Overview
A patient with difficult asthma presents with symptoms and exacerbations despite being prescribed high dose therapies and is potentially multifactorial.  Brittle asthma is quite rare and presents as two types.  Type 1 brittle asthmatics have wide diurnal variations in peak expiratory flow in the long term despite taking high dose therapies.  Type 2 brittle asthmatics appear to be well-controlled on asthma medication but have sudden, severe attacks.

Difficult asthma
It is important to understand that difficult asthma is defined quite broadly as asthma in which symptoms and exacerbations that occur despite the prescription of high dose therapy.  The patient has been diagnosed with asthma, and the appropriate therapeutics have been prescribed but the patient’s symptoms are not well-controlled.  This is not a pathophysiologically-distinct variant of asthma, but a presentation.  The BTS defines difficult asthma as persistent symptoms and/or frequent exacerbations despite being treated at step 4 or 5 of their treatment guidelines:

Possible factors contributing to difficult asthma
Poor adherence to medication
    Corticosteroid medication seems to be the main problem
Psychosocial factors
    Psychosocial dysfunction is a common feature in fatal and near fatal cases
    Difficult asthma is associated with coexistent psychological morbidity
Dysfunctional breathing
Allergy
    Sensitisation to moulds in particular seems to be associated with difficult asthma

Brittle asthma
Brittle asthma occurs in an estimated 0.05% of asthmatics.  Over the years two distinct patterns of brittle asthma have become clear, both of which occur in patients who show good adherence to their medications.

BTS definitions of brittle asthma types:
Type 1: Wide PEF variability (>40% diurnal variation for > 50% of the time over a period of >150 days) despite intensive therapy.
Type 2: Sudden severe attacks on a background of apparently well-controlled asthma.

Type 1 brittle asthmatics tend to require intensive hospital care for several days during an exacerbation, whereas Type 2 might only require care for several hours, since their exacerbations frequently remit as rapidly as they occur.  Type 1 brittle asthmatics tend to be female (3:1 female:male ratio), whereas Type 2 show no sex bias.

 

Further reading:
British guideline on the management of asthma: a national clinical guideline (132 pages!)
    Available at: http://www.brit-thoracic.org.uk/Portals/0/Guidelines/AsthmaGuidelines/sign101%20revised%20June%2009.pdf
Ayres, JG, Jyothish, D and T Ninan (2004) Brittle asthma.  Paediatric Respiratory Reviews, 5: 40-44

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