Introduction

Temporal lobe epilepsy (TLE) is a chronic, debilitating brain disorder which causes seizures due to abnormal electrical activity in the brain. It is the most common seizure disorder, affecting over fifty million people globally.¹ It is characterised by seizure activity from the mesial or lateral temporal lobes. Seizures in TLE include focal aware seizures, including auras, and focal impaired awareness seizures.² The impact of epilepsy, as well as its treatment, presents a significant burden on patients’ quality of life.

It is crucial to understand the pathophysiology of TLE to improve healthcare outcomes for individuals with this neurological condition. TLE pathophysiology mainly involves abnormal neuronal activity; in particular, the hippocampus plays a central role in seizure initiation. Hippocampal sclerosis is considered the most common pathological finding in TLE, although several other pathologies have been identified. However, the mechanisms underlying temporal lobe seizures are yet to be fully established.

Understanding Temporal Lobe Epilepsy 

TLE is the most common form of focal epilepsy, referring to seizures which begin in one region of the brain. The cause of temporal lobe seizures often remains unknown 

TLE can be divided into two main types: Mesial Temporal Lobe Epilepsy (MTLE) and Lateral Temporal Lobe Epilepsy (LTLE). MTLE is the most common form of temporal lobe seizures and involves the anatomy of the inner structure of the temporal lobe, including the hippocampus, parahippocampal gyrus and amygdala. LTLE, also known as neocortical temporal lobe seizures, is an incredibly rare form of temporal lobe seizures and is most commonly a result of genetic disorders.

Common clinical features of TLE include focal impaired seizures and focal onset aware seizures, often referred to as auras. Auras are phenomena in which individuals experience sensory symptoms, which may manifest as cognitive events such as ‘deja vu’ or a sense of anxiousness. During focal impaired awareness seizures, individuals lose awareness and often experience a loss of consciousness. They may also become unaware of their surroundings and experience difficulty speaking or understanding language. These seizures affect a larger section of one hemisphere of the brain in comparison to focal aware seizures. Symptoms of TLE are associated with several cognitive events, such as memory issues, as well as presenting an emotional and psychological impact, significantly affecting quality of life.

TLE presents a complex and extensive aetiology. The most common causes are Hippocampal Sclerosis (HS), infections and brain injuries.³ Recent studies have revealed a large number of TLE cases which are associated with genetic mutations. HS is a neurodegenerative process that causes severe neuronal loss and gliosis in the hippocampus and is the most common pathology found in TLE.⁴ The underlying pathophysiology of MTLE, which is secondary to HS (MTLE-HS), is distinguished from MTLE.⁵ Seizures appear to occur sporadically and have a gradual onset and offset of approximately 1-2 minutes. HS has a variety of pathological subtypes, characterised by the extent of neuronal loss and gliosis.⁶

Risk factors associated with TLE include a history of febrile seizures, infections, brain injuries and structural changes in the temporal lobe, including tumours. Understanding the underlying mechanism of TLE is crucial for the development of novel, effective therapies.

Neuroanatomy and Functional Role of the Temporal Lobe 

The temporal lobe of the brain, also referred to as the neocortex, is the second-largest lobe of the brain. It contains the cortical areas which process hearing, as well as sensory aspects of speech and memory. It plays a key role in an individual's ability to communicate with others, process memories and emotions and use language. Key structures of the temporal lobe include the hippocampus and the amygdala. 

The hippocampus is embedded deep in the temporal lobe and plays a crucial role in learning, memory and emotional processing.⁷ It is part of a larger group of structures known as the hippocampus formation, which includes the Entorhinal cortex, Subiculum, Hippocampus proper and Dentate gyrus. The hippocampus has a major function in cognitive functions, including short-term and long-term memory, visual-spatial memory and verbal memory. 

The amygdala is the major centre of the brain which processes emotions. It is an almond-shaped structure which lies in the temporal lobe. It is vital in regulating emotional responses, such as anxiety and fear responses.⁸ The amygdala and hippocampus also work together to link emotions to memories, learning and senses.

