You are in: eMedicine Specialties > Radiology > BRAIN/SPINE Mesial Temporal SclerosisArticle Last Updated: Sep 11, 2007AUTHOR AND EDITOR INFORMATIONSection 1 of 10
  Author: Scott Trepeta, MD, Director of Neuroradiology, Department of Radiology, Jamaica Hospital Scott Trepeta is a member of the following medical societies: American College of Radiology, American Medical Association, American Roentgen Ray Society, and American Society of Neuroradiology Coauthor(s): Stephen Chan, MD, MBA, MPH, Assistant Professor of Radiology, Columbia University; Consulting Staff, Department of Radiology, New York-Presbyterian Hospital Medical Center; Angela Lignelli-Dipple, MD, Assistant Professor of Radiology, Columbia University; Consulting Staff, Department of Radiology, Division of Neuroradiology, Columbia Presbyterian Medical Center Editors: Mahesh R Patel, MD, Chief of MRI, Department of Radiology, Santa Clara Valley Medical Center; Bernard D Coombs, MB, ChB, PhD, Consulting Staff, Department of Specialist Rehabilitation Services, Hutt Valley District Health Board, New Zealand; Robert M Krasny, MD, Consulting Staff, Department of Radiology, The Angeles Clinic and Research Institute; L Gill Naul, MD, Professor and Head, Department of Radiology, Texas A&M University College of Medicine; Chair, Department of Radiology, Chief, Section of Magnetic Resonance Imaging, Scott and White Memorial Hospital and Clinic Author and Editor Disclosure Synonyms and related keywords: MTS, hippocampal sclerosis, Ammon horn sclerosis, Ammon's horn sclerosis, complex partial seizures, temporal lobe epilepsy INTRODUCTIONSection 2 of 10
  BackgroundTemporal 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. PathophysiologyMesial 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 1990.1 Mesial temporal sclerosis is the pathologic abnormality most frequently seen in temporal lobe specimens. 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 follows2:
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:
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.2 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%.3 FrequencyUnited StatesIn 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/MorbidityNeurologic 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. AgeComplex 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. AnatomyThe 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. The hippocampal formation is part of the limbic system (see Image 2), which is believed to represent the anatomic substrate for memory and emotion. Clinical DetailsPatients 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.4 Preferred ExaminationCoronal 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 TechniquesMRI 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. DIFFERENTIALSSection 3 of 10
Other Problems to Be ConsideredOther causes of temporal lobe epilepsy, including brain tumors, hamartomas, migrational anomalies, and vascular malformations RADIOGRAPHSection 4 of 10
FindingsPlain radiography has no role in the evaluation of mesial temporal sclerosis. CT SCANSection 5 of 10
FindingsComputed tomography (CT) scanning is typically insensitive for evaluation of mesial temporal sclerosis and for the workup of medically refractory epilepsy. In part, this is a result of surrounding bone artifact from the base of the skull and of the plane of acquisition. In 1996, Bronen and colleagues concluded that CT scanning is not useful for the diagnostic evaluation of medically refractory epilepsy because of the relatively low sensitivity of CT scanning compared with that of MRI in detecting abnormalities in patients undergoing surgery for medically refractory epilepsy.5 In their study, a sensitivity of 32% was obtained for CT scanning, while MRI achieved a sensitivity of 95%. MRI was also demonstrated to be significantly better than CT scanning for the localization of mesial temporal sclerosis (98% vs 2%). MRISection 6 of 10
FindingsClassic MRI findings in mesial temporal sclerosis include decreased volume and an abnormally increased T2 signal of the hippocampus. The increased T2 signal is presumed to be a result of gliosis and the subsequent increase in free water content (see Images 10-11). Associated findings may include atrophy of the ipsilateral mammillary body, fornix, and other parts of the limbic system (see Images 12-13).6 On coronal T2 spin-echo views (see Image 6), the hippocampus is surrounded by hyperintensity from cerebrospinal fluid (CSF) in the temporal horn of the lateral ventricle, choroid fissure, and choroid plexus. This surrounding, high T2 signal somewhat limits detection of T2 signal abnormality in the hippocampus. Because fluid-attenuated inversion recovery (FLAIR) imaging nulls the CSF signal, abnormal signal intensity in the hippocampus is relatively more apparent.7 A pitfall