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TUESDAY, June 22 (HealthDayNews) -- People who experience a rare type of amnesia, which typically lasts less than 24 hours, develop brain lesions a day or two after the event, German researchers report.

The discovery gives researchers another clue about what may cause transient global amnesia (TGA).

Using a type of magnetic resonance imaging called diffusion-weighted imaging, the German team found small areas of brain damage in 26 of the 31 patients they followed. All of the lesions were located in the hippocampus, a part of the brain that assists with forming and storing memory.

The damage was not immediately evident. It took a day or two for the lesions to appear.

"It confirms the hippocampal involvement and hints to the underlying pathology of TGA," said Dr. Oliver L. Sedlaczek, a neurologist at the University of Heidelberg in Mannheim, Germany and lead author of the study, published in the June 22 issue of the journal Neurology.

It also may help clinicians confirm a diagnosis of TGA in patients with this sudden and mysterious memory loss.

"There has been a fair amount of controversy over whether imaging is useful in these patients," said Dr. David C. Tong, an associate professor of neurology at Stanford University School of Medicine, who wrote an accompanying editorial.

Previous studies reported conflicting results, with some showing no brain abnormalities and others reporting a high percentage of lesions. Delaying the imaging for 24 to 48 hours appears to be the key to detecting TGA.

People who experience an episode of TGA are suddenly unable to form new memories or recall the near past. Otherwise, they remain alert and have no other symptoms, and the amnesia ends as abruptly as it began, according to the Mayo Clinic.

"As a major medical center, we see one to two cases per year, so it's not super-common," said Stanford's Tong.

The authors said they were able to recruit 31 participants within 27 months, thanks to a regional stroke awareness program that makes doctors and patients very aware of symptoms that signal possible stroke.

While TGA often occurs after a stressful, emotional event, the exact cause of the condition isn't known. Sedlaczek suggests a "stroke-like mechanism" may be involved. The small lesions found with TGA, he said, suggest a possible link between stress and low-grade risk factors for stroke.

As a result, Sedlaczek said he and his and colleagues have begun to do a standard stroke assessment in all patients with TGA.

It is possible, Tong allowed, that the TGA cases in the German study were more ischemic in nature, meaning that the affected area experienced a temporary reduction in blood supply. If patients have a lot of risk factors for stroke -- high blood pressure, high cholesterol, or diabetes -- then clinicians should consider putting them on blood-thinning antiplatelet therapy, such as aspirin or Aggrenox, he agreed.

Still, the causes of TGA remain speculative.

"Some say it's vascular; some seizure. Most likely it's a mixture of different things all blended together," Tong suggested. What's needed, he said, is a prognostic study that compares TGA patients with abnormalities to those whose brain scans come up negative.

More information

Read more about memory loss from the National Library of Medicine.

(SOURCES: Oliver L. Sedlaczek, M.D., faculty member, department of neurology, University of Heidelberg, Mannheim, Germany; David C. Tong, M.D., associate professor, department of neurology and neurological sciences, Stanford University School of Medicine, Palo Alto, Calif.; June 22, 2004, Neurology)

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