Is smelling burnt toast a symptom of stroke?
Smelling burnt toast when there is none present is referred to as phantosmia. This phenomenon is known as olfactory hallucination and can occur in neurological conditions [1].
Some people can have changes in smell after a stroke. However, typically strokes cause a decrease or complete loss of smell. Olfactory hallucination is not a common symptom of stroke. Thus, smelling burnt toast is not a certain indication of a stroke [2, 3].
Olfactory hallucinations can occur in severe and extensive strokes. Although smelling burnt toast can be present, it is rarely limited to only one smell [4].
What are the most common symptoms of stroke?
A stroke is a medical emergency. It requires prompt medical attention. Fast diagnosis and treatment limit the potential complications of stroke. Therefore, a quick recognition of the stroke symptoms is important.
The top 3 most common symptoms of stroke are:
- Problems with speaking or language comprehension [5].
- Sudden numbness or weakness of extremities, often one-sided [5].
- Weakness in the face or face drooping [5].
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What are the causes of phantosmia?
Phantosmia is a phenomenon of olfactory hallucination. It means that you smell something that is not present in your vicinity.
The most common causes of phantosmia are problems in the sinuses. Even up to 72% of all cases of smell hallucination are caused by:
Other causes that can lead to phantosmia include neurotoxin exposure (like mercury intoxication) and radiotherapy of brain cancer [7, 8].
Less common causes that can induce olfactory hallucinations are often neurological. Among them are:
- head injuries,
- brain tumors,
- Alzheimer’s disease,
- Parkinson’s disease,
- epilepsy,
- strokes [7, 8].
Also, infection with COVID-19 can lead to phantosmia [9, 10].
In rare cases, migraines can also cause phantosmia [11].
Sometimes, the cause of olfactory hallucination is indeterminable and remains unknown [12].
How can a stroke lead to a smell disturbance?
A stroke can cause smell disturbance, either by interference with the sensing or perception of an odor.
The way a stroke can interfere with the sensing of smell is by the activation of the microglia in the olfactory bulb.
The olfactory bulb is the smell-sensing area in the brain. The microglia are immune cells that operate in the brain. When active, microglia secrete proinflammatory mediators. Secretion of these molecules in the olfactory bulb leads to a regional dysfunction [13].
The microglia activation happens especially in ischemic strokes [13].
Furthermore, a stroke can also interfere with the smell-sensing pathway in the brain. A stroke can lead to structural changes in the sensing region, impairing its function. These changes depend on the duration of the stroke [14].
A stroke can also affect a region in the brain that is responsible for smell recognition. Among those regions are:
- the piriform cortex [15],
- orbitofrontal cortex [16],
- hippocampus [17],
- amygdala [17],
- entorhinal cortex [18].
When a specific region that identifies the smell is affected by a stroke, it can lead to olfactory disturbance. For example, it can cause smell hallucinations [19, 20].