The concept of “insular lobe epilepsy” was first introduced in the 1950s when it was discovered that stimulation of the insula could produce symptoms similar to those of temporal lobe epilepsy, and the similarity in clinical symptoms between the two was confusing. The cross-regional, multicenter clinical case statistics conducted for many years afterwards also showed that about 70% of surgically treated temporal lobe epilepsy patients had a good prognosis, but 20% had poor surgical outcome, and 10% had ineffective surgical treatment; and among these ineffective patients, insula involvement may be a key reason for the treatment failure of some patients with refractory epilepsy after temporal lobectomy alone.
The insula belongs to the limbic system and, as the fifth lobe, it is closely associated with visceral activity and emotional centers and is adjacent to motor, sensory, and language centers. The insula has many different functions, such as memory, drive, emotion and higher autonomic control of taste and smell; tumors in the insula region can induce multi-system dysfunction, and improper diagnosis and treatment can be life-threatening. However, the deep anatomical location of the insula itself and the structural limitations of the dense vascular wall on the lateral side make it unlikely that seizures in the insula can be recorded on scalp electrodes, i.e., conventional EEG, which is extremely valuable for the diagnosis of epilepsy, plays a relatively weak role in the study of insular epilepsy, and it is very difficult to distinguish whether seizures originate in the medial temporal lobe and spread rapidly to the insula or whether they originate in the insula and spread rapidly to the medial temporal lobe. , is very difficult. Therefore, the study of insular lobe epilepsy was at a low point, and many even began to deny the existence of insular lobe epilepsy.
The insula has the following functional characteristics.
1, through a large number of anatomical and functional imaging as well as somatosensory, pain-evoked response and direct electrical stimulation of the insula cortical studies, all proved that the insula has the function of regulating its own somatosensory and nociceptive sensations.
2. the function of controlling visceral sensation and visceral movement
3. stimulation of the insula in patients with temporal lobe epilepsy during anterior temporal lobectomy is seen to produce changes in heart rate and blood pressure in about 50% of cases, leading to the consideration that it is the abnormal discharge effect of the insula that causes arrhythmias in patients during seizures, even resulting in sudden death
4. a combination of Penfield’s taste stimulation studies, neuroimaging studies and animal (monkey) experiments, which showed that electrophysiological changes induced by microelectrode stimulation on insula neurons are consistent with altered taste in patients with clinical insula damage
5. Hearing and language, especially aspects related to the localization of auditory attention and the reception of abnormal auditory stimuli, also involve the insula, and some studies have shown that both the left and right insula are involved in the production of language function.
Recent studies have reported that the insula may also have a neuronal system “mirroring” feature, i.e., when a subject performs an action, the brain reflects it when it observes another individual performing the same action, and the region integrates the subject’s sensations (or emotions), making itself responsive to them. The brain will respond when it observes another individual performing the same action. For example, the insula produces visceral sensory or visceral motor responses associated with the sight of a disgusted or pained expression on another person’s face.
In summary, the insula represents a higher level of tissue function than other lobes of the brain. Therefore, patients with epileptogenic foci involving this region are also bound to have specific symptomatic seizure characteristics.
The insula is a part of the cortical structure of the brain
Compared to other lobes of the brain, the insula has characteristic anatomical boundaries, with well-defined bounding sulci (periaqueductal fissures) and transitional cytoarchitectonic boundaries that connect to peripheral cortical areas and even more distant cortical areas, forming dense contact structures. The function of the insula, and its role in seizures, therefore cannot be viewed simply as an isolated functional center (island) as its name suggests, and Augustine’s review provides a complete description of the insula’s system of connections: the insula is connected to peripheral areas such as the amygdala, the nucleus basalis, and all cortices except the occipital lobe. Of the proven lateral fissure-insula, temporal lobe – limbic system – insula, and medial frontal orbital lobe – insula structural systems, it is the seizure symptoms produced by each of these epileptiform discharges that are ultimately attributed to insula lesions, which are actually only part of the full range of insula epilepsy symptoms that have attracted more attention from epilepsy researchers. This extensive structural linkage is what makes it difficult to identify cases of insular lobe epilepsy in epileptogenic disorders where the epileptogenic region extends beyond the insula.
