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| Module 4: Occupational Asthma - Lecture (Continued) |
3. DEFINITION:
3.1 Occupational asthma:
- variable airflow limitation and/or airway hyperresponsiveness;
- due to causes and conditions attributable to a particular occupational environment;
- not to stimuli encountered outside the workplace;
- 2 types depends on latency period between exposure and symptom development.
3.1.1 Immunological asthma:
This is characterised by work-related asthma appearing after a latency period:
- asthma caused by most high- and certain low-molecular weight agents for which an IgE-mediated mechanism has been proven;
- occupational asthma induced by specific agents for which neither an IgE- nor a non-IgE-mediated mechanism has been identified.
The diagnosis of occupational asthma relies on:
- a reliable diagnosis of asthma;
- occupational exposure to a known cause of asthma;and,
- a chronological relationship between asthma and the working environment.
The criteria for a diagnosis of occupational asthma requires all 4 factors listed in the Table below (A - D):
Diagnostic criteria for Occupational asthma (used by the Compensation Commissioner for COIDA-related cases - Circular instruction 176)
| (A) |
A medical practitioner’s diagnosis of asthma and physiological evidence of reversible airways obstruction or airways hyper-responsiveness |
| (B) |
An occupational exposure preceding the onset of asthmatic symptoms. |
| (C) |
An association between symptoms of asthma and work exposure. |
| (D) |
An exposure and/or physiological evidence of the relationship between asthma and the workplace environment (Diagnosis of occupational asthma requires D1 plus one or more of D2 – D5): |
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(1) |
Workplace exposure to agent reported to give rise to OA. |
| (2) |
Work-related changes in FEV1 or PEFR. |
| (3) |
Work-related changes in serial testing for non-specific bronchial hyperresponsiveness (e.g. methacholine challenge test). |
| (4) |
Positive specific bronchial challenge test. |
| (5) |
Positive skin prick test or raised specific IgE antibody level to the suspected agent. |
3.1.2 Non-immunological asthma:
This type occurs without a latency period = irritant-induced asthma or reactive airways dysfunction syndrome (RADS).
- may occur after single or multiple exposures to known irritants at high concentrations;
- recovery from the acute illness may be followed by a persistence of non-specific bronchial hyperresponsiveness and symptoms of asthma.
Diagnostic criteria for Irritant induced asthma - RADS (According to Brookes Classification):
- Absence of preceding respiratory complaints is documented. 2. The onset of symptoms occur after a single specific exposure incident or accident.
- The exposure was a gas, smoke, fume, or vapour was present in very high concentrations and has irritant qualities.
- The onset of symptoms occur within 24 hours after the exposure and persisted for at least 3 months.
- Symptoms simulate asthma, with cough, wheezing and dyspnoea predominating.
- Methacholine challenge test is positive.
- Other types of pulmonary diseases are ruled out.
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3.2 Related conditions:
3.2.1 Asthma-like disorder:
- symptoms of chest tightness associated with a cross-shift change in FEV1 but without persistent bronchial hyperresponsiveness or eosinophilia;
- cross-shift decline in FEV1 may predict later development of chronic airflow limitation;
- examples: byssinosis, which is common among workers exposed to cotton, flax, hemp, jute, sisal, and obstructive airflow limitation associated with grain dust.
3.2.2 Work-aggravated asthma:
- pre-existent asthma worsened by nontoxic irritants or physical stimuli in the workplace e.g. strenuous effort, dust or vapours, tobacco smoke.
- Note: a past history of childhood asthma or pre-existent adult asthma does not preclude the possibility that occupational asthma may develop after appropriate exposure to workplace agents.
Postgraduate Diploma in Occupational Health (DOH) - Modules 3 – 5: Occupational Medicine & Toxicology by Prof Rodney Ehrlich & Prof Mohamed Jeebhay is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.
Permissions beyond the scope of this license may be available at http://www.healthedu.uct.ac.za/
