eMedicine Specialties > Radiology > Brain/Spine

Mesial Temporal Sclerosis

Author: Scott Trepeta, MD, Director of Neuroradiology, Department of Radiology, Jamaica Hospital
Coauthor(s): Stephen Chan, MD, MBA, MPH, Consulting Staff, New York State Psychiatric Institute; Angela Lignelli-Dipple, MD, Assistant Professor of Radiology, Columbia University; Assistant Attending Physician, Department of Radiology, Division of Neuroradiology, Columbia Presbyterian Medical Center
Contributor Information and Disclosures

Updated: Aug 12, 2009

Introduction

Background

Temporal lobe epilepsy is the most common epilepsy syndrome in adults. Seizures usually begin in late childhood or adolescence. Virtually all patients have complex partial seizures, some of which generalize secondarily. In most patients, the epileptogenic focus involves the structures of the mesial temporal lobe. These structures include the hippocampus, amygdala, and parahippocampal gyrus. Antiepileptogenic drugs usually suppress secondary generalized seizures successfully, but 50% of patients or more will continue to have partial seizures. When seizures persist, anterior temporal lobectomy is the treatment of choice.¹

Pathophysiology

Mesial temporal sclerosis is the histologic substrate found in approximately 65% of temporal lobectomy specimens obtained in connection with temporal lobe epilepsy, as demonstrated in a study performed by the National Institutes of Health in 1990.⁶ Mesial temporal sclerosis is the pathologic abnormality most frequently seen in temporal lobe specimens.⁷

The hippocampus, or intralimbic gyrus, is formed by 2 cortical laminae embedded in each other: the cornu ammonis (CA), also called the hippocampus proper or Ammon horn, and the dentate gyrus. The CA can be divided into regions, or fields, depending on the appearance of pyramidal neurons.

The 4 fields (named by Lorente de No in 1934) are characterized as follows⁸ :

• CA1, or the Sommer sector, is the most vulnerable region; it is the most sensitive to hypoxia.
• CA2 is the most resistant and well-preserved sector.
• CA3, which enters the concavity of the dentate gyrus, is slightly vulnerable.
• CA4, sometimes called the endfolium, has intermediate vulnerability to insults.

To embryologic enthusiasts, CA4 is not part of the cornu ammonis but is a separate structure known as the dentate hilus.

The following 3 patterns of cell loss are described in the hippocampus:

• Classic Ammon horn sclerosis - Primary neuronal loss involves CA1 and CA4; occurs less often in C3 and least often in CA2
• Total Ammon horn sclerosis - Severe neuronal loss in all of the hippocampal zones, CA1 to CA4
• Endfolium sclerosis - Cell loss restricted to CA4

These patterns do not suggest clinical significance, and they are not correlated with the severity of seizures or with surgical outcomes. Neuronal loss is accompanied by fibrillary gliosis leading to hippocampal atrophy. In mesial temporal sclerosis, gliosis may also affect the amygdala, uncus, and parahippocampal gyrus.⁸

Other discrete structural epileptogenic lesions involving mesial temporal structures include hamartomas, gliomas, and vascular malformations. In addition, temporal lobe seizures may begin in an area of neocortex that preferentially projects to mesial temporal structures.

In nearly one third of all temporal lobectomy procedures, dual pathology (for example, a focal lesion and hippocampal sclerosis) is observed. Neuronal counts show mild cell loss when they are associated with a tumor consistent with hippocampal sclerosis as the primary abnormality.

Whether mesial temporal sclerosis is the cause or result of temporal lobe epilepsy is controversial. Some studies have shown a relationship between complex infantile febrile seizure and mesial temporal sclerosis. Patients with complex febrile seizures (duration >15 min, evidence of focal or lateralized convulsive activity, or >3 seizures within 24 h) have an increased incidence of mesial temporal sclerosis. Based on the number of complex features, the incidence of mesial temporal sclerosis in patients who have had complex febrile seizures is 4-50%.⁹

Frequency

United States

In the United States, epilepsy affects 0.5-1% of the population. Complex partial seizures account for approximately 35% of all cases of epilepsy. Epilepsy is refractory to medical intervention in 15-30% of patients with the condition.

Mortality/Morbidity

Neurologic morbidity from epilepsy can be associated with impaired intelligence and memory. At times, seizures of temporal lobe origin can be confused with a number of psychiatric conditions, such as hypomania or schizophrenia.

Age

Complex partial seizures can occur at any age; however, they demonstrate an increased incidence in adolescence and adulthood. Approximately 55% of all adult seizures are complex partial seizures.

Anatomy

The hippocampal formation is a complex structure that is composed of the CA, subiculum, dentate gyrus, parahippocampal gyrus, fimbria, and fornix. The hippocampal formation is located within the mesial temporal lobe (see Image 1) and protrudes into the medial wall of the temporal horn. Its superior border is the choroidal fissure. The CA is broken into the 4 previously discussed subfields, CA1-CA4.

Presentation

Patients with mesial temporal sclerosis usually present with complex partial seizures. Seizures arise in a single region of the brain, the temporal lobe, and they may become generalized secondarily. Because the seizures arise from a single region of the brain, they are termed partial or focal seizures. The term "complex" refers to impaired consciousness, which implies decreased responsiveness and a reduced awareness of self and of one's environment.

Seizures typically last 1-2 minutes; however, the postictal phase of confusion may be prolonged. Most mesial temporal seizures begin with an aura, usually visceral. Examples of auras include an unusual smell (such as that of burning rubber), a feeling of deja vu, a sudden and intense emotional feeling, or a sensory illusion, such as that of objects growing smaller or larger.

Complex partial seizures follow the aura. Typical behaviors include a motionless stare, oral alimentary automatisms (eg, lip smacking, swallowing, chewing, puckering), and upper extremity automatism (eg, fumbling or picking). Complex partial seizures may also be accompanied by the performance of highly skilled activities or a reaction to surroundings that are semiappropriate.

A definitive postictal period of variable duration is noted that may involve aphasia. The patient is usually amnesic for the events that took place during the seizure and may take minutes or hours to recover full consciousness.

When the seizure ends, the patient is usually amnesic for the events that occurred during the seizure, and the person may take minutes or hours to recover full consciousness.¹⁰

Preferred Examination

Coronal oblique magnetic resonance imaging (MRI) through the temporal lobes is the preferred modality.

Nuclear medicine scans (positron emission tomography [PET] scans or single-photon emission computed tomography [SPECT] scans) and electroencephalograms (EEGs) also play a role in localization.

Limitations of Techniques

MRI is contraindicated in patients with pacemakers, certain metal prostheses (eg, cochlear implants), or a large number of cerebral aneurysm clips. Metallic foreign bodies within the eyes, as well as shrapnel or bullets when they are located near vascular structures, also are contraindications.

Differential Diagnoses

Other Problems to Be Considered

Other causes of temporal lobe epilepsy, including brain tumors, hamartomas, migrational anomalies, and vascular malformations

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