Clinical manifestations

Presentation and course

Temporal lobe epilepsy is typically divided into mesial and neocortical (24). A distinct electroclinical constellation associated with a specific cause is rare for neocortical temporal lobe epilepsy but has been described for patients with autosomal dominant temporal lobe epilepsy presenting with focal seizures with prominent ictal auditory phenomena, normal MRI (42), and usually a good response to antiseizure medications (50). However, most neocortical temporal lobe seizures are characterized by their etiologic, semiologic, and electroclinical heterogeneity (39).

Neocortical temporal lobe seizure semiology varies considerably according to the area of the temporal lobe involved (24). The classical lateral temporal seizures are characterized by auditory, somatosensory, aphasia, or psychic auras followed by staring and contralateral facial and arm clonic movements, sometimes with secondary generalization. These “pure” temporal lateral seizures have an epileptogenic zone in the mid-posterior aspect of the superior temporal gyrus, usually involving primary or associative sensory areas for auditory, visual, or language functions (02). This region has few direct limbic connections, which may explain the fact that these auras may be prolonged with fewer automatisms as compared to seizures originating in other areas of the temporal lobe (30). By contrast, the lateral anterior and lateral inferior neocortical structures are highly interconnected with limbic structures (30; 02), explaining that seizures from these areas have semiology often very similar to mesial temporal seizures associated with hippocampal sclerosis (11).

The temporal pole is highly connected with the amygdala, the hippocampus, the parahippocampal gyrus, the cingulate gyrus, the orbitofrontal cortex, and the insula (14). The temporal pole cortex extends topographically to the insula (ventrally) and the entorhinal cortex (medial-inferiorly). A network analysis study revealed that the cortex extending from the temporal pole to the entorhinal area is one of the regions with the most association connections with other parts of the brain (08). Therefore, temporopolar seizures may present with different types of auras, often similar to mesial temporal seizures, including autonomic and rising epigastric sensations. In addition, tonic or clonic seizures, as well as hypermotor seizures, which are uncommon in temporal lobe epilepsy, may occur in patients with temporopolar seizures (14).

Mesial temporal lobe seizures have been studied more extensively than any other type of seizure and constitute a major manifestation of a recognized syndrome (19; 13). Because of these observations, mesial temporal lobe seizures can be used as a benchmark or gold standard against which other seizures can be compared.

Typical mesial temporal lobe seizures begin with an aura of a rising epigastric sensation and progress to altered consciousness with variable oro-alimentary (lip-smacking, chewing) and manual automatisms (picking, fumbling) (19). Manual automatisms often occur ipsilateral to the side of seizure origin, and dystonia of the hand may occur contralateral of the side of seizure origin (66). The patient may be partially responsive during a seizure and often has postictal confusion or aphasia.

Several reviews of temporal lobe epilepsy did not attempt to differentiate mesial from lateral temporal lobe seizures, implying there is no difference in clinical manifestations (02; 64). This implication is in agreement with another review that specifically examined this issue and concluded the two seizure types could not be distinguished (09). Extensive reciprocal connections between mesial and neocortical temporal structures could explain the similar clinical features from both regions (31; 07). Experiential and hallucinatory phenomena may require seizures to involve both mesial and neocortical structures (31; 07).

Nevertheless, some clinical features might help identify neocortical temporal lobe seizures.

Initial symptoms. Although mesial and neocortical temporal lobe seizures undeniably share common auras, several symptoms are more typical of seizure onset from the temporal neocortex.

Auditory hallucinations or distortions. Auditory hallucinations of any type should suggest seizure origin in the superior region of the posterior superior temporal gyrus, or at least in the lateral temporal neocortex. In a large, well-documented series of mesial temporal lobe seizures, there were no examples of auditory auras of any kind (25). On the other hand, a later report from that same group described auditory auras from both regions of seizure onset (56). A study using PET associated auditory auras with lateral neocortical onset (68). Simple auditory auras are thought to originate in the Heschl gyrus, the primary auditory area (46).

Vertiginous sensations. Vertigo has been associated with epilepsy since ancient times, but it almost certainly was over-interpreted (60; 05). True vertigo (tornado seizures) as a seizure symptom does occur, but only rarely (26). Over the years, this symptom has been equated with seizure origin in the posterior temporal neocortex or the temporoparietal junction (65); others have reported it with frontal seizure onset (41). There are, overall, few well-documented examples.

Complex visual hallucinations. Elementary visual hallucinations are a well-established symptom of seizures beginning in the occipital region, but the origin of complex visual hallucinations or visual flashbacks are generally thought to originate in the visual association cortex at the occipitotemporal or temporoparietal junctions, often including the posterior fusiform gyrus (31; 58; 07; 47). Therefore, auras of complex visual hallucinations should be equated with neocortical seizure activity unless there is strong evidence to the contrary. Overall, these complex visual epileptic auras are rare.

