Ischemic evidence of transient global amnesia: location of the lesion in the hippocampus

Abstract

Background and purpose: Transient global amnesia (TGA) is a rare amnestic syndrome characterized by the sudden onset of a selective anterograde and retrograde amnesia with a time course of up to 24 hours. Recent studies have found a high frequency of small high-signal abnormalities in the hippocampus on diffusion-weighted imaging (DWI), and accordingly ischemia has been proposed as an etiology of TGA. We hypothesized that TGA lesions occur preferentially in the CA1 region of the hippocampus, which is known to be susceptible to ischemia.

Methods: Twenty consecutive patients with a clinical diagnosis of TGA underwent DWI both within 24 hours of symptom onset and 3 days later. Twenty patients with high-signal abnormalities in the hippocampus on the initial DWI underwent subsequent DWI and T2-weighted imaging in the coronal plane to precisely localize the lesions.

Results: Seventeen patients had small high-signal abnormalities (with diameters of 1-3 mm) in the hippocampus unilaterally on DWI. One of these patients had two lesions in one hippocampus. Three of the 20 patients had lesions bilaterally in the hippocampus, 1 of whom had 3 bilateral lesions. A total of 25 lesions were identified: 5 in the hippocampal head, 19 in the body, and 1 in the tail. Six patients had unilateral lesions on the left,11 patients had them on the right, and 3 patients had bilateral lesions.

Conclusions: In this study, lesions associated with TGA were localized mostly to the lateral portion of the hippocampus, corresponding to CA1. This finding supports the ischemic etiology of TGA, but the underlying pathophysiologic mechanism requires further investigation.

Keywords: CA1; Hippocampus; Transient global amnesia.

Figure 1

Imaging results in patient 11 (64-year-old female). Initial DWI images obtained at 11 hours after TGA onset show no lesion for b=1000/slice thickness=5 mm and b=1000/slice thickness=3 mm, but a subtle high-signal lesion (grade +) in the right hippocampus for b=2000/slice thickness=3 mm and b=3000/slice thickness=3 mm (arrow, A). Three-day DWI images show a higher signal lesion (grade ++) for all sequences (B).

Figure 2

Coronal schematic drawing of the subfields in the right hippocampal body. The lower medial, lateral, and upper medial regions of the hippocampus correspond approximately to the subiculum, CA1, and remaining sectors (including CA2, CA3, CA4, and the dentate gyrus), respectively.

Figure 3

MRI findings of TGA patients. Transverse and coronal DWI show a small high-signal lesion (arrow) in the lateral region of the hippocampus (patients 1, 2, 4, 7, 10, 12, 14, 17, 18, and 19).

Figure 4

A 64-year-old woman (patient 11) with transient global amnesia. Transverse and coronal DWI show a small high-signal lesion in the lateral region of the posterior body of the hippocampus (arrow). An apparent diffusion coefficient map shows the low apparent diffusion coefficient of the lesion. Coronal T2-weighted image shows a small high-signal lesion in the same location as in DWI.