React condition, reflexes were observed in the EMG (mean onsets of 67 and 81 msec) and the EP was modified. Source derivation techniques revealed that the earliest cortical response (31 msec) across the central sulcus was not changed. Therefore the intention to react did not seem to affect afferent transmission to the primary sensorimotor cortex. Two periods of modulation were observed. In both, parietal and frontal potentials were modulated together, prior to the reflex components. After 70 msec, the pattern of potential gradients which occurred in the Passive case was accelerated and intensified in the React condition. The overall effect was to focus a larger zone of negativity over motor cortex at the time of triggered EMG output (109 msec). The earlier changes in cortical activity could be causally related to the appearance of the late stretch reflex. Since parietal and frontal areas were principally involved and not the motor area, it is suggested that the former exert a modulatory influence on spinal and brain-stem reflex centres.

Penicillin epileptogenic focus in the rat: requisites for transcortical reflex triggering

Epileptiform discharges elicited by natural or electrical stimulations, proprioceptive or cutaneous, were studied in the rat with an experimental acute focus induced by penicillin application in the motor area. EEG paroxystic spikes were easily triggered with restricted foci (0.5 to 1 mm2) located in the representation area of the stimulated region. However, despite the large overlap of sensory and motor cortical limb areas in the rat, EEG spikes, either spontaneous or triggered, were followed by muscular jerks only with much larger foci: at least 2 and 4 mm2, respectively, for anterior and posterior limb areas. Cutaneous stimulations were the most efficient in discharge production; however, discharges were triggered indifferently by muscular or cutaneous afferent fibers in about three-fourths of the cases. The temporal relation between EEG spike and myoclonic jerk were very close. A latency analysis (delay between triggered EEG spike and EMG response, parallel latency fluctuation of both phenomena, delay between spontaneous EEG spike and jerk) supported the hypothesis that a transcortical reflex mechanism, rather than a spinal excitability rebound, was involved in the jerk genesis. Iontophoretic ejection of penicillin within layers III-IV resulted in the development of electroclinical paroxysms. However, similar penicillin ejection within layer V, did not allow efferent discharge production. It is concluded that the involvement of a large surface or volume of cortical tissue is required to produce efferent discharges following EEG paroxysms. This observation is likely related to the unexpectedly wide representation of individual muscles at the motor cortical level.

Epilepsia partialis continua: active cortical spike discharges and high cerebral blood flow in the motor cortex and enhanced transcortical long loop reflex

We report a patient suffering from persistent myoclonic jerks in the right forearm without any definite EEG abnormality under routine recording conditions. By computer summation, using the jerk-locked averaging technique, a sharp spike was recognized as a precisely time-locked event in relation to myoclonic twitches. A cranial CT scan revealed a small cortical lesion, which was found very close to the sensorimotor cortex of the right arm. Cerebral blood flow study using the xenon inhalation method revealed a discrete focus of high flow, which corresponded well with the CT lesion. On electrical stimulation of the right median nerve, a large somatosensory evoked potential and an enhanced transcortical long loop reflex were observed. Electrocorticogram showed active focal spike discharges localized at the left precentral gyrus. We postulate that an epileptogenic focus in the motor cortex and an enhanced transcortical long loop reflex appear to be important for the occurrence of epilepsia partialis continua in this patient.

Myoclonus: relation to epilepsy

Myoclonus can be divided into those types which are fragments of epilepsy and those which are nonepileptic. Epileptic myoclonus is viewed as the effect of an isolated spike in neurons of the motor system. Cortical reflex myoclonus is a fragment of partial epilepsy and represents hyperactivity of a focal area of cerebral cortex. Reticular reflex myoclonus is a fragment of generalized epilepsy with hyperactivity of medullary brainstem reticular formation. Primary generalized epileptic myoclonus is a fragment of primary generalized epilepsy and may represent a generalized hyperactive response of cortex to subcortical input.

Electrophysiological studies of myoclonic jerks provoked by photic stimulation

A patient with a type of myoclonus produced by somatosensory and photic stimuli was studied electrophysiologically. Jerk-locked averaging showed cortical spikes in association with the spontaneous myoclonus. High amplitude somatosensory evoked potentials and high amplitude visual evoked potentials were observed. Somatosensory and photic stimuli produced 2 or 3 successive electromyographic (EMG) discharges. The time interval from the onset of the preceding EMG discharge to that of the following EMG discharge produced by photic stimulation corresponded to the latency of a long-loop reflex. It also corresponded to the time interval from the onset of the preceding EMG discharge to that of the following EMG discharge produced by somatosensory stimulation. Not only somatosensory but also photic stimuli might excite the loop of a long-loop reflex and cause myoclonic jerks.

Modulation of proprioceptive transcortical reflexes in the cat with a penicillin epileptic motor focus

Mechanisms responsible for the triggering of paroxysmal events by proprioceptive afferents, previously described in the monkey with a chronic epileptic focus, were studied in more detail in the cat with a penicillin focus. To analyse the topical organization of this reflex triggering, the focus was restricted to very small areas of the motor cortex; in this study only pericruciate areas were considered in which stimulation elicited a motor response in one of the several forelimb muscles tested, and which received afferents from that muscle. When the focus was located in the post-sigmoid gyrus, stimulation (usually by stretch) of the given (target) muscle first elicited a cortical spike following the evoked response, and secondly a late phasic EMG response (about 40 msec latency) quite distinct from purely spinal reflexes. Cortical spikes and late EMG responses were closely correlated, especially considering their probability of occurrence or their parallel latency fluctuations. In most cases, this effect was limited to the muscle whose motor area had been treated with penicillin: stretching muscles in the vicinity was ineffective, nor were these muscles activated when the target muscle was stimulated. Evidence is given for the participation of a transcortical reflex in the generation of the late phasic response and for the involvement of the pyramidal tract in this reflex.

Physiologic basis for focal motor seizures and the Jacksonian "March" phenomena

Mechanisms underlying focal motor seizures and the Jacksonian “march” have been explored on the basis of recent physiologic data concerning the “nested ring” spatial and functional organization of motor cortex. Focal motor seizures can be understood in terms of the focal representation at motor cortex of elements controlling movement of a limb part about a single joint. The preponderance of three foci of origin of motor seizures, first reported by Hughlings Jackson, are related to lower thresholds of excitability at these loci, as demonstrated for forelimbs in the case of subhuman primates, and to the resultant bias of peripheral and central input to activate these loci. The “march” phenomenon which typically involves specific patterns of spread in the forearm, is explained in terms of the recently discovered nested ring organization. Thus the centrifugal spread of excitation from a particular locus in motor cortex would demand the pattern of spread of convulsive movements generally observed in clinical situations. The latter involves spread from face or leg to shoulder, forearm, then to hand, or conversely.

The physiological basis of activation and/or inhibition of focal motor epilepsy by peripheral stimuli is also now available on the basis of current experimental evidence. Thus the point to point coupling of periphery and motor cortex as defined in terms of control of limb parts about single forelimb joints, provides a substantial understanding of many of the clinical events sometimes termed “reflex” epilepsy.