2.1 Study subjects
The study prospectively investigated patients with cILD and a control group of healthy controls (HCs). Study participants were consecutively recruited among patients with cILD followed at the Ruhrlandklinik between April 2017 and November 2019.
Inclusion criteria were an interstitial lung disease diagnosed according to American Thoracic Society (ATS)/ European Respiratory Society (ERS) criteria 2013 . Patients with unclassifiable ILD, incomplete data set or acute thoracic pain caused by any recent events (embolism, pneumothorax) or surgical interventions (including open lung biopsy) during the past six month, were excluded.
As a control group, we investigated HCs with age > 18 years, no pre-existing lung diseases, no chronical pain syndromes, no pre-existing analgesic medication or neurological conditions such as polyneuropathy. The study was approved by the local ethics committee of the Medical Faculty of the University Duisburg-Essen (16-7028-BO), and registered in the German register of clinical studies (DRKS00022978). Written informed consent was obtained from all participants.
All questionnaires were collected prior to performing QST. The Short Form 12 (SF-12) questionnaire was employed for assessing health-related QoL regarding physical and mental well-being . The painDETECT questionnaire was used to evaluate potential neuropathic pain . The painDETECT total score ranges between 0-38 and denotes the possibility of a neuropathic pain component being present (<13 very unlikely, 13-18 likely, >19 certainly). The number of pain areas was recorded by the body scheme of the painDETECT questionnaire.
2.3 Thoracic pain definition and quantitative sensory testing
TP was defined as persistent or intermittent pain ≥1 in the numeric rating scale (NRS), the most frequently used pain assessment scale .
QST is an established psychophysical test protocol for the quantitative evaluation of somatosensory function . The test is based on standardized somatosensory stimuli for which participant responses are recorded. 13 parameters can be obtained from seven separate test procedures involving nociceptive and non-nociceptive sensations . The same calibrated thermal and mechanical stimuli are always set in the same test sequence. For the present study, the following 10 QST-parameters were obtained in the given order: cold detection threshold (CDT), warm detection threshold (WDT), cold pain threshold (CPT), heat pain threshold (HPT), mechanical detection threshold (MDT), mechanical pain threshold (MPT), mechanical pain sensitivity (MPS), dynamic mechanical allodynia (DMA), wind-up ratio (WUR), and pressure pain threshold (PPT) (Extensive description in Supporting information). The correlation of the scores with impairment of sensitivity and pain perception can vary, for some scores being positive (WDT, HPT, MDT, MPS, DMA, WUR) and for others negative (CDT, CPT, MPT, PPT).
QST was performed by two trained examiners. Participants were trained in QST procedures by performing all tests on one hand, once. Subsequently, QST measurements were performed on both sides of the thorax. Patients were instructed not to look at the test area.
2.3.1 Thermal measurement
Thermal measurement was performed with a Medoc TSA 2001-II device (Medoc, Israel) [17, 18]. The contact area of the thermode was 30x30mm. Baseline temperature was 32°C for skin adaption. The subject was requested to stop the stimulus with linearly increasing intensity (1°C/s) immediately when perceiving the onset of cooling (CDT), warming (WDT), or the additional sensation of burning, stinging, drilling or pulling (CPT, HPT). The measurement was terminated by the patient through pressing a button, or when reaching the cut-off temperatures of 0°C and 50°C. Each temperature threshold was obtained three times per target area. Thresholds were calculated as the arithmetic mean temperature of the three consecutive measurements .
2.3.2 Mechanical detection threshold (MDT)
MDT was measured with modified von Frey filaments made of optic glass fibres (OptiHair2-Set, Marstock Nervtest, Germany) that exert forces between 0.25 and 512mN increasing by a factor of two from filament to filament [19, 20]. The contact area of the filaments was a small epoxy beat with a diameter of 0.30-0.45mm. Participants were asked to close their eyes, so that they could not observe the application of filaments, and were instructed to immediately report any perceived touch sensation within the target area. The force of the filaments was incrementally increased until the participant reported the first touch sensation. The force of the last filament used was noted as the first suprathreshold value. After this, filaments were applied in incrementally decreasing force until the patient did not report a touch perception. The force of the last filament was noted as the first infrathreshold value . This procedure was repeated five times per target area. The threshold was determined as the geometric mean of five supra- and infrathreshold values .
