Temporal lobe epilepsy
|Temporal lobe epilepsy|
|Classification and external resources|
Lobes of the brain
Temporal lobe epilepsy is a chronic neurological condition characterized by recurrent seizures (epilepsy) which originate in the temporal lobe of the human brain. The seizures involve sensory changes, for example smelling an unusual odour that is not there, and disturbance of memory. The most common cause is mesial temporal sclerosis. Treatment is through medication or surgery and prognosis is variable.
- 1 Introduction
- 2 Signs and symptoms
- 3 Causes
- 4 Pathology
- 5 Treatments
- 6 Complications and prognosis
- 7 Society, culture, religion and history
- 8 References
Over forty types of epilepsy are recognised and they are divided into two main categories: Partial seizures and generalized seizures. Partial seizures account for approximately sixty percent of all adult cases. Temporal lobe epilepsy (TLE) is the single most common form of partial seizure.
The International League Against Epilepsy (ILAE) recognizes two main types of temporal lobe epilepsy: mesial temporal lobe epilepsy (MTLE), arising in the hippocampus, the parahippocampal gyrus and the amygdala which are located in the inner (medial) aspect of the temporal lobe and lateral temporal lobe epilepsy (LTLE), the rarer type, arising in the neocortex at the outer (lateral) surface of the temporal lobe. The seizures of LTLE are characterized by auditory or visual features. Autosomal dominant Lateral Temporal Lobe Epilepsy (ADLTLE), is a rare hereditary condition.
In 2013, the ILAE published a new classification of MTLE based on abnormalities (hippocampal sclerosis) found at the microscopic level in the tissue of the hippocampus. However, the pathology changes used in the classification cannot easily be predicted by clinical features.
Signs and symptoms
When a seizure begins in the temporal lobe, the symptoms experienced by the patient, and the signs seen by bystanders, depend on the precise location of the seizure focus. In 1981, the ILAE recognized three types of seizures occurring in temporal lobe epilepsy. The classification was based on EEG findings.
Simple partial seizures
Simple partial seizures (SPS) involve small areas of the temporal lobe such as the amygdala and hippocampus. The term "simple" means that the level of consciousness of the patient is not altered during the seizure. In temporal lobe epilepsy, a simple partial seizure usually causes abnormal sensations only.
These may be mnestic sensations such as déjà vu (a feeling of familiarity), jamais vu (a feeling of unfamiliarity); amnesia; or a single memory or set of memories; auditory (an abnormal sound or tune); gustatory (an abnormal taste); olfactory (a smell that is not physically present); visual; or sensory (involving feelings on the skin or in the internal organs) sensations. Sensory disturbances may seem to move over the body. Dysphoric or euphoric feelings, fear, anger, and other emotions may also occur. Often, it is hard for the patient to describe the sensation.
Simple partial seizures may be called "auras" by lay people who mistake them for a warning sign of a subsequent seizure. Instead, the so-called aura is actually the partial seizure itself. Patients who only experience simple partial seizures may not recognize what they are, nor seek medical advice about them.
A simple partial seizure may or may not progress to any of the seizure types listed below.
Complex partial seizures
Complex partial seizures (CPS) are seizures which impair consciousness to some extent: they alter the person's ability to interact normally with his or her environment. They usually begin with a single partial seizure then spread to a larger portion of the temporal lobe resulting in impaired consciousness. Signs may include motionless staring, automatic movements of the hands or mouth, altered ability to respond to others, unusual speech, or other unusual behaviors.
Secondarily generalized tonic-clonic seizures
Seizures which begin in the temporal lobe but then spread to involve the whole brain are known as Secondarily Generalized Tonic-Clonic Seizures (SGTCS). The arms, trunk and legs stiffen (the tonic phase), in either a flexed or extended position, and then jerk (the clonic phase). Secondarily generalized tonic clonic seizures are known in the vernacular as convulsions or grand mal seizures. The word grand mal comes from the French term, meaning major affliction.
Post ictal period
Following each of these seizure types, there is some period of recovery in which neurological function is altered. This is the postictal state. The degree and length of postictal impairment directly correlates with the severity of the seizure type. Single partial seizures often last less than sixty seconds; complex partial seizures may last up to two minutes; and generalized tonic clonic seizures may last up to three minutes. The postictal state in the case of complex partial seizures and generalised tonic clonic seizures, the postictal state may last much longer than the seizure itself.
