Discussion

The aim of this study was to investigate the usefulness of measuring rhinomanometry in a supine body position in the assessment of patients with chronic nasal obstruction. The results from this study indicate that an increase in the total nasal airway resistance of more than 0.3 Pa/cm³/second in a sitting position, without decongestion, predicts significant nasal breathing problems when supine. The change in intranasal geometry assessed with acoustic rhinometry between sitting and supine was small, which confirms that a narrow nasal passage with high intranasal airway resistance in a sitting position is able to promote airway collapse beyond the nasal cavity itself when lying down. Patients with a total nasal airway resistance of more than 0.3 Pa/cm³/second when sitting are highly likely to have nasal breathing problems when lying down and should be evaluated for nocturnal nasal obstruction.

Rhinomanometry in a supine body position is useful when assessing patients with chronic nasal obstruction. The small change in intranasal geometry between sitting and supine, as measured with acoustic rhinometry in this study, is unable to explain the inability of the patients in the increased nasal airway resistance group to generate a nasal airflow to undergo rhinomanometry when supine and before decongestion. The results indicate that the airway collapse occurs beyond the nasal cavity. The dislocation of the tonsils, the tongue and soft palate, along with the lower jaw, could contribute. Changes in breathing pattern when lying down may also be important when a subject already has a critically high total nasal airway resistance when sitting up. This is further supported by the finding that 58 per cent of the patients in the increased nasal airway resistance group were able to perform rhinomanometry when supine after decongestion when the mean total nasal airway resistance when sitting before decongestion was reduced by half (from 0.89 to 0.4 Pa/cm³/second).

Nasal obstruction has been rated as the most troublesome symptom of allergic rhinitis when waking up in the morning.^{Reference Shedden1} Nasal obstruction is also associated with micro-arousals, upper airway resistance syndrome and, as a result, with daytime sleepiness and impaired health-related quality of life.^{Reference Young, Finn and Kim2} A recent study from Finland also showed that the impact of nasal obstruction on health-related quality of life was similar in patients with chronic rhinosinusitis and a nasal septal deviation.^{Reference Virkkula, Maasilta, Hytonen, Salmi and Malmberg3} This indicates that, regardless of whether the nasal obstruction is inflammatory or structural, the effect is the same.

In this study, we included patients referred to a tertiary otolaryngology clinic due to a longstanding, nasal steroid-refractory nasal obstruction, which was confirmed by more than 70 per cent reporting the use of nasal steroids, with remaining symptoms. All the patients were examined with both rhinomanometry and acoustic rhinometry by two trained examiners who were instructed to sample the data but had no prior knowledge of the analyses of the data. The cut-off level for total nasal airway resistance (more than 0.3 Pa/cm³/s) to define increased nasal airway resistance, according to the guidelines of the American Thoracic Society, was applied after the data were collected in the analyses. The results are therefore unbiased by the examiners and thus reliable. Since the study aimed to compare rhinomanometry data when sitting and supine, the total inability to perform rhinomanometry when supine in the increased nasal airway resistance group was unexpected. In two previous studies of rhinitis patients who were measured while sitting and supine, there were no reports of patients who were unable to undergo rhinomanometry when supine before decongestion due to the absence of nasal airflow.^{Reference Altissimi, Gallucci, Rossetti and Simoncelli14,Reference Stroud, Wright and Calhoun19} However, in these studies, the mean total nasal airway resistance was well below 0.3 Pa/cm³/second. An alternative cut-off of 0.25 Pa/cm³/second as the upper limit of normal total nasal airway resistance has been suggested.^{Reference Merkle, Kohlhas, Zadoyan, Mosges and Hellmich21} Adapting that cut-off level to this study would only have enabled 2 of 17 patients to undergo rhinomanometry when supine before decongestion and, as a result, 88 per cent would still not have had a measurable airflow when supine.

It is well recognised that one of the weaknesses of rhinomanometry, performed when sitting, is that patients with severe nasal obstruction are unable to complete the procedure due to non-measurable nasal airflow. Active anterior rhinomanometry requires one nostril to be blocked with a pressure-sensitive probe while the nasal airflow is assessed in the other nostril. This is a non-physiological setting, forcing the subject to increase inspiratory effort in order to inhale through the only nostril available. This increase in effort may in turn promote the collapse of the upper airway, resulting in the total loss of nasal airflow. In this study, patients exhibiting a severe nasal septal deviation or nasal polyps were excluded for that reason. The fact that all 41 patients included were able to perform rhinomanometry in the sitting position before decongestion according to standard procedure shows that, even though these patients had symptoms of nasal obstruction and many were unable to perform rhinomanometry when supine, the most severe cases of nasal obstruction were not present in the study.

Although rhinomanometry in the supine position is not a procedure recommended by the manufacturer or by the standardisation committee, the equipment can be used in this way. Different companies provide different devices for rhinomanometry that could be more or less feasible in this regard. We used the Rhinocomp® device because it is also easy to handle in the supine position. A pressure-sensitive probe is fitted to one nostril using adhesive tape, and a transparent face mask is then held over the nose and mouth. When the patient breathes through the unplugged nostril into the face mask, the airflow is assessed. Apart from poor fitting of the adhesive tape or of the face mask, there are a few other factors that need to be controlled for. The examiner is able to see if the patient is breathing through the mouth, which the patient must not do, thanks to the transparent face mask.

