Specificity of volumetric magnetic resonance imaging in detecting hippocampal sclerosis
C. Watson, F. Cendes, D. Fuerst, F. Dubeau, B. Williamson, A. Evans and F. Andermann
Department of Neurology, Wayne State University School of Medicine, Detroit, Mich, USA.
BACKGROUND: Magnetic resonance imaging (MRI)-based volumetric measurements
of the hippocampal formation are useful in detecting unilateral hippocampal
sclerosis (HS) in patients with temporal lobe epilepsy. In this pathologic
entity, volumetric MRI analysis shows the epileptogenic structure to be
atrophic when compared with the normal, nonepileptogenic side. Some authors
have suggested that the radiological features of atrophy of medial temporal
lobe structures are common in patients with complex partial seizures, but
also are seen frequently in other seizure types and can occur even in
patients without epilepsy. OBJECTIVE: To determine if seizures originating
in extrahippocampal sites cause gliosis, cell loss, and atrophy of medial
temporal lobe structures (i.e., HS). METHODS: We studied 110 patients with
chronic epilepsy using volumetric MRI measurements of the hippocampal
formation. Seventeen patients had pathologically proven HS, 27 patients had
seizures due to extratemporal structural lesions, 15 patients had seizures
caused by extrahippocampal temporal lobe lesions, 29 patients had primary
generalized epilepsy, and 22 patients had secondary generalized epilepsy.
RESULTS: All 17 patients with HS showed significantly reduced absolute
hippocampal formation volumes of greater than 2 SDs below the mean of the
control groups. The preoperative hippocampal formation volume measurements
correlated well with the severity of HS on pathological examination.
Hippocampal volumes were within the normal range in all patients with
primary generalized epilepsy, secondary generalized epilepsy, extratemporal
structural lesions, and extrahippocampal temporal lobe lesions.
CONCLUSIONS: Seizures originating at extrahippocampal sites do not cause
gliosis, cell loss, or atrophy of medial temporal structures. Significant
reduction in hippocampal volumes is a specific marker for HS.