Because a major function of the temporal lobe is short-term memory, complex partial seizures and generalised tonic clonic seizures cause amnesia for the period of the seizure. As a result, patients with temporal lobe complex partial seizures and generalised tonic clonic seizures may not remember having had a seizure.
Investigations which examine cerebral blood flow show a reduction in the perfusion of the ipsilateral temporal lobe between seizures in patients with intractable TLE. These investigations include H(2)(15)O positron emission tomography (PET) scanning and pulsed arterial spin labeling MR imaging.
The causes of TLE include mesial temporal sclerosis, traumatic brain injury, brain infections, such as encephalitis and meningitis, hypoxic brain injury, stroke, cerebral tumours, and genetic syndromes. Temporal lobe epilepsy is not the result of mental health disorders or fragility of the personality.
Although controversial at times, there is a link between febrile seizures (seizures coinciding with episodes of fever in young children) and subsequent temporal lobe epilepsy, at least epidemiologically.
Human herpes virus 6
In the mid 1990s, human herpesvirus 6 (HHV-6) was suggested as a possible causal link between febrile convulsions and mesial temporal lobe epilepsy. However, although the virus is found in temporal lobe tissue at surgery for TLE, it has not been recognised as a major factor in febrile seizures or TLE.
Dispersion of the granule cell layer in the hippocampal dentate gyrus is occasionally seen in temporal lobe epilepsy and has been linked to the downregulation of reelin, a protein that normally keeps the layer compact by containing neuronal migration. It is unknown whether changes in reelin expression play a role in epilepsy.
In TLE, there is loss of neurons in region CA1 and CA3 of the hippocampus.(p387–389) There is also damage to mossy cells and inhibitory interneurons in the hilar region of the hippocampus (region IV) and to the granule cells of the dentate gyrus. In animal models, neuronal loss occurs during seizures but in humans, neuronal loss is predates the first seizure and does not necessarily continue with seizure activity. The loss of the GABA-mediated inhibitory interneurons may increase the hyperexcitability of neurons of the hippocampus leading to recurrent seizures. According to the "dormant basket cell" hypothesis, mossy cells normally excite basket cells which in turn, inhibit granule cells. Loss of mossy cells lowers the threshold of action potentials of the granule cells.
Granule cell dispersion in the dentate gyrus
Granule cell dispersion is a type of developmental migration and a pathological change found in the TLE brain which was first described in 1990. The granule cells of the dentate gyrus are tightly packed forming a uniform, laminated layer with no monosynaptic connections. This structure provides a filter for the excitability of neurons.
In TLE, granule cells are lost, the structure is no longer closely packed and there are changes in the orientation of dendrites. These changes may or may not be epileptogenic. For instance, if the dendrites of granule cells reconnect, it may be in a way (through the laminar planes) that allows hyperexcitability. However, not all patients have granule cell dispersion.(p387–389)
Aberrant mossy fiber sprouting
In the TLE brain, where granule cells are damaged or lost, axons, the mossy fibres, 'sprout' in order to reconnect to other granule cell dendrites. This is an example of synaptic reorganisation. This was noted in human tissue in 1974 and in animal models in 1985. In TLE, the sprouting mossy fibres are larger than in the normal brain and their connections may be aberrant. Mossy fibre sprouting continues from one week to two months after injury.(p416–431) 
Aberrant mossy fibre sprouting may create excitatory feedback circuits that lead to temporal lobe seizures. This is evident in intracellular recordings. Stimulation of aberrant mossy fibre areas increases the excitatory postsynaptic potential response.
However, aberrant mossy fiber sprouting may inhibit excitatory transmission by synapsing with basket cells which are inhibitory neurons and by releasing GABA and neuropeptide Y which are inhibitory neurotransmitters. Also, in animal models, granule cell hyper-excitability is recorded before aberrant mossy fibre sprouting has occurred.
There are many oral medications available for the management of epileptic seizures. They were previously called anticonvulsants. However, this term is misleading because most TLE seizures are, in fact, not convulsions. The modern term is antiepileptic drugs (AEDs). Nearly all AEDs function by decreasing the excitation of neurons, for example, by blocking fast or slow sodium channels or by modulating calcium channels; or by enhancing the inhibition of neurons, for example by potentiating the effects of inhibitory neurotransmitters like GABA.
