Intracranial Stereo-EEG Resolves Discordant Clinical vs. Neuroimaging Findings in Patient With Tonic–Clonic Seizures

When clinical signs, electrophysiology results, and imaging data appear congruent in patients with refractory tonic–clonic seizures, it's reasonable to proceed directly to resection. However, when clinical and neuroimaging data are discordant, intracranial stereo-EEG (SEEG) may provide important localizing information.

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Massachusetts General Hospital physicians recently described such a case in Epileptic Disorders. The authors are Irfan S. Sheikh, MD, previously a fellow in the Department of Neurology, Mark Richardson, MD, PhD, director of the Functional Neurosurgery Program, Sydney Cash, MD, PhD, co-director of the Center for Neurotechnology and Neurorecovery, and colleagues.

Introduction to the Case

A 26-year-old, left-handed woman with refractory, focal to bilateral tonic–clonic seizures presented for evaluation for neurosurgery. She was having four seizures a day characterized by initial arousal, jerking movements, fidgeting of the hands, and progression to figure of four with left arm extension and mild clonic jerking, lasting one to two minutes with post-ictal fatigue.

Initial Evaluation

The initial surgical evaluation included the following:

• 3T MRI—Results were consistent with a focal cortical dysplasia (FCD) in the right inferior parietal lobule
• Neuropsychological testing—Showed left hemisphere dominance and intact language
• Magnetoencephalography—Revealed infrequent interictal epileptiform dipoles from the right posterior frontal cortex/central sulcus, with normal somatosensory and motor evoked potentials bilaterally
• Scalp EEG in an epilepsy monitoring unit—Within the first 24 hours of recording, 20 seizures were captured exclusively during sleep without aura. Only one seizure exhibited rhythmic 2- to 3-Hz activity, which was focused in the right frontocentral region. There were no interictal epileptiform discharges

Stereo-EEG Rationale and Findings

Although MRI suggested an FCD in the right inferior parietal lobule, clinical signs and scalp EEG findings were inconsistent with parietal onset: seizures occurring exclusively in sleep, intact awareness, late figure of four sign, and absence of a parietal aura. SEEG was performed to define the seizure onset zone, clarify its relationship to the primary motor cortex and rule out frontal lobe seizure onset.

Interictal SEEG in the vicinity of the lesion demonstrated a typical near-continous rhythmic fast spike burst (RFSB) pattern from the superficial contacts. Seizure onset co-localized with the area of greatest interictal activity from those contacts with rapid spread to the adjacent right lesion and ending with right frontal activity. No abnormalities were detected from the right frontal regions and right motor regions as a result of seizure propagation. However there were no independent ictal or intertidal abnormalities noted from the right frontal regions or motor cortex.

Treatment and Outcome

Based on SEEG and imaging findings, en bloc resection of the presumed FCD was performed. Pathological findings were diagnostic of FCD type 2b.

One year after surgery the patient remains seizure-free, consistent with an earlier report in Seizure in which resection of FCD type 2b with an RFSB pattern was associated with seizure freedom in 67% of cases. It remains unclear whether this pattern is a reliable predictor of surgical outcome.

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