History and exam

Key diagnostic factors

common

risk of carbon monoxide exposure

Take a thorough history to assess the likelihood of carbon monoxide exposure.[22]

Use the COMA questions:[21]

  • C: Cohabitees/companions - Is anyone else in the property affected (including pets)?

  • O: Outdoors - Do the symptoms improve when out of the building?

  • M: Maintenance - Are any fuel-burning appliances and vents properly maintained?

  • A: Alarm - Does the affected building have a carbon monoxide alarm?

headache

The most common symptom, reported by 90% of patients with carbon monoxide poisoning.[7]​[10][18]​​[21]

  • Often a tension-type headache.[21]

nausea

A common symptom.[10][18]

vomiting

A common symptom.[10][18]

vertigo

A common symptom.[10][18]

altered consciousness

A common symptom.[10][18]

  • Ask about drowsiness, tiredness, syncope, or confusion.[1][10]​​​ Lethargy/mild tiredness can be a feature of chronic poisoning.[10][22]

  • Coma may be present in people with severe poisoning.​[10][17][22]

  • Carbon monoxide poisoning may cause inattention and memory change.[1]​[19][22]​ Memory change is a feature of chronic poisoning in particular.[10]

  • Use the mini-mental state examination or a similar test of short-term memory and cognitive function.​[18][19]

weakness

Subjective weakness is present in around 20% of patients with acute carbon monoxide poisoning.[10]

dizziness

Ask about dizziness.[2][10][17]​[18]​​​​​

dyspnoea

Ask about breathing difficulties or shortness of breath.[21][22]

Respiratory failure and Cheyne-Stokes breathing are features of severe poisoning.[1][10]​​[24]

pain

Ask about chest or abdominal pain.

  • Cardiovascular symptoms, such as angina, or cardiac ischaemia or infarction may be present.​​[1][19][22]

  • Non-specific physical pain may be a symptom of chronic poisoning.[22]

hypotension or hypertension

Assess blood pressure as the patient may be hypotensive or hypertensive.[1]​[19][22]​[31]

sleep changes

Ask about tiredness, fatigue, and sleep disturbance.[10]

irritability

Ask about irritability.​[10][18]

flu-like symptoms

A feature of chronic carbon monoxide poisoning.[10]

Other diagnostic factors

common

delayed neuropsychiatric features

Consider in a patient previously exposed to carbon monoxide.

  • These include memory impairment, disorientation, apathy, mutism, irritability, inability to concentrate, emotional lability, peripheral neuropathy, incontinence, choreoathetosis, apraxia, psychosis, dementia, Parkinsonism, and hearing loss.[1][10][17][18]​​​​[28]​​​​

other severe neurological symptoms

Seizures, coma, cortical blindness, or cerebral oedema may develop in people with severe poisoning.[1]​[10][17]​​[19][22]

uncommon

focal neurological abnormalities

Perform a neurological examination and test the patient’s coordination and balance. Carbon monoxide poisoning may cause sensory changes and ataxia.[22]

Request a head CT if there is concern about neurological abnormalities.[9] Images may reveal intracranial pathology.

cardiac arrest

A feature of severe carbon monoxide poisoning.

pulmonary oedema

Non-cardiogenic pulmonary oedema is a feature of severe carbon monoxide poisoning.[10][19]​​[22]

disseminated intravascular coagulation

May be a feature of carbon monoxide poisoning.[26]​ See our topic Disseminated intravascular coagulation.

skin lesions

Skin lesions such as blisters and sweat gland necrosis can be features of carbon monoxide poisoning.[10][27]​​

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