Temporal lobe seizures severely affect hippocampus function, meaning that individuals may experience strange feelings and sensations during seizures, including anxiety, and may not be able to remember the seizure after it has happened. Additionally, frequently recurring temporal lobe seizures can decrease the size of the hippocampus, resulting in cognitive decline, memory impairment and emotional changes. Therefore, damage to the temporal lobe can cause difficulty in understanding language and long-term memory dysfunction.

Pathophysiology and Mechanisms of TLE

The pathophysiology of TLE is complex and involves several structural and molecular changes which contribute to the production and progression of seizures. Neuronal hyperexcitability is the main cause of TLE as a result of pathological changes in the temporal lobe. These pathological changes can include neuronal loss, granule cell dispersion and genetic mutation. When seizures occur, there is a distortion between the normal balance of excitation and inhibition in the brain due to acquired or genetic factors.

The predominant and most examined pathology of MTLE is HS, characterised by a loss of nerve cells in the hippocampus, gliosis and the reorganisation of neuronal circuits. As discussed, seizures arise through a myriad of etiologies. LTLE has been poorly studied in comparison to MTLE, including lesional and nonlesional cases.⁹ However, the pathological processes underlying the formation of these lesions remain elusive. Several other pathologies identified include genetic mutations, interconnected epileptic networks and the role of astrocytes in epilepsy pathogenesis.

Genetic and Molecular Influences in TLE

TLE is a complex and polygenic neurological disorder. Several linkage analyses of familial TLE cases have identified various genes which may be involved in the aetiology of TLE.¹⁰ TLE is a heterogeneous condition and may be caused by multiple rare mutations. Various susceptibility genes of interest have been found, which may be involved in the pathology of TLE. However, ongoing research is vital to determine genes which are associated with TLE.

Implications for Treatment and Management 

The pathophysiology of TLE significantly affects its treatment and clinical management. HS is the primary pathology targeted in the treatment of TLE.¹¹ Patients living with TLE with HS may respond poorly to first-line medications, including antiseizure medications such as Brivaracetam and Cenobamate, which are used to regulate seizures. However, these patients often benefit from temporal lobe epilepsy surgery when medications prove to be unsuccessful. The most common TLE brain surgery involves removing a section of the temporal lobe, although there are various types of surgeries available. Following surgery, the frequency of temporal lobe seizures is significantly reduced. Therefore, surgical treatment is often offered as a safe, highly effective therapeutic option to more patients who are suffering from drug-resistant TLE.^12,13

An alternative TLE treatment option is neurostimulation if first-line therapies are unsuccessful and surgery is not an option. This includes delivering electrical impulses to the brain using implanted devices, for example, by vagus nerve stimulation.¹⁴ Responsive neurostimulation is a more targeted approach for patients with mesial TLE.  This has proven to be effective in reducing the number of seizures over time for patients with medically intractable epilepsy.¹⁵

Many patients can successfully manage temporal lobe seizures with antiseizure medications in addition to lifestyle changes such as a ketogenic diet. Management of TLE varies based on the pathology of the disease, meaning that the development of personalised treatment strategies is vital. Further research into the mechanisms underlying TLE is critical in the development of new TLE treatments. 

Summary 

TLE is a debilitating neurological condition and the most common form of focal epilepsy. Temporal lobe seizures are characterised by abnormal electrical activity in the mesial or lateral temporal lobes of the brain, leading to impaired awareness.

The temporal lobe, particularly the hippocampus and amygdala, plays a critical role in memory, learning and emotional processing. Therefore, damage to these structures can lead to severe cognitive decline.

The pathophysiology of temporal lobe epilepsy is complex and not yet fully understood. 

Its primary pathology involves hippocampal sclerosis, which is typically associated with MTLE. While MTLE, particularly associated with HS, has been extensively studied, LTLE is much rarer and less understood. Other factors leading to TLE include brain injuries, infections and tumours. Genetic mutations also contribute to the formation and progression of temporal lobe seizures, with ongoing research identifying potential genes associated with TLE. 

Treatment and management of TLE are largely dependent on its underlying pathology. First-line therapies include antiseizure medications, although surgery provides a safe, effective therapeutic option for patients with drug-resistant epilepsy. 

Further research into TLE pathophysiology and the advancement of personalised therapeutic strategies holds significant promise for improving patient outcomes. It is essential to uncover the underlying pathology of TLE to develop effective therapies which can improve the quality of life for patients living with epilepsy.