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We read with interest the article by Hirayama et al1 in which the authors, using an original imaging method, low field magnetic resonance fluoroscopic study, proposed that upper airway obstruction precedes laryngeal occlusion causing the stridor in patients with multiple system atrophy (MSA). This issue of nocturnal stridor in MSA is of great importance since it is a common cause of sudden death and a recognised prognostic factor in this disease.2 It affects about 19% of patients as shown in our series and by others.3 We feel that the relationship proposed between obstructive apnoeic respiratory events and stridor is not as simple as suggested by the authors and must be considered in light of classical standardised polysomnographic (PSG) data.
In our own series, 18 consecutive patients with MSA were assessed for night-time disturbances by all-night standard PSG with continuous synchronised audiovisual recording. Nocturnal stridor occurred in 10 patients and, except in one patient, was always accompanied by breathing disorders, mostly apnoeic, with or without significant oxygen desaturation. In four patients, obstructive sleep apnoeas (OSA) occurred without stridor, and one of these patients presented predominantly with central apnoea that also occurred while awake. Among the patients with stridor, four presented predominantly OSA and one mainly central apnoea. Mixed and prolonged apnoea, up to 53 s, was seen along with stridor in five patients and was isolated in two others. Episodes of mixed apnoea were typical in their occurrence as they were always preceded by heavy, prolonged inspiratory effort and stridor, indicating upper airway obstruction. Such episodes were not detected in OSA patients without stridor. Apnoeic events of any type were in most cases followed by the recurrence of snoring and not by an inspiratory stridor sound.
Thus, there seems to be a wide variety of combined sleep-related breathing disorders ranging from a majority of obstructive apnoeas to stereotyped mixed apnoeas of very long duration and sometimes preceded by stridor in MSA.
Nocturnal breathing disturbances in MSA are due to the complex involvement of multiple brainstem nuclei, leading to a defect in the respiratory control system independently of the occurrence of stridor.2 Among these breathing disorders, OSA are the most common and may occur in non-obese MSA patients even in the absence of stridor, thus indicating that the mechanism underlying the two events is different. The higher incidence of OSA observed in MSA patients may also be due to the severity of bradykinesia and the fact that patients with severe MSA lie predominantly, if not always, in the supine position while asleep. The reduction of nocturnal obstructive events during lateral position in patients with OSA has already been reported.4 In our patients, who were audio monitored, stridor was not followed by typical obstructive apnoea nor was the apnoea ended by a stridor. Thus, we believe that stridor and OSA in MSA are different and independent events. We also found that mixed apnoea occurred stereotypically and was very prolonged and often preceded by a harsh sound typical of stridor, as documented by audio monitoring.
Non-invasive continuous positive air pressure (CPAP) should be proposed for relief of sleep breathing disorders. It has been used successfully to treat stridor and OSA in MSA patients.5 In our series, nine patients accepted CPAP treatment (six with stridor and sleep apnoea and three with isolated OSA). One patient died before initiation of the treatment and two patients without sleep complaints dropped out after one week because of lack of tolerance despite having a severe apnoea/hypopnoea index. Since the onset of CPAP treatment, both patients and their spouses reported better sleep, improved daytime alertness and wellbeing. For some patients, getting used to CPAP took up to a month, after which it was generally well tolerated. After a mean follow up period of 10 months, the patients’ compliance with the continued use of CPAP was satisfactory and their relatives did not report any recurrence of stridor.
Thus we feel that the relationship between stridor and sleep apnoea is far from clear, especially considering the polysomnographic association of stridor and mixed apnoea that we found. Complex supranuclear neurological dysfunction may account for this association, but further studies are however needed to clarify this issue and better establish the indications for CPAP.
The study was supported by a PHRC 97 grant and a CHU de Bordeaux-CNRS fellowship.
We would like to thank Dr Ghorayeb et al for their interest in our paper and their comments. We agree with the view that the relation between stridor and apnoea in MSA is very important.1 Unfortunately, in our experimental procedure, we could not establish a correlation between the image of the vocal cords and upper airway and stridor symptoms because it is difficult to record airflow and vocal sound simultaneously in a magnetic resonance imaging (MRI) recording.2
In obstructive sleep apnoea syndrome (OSAS), even when the upper airway was obstructed completely, the vocal cords were not obstructed. Therefore, stridor does not develop although snoring may occur in OSAS. In contrast, MSA patients had an obstructed upper airway, which was frequently accompanied by stenosis of the vocal cords. However, we did not find stenosis of the vocal cords without stenosis of the upper airway. If the stridor is produced by stenosis of the vocal cords and snoring is produced by stenosis of the upper airway, snoring should be accompanied by stridor in all MSA patients. In fact, we observed that the initial narrowing of the larynx and pharynx produced snoring. Ghorayeb et al point out that OSA (SAS with upper airway obstruction) can commonly occur even in non-obese patients with MSA without the presence of stridor. We agree with this observation, but in this study, we did not find stenosis of the vocal cords without upper airway stenosis, so none of our patients developed stridor without snoring. We observed the patients in the MRI room to identify the sleep state and the presence of snoring and stridor, and we found that the highest pitch vocal sound appeared after heavy and prolonged inspiratory effort. This phenomenon is very similar to Ghorayeb et al’s observation of apnoea and stridor. We also suppose that the phenomenon of apnoea in MSA patients occurs with confinement of stenosis of the upper airway. Therefore, we think that some patients in MSA with SAS can be treated with CPAP similar to OSAS patients. However, the effect of CPAP could be diminished, since the respiratory centre is eventually involved with the progression of disease in MSA, and central apnoea and abnormal respiration may appear. Further study is required to clarify the indication of CPAP in patients with MSA.
The mechanism of SAS in MSA is unclear. Our study showed complete obstruction of the upper airway and vocal cords occurred in MSA even with the presence of tongue atrophy and without narrowing of the larynx. Thus, we suggest that there is another mechanism involved distinct from that of OSAS. Some reports have stated that a dystonia-like phenomenon was present in the vocal cords in the stridor through electromyographic study, suggesting a similar mechanism to be present in the progression of upper airway obstruction.3,4