Your next patient has had headache, dizziness, nausea and feeling run down for 7 days and wants to know why. Here's how the carbon monoxide poisoning case unfolds and what to pick up on the way.

This carbon monoxide poisoning case is easy to miss because it mimics viral illness or migraine: a week of headache, dizziness, nausea and lethargy. Symptoms are worst at home and ease when he is out. His partner has headaches and the dog is unwell. There was one near faint at home and mild chest tightness on exertion. There is no neck stiffness, photophobia or rash.

This page walks through how the case presents, what to pick up in the history, the tests and the teaching points. It covers why pulse oximetry can be falsely normal and why CO-oximetry on a blood gas measures carboxyhaemoglobin, how the level supports the diagnosis yet correlates poorly with severity and falls with oxygen and time since exposure, and how to assess cardiac and neurological effects with ECG and troponin, given risks of myocardial ischaemia and later neuropsychiatric sequelae.

How the case presents

Your next patient has had headache, dizziness, nausea and feeling run down for 7 days and wants to know why. In their own words: "About a week of headache, dizziness, nausea and feeling generally run down."

Observations on arrival:

Observation	On arrival
Heart rate	98 beats per minute
Blood pressure	128/80 mmHg
Respiratory rate	18 breaths per minute
Oxygen saturation	99%
Temperature	36.9 °C

What to pick up in the history

These are the findings the case is built around. Each one moves you towards or away from carbon monoxide poisoning, and the scorecard checks whether you found it.

A week of headache, dizziness, nausea and lethargy: A non-specific cluster routinely mislabelled as viral illness, so the exposure history must be sought.
Symptoms worst at home and improve when he is out: environmental patterning is the strongest single pointer to carbon monoxide and to a source in the flat.
Partner has headaches too and the dog is unwell: whole-household illness including a pet effectively confirms a shared environmental toxin rather than infection.
One episode of feeling faint and nearly passing out at home.
Mild chest tightness on exertion: suggests carbon monoxide-induced myocardial ischaemia, so check an ECG and troponin.
No neck stiffness, photophobia or rash: weighs against meningitis, the main alternative for a week of headache with malaise.

Red flags you must not miss

The case marks these as must-elicit. They only come out if you ask.

Symptoms worst at home and improve when he is out.
Partner has headaches too and the dog is unwell.
One episode of feeling faint and nearly passing out at home.
Mild chest tightness on exertion.

Investigations and what they showed

Carboxyhaemoglobin: Elevated carboxyhaemoglobin (well above the smoker/non-smoker baseline), confirms carbon monoxide poisoning. Why it helps: the specific diagnostic test, measured by co-oximetry on a blood gas; the level supports the diagnosis (though it correlates poorly with severity and falls with oxygen/time since exposure).
Arterial blood gas: A metabolic acidosis with raised lactate; the measured (co-oximetry) oxygen saturation is low despite a normal calculated/pulse-oximeter SpO2. Why it helps: provides co-oximetry (carboxyhaemoglobin) and reveals the tissue hypoxia/acidosis, and demonstrates why the pulse oximeter reads falsely normal in CO poisoning.
Lactate: Raised (tissue hypoxia); a very high lactate should also raise concern for cyanide co-exposure in smoke inhalation. Why it helps: marks tissue hypoxia and severity; a disproportionately high lactate in a fire/smoke context suggests concurrent cyanide toxicity.
High-sensitivity troponin: May be raised, CO causes myocardial injury/ischaemia. Why it helps: CO poisoning can cause myocardial ischaemia/injury even in young hearts, check troponin, especially with chest symptoms or ECG changes.
12-lead ECG: May show ischaemia (ST depression/T-wave changes) or arrhythmia from CO-induced myocardial hypoxia. Why it helps: screens for the cardiac effects of CO (ischaemia/arrhythmia), which influence severity assessment and disposition.
CT head: In significant poisoning, bilateral symmetric low density in the globus pallidus; often used first to exclude other acute intracranial causes. Why it helps: excludes other causes of headache/altered consciousness and can show the characteristic bilateral globus pallidus changes of CO toxicity.
MRI brain: Bilateral, symmetric globus pallidus lesions (and white-matter change), the classic neuro-imaging correlate; predicts delayed neuropsychiatric sequelae. Why it helps: more sensitive than CT for the characteristic globus pallidus/white-matter injury and for prognosticating delayed neurological effects.
Inspect the patient's level of alertness and demeanour: He is awake and oriented but slowed and vague, he loses the thread of his own story, asks for questions to be repeated, and describes feeling 'foggy' and unable to concentrate at work. Why it helps: subtle cognitive slowing is often the only abnormal sign in chronic low-level CO exposure, and it is exactly what gets labelled as a viral illness or stress.
Inspect the bedside and surroundings: The pulse oximeter on his finger reads 99% on air, sitting next to a young man who is symptomatic and unwell. Why it helps: the oximeter is lying. It measures the ratio of two wavelengths and cannot tell carboxyhaemoglobin from oxyhaemoglobin, so it reads high or normal in CO poisoning. A normal SpO2 must never be used to exclude it, you need CO-oximetry on a blood gas.
Inspect the skin: The skin is of normal colour, with no bullae. Cutaneous bullae over pressure points are described in severe CO poisoning with prolonged unconsciousness, but he has none. Why it helps: bullae and rhabdomyolysis mark a patient who has been down for hours. Their absence fits a subacute exposure over a week rather than a single overwhelming one, but does nothing to make the poisoning less real or less urgent.
Inspect the patient (general appearance): A well-nourished, well-kempt young man who does not look acutely ill, no distress, no work of breathing, no fever, no rash. Why it helps: looking well is the point. CO poisoning has no reliable physical signs at all; the entire diagnosis lives in the environmental history and in remembering to ask about it. If you have not asked about the boiler and the household, you have not looked.

