Thank you for your recent inquiry.
Actually, restrictive findings on pulmonary testing as manifested by a decline in forced vital capacity are not unusual. Because the decline in FEV1 usually exceeds the decline in FVC, the literature has focused on FEV1 and FEV1/VC ratios (1). However, accompanying the more exaggerated decline in FEV1, there is often an associated decline in FVC.
In addition, children with early onset asthma oftentimes do not reach full lung growth and therefore have diminished FVC measurements as well. So, to find a diminished vital capacity and forced vital capacity in asthma is not rare.
As long as you feel comfortable that other causes of restrictive lung disease such as those produced by external causes (e.g., obesity) or by interstitial lung disease (e.g., sarcoidosis), I would not usually be concerned over restrictive findings in an asthmatic.
Diagnosing external causes usually can be done on the basis of history and physical examination. Other lung diseases, especially interstitial diseases, can usually be adequately ruled out with performance of a DLCO or high resolution CT.
Such restrictive findings can even predominate in some asthmatics when you have controlled the obstructive disease with corticosteroids and long-acting beta-agonists. So restriction alone would not ordinarily in my mind prompt a visit to a pulmonologist. Restriction accompanied by an abnormal DLCO, characteristic findings on CT scan or MRI of significant interstitial disease, or clinical findings which indicate the presence of external restriction or neurological disorders would prompt a visit.
The restriction itself in asthma is probably due to the deposition of collagen and ground substance within the lung parenchyma, and results not only in a restrictive defect but also a decrease in compliance. It has been hypothesized that this phenomenon may be protective against severe bronchoconstriction since it is an imposing force to such constriction (2, 3, 4).
Thank you again for your inquiry and we hope this response is helpful to you.
Ulrik CS. Outcome of asthma: longitudinal changes in lung function. European Respiratory Journal 1999; 13(4):904-918.
Sheharyar R, et al. What effect does asthma treatment have on airway remodeling? Current Perspectives. J Allergy Clin Immuno, July 13, 2011; available online.
Al-Muhsen S, et al. Remodeling in asthma. J Allergy Clin Immunol, June 2011; available online.
Mauad T, et al. Asthma therapy and airway remodeling. J Allergy Clin Immunol 2007 November); 120(5):907-1009.
Phil Lieberman, M.D.