Introduction
Objective confirmation of asthma usually requires evidence of variable expiratory airflow limitation. Spirometry is central to assessment, but results may be normal between symptomatic episodes and may be relatively insensitive to distal airway abnormalities [1]. Oscillometry measures respiratory impedance during quiet breathing and can provide complementary information about frequency‐dependent resistance and reactance [2].
We report a patient with CT‐confirmed small‐airway involvement and asthma whose detailed spirometry and mannitol challenge were negative by the conventional FEV1 criterion, but whose peak‐challenge oscillometry showed marked bronchodilator‐responsive abnormalities. We use “small airway‐predominant asthma” as a descriptive asthma phenotype rather than a separate diagnostic category.
Case Report
A 58‐year‐old woman was referred with episodic exertional dyspnoea, chest tightness, increased breathing effort and difficulty completing sentences during episodes. Examination was normal. No blood eosinophil count, total or specific immunoglobulin E, fractional exhaled nitric oxide or allergy testing was available from the initial assessment.
Ibuprofen‐associated chest tightness and dyspnoea were documented; no other allergic comorbidity was recorded.
On 26 February 2026, spirometry was normal (Table 1). FEV1 was 2.12 L (89% predicted), forced vital capacity (FVC) 2.60 L (87%), FEV1/FVC 81.5% and peak expiratory flow (PEF) 5.60 L/s (97%). Forced expiratory flow at 25%–75% of FVC (FEF25–75) was 2.42 L/s.
Baseline oscillometry showed normal resistance at 5 Hz (R5; 0.430 kPa s L−1; Z + 0.88), a low R5 minus resistance at 20 Hz (R5–R20; 0.030 kPa s L−1) and reactance area (AXE) within its reference range (1.030 kPa/L; Z + 1.44), with mildly abnormal reactance at 5 Hz (X5; −0.230 kPa s L−1; Z − 1.80).
Functional CT on 17 February showed widespread multifocal lobular air trapping and large‐ and small‐scale ventilation heterogeneity, with minimal scattered centrilobular emphysema (< 5%) and no bronchiectasis, interstitial lung disease, endobronchial lesion or tracheobronchomalacia. The widespread air trapping and ventilation heterogeneity provided objective confirmation of small‐airway involvement.
On 3 March 2026, pre‐challenge spirometry again was normal: FEV1 2.18 L (92%), FVC 2.68 L (90%), FEV1/FVC 81.3%, FEF25–75 2.44 L/s (108%) and PEF 6.68 L/s (115%) (Table 1). Across 635 mg mannitol, FEV1 was 2.04–2.23 L and FEF50 was 3.29–4.41 L/s; the challenge was negative by the conventional FEV1 criterion (Table 2) and the overlaid flow–volume curves showed no consistent dose‐related obstructive change (Figure 1A). FEF25/MEF25 was not reported.
Symptoms during provocation and bronchodilation were not prospectively recorded.
At peak exposure, oscillometry was abnormal: R5 0.958 kPa s L−1 (Z + 3.93), R5–R20 0.252 kPa s L−1 (Z + 7.08), X5–0.368 kPa s L−1 (Z − 4.18) and AXE 7.433 kPa/L (Z + 4.29). After bronchodilator, R5 fell to 0.666 kPa s L−1 (30%), R5–R20 to 0.110 kPa s L−1 (56%) and AXE to 2.012 kPa/L (73%); X5 improved to −0.244 kPa s L−1 (Figure 1B). Values remained outside their reference ranges.
Peak within‐session coefficients of variation were 14% for R5 and 32% for AXE, so the precise magnitudes require caution.
Treatment with beclometasone/formoterol/glycopyrronium 100/6/10 μg, two inhalations twice daily through a spacer, was commenced. By 9 April, R5, R5–R20 and AXE were within reference ranges. By 2 July, she reported no chest tightness or exercise limitation.
Spirometry remained normal (FEV1 2.28 L, 96% predicted; FVC 2.80 L, 94%; FEV1/FVC 81.4%) without a significant bronchodilator response; oscillometry was normal. The concordant symptoms, CT, challenge oscillometry, bronchodilator response and longitudinal treatment response established the diagnosis of small airway‐predominant asthma.
Discussion
Detailed spirometry did not disclose airflow obstruction or an abnormal mid‐expiratory flow in this patient. Although FEF25–75, also termed mean mid‐expiratory flow, is sometimes used as a small‐airway surrogate, it is highly variable, depends on FVC and is not sufficiently specific to diagnose small airway disease in an individual [3]. Its normal value here therefore neither excluded distal dysfunction nor provided an alternative positive spirometric signal.
Mannitol is an established indirect bronchial challenge, but conventional interpretation is based on the fall in FEV1 [4]. The discordance in this case does not establish general superiority of oscillometry over spirometry because the methods assess different mechanical properties. Small airway dysfunction in asthma is multidimensional and discordance among physiologic measures is recognised [5].
In this patient, however, oscillometry disclosed marked, bronchodilator‐responsive impedance abnormalities at peak challenge despite normal FEV1, FEV1/FVC, FEF25–75, PEF and flow‐volume curves. CT independently confirmed widespread small‐airway involvement and symptom resolution with normalisation of oscillometry during controller treatment provided longitudinal corroboration.
Important limitations are the single‐patient design; lack of symptom recording during challenge; absent Type 2 biomarker and formal allergy data; variability of the peak oscillometric measurements; and no untreated control or repeat provocation. Response to triple therapy cannot identify which component was effective. Consequently, this observation does not define diagnostic thresholds for oscillometry during mannitol challenge.
Standardised prospective studies are needed. These limitations constrain generalisability, but do not diminish the concordance of the clinical, CT, physiological and longitudinal findings in this patient. Oscillometry was a clinically decisive adjunct to CT in identifying small airway‐predominant asthma despite normal detailed spirometry and a negative conventional mannitol response.
Author Contributions
Cheng Long Lu: conceptualization, writing – review and editing, resources, supervision. Mustafa Dhahir: writing – original draft, writing – review and editing, methodology. Anthony Muller: data curation.
Funding
The authors have nothing to report.
Ethics Statement
Ethics approval was not required for this single‐patient case report according to local institutional requirements.
Consent
Written informed consent was obtained from the patient for publication of this case report and accompanying images using the journal consent form.
Conflicts of Interest
The authors declare no conflicts of interest.