These are disorders caused by allergic substances or particles which stimulate hypersensitivity of the lungs. They are of great medical significance to man because these disorders affects a good percentage of the world population today.
Extrinsic allergic alveolitis
These are a group of disorders characterized by the development of hypersensitivity pneumonitis. Many of them are caused by organic dusts. Fungal spores contaminating by penetrating the epithelium of the alveoli can produce a type III hypersensitivity reaction, leading to bronchioloalveolitis. Several distinct entities are included in this group.
Some common types of bronchioalveolitis
Farmer's lung (mouldy hay), Bagassosis (Sugarcane dust), Mushroom worker's lung (Mushroom compost), Malt-worker's lung ( mouldy barley, malt dust), Maple-bark stripper's lung (mouldy bark), Bird fancier's lung (bird's droppings).
Clinically all these disorders cause dyspnea and systemic disturbances on exposure to the antigen. Physical examination may reveal wheeze and crepitations. Skiagram of the chest may reveal small nodular shadows of diffuse honeycombing. Avoidance of the allergen causes relief of symptoms and recovery in the early stages, but in advanced cases the symptoms continue. Diffuse pulmonary interstitial fibrosis develops in those cases which continue a chronic pulmonary hypertension and cor pulmonale. Treatment consists of avoidance of the allergen and bronchodilators. In some cases, corticosteroids are beneficial.
Bronchial Asthma
This is perhaps the most common chronic respiratory ailment seen all over the world. Around 0.5-2% of the population suffer from asthma. Bronchial asthma is clinically characterized by widespread reversible functional narrowing of the airways that varies in severity. The tracheobronchial tree reveals increased responsiveness to several factors, both immunological and non-immunological. Immunological mechanisms are precipitated by several organic and other dusts, fumes, chemicals and so on. Non-immunological stimuli include thermal, chemical or psychological factors.
Bronchial asthma is broadly classified into the extrinsic and intrinsic (cryptogenic) types. In the former an external precipitating factor is identifiable, whereas in the latter it is not. The antigens include ingested, inhaled or parenterally administered substances. The serum of such individuals may show elevated levels of specific antibodies belonging to the IgE and sometimes IgG classes. Persons developing extrinsic asthma have other atopic manifestations like eczema. The dermatological and respiratory manifestation show a see-saw relationship. In many cases family history of bronchial asthma may be present. Extrinsic asthma generally sets in by the age of 10-15 years. This type has a better prognosis from the point of response to therapy and mortality. The age of onset for instrinsic asthma is after 30 years. Precipitating causes or raised antibody levels are not evident but these patients show a higher frequency of eosinophilia, aspirin sensitivity, and nasal polyposis.
Pathogenesis
Common stimuli which precipitate extrinsic asthma are inhaled allergens-like house dust, pollens, fungi, animal hairs, insect scales and industrial fumes, and foods and drugs which are consumed in day-to-day life. Once sensitization occurs, these antigens release chemical mediators from the mast cells by interacting with the IgE molecules on their surface. Type I hypersensitivity reaction ensues. Asthma can also be caused by type III (delayed) hypersensitivity mechanism mediated by IgG. In some individuals both type I and type III reactions occur, the former leasing to an immediate asthmatic paroxysm and the latter leading to a delayed episode.
Exercise-induced asthma is a condition in which bronchospams is provoked by various forms of exercise such as running or climbing stairs, but others such as swimming may not do so. Provocation of bronchospasm by cold inspired air is a possibility in such cases. The mechanism is a type I hypersensitivity reaction. Some subjects with intrinsic asthma develop the symptoms on taking aspirin. Respiratory infection and psychological stress play important roles in precipitating asthmatic paroxysms in both types. Both viral and bacterial infections may trigger off a paroxysm and the episodes tend to recur as long as the infections persist. In children asthma may be the presenting symptom in primary tuberculosis, so also in adults asthma may be aggravated by coexisting pulmonary tuberculosis. Cigarette smoking and air pollution act as aggravating factors in many affected individuals. The role of psychological stress is more in perpetuating the asthma than initiating the condition.
Pathology
The main lesion is in the bronchi and other small air passages. The bronchi are obstructed by inspissated mucus. The bronchial mucosa is edematous. The submucosa also shows edema and infiltration by eosinophils and basophils. The smooth muscle layers show marked hypertrophy. The final picture of bronchial obstruction is brought about by a combination of bronchial muscle spasm, mucosal edema, and thick secretions. The lungs show hyperinflations which is reversible in the early stages.
Clinical features
The attacks start with dyspnea (often at rest), expiratory wheeze and cough. The onset is abrupt in most cases. These attacks may occur seasonally or during all times of the year (perennials asthma). The attacks may last for several hours if untreated. Severity of the paroxysm varies. In the moderately severe case the patient is orthopneic and cyanosed, and the accessory muscles of respiration are active. There may be ineffective cough with only very scanty and tenacious mucoid expectoration. The asthmatic paroxysm in many individuals is ushered in by coughing and sneezing on exposure to the allergen. The pulse is rapid. Blood pressure is normal or elevated. In severe cases pulsus paradoxus may occur. Expansion of the chest is considerably diminished, often to less than 2cm during the attack. The diagnostic feature of bronchial asthma is the presence of expiratory wheeze heard all over the chest.
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