Whereas severe, acute CO poisoning typically is detected immediately after exposure, symptoms of chronic low-level CO exposure are easily mistaken for a primary depressive (Table 1) or other neuropsychiatric disorder—or overlooked altogether. Some cases persist for months before CO exposure is diagnosed. Clinicians often give unnecessary—sometimes costly—medical treatment while ignoring the underlying poisoning.
Mechanism of action. CO binds with hemoglobin (with an affinity >200 times that of oxygen) to form carboxyhemoglobin (COHb), which causes cellular anoxia by blocking transport of oxygen to the tissues, including the brain.4,6,8
CO poisoning symptoms vary depending on COHb concentration (Table 2). COHb >5% in a symptomatic nonsmoker may indicate chronic low-level CO poisoning and require further evaluation.9 Levels >10% are common in heavy smokers (2 to 4 packs/day). It should be noted that Mrs. A does not smoke.
Presentation. Patients with chronic low-level CO poisoning often present with vague, nonspecific symptoms, such as weakness and fatigue, abdominal pain, nausea, vomiting, diarrhea, decreased concentration, diminished cognitive abilities, persistent headaches, and trouble sleeping.4,8,10,11 Patients age >65 especially may present with multiple cognitive and somatic complaints that suggest Parkinson’s disease, chronic fatigue syndrome, dementia, or—in Mrs. A’s case—depression.5,10,12
Table 2
Signs, symptoms of CO poisoning that emerge at different carboxyhemoglobin levels
| Carboxyhemoglobin level (% HgB) | Signs, symptoms |
|---|---|
| 5-10 % | Exacerbates angina in some patients with heart disease |
| 10-20 % | Mild headache, breathlessness on exertion |
| 20-30 % | Throbbing headache, irritability, mental status changes, fatigue |
| 30-40 % | Severe headache, weakness, nausea, dizziness, visual problems, confusion |
| 40-50% | Increased confusion, hallucinations, severe ataxia, rapid breathing |
| 50-60 % | Syncope or coma with convulsions, tachycardia with weak pulse |
| 60-70 % | Deep coma, incontinence |
| 70-80% | Profound coma, depressed respiration, absent reflexes |
| >80 % | Rapid death from respiratory arrest |
| Source: Adapted from Gilman AG, Rall TW, Nies AS, Taylor P (eds). Goodman and Gilman’s the pharmacological basis of therapeutics (8th ed). New York: Pergamon Press, 1990. | |
Health effects of CO exposure range from subtle cardiovascular and neurobehavioral sequelae at low concentrations to loss of consciousness and death after acute exposure to higher concentrations.3,5
Hypoxia of the brain and other organs resulting from low-level CO poisoning can cause a range of physiologic effects, including mental status changes.10,11 Low-level CO exposure is particularly dangerous to pregnant women and to patients with a pre-existing ischemic illness.
Pregnancy. Chronic low-level CO exposure during pregnancy can harm the fetus, leading to low birth weight, short neonatal length, prematurity, perinatal death, and increased risk of developmental dysfunction.13
Ischemic illnesses. Because COHb cannot transport oxygen, the tissues that demand the most oxygen—such as the brain, heart, and skeletal muscles—are most affected. Because cardiac muscles extract approximately 75% of available oxygen from blood, patients with cardiac and pulmonary ischemic illnesses face a high risk for tissue injury with CO poisoning. At COHb levels >10%, patients with pre-existing cardiac disease experience increased severity and duration of angina; concentrations >15% place them at risk of myocardial infarction.6
Length of recovery from chronic CO exposure varies widely depending on severity of exposure and the patient’s general health.3,5 CO has a 4- to 6-hour half-life and is excreted via the lungs fairly rapidly, so recovery can be swift once CO exposure is stopped. Emergency room referral depends upon severity of symptoms and CO exposure duration and nature (accidental or intentional).
The authors’ observations
CO poisoning can lead to long-term mental status changes. In a 3-year follow-up of patients repeatedly exposed to low CO levels:
- 43% developed neurologic sequelae including memory impairment
- 33% experienced personality changes including irritability, verbal aggression, violence and impulsivity, moodiness, distractibility, and sexual promiscuity
- 11% suffered gross neuropsychological effects, including psychosis, disorientation, and blindness.4
Primary care physicians and psychiatrists should monitor patients who have recovered from CO poisoning for symptoms of these disorders.
DETECTING CHRONIC CO EXPOSURE
Mrs. A’s case illustrates the seriousness and diagnostic complexity of chronic low-level CO exposure in older patients, especially during the fall and winter with increased home heating appliance use.7 CO exposure was not considered as a cause of Mrs. A’s symptoms until heating contractors found the water heater leak.
Watch for patients whose neuropsychiatric symptoms do not respond to treatment. Ask them about possible environmental, seasonal, or diurnal variations in symptoms. Also ask if the patient’s home heating system or water heater is ≥10 years old or has been malfunctioning (Box 2).
Checking COHb blood levels is the simplest way to confirm CO poisoning.6,14
Seven questions to ask patients when you suspect chronic CO poisoning
- Is your home heating system or water heater 10 or more years old or malfunctioning?
- Do you use a gas range or stove for supplemental heat?
- Do symptoms improve or worsen in certain environments or at a certain time of day?
- Have fireplace flues and/or chimney vents been checked within the past year?
- Has another household member—including a pet—also been ill?
- Is a family member who remains at home persistently ill, whereas others who leave periodically improve?
- Do symptoms improve or worsen during certain months or seasons?