In TLE, the most commonly used older AEDs are phenytoin, carbamazepine, primidone, valproate and phenobarbital. Newer drugs, such as gabapentin, topiramate, levetiracetam, lamotrigine, pregabalin, tiagabine, lacosamide, and zonisamide promise similar effectiveness, with possibly fewer side-effects. Felbamate and vigabatrin are newer AEDs, but can have serious adverse effects so they are not considered first-line treatments.
Up to one third of patients with medial temporal lobe epilepsy will not have adequate seizure control with medication alone. For patients with medial TLE whose seizures remain uncontrolled after trials of several AEDs (that is, the epilepsy is intractable), surgical excision of the affected temporal lobe may be considered.
Epilepsy surgery has been performed since the 1860s and doctors have observed that it is highly effective in producing freedom from seizures. However, it was not until 2001 that a scientifically sound study was carried out to examine the effectiveness of temporal lobectomy.
Temporal lobe surgery can be complicated by decreased cognitive function. However, after temporal lobectomy, memory function is supported by the opposite temporal lobe; and recruitment of the frontal lobe. Cognitive rehabilitation may also help.
If a person is not an optimal candidate for epilepsy surgery, further management options include, new (including experimental) antiepileptic drugs, vagus nerve stimulation and in children, the ketogenic diet. Other avenues of treatment include brain cortex responsive neural stimulators, deep brain stimulation, and stereotactic radiosurgery, such as the gamma knife.
Complications and prognosis
After childhood onset, one third will "grow out" of TLE, finding a lasting remission up to an average of 20 years. The finding of a lesion such as hippocampal sclerosis (a scar in the hippocampus), tumour, or dysplasia, on magnetic resonance imaging (MRI) predicts the intractability of seizures.
The effect of temporal lobe epilepsy on personality is an historical observation dating to the 1800s. Sigmund Freud stated,
- "We know that epilepsy produces these remarkable changes in the personality."
Personality and behavioural change in temporal lobe epilepsy is seen as a chronic condition when it persists for greater than three months. A particular neural location (especially on the surface of the temporal lobe) may be the origin of such changes.
Individuals with temporal lobe epilepsy have a high prevalence of depression. Although the psychosocial impacts of epilepsy may be causative, there are also links in the phenomenology and neurobiology of TLE and depression.
The temporal lobe and particularly the hippocampus plays an important role in memory processing. Declarative memory (memories which can be consciously recalled) is formed in the area of the hippocampus called the dentate gyrus.
Temporal lobe epilepsy is associated with memory disorders and loss of memory. Animal models and clinical studies show that memory loss correlates with temporal lobe neuronal loss in temporal lobe epilepsy. Verbal memory deficit correlates with pyramidal cell loss in TLE. This is more so on the left in verbal memory loss. Neuronal loss on the right is more prominent in non-verbal (visuospatial memory loss).
Society, culture, religion and history
The first to record and catalog the abnormal symptoms and signs of TLE was Norman Geschwind. He found a constellation of symptoms that included hypergraphia, hyperreligiosity, collapse, and pedantism, now called Geschwind syndrome.
Vilayanur S. Ramachandran explored the neural basis of hyperreligiosity in TLE using galvanic skin response (correlating with emotional arousal) to determine whether it was due to an overall heightened emotional state or was specific to religious stimuli. By presenting subjects with neutral, sexually arousing and religious words while measuring galvanic skin response, Ramachandran was able to show that patients with TLE showed enhanced emotional responses to religious words, diminished responses to sexually charged words, and normal responses to neutral words. These results suggest that the medial temporal lobe is specifically involved in generating some of the emotional reactions associated with religious words, images and symbols.
Temporal lobe epilepsy may be a less likely explanation for the revelational experiences of prominent religious figures such as Abraham, Moses, Jesus, Mohammed and Saint Paul because they had complexity of their interactions with their visions, a lack of reported stereotypy, an absence of amnestic periods and an absence of reported automatisms or generalized motor events with their revelatory experiences. Psychiatric conditions with psychotic spectrum symptoms were reasoned to be a more plausible explanation of their revelatory experiences.
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