Asthma

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Chronic obstructive airways disease} (COAD) is a term used to describe conditions in which there is chronic limitation to air flow in the lungs.
Flow is reduced for one of two main reasons : either because airways resistance is increased (normally by narrowing of airways) or because the outflow pressure is reduced (elastic recoil of lungs is lost).

The main diseases in this category are:

chronic bronchitis

asthma (narrowing of airways) and

emphysema (loss of elastic recoil)

Heavy smokers who have persistent cough with sputum, breathlessness on exertion, and airways obstruction

The main risk factors for this type of disease are lifetime smoking exposure, and asthma in childhood.
Acute episodes of infection superimposed on COPD cause acute decline in lung function and may precipitate acute deterioration of chronic cor pulmonale.
For this reason, prophylaxis with pneumococcal and influenza vaccines is advisable in this group of patients.
Although often suggested, there is no strong epidemiological evidence that such repeated infection is related to the long-term progression of airways obstruction seen in these cases.

Asthma is characterized by reversible airways obstruction in small airways

Asthma is the most common cause of recurrent breathlessness, cough and wheeze.
It is characterized by obstruction of small airways by a combination of bronchospasm and mucus plugging, which fluctuates with time and is frequently partially reversible with bronchodilator drugs.
It is a common disorder, affecting around 10% of children and 5% of adults.

Asthma is due to a complex chronic inflammatory response in bronchial mucosa

The pattern of responses is seen in long-standing asthma, whatever the cause, and it has been difficult to separate distinct mechanisms in asthma triggered by different causes. The prevailing view is that asthma is a form of low-grade chronic inflammation of the airways, with a variety of triggers causing acute exacerbations.

In severe disease (chronic asthma), airway obstruction is persistent despite drug therapy; chronic alveolar hypoventilation may cause pulmonary vasoconstriction and pulmonary hypertension.
Status asthmaticus refers to severe, acute disease that does not respond to drug therapy and may cause death from acute respiratory insufficiency.

There have been recent suggestions that the incidence of asthma is rising, with speculation that this increase might relate to environmental atmospheric pollution.

Most people with asthma have mild disease with acute episodes of bronchospasm that are triggered by well-recognized causes.

Allergy, e.g. to house dust mites.

Infection. Viral infection triggers bronchoconstriction particularly in children.

Occupational exposure. Some agents act as allergens, others by direct irritation of airway.

Drug-induced, e.g. -antagonists and aspirin.

Irritant gases, e.g. sulphur dioxide, nitric oxide, ozone in smog.

Psychological stress. Exertion. Cold air.

The concept that there are two types of asthma, extrinsic (due to allergy) and intrinsic (due to constitutional factors), has not been borne out by research; there is much overlap between asthma with different triggers.

Disease can be controlled by drug therapy with 2-adrenoceptor agonists and corticosteroids.

The main events that take place in the airways in asthma are:

Bronchoconstriction due to increased responsiveness of bronchial smooth muscle.

Hypersecretion of mucus leading to plugging of airways.

Mucosal oedema leading to narrowing of airways.

Extravasation of plasma in sub-mucosal tissues due to leakage from vessels.

Infiltration of bronchial mucosa by eosinophils, mast cells, lymphoid cells and macrophages.

Focal necrosis of airway epithelium.

Deposition of collagen beneath bronchial epithelium in long-standing cases.

Sputum contains Charcot-Leyden crystals (derived from eosinophil granules) and Curschmann spirals (composed of mucus plugs from small airways).

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