In primate experiments, insula cortico-tissue projection areas are generally associated with different cortical structures. In monkeys, the insula is divided into three cytoarchitectonic regions: (i) a ventral-oral no-granular region (Ia); (ii) a transitional no-granular region (Id); and (iii) a dorsal-caudal granular region (Ig). These different cytoarchitectonic regions are associated with different functional areas within the insula: (i) Ia is associated with olfactory and autonomic functions; (ii) Id is associated with gustatory functions; and (iii) Ig is associated with somatosensory, auditory and visual functions. The few clinical cases and electrophysiological data confirm the existence of a similar topographical organization in the human insular cortex, and Dupont showed by PET studies that emotional sensations (fear, distress, restlessness) or visceral symptoms (epigastric rising sensation, thoracic pressure, etc.) are closely related to the distribution of different areas of the insula.
The insula is an epileptic symptomatic seizure area
In the 1940s and 1950s, Penfield and Faulk studied insula stimulation in temporal lobe epilepsy patients undergoing surgery with craniotomy under local anesthesia for cortical electroencephalography (EcoG) monitoring, and the EcoG recordings showed that about half of these patients continued to have a large number of paroxysmal spike-like waves after temporal lobe surgery. They reasoned that the majority of positive responses to stimulation of the insula consisted of either sensations similar to those produced by stimulation of its superior lateral fissure (area SII) or alterations in gastrointestinal sensation secondary to gastrointestinal motility in the gastrointestinal tract. Epilepsy of insular cortical origin behaves very similarly to temporal lobe epilepsy in this respect, which partly explains the unsatisfactory outcome of patients diagnosed with “temporal lobe epilepsy” after simple temporal lobe resection. Although EcoG can be used as a systematic approach to explore the insula, Penfield was also unsuccessful in recording clear localized epileptiform discharges in the insular cortex. Guillaume, who followed him, began to call the attention of his colleagues to the concept of “insula epilepsy”. However, none of them could provide more evidence that insula surgery was more effective in improving symptoms in patients with intractable temporal lobe epilepsy than conventional temporal lobe surgery. Thus this particular symptom presentation that could reflect insula discharge was gradually faded and forgotten. It was only recently that several new case reports clearly suggested that “epileptiform discharges can be terminated after removal of the damaged insula”, and the insula as a seizure zone of epilepsy symptoms came back to the forefront.
Clinical manifestations of insular lobe epilepsy
French scholar J. Isnard et al. observed and summarized the common characteristics of individuals with insula-like seizures by directly stimulating the insula.
1. consciousness.
2. all have sensory aura symptoms before the seizure, mostly seen as a current or burning sensation limited to the perioral area or a wide range (face-shoulder-hand and trunk, upper limb-trunk-lower limb).
3. before or during an attack, there may be retrosternal pain, abdominal cramping, nausea and vomiting, dysphonia and other sensory abnormalities, and there is often dysphonia or dysarthria in the pharynx that tends to be mute.
4. There are motor and sensory symptoms of the pharynx during the attack, and they are accompanied by grasping and scratching movements of the hand or hands reaching for the neck on the opposite side of the discharge area.
5. there are often motor symptoms on the ipsilateral or contralateral side of the discharge, such as spasms of the face or upper extremities, rotation of the head and eyes, and generalized dystonia.
Being able to summarize one or more of the shared symptoms is very useful for the localization of the epileptic discharges: in the event of another similar seizure, its location can be quickly identified. On this basis, surgical excision of the lesion is performed, and postoperative control of the epileptiform discharges is satisfactory.