In about 80% of cases, asthma can be treated appropriately, with an appropriate level of therapy and with good patient compliance. However, there is a substantial percentage of asthmatics (about 10-20%) in which the control of asthma is achievable only with high doses of drug therapy or, more frequently, is not reachable for the severity of the disease or for the presence of comorbidity that make it difficult to treat asthma.

The definition of severe asthma according to the ERS / ATS 2014 document is as follows: "asthmatic patients who require high doses of inhaled corticosteroids combined with control drugs such as LABA, anti-leukotrienes or theophylline or those that require oral corticosteroids for at least 6 months in the previous year. Such patients may remain uncontrolled despite the high level of the therapeutic regime (Step 5 GINA) or may achieve an adequate control of symptoms which, unfortunately, it gets lost with the reduction of corticosteroid doses ".

Before defining a patient suffering from severe asthma, it is necessary a more deepened evaluation for other types of asthma in order to estimate: aggravating factors, comorbidities, adherence to the therapy, disease control over time and alternative diagnosis to asthma.

In recent years much attention has been paid to the concept of heterogeneity of asthma based on several clinical functional and biological factors. These different forms of asthma are commonly defined phenotypes and for phenotype we mean the observable characteristics of an organism due to interaction between genetic heritage and environmental factors which are relatively stable over time. The phenotypes can be defined according to clinical factors such as lifestyle (e.g. smoking asthmatics) or comorbidities (obesity, rhino sinusitis, gastro esophageal reflux disease), functional assessment (such as asthma with airway obstruction no longer reversible) and the type of measurable inflammation with non-invasive/invasive methods.

In recent years several studies using statistical methods of factor analysis ("Cluster analysis") were performed to identify groups of patients on the basis of categorical and parametric variables to clinical, functional and biological indicators.

In particular, two major studies carried out one in the UK ( "Leicester study") and one in the USA ( "SARP study") have identified specific asthma phenotypes with different severity: 1) allergic asthma mild to moderate grade in atopic subjects; 2) severe allergic asthma in early-onset atopic, severe decline of FEV1; 3) severe asthma in advanced age obese, late-onset, less atopic with non-eosinophilic inflammation; 4) severe asthma in adults with intolerance to aspirin / NSAIDs, delayed onset, even with rhinitis/sinusitis polypoid and highly eosinophilic inflammation; 5) severe asthma with fixed obstruction in advanced age, reduced FEV1 with poor reversibility, high doses of inhaled corticosteroids or oral use of CS and inflammation mixed eosinophilic/neutrophilic.

Such patients should have regular and frequent clinical and functional checks in specialist centres (at least every 4-6 months) in order to adjust the treatment plan.