Symptomatic Focal Epilepsies

Nancy Foldvary-Schaefer

Certain epileptic disorders are characterized by clusters of signs and symptoms that are considered epileptic syndromes. Because most of these syndromes have numerous etiologic factors, few are defined as specific diseases. The 1989 International League Against Epilepsy (ILAE) classification system subdivides epilepsies and epileptic syndromes into three categories based on clinical history, electroencephalographic manifestations, and etiologic factors (Table 23.1) (1). Localization-related epilepsies and syndromes involve seizures that originate from a localized cortical region. Symptomatic epilepsies have an identifiable cause, such as mesial temporal sclerosis (MTS). Cryptogenic syndromes are presumed to be symptomatic, but they have no known cause and occur with or without accompanying neurologic abnormalities. Idiopathic epilepsies and syndromes are presumed to be inherited.

TEMPORAL LOBE EPILEPSY

Temporal lobe epilepsy (TLE) constitutes nearly two thirds of localization-related epilepsies of adolescence and adulthood (2). Although mesial temporal lobe epilepsy (MTLE) is believed to represent most cases, recent advances in neuroimaging increasingly identify seizures arising from the temporal neocortex. The natural history of TLE is variable, with as many as 30% to 40% of patients continuing to have seizures despite appropriate medical management (2).

Mesial Temporal Lobe Epilepsy

The seizures of MTLE, the most widely recognized symptomatic focal epilepsy, arise from the hippocampus, amygdala, and parahippocampal gyrus. Birth and development are normal, and febrile seizures in infancy or childhood are the most common risk factor. In one series (3), 45 (67%) of 67 patients had febrile seizures without a recognized central nervous system (CNS) infection prior to onset; seizures were complicated in 33 (73%) of 45 cases. Other risk factors include CNS infection, head trauma, and perinatal injury. Onset ranges from the latter half of the first decade of life to early adulthood, typically after a latency period following the presumed cerebral insult.

Most patients report auras, such as rising abdominal sensations and fearful feelings. Anxiety, olfactory disturbances, psychic phenomena such as déjà vu, and nonspecific cephalic sensations are frequently experienced. Virtually all patients have complex partial seizures with oral or manual automatisms (3, 4, 5); unilateral dystonic posturing of a contralateral extremity occurs in approximately 25% of patients (6). Postictal language disturbances are highly predictive of epilepsy arising from the dominant temporal lobe, whereas interpretable speech during the ictus strongly suggests nondominant temporal lobe origin (7), as do the less common ictal vomiting and automatisms with preserved responsiveness (8,9). Motor manifestations, such as facial clonic activity and tonic posturing of one or more extremities, appear 10 or more seconds after clinical onset, once the ictal discharge has spread to extratemporal or neocortical temporal structures (10). Autonomic manifestations include hyperventilation and changes in blood pressure and heart rate. Postictal coughing or nose wiping can occur; both are more common in patients with MTLE than in those with neocortical temporal lobe seizures (11, 12, 13). The hand used for nose wiping is usually ipsilateral to the seizure origin. In addition to complex partial seizures, about half of patients with MTLE also experience secondary generalized tonic-clonic seizures (GTCSs), which may be preceded by version of the head or eyes, or focal clonic activity of the
face or upper extremity contralateral to the epileptogenic temporal lobe. Stress, sleep deprivation, alcohol ingestion, and menstruation may all precipitate seizures. Status epilepticus occurs in a minority of patients.

TABLE 23.1 SYMPTOMATIC FOCAL EPILEPSIES

Temporal lobe
	Mesial
	Neocortical
Frontal lobe
	Supplementary motor
	Cingulate
	Anterior frontopolar
	Orbitofrontal
	Dorsolateral
	Opercular
	Motor cortex
Parietal lobe
Occipital lobe

