The recent statement on interpretive strategies for lung testing uses the acronym PRISm for preserved ratio impaired spirometry. PRISm identifies patients with a normal forced expiratory volume in 1 second/forced vital capacity ratio but abnormal FEV1 and/or FVC (usually both). Most medical students are taught to call this a “restrictive pattern,” and every first-year pulmonary fellow orders full lung volumes when they see it. If total lung capacity (TLC) is normal, PRISm becomes the nonspecific pattern. If TLC is low, then the patient has “true” restriction, and if it’s elevated, then hyperinflation may be present.
The traditional classification scheme for basic spirometry interpretation (normal, restricted, obstructed, or mixed) is simple and conceptually clear. It turns out that many with this pattern won’t have an abnormal TLC, so the name is, in some ways, a misnomer and can be misleading. Enter PRISm, a more descriptive and inclusive term. The phrase also lends itself to a phonetic acronym that is fun to say, easy to remember, and likely to catch on with learners.
Information on occurrence and clinical behavior comes from large cohorts with basic spirometry, but without full lung volumes because PRISm no longer applies once TLC is determined. As may be expected, prevalence varies by the population studied. Estimates for general populations have been in the 7%-12% range; however, one study examining a database of patients with clinical spirometry referrals found a prevalence of 22.3%. Rates may be far higher in low- and middle-income countries. Identified risk factors include sex, tobacco use, and body mass index; the presence of PRISm is associated with respiratory symptoms and mortality. Thus, PRISm is common and it matters.
Along with PRISm, the nonspecific pattern is a new addition, if not a new concept, to the 2022 interpretative strategies statement. As with PRISm, the title is necessarily broad, though far less imaginative. Defined by reductions in FEV1 and FVC and a normal TLC, the nonspecific pattern has classically been considered a marker of early airway disease. The idea is that early, heterogeneous closure of distal segments of the bronchial tree can reduce total volume during a forced expiration before affecting the FEV1/FVC. The fact that the TLC is not a forced maneuver means there is proportionately less effect from more collapsible/susceptible smaller units. More recent data suggest that there are additional causes.
Because the nonspecific pattern requires full lung volumes, we have less population-level data than for PRISm. Estimated prevalence is approximately 9.5% in patients with complete test results. The two most common causes are obesity and airway obstruction, and the pattern is relatively stable over time. Notably, an increase in specific airway resistance or TLC minus alveolar volume difference predicts progression to frank obstruction on spirometry.
The physiologic changes that obesity inflicts on the lung have been well described. Patients with obesity breathe at lower lung volumes and are therefore susceptible to small airway closure at rest and during forced expiration. There is no doubt that the increased recognition of PRISm and the nonspecific pattern is in part related to the worldwide rise in obesity rates.
Key takeaways
In summary, PRISm and the nonspecific pattern are now part of the classification scheme we use for spirometry and full lung volumes, respectively. They should be included in interpretations given their diagnostic and predictive value. Airway disease and obesity are common causes and often coexist with either pattern. Many will not have a true, restrictive lung deficit, and a reductionist approach to interpretation is likely to lead to erroneous diagnoses. There were many important updates included in the 2022 iteration on lung testing interpretation that should not fly under the radar.
Dr. Holley is professor of medicine at Uniformed Services University in Bethesda, Md., and a pulmonary/sleep and critical care medicine physician at MedStar Washington Hospital Center in Washington. He disclosed ties with CHEST College, Metapharm, and WebMD.
A version of this article appeared on Medscape.com.