Invisible, odorless, and tasteless, carbon monoxide gas (CO) is often referred to as a quick and silent killer. Now it's also recognized for causing permanent injury to the brain-in concentrations once thought too low for concern. Victims of CO poisoning may suffer brain damage that results in memory loss and other cognitive impairments. The hippocampus, a brain area critical to forming new memories, is particularly vulnerable to the effects of the gas.

"CO poisoning is very common," says Ramona Hopkins, Ph.D., an assistant professor of psychology and neuroscience at Brigham Young University and a senior research associate of pulmonary and critical care medicine at LDS Hospital in Salt Lake City. Yet as common as it is, she adds, "people are not aware of how serious CO poisoning can be. Almost everyone we see who's been exposed to CO is surprised to find out that it could be this devastating."

CO's attack on the brain

As CO is inhaled, it enters the bloodstream and attaches to hemoglobin, the protein in red blood cells that carries oxygen. CO, however, displaces oxygen because it binds to hemoglobin 200 times tighter than oxygen does. The body can't get enough oxygen-a condition called anoxia.

Besides causing anoxia, CO is also toxic to cells. It triggers a biochemical cascade that, if uninterrupted, can continue to damage the cells even after the exposure to the gas has stopped, according to Hopkins. That may be why current research shows that individuals with anoxic brain injury tend to have less recovery than those with physical injuries to the brain.

Starving the brain

In the brain, which consumes about 20 percent of the body's total oxygen, there appears to be a hierarchy of ways in which anoxia exerts its damaging effects. Three brain structures, the hippocampus, the cerebellum, and the basal ganglia, "seem to be more vulnerable than other parts of the brain," Hopkins explains. The hippocampus, the region of the brain responsible for transferring new information to long-term memory, is most vulnerable because it sits deep in the middle of the temporal lobe, at the very end of the blood's circulation route. In addition, during low oxygen conditions the neurons in the hippocampus release a chemical called glutamate at higher-than-normal levels. This is toxic to brain cells.

Firsthand experience

Hopkins has firsthand experience with the effects of anoxia on the brain. In 1981, her eldest son, four years old at the time, was pinned across his chest by a garage door. He was comatose for several weeks and suffered serious anoxic brain injury. "We spent years doing therapy, and I kept asking, 'Where's the research? What can we expect?' And I really wasn't getting any answers." Hopkins, a nurse by profession, decided to find the answers herself and went to graduate school to investigate the effects of anoxia on the brain.

In 1996, Hopkins received her Ph.D. She has found her research on anoxia very satisfying personally and professionally. "I love what I do," she says. "Just getting information out regarding the effects of anoxia and the long-term outcome has really helped a lot of people. When I tell patients about my son, they realize that I understand what they're going through and they're willing to be involved in my research." (Her son, now 25, graduated from high school and works for a local business.)

Consequences of CO poisoning

In her studies, Hopkins has documented a variety of damaging effects on the brain after CO exposure. Initially, brain scans may reveal small spots of damaged brain cells. "Those tend to occur within the first 24 to 48 hours after exposure," she notes. Over time, signs of cell loss, or atrophy, appear. "It takes a while for the cells to die and be broken down by other cells and removed. That process doesn't happen immediately. We see atrophy at two weeks up to 12 months after CO exposure."

On average, 25 to 50 percent of the CO-poisoning victims in her studies have cognitive impairments. Some experience a memory impairment known as anterograde amnesia: the inability to form new memories. While memories from before the brain injury remain intact, the person has trouble learning and remembering new information, such as lists of information and the names of new acquaintances. In some cases, the impairments interfere with the person's ability to hold a job or live independently.

The degree of memory impairment appears to be directly related to the amount of damage sustained by the hippocampus. In fact, the extent of damage visible on magnetic resonance imaging (MRI) roughly predicts the amount of impairment. Although some impairments may improve as the person recovers from the incident that caused the anoxia, memory function may continue to decline. Thus, even if a person survives CO poisoning, the resulting memory impairments can last a lifetime.

Hidden victims

Each year, approximately 50,000 people are seen at hospitals for CO poisoning, says Hopkins. She suspects many more cases are not diagnosed because the symptoms often resemble those of the flu, including nausea, headache, lethargy, and dizziness.
Poisoning can occur even with small amounts of CO, if inhaled over a long time. "Any kind of incomplete combustion gives off carbon monoxide," explains Hopkins. For example, "if people have an attached garage and warm their cars up in the garage, they may be getting carbon monoxide pulled into the house." Just the exhaust from a gas stove that's not burning efficiently can cause CO poisoning. "We've seen a few cases around Thanksgiving when people cook a lot." Other sources of carbon monoxide include faulty furnaces, gas appliances, gas fireplaces, kerosene or propane space heaters, and charcoal grills.

Act quickly

If you suspect you're suffering from CO poisoning, immediately get to a well-ventilated area and seek medical attention. A blood test can determine whether you've been exposed. The treatment for CO exposure is oxygen, which, stresses Hopkins, "is important to get as soon as possible." Since CO itself has no odor or color, Hopkins advises, "the best thing to do is to buy a carbon monoxide detector. They cost $30 to $50, which is much less than it costs to go to the emergency room, even if you have a good outcome."

Further Reading:

• "MRI, quantitative MRI, SPECT, and neuropsychological findings following carbon monoxide poisoning brain injury," by Shawn D. Gale, Ramona O. Hopkins, Lindell K. Weaver, Erin D. Bigler, Edgar J. Booth, and Duane D. Blattner. (Brain Injury, 1999, Volume 13, Number 4, 229-243.)
• "Continuing decline of memory skills with significant recovery of intellectual function following severe carbon monoxide exposure: clinical, psychometric, and neuroimaging findings," by Cecil R. Reynolds, Ramona O. Hopkins, and Erin D. Bigler. (Archives of Clinical Neuropsychology, 1999, Volume 14, Number 2, pp. 235-249.)

Contact Information:

National Institute of Neurological Diseases and Stroke
Anoxia/Hypoxia Information Page
http://www.ninds.nih.gov/health_and_medical/disorders/anoxia_doc.htm

Brain Injury Association
105 North Alfred Street
Alexandria, VA 22314
Email: [email protected]
Website: http://www.biausa.org
Tel: 703-236-6000 800-444-6443
Fax: 703-236-6001

National Rehabilitation Information Center (NARIC)
1010 Wayne Avenue
Suite 800
Silver Spring, MD 20910-5633
Email: [email protected]
Website: http://www.naric.com
Tel: 301-562-2400 800-346-2742
Fax: 301-562-2401

Head Injury Hotline
600 First Avenue
Suite 212
Seattle, WA 98104-2221
Email: [email protected]
Website: http://www.headinjury.com
Tel: 206-621-8558
Fax: 206-624-4961