TUESDAY, March 31, 2015 (HealthDay News) -- People with chronic fatigue syndrome show a distinct pattern of immune system proteins in their spinal fluid -- a finding that could shed light on the "brain fog" that marks the condition, researchers say.

The new study found that, compared with healthy people, those with chronic fatigue syndrome had lower levels of certain immune-system proteins called cytokines in the fluid that bathes the spinal cord and brain.

The exception was one particular cytokine, which was elevated in not only people with chronic fatigue, but also those with multiple sclerosis.

The finding could offer clues as to why people with chronic fatigue syndrome typically have problems with memory, concentration and thinking, said lead researcher Dr. Mady Hornig, a professor at Columbia University's Mailman School of Public Health in New York City.

The study also bolsters evidence that some type of immune dysfunction underlies the puzzling disorder, Hornig said.

Chronic fatigue syndrome is known medically as myalgic encephalomyelitis/chronic fatigue syndrome, or ME/CFS. In the United States, it affects up to 2.5 million people, according to the Institute of Medicine, a scientific panel that advises the federal government.

In February, the IOM released a report affirming that chronic fatigue syndrome is a legitimate medical condition that many health professionals still misunderstand -- or even dismiss as a figment of patients' imagination.

The term "chronic fatigue syndrome" was coined back in 1988, and in hindsight, it was a "lousy" choice, said Suzanne Vernon, a virologist and scientific director of the Solve ME/CFS Initiative, based in Los Angeles.

"People hear it and think, 'Oh, you're tired. I'm tired, too,'" said Vernon, who was not involved in the study. "But this is debilitating fatigue. It's like having a case of the flu that never goes away."

Plus, symptoms go beyond fatigue, and include what's been dubbed "brain fog" -- a collection of thinking-related problems such as confusion and difficulty with concentration and short-term memory.

For the new study, reported March 31 in the journal Molecular Psychiatry, Hornig's team studied spinal-fluid samples from 32 people with chronic fatigue syndrome, 40 with multiple sclerosis, and 19 healthy people.