2.3.3 Mechanical pain threshold (MPT)
MPT was measured with a PinPrick-Set (MRC Systems GmbH, Germany) that exert forces between 8 and 512mN increasing by a factor of two from pinprick to pinprick . The contact area of the pinpricks is 0.2mm diameter. The sensation of pinpricks is produced by the weight of the needle resting on the skin of patient. Patients were instructed to report whether the touch of a pinprick evoked the sensation sharpness, or not. In increasing forces, the force of the first pinprick described as sharp had to be noted as the first suprathreshold value, followed by descending stimuli until the first pinprick is only a touch, noted as infrathreshold value. As for the MDT, five ascending and descending series of stimuli were performed per target area. The MPT was calculated as the geometric mean of five infra- and suprathreshold values .
2.3.4 Mechanical pain sensitivity (MPS)/ Dynamic mechanical allodynia (DMA)
Using needle stimulators of different intensities, a stimulus-response curve of MPS was generated . Seven different stimulus intensities were applied in a randomized sequence including each stimulus intensity five times per area. The patient evaluated the individual pain intensities directly after each individual stimulus according to a numeric rating scale between 0 and 100. DMA was examined according to the same test scheme as described for MPS. A moving touch stimulus (cotton swab, Q-tip, brush), which normally does not lead to painful perception, was applied between the needle stimuli. Each of these three non-noxic stimuli was applied five times per area. A total of 50 stimuli (touch and needle stimulus) were applied on both sides of the thorax and the painfulness was recorded numerically. As a measure for the sensitivity to pain, the geometric mean value of pain ratings for needle (MPS) and touch (DMA) stimuli was calculated .
2.3.5 Wind-up ratio (WUR)
WUR was determined with a pinprick of 128mN. A single stimulus alternated with a train of ten pinprick stimuli (1/s) within an area of 1cm2. The single stimulus and the stimulus train were rated by the patient on a numeric rating scale between 0 and 100, separately. The procedure was repeated five times. The wind-up ratio was calculated as the arithmetic mean pain rating of the five trains divided by the arithmetic mean pain rating of the five single stimuli .
2.3.6 Pressure pain threshold (PPT)
PPT was measured using a pressure gauge device (FDN 200, Wagner Instruments, USA) with a contact area of 1cm2 and pressure limit of 20kg/cm2, equivalent to 2000kPa. The algometer was applied to the thenar of the respective test side, as testing on the chest is not possible due to the insufficiently large contact area to the muscles in the intercostal space. The application was made manually, with an increasing force of 50kPa/s, corresponding to 0.5kg/cm2/s. Participants were asked to indicate the onset of a burning, stinging, drilling or pulling sensation. Application of pressure was stopped on feedback and the force reached was recorded as the threshold [23, 24]. The procedure was repeated three times per target area. Pressure pain threshold was calculated as the arithmetic mean of these three measurements .
2.4 Pulmonary function tests and blood gas analysis
Measurements including FVC, forced expiratory volume in 1s (FEV1), TLC, DLCO, arterial oxygen tension, arterial carbon dioxide tension, arterial oxygen saturation, and alveolar-arterial oxygen tension difference were performed at the time of QST.
Variables distribution was calculated by using Kolmogorov-Smirnov test. Descriptive statistics (frequency, mean ± standard deviation) were performed.
Sample size was calculated based on a number of 360 patients with a new diagnosis of ILD per year at our institution and the fact that up to 10% of them are expected to have TP not dependent on surgical procedures (estimated population size 36). The minimum sample size of ILD patients with TP is 33 with a confidence level of 95% (95%CI) and a margin of error of 5%.
Comparison between cILD patients and HCs were tested using the Mann-Whitney U-test or Student’s t-test for continuous variables and chi-square test for categorical variables. Correlations between continuous variables were calculated by using Pearson or Spearman correlation tests. We considered p < 0.05 to be statistically significant. Statistical analysis was performed with SPSS 27.0 (SPSS Inc., Chicago, USA).