Pulmonary function in patients with Huntington’s Disease

To determine the number of participants required for this study, we performed a pilot study in which spirometry, maximum expiratory (MEP) and inspiratory pressures (MIP) were compared between manifest HD patients and age-matched healthy volunteers. Given that in HD, the main cause of death is an aspiration pneumonia event that is associated with an inability to clear airway secretions in which expiratory muscles have a major role, [11] the sample size calculation was based on MEP. It was estimated that there would be a difference of at least 30 cmH₂O (SD = 30) in MEP between HD patients and healthy volunteers. With alpha 0.05 and 80% power, an adequate sample size was calculated to be at least 13 participants per group.

Eighteen patients with manifest HD (11 men, 7 women) were recruited using the Huntington’s Enrichment Research Optimisation scheme database [12], with assistance from the Huntington’s Western Australia Association. Inclusion criteria were a positive genetic test, clinically verified disease expression (Unified Huntington’s Disease Rating Scale–Total Motor Score [UHDRS-TMS] ≥5), and the ability to understand and respond to instructions. Exclusion criteria were other confounding neurological disorders, current smokers, occupational or ambient exposure to pollutants that could have affected their pulmonary function, patients with a history of cardiovascular pathology, lung disease, chronic obstructive pulmonary disease (COPD) or the presence of respiratory symptoms such as cough, phlegm, wheezing or dyspnoea at the time of assessment. Patients with HD at later stages of the disease were not considered for this study due to their incapacity to perform a satisfactory pulmonary function tests. The control group (10 men, 8 women) consisted of mainly partners of the recruited HD patients, and some people with similar life-style characteristics to the recruited HD patients. Healthy volunteers were age and gender-matched with the recruited HD patients. The same exclusion criteria as those for the HD patients were applied to the control group. Nine patients with HD and 6 healthy volunteers had a previous history of smoking. Informed written consent was obtained from participants and the study was approved by the Human Research Ethics Committee of Edith Cowan University. Characteristics of the participants are presented in Table 1.

For MIP and MEP measurements, each participant was asked to sit upright with a nose clip in place to prevent nasal air leakage. A flanged rubber mouthpiece was connected to a pressure manometer (Micro RPM, Micro Medical-Care Fusion, Kent, United Kingdom) and placed in the mouth. Participants were asked to hold the pressure manometer with both hands and to create a tight lip seal around the flanged mouthpiece. A flanged mouthpiece was used as recommended by the American Thoracic Society/European Respiratory Society (ATS/ERS), because it ensures the least air leakage at the mouthpiece [13]. This method was used, since the portability of the manometer is advantageous for clinical use and its reliability has been assessed [14].

For MEP assessment, participants were asked to breathe in to total lung capacity and then to blow hard into the mouthpiece. For MIP assessment, participants were instructed to breath out to residual volume and then to breath in with maximum effort through the mouthpiece. Inspiratory and expiratory efforts were required to be maintained for more than one second. The order of the procedures were first the inspiratory followed by the expiratory effort [13]. Both manoeuvres were repeated a minimum of 5 times with a 30-s rest between measures to minimize the effects of fatigue until three trials showed values within 5% variation of each other. The best result from the three respiratory manoeuvres was used for further analysis as described by Black and Hyatt [15] and the ATS/ERS guidelines [16]. Results of MIP and MEP are expressed as absolute values, because no suitable predictive equations are available.

Spirometric assessment was performed using a spirometer (Medgraphics, model CPFS/D, St. Paul MN, USA) connected to a laptop computer (Dell, Latitude E6510, USA). The spirometer met all the quality control requirements of the ATS and was calibrated before each testing session with a Hans Rudolph 3.0 syringe, based on the manufacturer’s recommendations. In accordance with the ATS/ERS guidelines [17], each participant was asked to sit upright with a nose clip in place to prevent nasal air leakage. Participants were instructed to perform a slow vital capacity (SVC), a forced vital capacity (FVC) and a maximal voluntary ventilation (MVV) manoeuvre. From these measurements, forced expiratory volume in one second (FEV₁), peak expiratory flow (PEF) and the ratio of FEV₁ to FVC (FEV₁/FVC) from the largest FEV1 and FVC were calculated. Predicted values were calculated for all participants using Stanojevic 2009 [18] prediction equations, which include age, height, weight and gender. The equations include reference values for FVC and FEV₁, and have been validated for Caucasian Australasian population [19]. Peak expiratory flow predicted values were calculated using Nunn and Gregg [20] regression equations. Given that none of the participants in the study had a confirmed diagnosis of COPD or presented with any known symptoms of COPD, such as cough, sputum production or dyspnea at the time of the assessment, a reversibility test with bronchodilator was not performed.

All the participants of the study were interviewed and completed a general health questionnaire, in which they reported their cardio-vascular and smoking history as well as their respiratory condition. The body mass and height were recorded for each participant using an accurate and calibrated scale (HW200, A&D Mercury Pty, Ltd, Thebarton, SA) and a wall-mounted stadiometer (Model 220, SECA, Hamburg, Germany), followed by MIP and MEP measurements performed over 3 testing sessions with a week apart between sessions.

Thorough explanation, demonstration and practice were provided before attempts at MIP, MEP and spirometric measures. Each participant was instructed to perform spirometric maneuvers in the following order: SVC, FVC and MVV. Each manoeuvre was repeated at least 3 and maximum of 8 times, with 1–2 min rest between attempts. The best result from the three technically acceptable manoeuvres was used for further analysis. For those participants who had difficulties achieving acceptable and reproducible spirograms on the first testing day, the measurement was repeated on different days, to ensure an accurate spirometry. The weekly measurements were taken at the same place and time of day for each participant.

Descriptive data are given as mean and standard deviation. Normality assumption for all continuous variables was tested using the Shapiro-Wilk test. To assess the test-retest reliability of MIP and MEP measurements for each group (HD, control), the interclass correlation coefficient (ICC) and the coefficient of variation (CV) were calculated. To compare spirometric variables, MIP and MEP between HD patients and healthy volunteers, an independent t-test was performed. Relationships between each pulmonary function measurement and UHDRS-TMS were analyzed by a Pearson correlation coefficient. Statistical significance was set at p ≤ 0.05. All statistical analyses were performed using STATA version 9.1.