During prolonged electroencephalographic monitoring, more than 90% of patients with MTLE demonstrate epileptic discharges localized to the sphenoidal or anterior temporal electrodes (14,15). Bitemporal independent spikes or sharp waves, maximal on the side of seizure origin, occur in 25% to 50% of cases (14, 15, 16). Additional closely spaced electrodes designed to record from the basal or mesial temporal regions facilitate definition of anterior temporal discharges. Surface electrodes placed 1 cm above a point one-third the distance between the external auditory meatus and the external canthus (T1 and T2 placements), FT9 and FT10 in the International 10-10 Electrode Placement System, or sphenoidal electrodes may be used (17). Sphenoidal electrodes are positioned beneath the zygomatic arch approximately 2.5 cm anterior to the incisura intertragica, roughly 10 degrees superiorly from the horizontal plane and posteriorly from the coronal plane (18); they are well tolerated and relatively free of artifacts and complications. Mesial temporal spikes have maximal negativity at anterior temporal or sphenoidal electrodes and widespread positivity over the vertex (8,19,20). Epileptic discharges are absent on serial electroencephalograms (EEGs) in less than 10% of cases (15). One-third of patients have temporal intermittent rhythmic delta activity, consisting of repetitive, rhythmic, saw-toothed or sinusoidal 1- to 4-Hz activity, 50 to 100 μV in amplitude, occurring in the anterior regions (21).

Most temporal lobe seizures involve a gradual buildup of lateralized or localized rhythmic alpha or theta activity, which may be preceded by diffuse or lateralized suppression or arrhythmic activity (11,16,22). The rhythmic activity occurring in the ipsilateral temporal region within 30 seconds of clinical or electrographic seizure onset (8,15,22) correctly predicts an ipsilateral temporal onset in more than 80% of cases, as confirmed by invasive recordings (22). An initial, regular 5- to 9-Hz inferotemporal rhythm is more specific for seizures of hippocampal origin, although this pattern requires the synchronous recruitment of adjacent inferolateral temporal neocortex (23). Seizures confined to the hippocampus, as revealed on intracranial EEG, produce no change on scalp recordings (24). Because of the vertical orientation of dipole sources, seizures of mesiobasal temporal origin may produce rhythmic activity of positive polarity at the vertex coincident with a negative rhythm at the sphenoidal or temporobasal surface electrodes (23,24). Lateralized postictal slowing or background attenuation correctly predicts the side of origin in 96% to 100% of seizures (15,25). False lateralization is observed in 3% to 13% of patients with MTLE (8,11,15) and, like nonlateralized patterns, is more common in patients with bitemporalindependent epileptic discharges (26). In a study of 184 patients with TLE, all patients with unilateral hippocampal atrophy had concordant interictal and ictal EEG lateralization (27).

High-resolution magnetic resonance imaging (MRI) frequently reveals hippocampal atrophy and abnormal signal intensity in the mesial temporal region suggestive of MTS. ¹⁸F-fluorodeoxyglucose positron emission tomography (PET) demonstrates temporal lobe hypometabolism that usually extends to the ipsilateral frontoparietal cortices and basal ganglia. Both ictal and interictal single-photon emission computed tomography (SPECT) scans show unilateral abnormalities, although the spatial resolution is inferior to that of PET and time of injection relative to seizure onset dramatically influences results.

MTS is the most common pathologic substrate of MTLE (28), with marked neuronal loss in CA1, CA3, CA4, and the dentate granule cells, and relative sparing of the CA2 pyramidal cells, subiculum, entorhinal cortex, and temporal neocortex. Loss of granule cell innervation leads to reactive synaptogenesis and a subsequent excitatory process capable of initiating and propagating seizures. The co-existence of MTS and extralimbic lesions is observed in approximately 30% of surgical specimens (29). Neoplasms, vascular malformations, and developmental malformations of the mesial temporal structures can also produce epilepsy with clinical and electrophysiologic features similar to those of MTLE caused by MTS.

Neocortical Temporal Lobe Epilepsy

The clinical, neuroradiologic, and electrophysiologic manifestations of neocortical temporal lobe epilepsy (NTLE) are less well defined than in MTLE. Because its pathologic substrates are more diverse than those in MTLE, age of onset is variable, with seizures often beginning in the third decade of life or later (30). Febrile seizures in infancy or early childhood are relatively rare (25,30,31, 32, 33), but head trauma, birth injury, and CNS infection are found more frequently in these patients than in those with MTLE (30,33).

Ictal activation of identical structures may render the clinical symptoms of neocortical temporal seizures
indistinguishable from those observed with seizures originating from mesial temporal structures. Abdominal auras, psychic phenomena, and nonspecific sensations are frequently reported. In one study (34), abdominal auras were common in patients with MTLE, whereas patients with NTLE described psychic phenomena. However, abdominal auras did occur in patients with NTLE, particularly when the ictal discharge propagated to the mesial temporal structures. Auditory, vestibular, and complex visual phenomena caused by activation of the Heschl gyrus and visual and auditory association cortices are relatively rare, but reliably predict temporal neocortical activation. Complex partial seizures occur in most patients, and GTCSs are also observed. Automatisms and contralateral dystonic posturing occurring early in the attack are less common in patients with NTLE than in those with MTLE; early clonic activity of the contralateral upper extremity or facial grimacing is suggestive of a neocortical temporal origin (30,31,33, 34, 35, 36). Autonomic manifestations were notably absent in a small study of NTLE seizures (30).

