Little or no effect of steroids

Steroids inhibit inflammation

COPD, chronic obstructive pulmonary disease; FEVj, forced expiratory volume in 1 s; FVC, forced vital capacity.

COPD, chronic obstructive pulmonary disease; FEVj, forced expiratory volume in 1 s; FVC, forced vital capacity.

ditions is considerable. Both are common — asthma affects up to 5% of the adult population across all age groups [15], and symptomatic COPD affects 5% of the population aged over 65 [16].

In younger patients, the classical asthma history of acute breathlessness and wheezing interspersed with periods of complete wellness, especially if coupled with a history of waking coughing and wheezing in the night, makes the diagnosis relatively straightforward. In asthma patients over the age of 40, there is often chronicity secondary to airway remodelling that has led to a nonreversible element, and the symptom pattern is less clear cut. Even the classical nocturnal worsening of symptoms is a poor discriminator. Breathlessness on exertion, wheezing, cough productive of sputum (especially in current smokers), are common in both COPD and asthma. Perhaps because of this lack of specificity, the descriptive definitions used in management guidelines make little reference to symptoms.

Table 3.1 (adapted from the GOLD document [17]) shows that while there are symptom and history features that may point to either asthma or to COPD, there is no cardinal feature that differentiates between the two. Because atopy and asthma are common, inevitably a significant number of COPD patients will also have a similar history. Similarly, a significant number (25% or so) of asthma patients smoke.

COPD was defined by the British Thoracic Society Guidelines [18] in terms of an abnormal airflow obstruction — a reduced FEV1 and FEV1/forced vital capacity (FVC) ratio—that remains largely unchanged over time. The more recent GOLD guideline from the WHO [17] includes additional reference to the inflammation present in COPD. While there is no dispute that the inflammations in asthma and COPD are entirely different, the distinction in clinical practice is academic, as it is not practical to collect and examine tissue routinely. The procedures are too invasive to be justifiable other than for research studies, since the relevance to better clinical care has yet to be demonstrated.

Because it is unusual to have a pathological sample, clinical medicine has to rely on physiology. But here too, there is no absolute definition of reversibility. If a patient's obstructed lung function is shown to return to the normal range simply by administering a bronchodilator, then the diagnosis of asthma is almost certain. COPD is effectively excluded by normal values. Unfortunately, in older patients, the reversibility of the airway obstruction is partial even in asthma, and the unanswered question is what the smallest level of reversibility that diagnoses asthma to be present.

The differentiation is further complicated as COPD varies so much in its clinical manifestations over the 40-50-year time course. Most patients with mild disease (an FEV1 of above 60% of that predicted) are likely to report few or no symptoms, because they have simply lost their respiratory reserve of function. They rarely present to a doctor. In contrast, a fall of FEV1 from 100% to 70% in an asthmatic over the course of a few hours will almost certainly be reported as tightness and wheezing. The difference probably reflects the accommodation to the chronic situation in COPD. Thus, if the FEV1 in a symptomatic patient is near normal, the diagnosis is more likely to be asthma than COPD. If a reversibility test returns the lung function to the normal range, then asthma is confirmed—although an element of coincident COPD cannot be excluded.

As COPD progresses to moderate impairment, with an FEV1 of around 50% of predicted, chronic symptoms of exertional dyspnoea are likely, although this will be modified by the demands placed on the person concerned — e.g. by their occupation — and the opportunity to use mechanical aids or avoid the activity involved. Over half of patients with moderate COPD will exhibit a significant response of 200 mL to bronchodilators. In a few, FEV1 values will not return to normal, but changes of 300-400 mL are strongly suggestive of an asthmatic component. Up to 20% of patients will also show a 200-mL or more response to an oral steroid trial (30 mg/day for 2 weeks). Does this make them asthmatic — or is the term 'COPD with an asthma element' more appropriate? Chronic asthma patients can show an identical picture, and it is extremely difficult to separate patients on clinical or physiological grounds.

At the severe end of the COPD spectrum, with an FEV1 in the range of 35% of predicted, chronic symptoms are always present. A patient with acute asthma and an FEV1 that has fallen to this level over a few hours is likely to be in ex tremis and is easily differentiated. A few patients with chronic asthma have values in this range, but they are relatively few in number, and the probability is that a patient with an FEVj of 35% predicted is likely to have COPD rather than asthma. In severe disease, the level of response to bronchodilators is often small and the potential for confusion with the 'asthmatic element' is less.

Thus, in patients with chronic symptoms, the lower the FEVj, the less the response to bronchodilators, the older the patient, and the heavier the smoking history, the more likely is the diagnosis to be COPD. But this is an inexact science, and there are as yet no figures to help make this an objective exercise. But until a simple pathology test is found, it is the best that can be done in the practical clinical situation.

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