🧴 For some people, walking through the air freshener aisle in a supermarket or standing near someone wearing a strong perfume can trigger MCAS symptoms very quickly. These can include headaches, nausea, dizziness, brain fog, burning eyes, nasal itching, flushing and sudden exhaustion. Sometimes the reaction is strong enough that the person has to escape from the area and find somewhere with cleaner air.
👃 It is easy to describe this as being overly sensitive to smell, but that is not the whole picture. Fragranced products release lots of volatile organic compounds, generally shortened to VOCs. Some are detected by olfactory receptors, which create the perception of smell. While sensory endings of the trigeminal nerve detect chemical irritation and that can trigger a range of wider responses.
What happens when smells enter the nose?
🌫️ The nasal lining contains some branches of the trigeminal nerve. This nerve helps detect irritating chemicals and produces protective responses such as burning, watering eyes, sneezing and the urge to move away from an irritant.
⚡ Some airborne chemicals can activate sensory receptors of the trigeminal nerve known as TRP channels. TRPA1 is particularly involved in detecting chemical irritants, while TRPV1 also contributes to responses to some chemical and inflammatory stimuli. Once these receptors are activated, warning signals can travel rapidly into the rest of the nervous system.
How can odours trigger migraine?
🧠 The trigeminal system is thought to play an important role in migraine. Strong odours and chemical irritants are common triggers for migraine patients. Trigeminal activation can lead to the release of neuropeptides such as CGRP and increase the sensitivity of pain-processing pathways.
💥 This helps explain why a fragrance can lead to much more than an unpleasant smell. Someone who is susceptible may experience headaches, nausea, dizziness, light sensitivity, difficulty concentrating (brain fog) or a full-blown migraine attack.
What role do mast cells and MCAS have?
🧬 Mast cells are immune cells found throughout the body. This includes the skin, airways, digestive system and tissues around blood vessels. When activated, they release histamine, prostaglandins, leukotrienes, cytokines and many other inflammatory substances.
🔥 In some people, mast-cells become unusually reactive. Mast Cell Activation Syndrome, or MCAS, is a condition in which mast-cell inflammatory mediators are released inappropriately. This produces symptoms in more than one body systems. These could include flushing, itching, hives, nasal congestion, gastrointestinal cramping, a rapid heart rate, headache, fatigue and other symptoms.
🌫️ Those with MCAS often have reactions to a range of triggers. These can include fragrances, odours, alcohol, heat or temperature changes, medications, stress and hormonal changes are commonly reported. Fragrance sensitivity on its own does not diagnose MCAS, but repeated reactions involving several body systems may be relevant when the wider clinical picture is considered.
🔄 Mast cells and sensory nerves also communicate with each other. Mast-cell inflammatory mediators can make sensory nerves in the area more reactive. At the same time, neuropeptides released from nerves can influence mast-cell activity. This bi-directional interaction between the nervous and immune systems helps explain why some fragrance reactions include both neurological and physical symptoms.
🧠 There can also be some overlap between MCAS, migraine and conditions involving autonomic or sensory symptoms. This means headache, dizziness, flushing, itching, palpitations or tachycardia, gastrointestinal symptoms and fatigue can sometimes occur together rather than as separate problems.
Why can allergy tests fail to find anything?
🧪 Standard allergy tests mainly look for specific eg IgE-mediated allergic reactions. IgE means allergic reactions involving an antibody called immunoglobulin E. In this type of allergy, IgE recognises a specific substance, such as pollen, dust mite faeces or a food protein. A negative result therefore does not rule out every possible reaction to fragrances or airborne chemicals, since one fragrance will consist of many chemicals.
🔬 Symptoms may be triggered via the irritation of sensory nerves, migraine pathways, non-IgE immune responses, asthma, contact reactions or abnormal mast-cell activation. Negative allergy testing does not automatically point towards MCAS, but equally it does not exclude the other mechanisms that can produce symptoms.<
What does this mean in practice?
🩺 Reactions to fragrance or other odours can be significant even when routine tests are normal. Persistent or multisystem reactions deserve proper medical assessment, particularly where there are many body systems involved eg breathing problems, recurrent hives, marked changes in heart rate or blood pressure, throat symptoms or significant gastrointestinal reactions.
🧠 Repeated reactions can also lead to understandable anxiety about shops, workplaces, public transport or other places where odour exposure is difficult to control. Therapy can help with the anticipatory anxiety, stress and avoidance that sometimes develops around these experiences. This does not mean the original physical reaction is imagined or caused by anxiety. Learning to relax can also have a positive effect on the immune system and in turn, may allow mast cells to become less reactive.