Eosinophilic asthma: what it is, how it is identified and how it is treated

Asthma is an inflammatory condition, and the inflammation can take different forms. Asthma + Lung UK explains that most people with difficult-to-control or severe asthma have a more severe type of inflammation called Type 2 inflammation, which is linked to allergies or to higher levels of a blood cell called an eosinophil. Eosinophils are a type of white blood cell that helps the immune system work properly, but in some people with severe asthma there are too many in the blood, which can make airway inflammation worse.

Asthma + Lung UK describes three phenotypes of severe asthma that specialist teams may use: severe eosinophilic asthma, severe allergic asthma (high IgE levels, a strong response to allergies) and non-eosinophilic severe asthma (tests do not show high eosinophils). Knowing the phenotype helps in choosing treatments, because some target Type 2 inflammation, a feature of severe eosinophilic or severe allergic asthma.

NICE's technology appraisals use the term severe refractory eosinophilic asthma for people whose asthma remains uncontrolled on standard therapy, and describe it as a type of severe asthma that can be difficult to control.

Not everyone with poorly controlled asthma has severe asthma, and not everyone with severe asthma is eosinophilic. Asthma + Lung UK says a GP may describe asthma as difficult or hard to control if, in the last 12 months, you have needed two or more courses of steroid tablets, gone to hospital or A&E at least once, been prescribed six blue relievers, had lots of symptoms despite high-dose treatment, or been on maintenance steroid tablets.

It adds that there can be many reasons for difficult asthma, including allergies, stress and other medical conditions, and that most people with uncontrolled or difficult asthma do not have severe asthma. Not taking an inhaler regularly and incorrect technique are common causes of uncontrolled Type 2 inflammation. That is why a specialist team will check inhaler technique, adherence, triggers such as smoking or vaping, and other conditions like obstructive sleep apnoea, obesity, acid reflux, allergies and anxiety or depression before confirming severe asthma.

Eosinophilic asthma does not have a unique symptom set. The NHS describes the main symptoms of asthma as wheezing, coughing, shortness of breath and a tight chest, which usually come and go and may be worse at night and early in the morning. Asthma + Lung UK says that in severe asthma you may have lots of symptoms most days, frequent asthma attacks that can happen without warning, and difficulty controlling symptoms even with higher doses of medicine.

Having these symptoms does not on its own mean you have severe asthma, and a very bad attack does not either. Tests, not symptoms, identify the eosinophilic pattern.

In general asthma care, NICE guideline NG245 (jointly developed with the British Thoracic Society and the Scottish Intercollegiate Guidelines Network) recommends measuring blood eosinophil count or FeNO in adults with a history suggestive of asthma. For people aged 12 and over whose asthma is not controlled on moderate-dose maintenance and reliever therapy despite good adherence, it advises checking FeNO and blood eosinophil count, and referring to a specialist in asthma care if either is raised. The guideline notes that it does not cover managing severe asthma itself.

Asthma + Lung UK lists the tests a specialist team may use for severe asthma. A blood test checks for inflammation or allergies and can measure eosinophils; a sputum test examines phlegm for eosinophils; FeNO shows how inflamed the airways are, and if levels fall with regular, correct use of a steroid preventer you do not have severe asthma; allergy blood tests measure IgE; spirometry with a bronchodilator reversibility test shows lung function; and a CT scan may be used to look for causes of difficult symptoms. Results can vary with the time of day, oral steroids or smoking, so some tests may be repeated.

The foundation is the same as for other asthma: inhalers used correctly, an asthma action plan and trigger management. The NHS says that if inhalers are not enough your team may recommend a stronger inhaler or tablets, and for severe asthma not controlled by these, biological therapies given by injection.

Asthma + Lung UK explains that biologics are based on antibodies made in a laboratory (monoclonal antibodies) which block the changes that cause inflammation in the lungs. They are a specialist, targeted treatment with strict criteria and only work for some people. To decide, specialists may do a blood eosinophil test and a FeNO test, check other conditions, and make sure existing medicines and inhaler technique are right. Six biologic medicines are listed for severe asthma: mepolizumab, reslizumab, benralizumab, omalizumab, dupilumab and tezepelumab. The NICE appraisals read for this page cover mepolizumab, benralizumab and reslizumab for severe eosinophilic asthma.

NICE's criteria are specific. For mepolizumab, recommended as an add-on for severe refractory eosinophilic asthma, an adult must have followed the optimised standard treatment plan and have either a blood eosinophil count of 300 cells per microlitre or more with at least 4 exacerbations needing systemic corticosteroids in the previous 12 months (or continuous oral corticosteroids of at least prednisolone 5mg a day over 6 months), or 400 cells per microlitre or more with at least 3 such exacerbations. Treatment is stopped at 12 months if there is not an adequate response, and response is assessed each year thereafter. Benralizumab has similar criteria. These are criteria for NHS access, not a statement about who could benefit.

Asthma + Lung UK says biologics can make severe asthma symptoms easier to manage, lower the amount of steroid tablets you need, which reduces steroid side effects, and reduce asthma attacks. It describes review schedules: every four to eight weeks for people who have been using long-term steroid tablets, and every three to six months otherwise, with a six-month review of how well the biologic is working. Some people can inject at home once trained.

Like all medicines biologics have risks. Asthma + Lung UK notes that severe allergic reactions (anaphylaxis) can happen in some people, usually within two hours of a dose and uncommon with most biologics, and that if lips, mouth, throat or tongue swell suddenly, or you struggle to breathe, you should call 999. Biologics may be continued in pregnancy, but that should always be discussed with the specialist.