Why the diagnosis is carbon monoxide poisoning

The decisive diagnosis is carbon monoxide poisoning. above all, pulse oximetry reads falsely normal because it cannot distinguish carboxyhaemoglobin from oxyhaemoglobin, so measure carboxyhaemoglobin by CO-oximetry on a blood gas.

Carbon monoxide poisoning at a glance

What it is

Carbon monoxide poisoning is toxicity from inhaling carbon monoxide, a colourless, odourless gas produced by incomplete combustion of fuels such as gas, wood, coal and petrol. Because the gas cannot be seen, smelt or tasted, exposure is often unrecognised until symptoms are already present, and it is frequently mistaken for a viral illness or migraine.

Pathogenesis

Carbon monoxide binds haemoglobin roughly 200 to 250 times more avidly than oxygen, forming carboxyhaemoglobin, which both reduces the blood's oxygen-carrying capacity and shifts the oxygen dissociation curve so that any oxygen still bound is released to tissues less readily. Carbon monoxide also binds directly to mitochondrial cytochrome oxidase, impairing cellular respiration independent of blood oxygen content, which is why symptoms can be severe even before hypoxia is obvious.

Who it affects

Anyone exposed to fumes from a faulty fuel-burning appliance, a blocked flue or chimney, or an idling engine in an enclosed space, especially in winter when heating use rises. Whole households and even pets are often affected together, which is itself a diagnostic clue. Unborn babies and people with pre-existing heart or lung disease are at highest risk of harm from a given exposure.

How serious it is

Carbon monoxide poisoning can cause myocardial ischaemia, loss of consciousness, seizures and death, and up to 40% of survivors of significant poisoning develop delayed neuropsychiatric sequelae, memory loss, personality change or parkinsonism, appearing weeks later. It is a leading cause of poisoning death worldwide, precisely because it is so easily missed.

Causes and risk factors

There is no personal susceptibility factor beyond pre-existing cardiorespiratory disease and pregnancy.

Investigations and lab tests

Diagnosis rests on measuring carboxyhaemoglobin by CO-oximetry on a blood gas, the level supports the diagnosis but correlates poorly with severity and falls once on oxygen or with time since exposure. Every patient needs an ECG and troponin for myocardial ischaemia, and a careful neurological exam.

At a glance: Confirm exposure: carboxyhaemoglobin by CO-oximetry. Assess harm: ECG and troponin, neurological exam. Do not rely on: pulse oximetry, which reads falsely normal.

Teaching points

Carbon monoxide poisoning causes non-specific symptoms (headache, dizziness, nausea, lethargy, poor concentration, faintness) and is frequently misdiagnosed as a viral illness or migraine.
Pulse oximetry is falsely normal, it cannot distinguish carboxyhaemoglobin from oxyhaemoglobin; measure carboxyhaemoglobin by CO-oximetry on a blood gas.
The COHb level supports the diagnosis but correlates poorly with severity and falls with oxygen and time since exposure.
Assess cardiac (ECG, troponin) and neurological effects; CO can cause myocardial ischaemia and later neuropsychiatric sequelae.
The classic imaging finding is bilateral, symmetric globus pallidus injury on CT/MRI.
In smoke/fire exposure, consider concurrent cyanide poisoning (severe acidosis/very high lactate).
Above all, make the source safe, fit a CO alarm, and warn/screen co-habitants, do not send the patient back to an unsafe environment.

Practise a similar case

Reading a case is never the same as feeling the clock ticking and choosing your next question. Would you have asked about others at home when he said the headache eased outside, and caught the partner and the unwell dog clue? Now take a history from a different simulated patient in acute medicine and see what you notice.

Here's one from the same specialty, with the diagnosis left for you to find. Your next patient has been weak and breathless for 3 days, with swollen legs and struggling to sit up. Take this patient's history in Diagnosica. It's free to start, no card needed.

Carbon monoxide poisoning case presentation: history, findings and tests