A 70-year-old woman with signs of early Alzheimer's disease turned out to have a treatable condition, French doctors say.

In July 2007, the then 70-year-old came to hospital with a four-month history of confusion and episodes of memory loss but not previous medical history.

Her physical, blood and neurological exams were normal but tests showed memory and learning problems.

When an MRI showed a brain abnormality usually tied to limbic encephalitis — a brain disorder that attacks the limbic system, a collection of structures at the base of the brain responsible for emotions and many other basic functions.

The symptoms of limbic encephalitis include short-term memory problems, temporal seizures, and psychiatric disorders that progress over a few weeks.

Could be mistaken for Alzheimer's

Data on limbic encephalitis is scarce for older patients, which makes it difficult to diagnose, Dr. Nathalie Costedoat-Chalumeau, of the Centre Hospitalier Universitaire Pitié-Salpêtrière in Paris and co-authors say in Saturday's issue of the medical journal The Lancet.

"In our case, the diagnosis could have been mistaken for Alzheimer's disease, especially since the brain CT scan was normal. However, the short period of evolution and associated MRI features were atypical for Alzheimer's disease, and led us to look for limbic encephalitis," the authors wrote.

Previously, limbic encephalitis was considered a rare disease that did not respond to treatment.

After antibodies and autoimmune mechanisms were discovered, doctors tried immunosuppressive treatments and the woman's prognosis improved, the researchers said.

Limbic encephalitis should now be considered as a non-exceptional and treatable cause of rapidly progressive dementia, the authors concluded.