The typical epigastric aura with a rising sensation in the abdominal area is rare in patients with neocortical temporal lobe seizure onset and should suggest mesial temporal seizure origin (33; 48). By contrast, auditory auras can occur rarely in seizures originating in the mesial temporal region (44).

Signs (objective seizure manifestations). There are many similarities between the objective manifestations of mesial and neocortical temporal lobe seizures, but there may be some trends that help separate them. Several studies have noted that the motor manifestations (automatisms) are less common in lateral neocortical seizures (29; 23; 56). A study found that lateralized motor features (contralateral dystonic posturing, ipsilateral automatisms) seen in mesial temporal lobe seizures occurred with opposite laterality in neocortical temporal seizures (ipsilateral dystonic postures and contralateral automatisms) (16). Neocortical temporal lobe seizures generally lacked the motor activity associated with mesial temporal lobe seizures (52). One study compared patients with mesial temporal lobe epilepsy and hippocampal sclerosis with patients with neocortical temporal lobe epilepsy successfully treated by neocortical resections sparing the hippocampus. Epigastric rising sensation (6% vs. 40%), oral automatisms (29% vs. 80%), gestural automatisms (47% vs. 80%), and dystonic posturing (0% vs. 40%) were significantly less frequent in neocortical temporal lobe epilepsy (48).

Prognosis and complications

Prognosis depends on the etiology, eg, slow-growing neoplasms; vascular malformations carry their own prognostic and complication factors. The overall prognosis of neocortical temporal lobe seizures without detectable lesions has not been established.

Biological basis

Localization

By definition, neocortical temporal lobe seizures have their origin in the temporal neocortex, excluding all mesial structures (24). This would include all of the lateral temporal neocortex, the inferior temporal neocortex (excluding the parahippocampal gyrus), the temporal polar region, and the superior plane of the temporal lobe.

Etiology and pathogenesis

Most etiologies of neocortical epilepsies are low-grade tumors, malformations of cortical development, posttraumatic, ischemic, and inflammatory-infectious scars, and cavernous angioma (03; 10; 06).

Pathophysiology

There is no unique pathophysiology to neocortical temporal lobe seizure origin. Any destructive, neoplastic, vascular, or congenital epileptogenic lesion can result in seizures from these regions. As previously noted, preferential spread patterns may be to mesial structures, producing seizures similar to mesial temporal lobe seizures. Other potential spread patterns have not been extensively studied.

Dipole modeling of ictal and interictal patterns have shown differences between neocortical and mesial temporal lobe seizures (01). Ictal scalp EEG patterns may also help differentiate the two types of temporal lobe seizures (17; 51; 37; 45), but intracranial EEG recording may be required to differentiate between mesial temporal and lateral neocortical seizure onset. Coherence analysis of ictal data from intracranial studies can identify different subtypes of temporal lobe seizures and assist in distinguishing lateral from mesial temporal lobe seizures (02).

In a study with intracranial stereo-electroencephalography (SEEG), the cortico-cortical evoked potentials responsiveness triggered by single pulse electrical stimulation was abruptly reduced in the onset of mesiotemporal seizures but preserved in neocortical temporal seizures (55). These findings may help to explain the clinical differences between mesial and neocortical temporal lobe seizures.

Differential diagnosis

Confusing conditions

The main differential diagnosis of neocortical temporal lobe seizures is mesial temporal lobe seizures. As noted previously, preferential spread patterns to mesial structures would produce seizures similar to those beginning medially. This is true of some, but not all, temporal neocortical seizures.

As mesial temporal lobe seizures mainly occur in association with mesial hippocampal sclerosis, the presence or absence of certain risk factors such as complicated febrile seizures can help differentiate the two conditions (51; 53; 39; 48), but in clinical practice, the presence of MRI findings of hippocampal sclerosis or a neocortical lesion are the most helpful findings to differentiate mesial from neocortical temporal lobe epilepsies (11; 13). One study suggested some of the lateralizing findings associated with mesial temporal lobe seizures (contralateral dystonic posturing, ipsilateral automatisms, etc.) present in a mirror image fashion in neocortical temporal lobe seizures (16). An auditory aura of any type, complex visual auras, or complex memory flashbacks should suggest neocortical temporal seizure onset, but this needs better documentation. A study analyzing intracranial video-EEG recordings showed that a generalized tonic phase occurred in all seven patients with mesial temporal lobe epilepsy who had focal onset bilateral motor (convulsive) seizures, but in only 7 of 13 (54%) of those of neocortical temporal lobe epilepsy (54).

