A Study of Medial and Lateral Temporal Lobe Epilepsy Based on Stereoelectroencephalography
Temporal lobe epilepsy (TLE) is the most prevalent form of drug-resistant epilepsy, accounting for 60%–80% of cases. Despite advances in neuroimaging and neurophysiology, distinguishing between subtypes of TLE, particularly medial temporal lobe seizures (MTLS) and lateral temporal lobe seizures (LTLS), remains a clinical challenge. This study leverages stereoelectroencephalography (SEEG) to explore electrophysiological characteristics, surgical outcomes, and prognostic factors in patients with TLE, offering insights into the heterogeneity of this condition.
Study Design and Patient Cohort
The study enrolled 30 patients with drug-resistant TLE who underwent anterior temporal lobectomy at Xuanwu Hospital, Capital Medical University, between 2016 and 2017. Preoperative evaluations included video electroencephalography (EEG), magnetic resonance imaging (MRI), and SEEG for cases where non-invasive methods failed to localize epileptic foci. Patients were classified into MTLS (60%, n = 18) or LTLS (40%, n = 12) groups based on SEEG-defined seizure onset zones. SEEG electrodes (6 per patient on average, with 5–16 contacts) targeted deep structures like the hippocampus and amygdala, enabling precise localization of epileptogenic activity.
Clinical and Demographic Findings
Patients with LTLS exhibited distinct clinical profiles compared to those with MTLS. The LTLS group had a significantly higher mean age at surgery (29.9 ± 12.5 vs. 26.9 ± 6.9 years, P = 0.009) and longer epilepsy duration (17.9 ± 12.1 vs. 11.9 ± 6.0 years, P = 0.038). These differences suggest divergent pathophysiological timelines, with LTLS potentially reflecting a more complex or insidious epileptogenic process. In contrast, MTLS patients demonstrated a prolonged latency between SEEG-recorded ictal onset and clinical symptom manifestation (67.3 ± 59.1 vs. 29.3 ± 24.4 seconds, P = 0.008), highlighting slower propagation of epileptic activity in mesial temporal networks.
SEEG Ictal Patterns and Localization
SEEG revealed distinct ictal onset patterns between groups. In MTLS, sharp/spike-wave rhythms predominated (55.6%), while low-voltage fast activity (58.3%) characterized LTLS. These patterns align with prior studies linking low-voltage fast activity to neocortical epilepsy and rhythmic spikes to mesial temporal sclerosis. The SEEG methodology proved critical for differentiating subtypes, as hippocampal sclerosis (HS)—a hallmark of MTLS—was absent in 27.8% of MTLS patients, emphasizing the limitations of relying solely on MRI findings.
Surgical Outcomes and Prognostic Factors
Engel classification outcomes at a mean follow-up of 25.7 ± 4.8 months demonstrated superior efficacy in MTLS patients. Among MTLS patients, 77.8% (n = 14) achieved Engel I (seizure-free), compared to 41.7% (n = 5) in the LTLS group (P = 0.049). The overall cohort showed a 63.3% Engel I rate, with higher recurrence in LTLS (33.3% Engel IV vs. 11.1% in MTLS). Kaplan-Meier analysis revealed that 37.3% of recurrences occurred within 3 months post-surgery, stabilizing thereafter, suggesting an early critical window for intervention.
Pathological Correlations
Pathological examination corroborated SEEG findings. HS was identified in 72.2% of MTLS patients, often coexisting with focal cortical dysplasia. In LTLS, only 8.3% exhibited HS, with most showing non-specific cortical abnormalities. This disparity underscores HS as a common but not obligatory feature of MTLS and highlights the need for multimodal evaluation in TLE.
Mechanistic and Clinical Implications
The prolonged SEEG-to-symptom latency in MTLS points to a “silent phase” of electrical propagation before clinical manifestation. This delay may reflect the involvement of the limbic system or inhibitory mechanisms in mesial temporal structures. Conversely, rapid symptom onset in LTLS aligns with direct neocortical involvement and faster spread to motor or sensory regions.
The prognostic advantage of MTLS may arise from anatomical constraints limiting epileptogenic networks to resectable mesial structures. In contrast, LTLS often involves broader networks, increasing the risk of incomplete resection. The predominance of low-voltage fast activity in LTLS—a pattern associated with epileptogenicity—further underscores the aggressive nature of neocortical epilepsy.
Methodological Insights and Limitations
SEEG emerged as an indispensable tool for delineating epileptogenic zones, particularly in MRI-negative cases. Its ability to sample deep structures like the hippocampus provided diagnostic clarity unobtainable via subdural electrodes. However, the study’s small sample size (n = 30) and single-center design limit generalizability. Additionally, the lack of scalp EEG-SEEG correlation analysis and detailed seizure semiology restricted deeper mechanistic insights.
Conclusions
This study delineates MTLS and LTLS as distinct TLE subtypes with unique clinical, electrophysiological, and prognostic profiles. SEEG-defined localization of seizure onset zones enables personalized surgical planning, with MTLS patients achieving superior outcomes. Early post-surgical monitoring is critical, as most recurrences manifest within 3 months. Future research should explore longitudinal SEEG biomarkers and refine resection strategies for neocortical TLE.
doi.org/10.1097/CM9.0000000000001256
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