The electrophysiologic manifestations of NTLE may also be indistinguishable from those of MTLE. The distribution of epileptiform activity in NTLE varies with the location of the epileptogenic zone. Interictal spiking may be maximal at lateral or posterior temporal electrodes; however, sphenoidal and anterior temporal interictal discharges also are observed (24,30,33). Ictal patterns include temporal rhythmic theta or alpha activity, as seen in seizures of mesial temporal origin; irregular, polymorphic 2- to 5-Hz lateralized or localized patterns; and nonlateralized arrhythmic activity (24,30,33). Bilateral ictal patterns are more common and appear earlier in patients with NTLE than in those with MTLE, and rhythmic activity is hemispheric rather than temporal in distribution (11,30,33).

FRONTAL LOBE EPILEPSY

Frontal lobe epilepsies (FLEs) are subclassified according to the presumed location of the epileptogenic zone. Several distinct syndromes have been described, but in some cases, the clinical, electrographic, and neuroimaging features are variable and localization is not possible. After TLE, FLE is the most common type of epilepsy, accounting for 20% to 30% of cases in surgical series (34,37). Frontal lobe seizures take the form of simple partial, complex partial, atonic, tonic, myoclonic, and tonic-clonic attacks. Simple partial seizures involve motor, sensory, autonomic, affective, and cognitive phenomena. Complex partial seizures differ from those originating in the temporal lobe by their abrupt onset, short duration, rapid secondary generalization, and minimal or no postictal state (37). Status epilepticus and seizure clusters are relatively common. Generalized motor activity, version, and focal clonic and tonic activity are frequently observed. Complex motor automatisms, including bicycling and thrashing movements, sexual automatisms, laughter, and vocalizations, may result in misdiagnosis as psychogenic seizures and, when present early in the attack, distinguish FLE from TLE (5,38). Complex partial status epilepticus occurs in 40% of patients with FLE (37). Prolonged absence seizures with generalized 3-Hz spike-wave complexes and focal motor features or language disturbances also have been observed (39).

Ictal symptomatology may be more helpful than an EEG in localizing frontal lobe seizures. The inaccessibility of a large portion of the frontal lobes to surface electrodes, the rapid spread of seizures within and outside this area, secondary bilateral synchrony and bilateral epileptogenesis as a consequence of bifrontal injury, and variably sized seizure onset zones all contribute to the lack of electroencephalographic localization or mislocalization in patients with FLE (40,41). The interictal EEG may be entirely normal or may reveal generalized or lateralized slowing, focal, hemispheric, multiregional, or generalized spikes or polyspikes, or low-voltage fast activity. In a large study of patients with FLE, interictal epileptiform activity appeared bilaterally synchronous in 37%, lobar in 32%, multilobar in 24%, focal in 12%, and hemispheric or bifrontal and independent each in 9% of cases (42). Epileptiform activity is restricted to the frontal lobes in only 25% of patients and to other areas in 20% to 50%; in 20% to 70% of patients, epileptiform activity is absent (37,43). Supra- and infraorbital, sphenoidal, and additional closely spaced scalp electrodes enhance spike detection in patients with suspected orbitofrontal or mesial frontal seizures. Midline epileptiform activity may be mistaken for vertex sharp transients that appear spike-like in children or may escape detection if viewed on montages not incorporating midline electrodes. Secondary bilateral synchrony, in which a unilateral epileptogenic focus near the midline produces seemingly bilaterally synchronous generalized epileptiform activity, is observed in up to 20% of patients (37). Unilateral discharges that precede and initiate the bilateral epileptiform activity, distinct morphology of focal and bilateral discharges, isolated unilateral discharges, and focal slowing distinguish secondary bilateral synchrony from the generalized epileptiform activity characteristic of idiopathic generalized epilepsies. Bilaterally synchronous bursts of spike-and-wave complexes occurred in 26 of 31 patients whose epilepsy was associated with frontal parasagittal lesions (44).

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