Diagnosis
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The following steps should be followed in investigating suspected occupational asthma.
Step 1. | Obtain history of asthma following exposure to a Scheduled substance at work, or history highly suggestive of specific work-related asthma irrespective of Schedule. |
* | Inquire about the person’s job. Tables 1 and 2 can be used as guides.
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* | Make contact with the employer, supplier, company doctor or a union safety representative to obtain or confirm this information.
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* | Ascertain the relationship between work and symptoms through a detailed history. Occupational asthma, depending on the substance, typically has its onset after a sensitising interval. The patient may give a clear history of symptoms precipitated by working with a particular substance, and improving on weekends and when on leave.
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* | Certain issues need to be remembered in differentiating between general and occupational asthma. First, childhood history of asthma or a history of atopy does not by itself rule out occupational asthma. Second, symptoms may come on after a delay of several hours after leaving work, producing nocturnal asthma. Further, once the asthma has set in, symptoms may persist through weekends, show early morning “dipping”, or be aggravated in a range of situations, and thus resemble general asthma.
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* | Some of the risk factors that have been identified in occupational asthma include: older age; male gender; atopy; smoking; and exposure to high levels of sensitising agents.
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* | At the stage of suspecting occupational asthma, it is advisable to contact a centre with facilities for diagnosing occupational asthma. These are listed in Appendix 1. Where the patient does not have access to these services, the doctor should take the investigation as far as possible and manage the case on the basis of reasonable suspicion.
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Step 2. | Confirm diagnosis of asthma using spirometry or peak expiratory flow rate (PEFR). Histamine or methacholine challenge test if necessary. |
* | Reversibility should be sought with spirometry. As a rule of thumb, an improvement in FEV1 of greater than 12 to 15% following administration of a bronchodilator suggests asthma. If a peak flow meter is used, an improvement of 20% or more in PEFR is required.
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* | Since airways obstruction is variable, it may not be present at the first examination. Measurement of non-specific bronchial responsiveness in a specialised centre, using histamine or methacholine as a provocative agent, may assist in making a diagnosis of asthma in such cases.
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* | Should fixed obstruction be present, a trial of corticosteroids over a few weeks may be required to show reversibility.
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Step 3. | Establish work-relatedness: serial PEFR testing, augmented by serial FEV1 or non-specific bronchial responsiveness testing |
* | Serial PEFR testing: This is currently the most practicable test of work-relatedness. Its main potential drawbacks are its requirement for daily recording of PEFR and dependence on the individual’s honesty. Although tailoring to the individual case is required, a suggested approach is as follows:
a) Schedule: 2 weeks at work, 2 weeks off work, 2 weeks back at work, doing usual work. b) Instrument: Portable peak flow meter. c) Recording: 4 times daily, the first on waking and at same time each day, plus at time of acute chest tightness. Always done before medication. 3 blows each time, with the best 2 readings within 20 ml of each other. The highest of the 2 readings written as a number (not plotted). Specially designed chart is most appropriate. Record daily responses as (yes/no): exposure, tight chest, respiratory infection; also, use of inhaled bronchodilator (number of times). d) Medication: Stabilise medication and determine best PEFR (from flow-volume loop) in advance. Avoid varying regimen during trial. e) Quality control: Check blowing technique, understanding of procedure, completion of chart. See after one week, then every 2 weeks – retain completed chart at each visit. Factitious records suggested by lack of variability between blows and times, or lack of increased variability when peak flow deteriorates.
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* | Cross-shift FEV1 measurement: This test may be useful if positive, but is insensitive (e.g. to the late asthmatic response) and is confounded by normal diurnal improvement in FEV1.
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* | Change in non-specific bronchial responsiveness: This test could be done during the period of work exposure and again at the end of the off-work period, as an adjunct to serial PEFR testing. A decrease in responsiveness in the off period suggests a work-related effect.
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Step 4. | Identify aetiologic agent if possible |
* | Identify the specific aetiologic agent. This step is based on the list in Table 1, which has the status of a presumptive list. In other words, the occurrence of asthma in someone exposed to a substance on the list creates a high index of suspicion that the asthma is caused by the substance.
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* | If the suspected substance is not on the list (or not identifiable in a multi-exposure setting), the burden of proof required under the COIDA is in theory stronger, and more stringent objective tests, such as a bronchial challenge test or a specific immunological test, to establish the cause may be needed.
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Step 5. | Establish specific hypersensitivity: skin prick tests; RAST (if available); specific bronchial challenge (if exposure unscheduled, or association doubtful). |
* | Immunological tests such as skin prick tests and the RAST are useful in identifying specific agents by confirming both exposure and immune reactivity. The list of RAST tests available at the Allergology Unit, UCT Medical School, is included in Appendix 2.
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* | Specific bronchial challenge testing, i.e. with the agent in question in a specialist hospital lung function laboratory, offers the firmest objective evidence of occupational asthma. However, this procedure has a number of disadvantages including some risk to the patient; the difficulties in many cases of reproducing an appropriate antigen challenge outside the workplace; and the inaccessibility of specialist centres outside the main cities.
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* | These tests may also useful in making a retrospective diagnosis i.e. when the patient presents to the physician after having left the job. In such a case, no opportunities exist for conducting spirometry or serial PEFR while the person is still exposed to the putative agent. This may make it very difficult to reach a conclusion. However, if the history is compatible with occupational asthma associated with a known exposure to a known agent, a presumptive diagnosis could be made.
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Step 6. | Estimate the degree of functional impairment and work disability |
* | The degree of impairment of the patient should be assessed using guidelines generally agreed upon. The American Thoracic Society (ATS) has published useful guidelines in this regard. The ATS recommends that impairment /disability should take place on two occasions:
i. Temporary disability: once the diagnosis of occupational asthma is made, the person should be considered 100% impaired for the job that caused the illness and should be removed from the job. ii. Permanent disability: assessment should be carried out two years after removal from exposure when impairment has been shown to plateau.
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* | The Compensation Commissioner specifically requests the following details to make an assessment of disability: the actual lung function tests (spirometry) while the worker was exposed and after prolonged removal from exposure to the agent (worst and best lung function); usual asthma medication; and details of treatment for acute attacks over the preceding year. The FEV1 should be done at least 12 hours after the use of a bronchodilator, and three weeks after removal from the causative agent. A score is assigned to both functional loss and the necessity for medication as illustrated in Table 3. The percentage permanent disablement is then assessed according to the total score as it appears in Table 4.
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Table 3. Pulmonary disability criteria and scores
FEV1 |
Score |
Medication |
Score |
>80% of predicted |
0 |
Nil |
0 |
71 to 80% of predicted |
1 |
One or more drugs |
1 |
56 to 70% of predicted |
2 |
(including inhaled steroids) |
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40 to 55% of predicted |
3 |
Systemic steroids |
4 |
<40% of predicted |
4 |
* Source: Rautenbach B. Permanent disability assessment in occupational diseases. Office of the Compensation Commissioner, Pretoria, in Occupational Health Southern Africa, 1996; 2:30-31
Table 4. Percentage permanent disability
Total score |
Permanent disability % |
6 to 8 |
100 |
5 |
80 |
4 |
60 |
3 |
45 |
2 |
35 |
1 |
25 |
0 |
15 |
* Source: Rautenbach B. Permanent disability assessment in occupational diseases. Office of the Compensation Commissioner, Pretoria, in Occupational Health Southern Africa, 1996; 2:30-31
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