Similar conditions apply to acoustic rhinometry. We used the acoustic rhinometer from Rhinometrics® that samples a constant flow of sound impulses, thus enabling the examiner to correct for reading errors in real time due to leakage at the nostril. These conditions are the same when both sitting and supine.

Of 41 consecutive patients referred to the otolaryngology department for chronic nasal obstruction, only 12 (29 per cent) had an increased total nasal airway resistance before decongestion, which is surprising. There are no widely accepted normal values for total nasal airway resistance and several cut-off levels have been suggested, including the American Thoracic Society guidelines cut-off level of less than 0.3 Pa/cm³/second used in this study.^{Reference Chaaban and Corey16} Using this cut-off, 29 (71 per cent) of the patients suffering from chronic nasal obstruction had a total nasal airway resistance in the normal range. This effectively illustrates the complicated relationship between subjective symptoms of nasal obstruction and objective measurements of nasal patency. Several studies have shown that this correlation is inconclusive.^{Reference Andre, Vuyk, Ahmed, Graamans and Nolst Trenite22}

The patients in this study had twice as much asthma and 2.5 times more hay fever than the general Swedish population.^{Reference Bjerg, Ekerljung, Middelveld, Dahlen, Forsberg and Franklin23} Chronic rhinosinusitis was present in 51 per cent compared with 11 per cent in the population, and 95 per cent reported nasal obstruction for more than 12 weeks. Women were over-represented (61 per cent) and present smoking was more prevalent compared with Swedish population data (17 vs 7–11 per cent). Self-reported symptoms of sleep disturbance were common and 17 per cent were diagnosed with mild sleep apnoea syndrome. There were, however, no significant differences between the increased nasal airway resistance group and the normal nasal airway resistance group with regard to these factors. There was a tendency towards more women in the increased nasal airway resistance group (75 per cent vs 55 per cent). Women generally have smaller noses than men and a smaller intranasal cross-sectional area is associated with higher total nasal airway resistance.^{Reference Samolinski, Grzanka and Gotlib24} Smaller noses have less leverage if nasal inflammation or structural changes appear, leading to nasal obstruction at an earlier point than when the nose is larger.

In the increased nasal airway resistance group, there was also a tendency towards more asthma (41.7 per cent vs 10.3 per cent) and hay fever (92 per cent vs 66 per cent). Asthma and hay fever are closely associated with inflammation of the nasal mucosa, which can reduce the minimal cross-sectional area and increase total nasal airway resistance.^{Reference Hellgren, Toren, Balder, Palmqvist, Lowhagen and Karlsson25,Reference Gaga, Lambrou, Papageorgiou, Koulouris, Kosmas and Fragakis26} Patients with mild asthma and rhinitis have also exhibited impaired regulation of the nasal mucosal swelling when going from sitting to supine.^{Reference Rimmer, Greenwood, Bartlett and Hellgren27} In a Danish study of 230 individuals from a random adult population, 14 per cent reported nasal obstruction. The authors found that a nose with small intranasal dimensions, symptoms of allergy and a large degree of swelling of the mucosa were some of the important factors when it came to predicting nasal obstruction in that group and this is in accordance with the findings from our study.^{Reference Grymer, Hilberg and Pedersen28}

Snoring was reported by 56 per cent of all the patients (mean age: 33 years). In the increased nasal airway resistance group, it was higher at 75 per cent, but the difference compared with the normal nasal airway resistance group was not statistically significant. According to the Wisconsin Sleep Cohort, nasal obstruction when supine is considered to be a risk factor for snoring.^{Reference Young, Finn and Kim2}

In this study, there was no difference in apnoea-hypopnea index or oxygen desaturation index between the two groups. More than 80 per cent of the patients in the increased nasal airway resistance group had an apnoea-hypopnea index of less than 5, which is in line with current research and indicates that there is no linear correlation between nasal obstruction and the severity of sleep-disordered breathing.

This study has several strengths and weaknesses that have to be considered. The cut-off level for total nasal airway resistance of more than 0.3 Pa/cm³/second used in this study may not fully reflect the difference between an obstructed and a patent nose. The patients in the normal nasal airway resistance group were all suffering from nasal obstruction, despite having a total nasal airway resistance of less than or equal to 0.3 Pa/cm³/second. Sensory nasal obstruction without any increase in the total nasal airway resistance has been described and may be present in some of our study subjects.^{Reference Eccles and Jones29} Inflammatory nasal obstruction typically varies over time and some of the measurements may therefore have been made during a period of improvement. Rhinomanometry depends on patient effort, and breathing through only one nostril at a time can increase the tendency towards airway collapse compared with normal tidal breathing. These and other methodological factors, such as leakage at the pressure catheter or facial mask, may also produce false readings.

The NOX T3 sleep monitor is easy to use in the home environment. However, the T3 device was not equipped with electroencephalogram that measures whether the patient is asleep or awake. Polysomnography would have been preferred, but it was not available to us.