of coronal FLAIR imaging is the slight hyperintensity of all limbic structures relative to the neocortex; therefore, experienced neuroradiologists who have knowledge of the normal signal intensity of the hippocampus are needed (see Image 5). Thin-section volumetric T1-weighted imaging is occasionally used to calculate hippocampal volume; however, because it does not depict abnormal signal intensity, it is less useful than FLAIR and T2-weighted spin-echo imaging for visual detection of mesial temporal sclerosis. Magnetic resonance spectroscopy (MRS) can help in lateralizing temporal lobe epilepsy. Lateralization is useful in a patient with clinical temporal lobe epilepsy but no localizing findings on MRI. As many as 20% of patients with clinical temporal lobe epilepsy have no such MRI findings. Hydrogen-1 MRS demonstrates the anatomic distribution of metabolite signals. The metabolites frequently studied include N-acetylaspartate (a neuronal marker), creatine (a relatively stable marker found in the brain that is often used as a reference to compare with other metabolites), and choline (a marker related to cell membrane synthesis). Studies have shown that interictal N-acetylaspartate is reduced in the ipsilateral temporal lobe compared with the uninvolved temporal lobe.8 Degree of ConfidenceRoutine MRI is typically insensitive to findings of mesial temporal sclerosis. In 1998, McBride and colleagues compared findings of standard MRI performed outside an epilepsy center with the findings of special temporal lobe seizure protocols performed at major epilepsy centers.9 Although routine MRI readily depicted low-grade tumors and vascular malformations, it was inadequate for diagnosing hippocampal sclerosis. This difference occurred because the hippocampal structures are relatively flat and lie predominantly in the axial plane (in which most routine sequences are performed); therefore, subtle lesions of the hippocampus may be missed (see Images 7-9). Optimized high-resolution MRI of the temporal lobes is required for reliable detection of mesial temporal sclerosis.10 Special oblique coronal thin sections perpendicular to the plane of the hippocampus (see Images 3-4) have high sensitivity and specificity for mesial temporal sclerosis. Both thin-section T2-weighted spin-echo and FLAIR imaging have been useful for the diagnosis. T2-weighted spin-echo imaging is somewhat better than FLAIR imaging for demonstrating the internal architecture of the hippocampus; however, the degree of signal abnormality is somewhat more obvious on FLAIR imaging. The advantage of FLAIR imaging is derived from the decreased background signal intensity that originates in extrahippocampal structures. In a study performed by Berkovic and colleagues in 1995, sensitivity of MRI for mesial temporal sclerosis was as high as 97%, and specificity was 83%.11 (Other studies have determined values of 80-90% sensitivity.) The authors reported on patients who underwent MRI and who later received anterior temporal lobectomy. Radiologic findings were correlated with pathologic findings. MRI findings of mesial temporal sclerosis have also been correlated with surgical outcome. Patients with mesial temporal sclerosis that was visible on magnetic resonance images (and that was subsequently confirmed on pathology) were found to have improved postsurgical outcomes, with high seizure-free rates or substantial improvement in seizures in comparison with patients who had normal MRI findings.11 ULTRASOUNDSection 7 of 10
FindingsAlthough ultrasonography is useful for the evaluation of the neonatal brain, it plays no role in the evaluation of temporal lobe epilepsy and mesial temporal sclerosis. NUCLEAR MEDICINESection 8 of 10
FindingsSPECT scanning and PET scanning with 18-fluorodeoxyglucose (FDG) provide functional information about the temporal lobe. PET scans show glucose metabolism in the brain by using a positron-emitting substance. Patients with temporal lobe epilepsy have decreased glucose metabolism in the affected lobe during the interictal period. SPECT scans show the distribution of blood flow in the brain at the time of the injection of a radiotracer, which is injected ictally or interictally. If the radiotracer is injected ictally, focally increased uptake is identified in the affected temporal lobe (hot focus). If the radiotracer is injected interictally, the effected temporal lobe demonstrates decreased uptake compared with that of the rest of the brain (cold focus). Degree of ConfidenceSensitivity for detection of interictal seizure foci is 65-75% for both SPECT scans and PET scans. When the source of seizures is lateralized on PET scans or SPECT scans, 94% of patients improve after surgical resection. MULTIMEDIASection 9 of 10
REFERENCESSection 10 of 10
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