When the MRI reveals potentially epileptogenic structural lesions in mesial or neocortical temporal regions, the differential diagnosis is greatly simplified. Patients with normal MRI and suspected mesial or neocortical temporal origin are much more challenging. Ictal SPECT scanning and coregistration with MRI, although logistically difficult, can provide diagnostic data not otherwise obtainable (61; 12). In addition, proton MR spectroscopy and PET studies can be helpful to distinguish mesial from neocortical temporal lobe seizures (40; 32).

Patients with MRI-negative temporal lobe epilepsy may have neocortical or mesial temporal lobe seizure onset, and the differential diagnosis of the seizure onset zone in these patients may be difficult without invasive EEG monitoring (15).

In addition, seizures originating outside of the temporal lobe can secondarily invade the mesial or neocortical temporal regions or both. This has been documented for orbitofrontal and occipital seizure onset (36; 62). Clinical seizure characteristics and other diagnostic studies will help identify some, but not all, of these patients (66).

Even in some patients with clearly MRI defined hippocampal sclerosis, seizure semiology may suggest neocortical temporal lobe onset, and intracranial EEG investigation may reveal seizures originating in temporal pole or other neocortical regions, either independently or simultaneously with the mesial temporal structures (14). Based on stereo-EEG recordings, Kahane and Bartolomei proposed the mesial, temporopolar, mesiolateral, lateral, and temporal “plus” subtypes of temporal lobe epilepsy associated with hippocampal sclerosis (38).

Associated and underlying disorders

An inherited epilepsy syndrome with predominant auditory seizures has been identified in several families. These seizures are thought to originate in the lateral neocortical temporal cortex (49; 67; 42). The syndrome was termed autosomal dominant partial epilepsy with auditory features or familial lateral temporal lobe epilepsy (42). A mutation in one copy of the leucine-rich, glioma-inactivated 1 gene (LGI1) in the 10-cM region on chromosome 10 q24 is present in approximately 50% of families associated with the syndrome (04; 42). Seizures are characterized by prominent auditory hallucinations such as auras. Secondarily generalized seizures are common (53), and MRI findings are usually normal (67). However, others describe abnormalities of the temporal neocortex that can be visualized on MRI (43).

Most cases of temporal neocortical epilepsy are associated with cortical lesions such as focal cortical dysplasias, neoplasms, or vascular malformations and do not represent a specific syndrome.

Diagnostic workup

The diagnostic work will vary greatly in complexity, depending on the response to medical treatment and consideration of surgical therapy. A careful history by an experienced examiner should establish the diagnosis of epilepsy and help define the seizure type. A routine EEG may or may not be helpful, as interictal abnormalities may be lacking. Interictal EEG would not be able to differentiate mesial from neocortical temporal origin, or even extratemporal origin. As noted previously, special EEG analysis techniques might help (01).

In the case of new onset seizures, patients should have MRI scans early in the course of evaluation to look for structural lesions as the cause of seizures. Structural lesions, depending on their location and appearance, can have a significant impact on management and evaluation. In the case of documented pharmacoresistance and possible surgical intervention, the diagnostic evaluation proceeds along well-established lines. In addition, magnetic resonance spectroscopy might help differentiate mesial temporal lobe seizures from neocortical temporal lobe seizures (63). Further diagnostic evaluation in the case of pharmacoresistance would include video EEG monitoring, neuropsychological evaluation, and possible intracranial EEG monitoring with depth, strip, and grid electrodes, separately or in combination. FDG-PET or ictal SPECT can be very helpful in MRI-negative temporal epilepsies (12).

An 18F-FDG-PET study showed that the hypometabolism in mesial temporal lobe epilepsy was limited to the ipsilateral temporal and insular lobes, whereas in neocortical temporal lobe epilepsy the hypometabolism was more widespread, involving more frequently the ipsilateral temporal, frontal, and parietal lobes (69).

Management

Depending on the results of the evaluation, the management can take one of several directions. If the MRI is normal and neocortical temporal lobe seizures are suspected, medical management should be undertaken. If, after a reasonable trial with appropriate antiseizure medications, seizures remain inadequately controlled, surgery should be considered, recognizing that seizure onset localization can be challenging, particularly on the language-dominant side (66). However, with careful presurgical evaluation through experienced epilepsy surgery programs, surgical therapy can be successful in lateral neocortical temporal epilepsy, even in patients with no obvious MRI abnormalities (57; 18). Despite the differences in EEG and clinical characteristics between mesial temporal lobe epilepsy with hippocampal sclerosis and lesional neocortical temporal lobe epilepsies, surgical outcomes are similar (64).