Carbon Monoxide Toxicity Clinical Presentation

Updated: Dec 31, 2020
  • Author: Guy N Shochat, MD; Chief Editor: Gil Z Shlamovitz, MD, FACEP  more...
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Misdiagnosis of carbon monoxide (CO) toxicity commonly occurs because of the vagueness and broad spectrum of complaints; symptoms often are attributed to a viral illness. Specifically inquiring about possible exposures when considering the diagnosis is important (see Background and Causes).

For nonfatal nonintentional non–fire-related exposures, the most common symptom was headache (37%) followed by dizziness (18%) and nausea (17%). [11] However, any of the following symptoms should alert suspicion in the winter months, especially when the patient has a history compatible with CO exposure and when more than one patient in a group or household presents with similar complaints:

  • Malaise, flulike symptoms, fatigue
  • Dyspnea on exertion
  • Chest pain, palpitations
  • Lethargy
  • Confusion
  • Depression
  • Impulsiveness
  • Distractibility
  • Hallucination, confabulation
  • Agitation
  • Nausea, vomiting, diarrhea
  • Abdominal pain
  • Headache, drowsiness
  • Dizziness, weakness, confusion
  • Visual disturbance, syncope, seizure
  • Fecal and urinary incontinence
  • Memory and gait disturbances
  • Bizarre neurologic symptoms, coma

Chronic exposure also produces the above symptoms; however, patients with chronic CO exposure may present with loss of dentition, gradual-onset neuropsychiatric symptoms, or, simply, recent impairment of cognitive ability.



Physical examination is of limited value. Inhalation injury or burns should always alert the clinician to the possibility of CO exposure.

Vital signs may include the following:

  • Tachycardia

  • Hypertension or hypotension

  • Hyperthermia

  • Marked tachypnea (rare; severe intoxication often associated with mild or no tachypnea)

Although so-called cherry-red skin has traditionally been considered a sign of CO poisoning (ie, "When you're cherry red, you're dead"), it is in fact rare. [3] Pallor is present more often

Ophthalmologic findings include the following:

  • Flame-shaped retinal hemorrhages

  • Bright red retinal veins (a sensitive early sign)

  • Papilledema

  • Homonymous hemianopsia

  • Noncardiogenic pulmonary edema

Neurologic and/or neuropsychiatric findings may include the following;

  • Memory disturbance (most common), including retrograde and anterograde amnesia with amnestic confabulatory states

  • Emotional lability, impaired judgment, and decreased cognitive ability

  • Stupor, coma, gait disturbance, movement disorders, and rigidity

  • Brisk reflexes, apraxia, agnosia, tic disorders, hearing and vestibular dysfunction, blindness, psychosis

Long-term exposures or severe acute exposures frequently result in long-term neuropsychiatric sequelae. Additionally, some individuals develop delayed neuropsychiatric symptoms, often after severe intoxications associated with coma.

After recovery from the initial incident, patients present several days to weeks later with neuropsychiatric symptoms such as those just described. Two thirds of patients eventually recover completely.



See the list below:

  • Most unintentional fatalities occur in stationary vehicles from preventable causes such as malfunctioning exhaust systems, inadequately ventilated passenger compartments, operation in an enclosed space, and utilization of auxiliary fuel-burning heaters inside a car or camper.

  • Most unintentional automobile-related CO deaths in garages have occurred despite open garage doors or windows, demonstrating the inadequacy of passive ventilation in such situations.

  • Colorado state datafrom 1986-1991 revealed that leading sources of 1149 unintentional nonfatal CO poisonings were residential furnaces (40%), automobile exhaust (24%), and fires (12%); however, furnaces were responsible for onlly 10% of fatal poisonings [22]

  • In the setting of structure fires, CO presents greater risk than thermal injury or oxygen deprivation, both for firefighters and victims [23]

  • In most developing countries, cooking or heating is often done with unvented cookstoves, wood, charcoal, animal dung, or agricultural waste,  which has been linked with elevated HbCO levels

  • Boats and houseboats represent a significant and underappreciated source of exposure, with multiple case reports and studies [5]