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Kwong Wah Hospital, Hong Kong, People’s Republic of China
Correspondence to: Daniel K. Ng, Consultant Pediatrician, Department of Pediatrics, Kwong Wah Hospital, Waterloo Rd, Hong Kong, SAR, People’s Republic of China; e-mail: dkkng{at}ha.org.hk
To the Editor:
We read with interest the article in CHEST "Is a 2-Night Polysomnographic Study Necessary in Childhood Sleep-Related Disordered Breathing?" by Li et al (November 2004)1 and the accompanying editorial by Guilleminault and Lee.2 As illustrated by Flemons et al,3 access to diagnosis for patients with suspected obstructive sleep apnea syndrome (OSAS) is a global problem. The access to pediatric sleep studies was probably even more limited.4 It is fortunate that Li et al1 showed that 2-night polysomnographic studies were not necessary for the diagnosis of childhood sleep-disordered breathing (SDB). However, several points in this study warrant further discussion. The studied population was biased, as > 50% of subjects were obese. The incidence of obesity was much higher than that in the previous 2-night polysomnography study (7 of 30 patients, 23.3%)5 and in the population of obese children in Hong Kong (12 to 14%).6 Hence, application of the findings by Li et al1 to the general population might be limited as obese subjects are more prone to developing OSAS.7 This application was not helped by the lack of subset analysis with regard to obesity in the study by Li et al.1
Apnea-hypopnea index (AHI) was a better diagnostic parameter for the diagnosis of OSAS in children than was obstructive apnea index as complete airway obstruction was uncommon in children8 and the normal value of AHI was recently published9 using the old data. We were surprised that AHI was not used as the diagnostic criteria in the study by Li et al1 despite the fact that it was reported by them. Using an AHI of > 1.5 as the diagnostic criteria for OSAS, we suspect that a significant proportion of the so-called normal group would be classified as having OSAS on the second night as the mean hypopnea index rose from 1.2 to 1.59 on the second night (p < 0.005). If one uses an AHI of > 1.5 as the criterion for OSAS in the study by Li et al,1 the exact opposite conclusion could be drawn (ie, that the first-night effect was significant even in children). To further complicate the picture, this trend for the second night was reversed in the SDB group, which actually saw a significant decrease in hypopnea index on the second night (ie, from 9.71 to 6.68). This difference in the direction of trends of the healthy and SDB groups could not be explained by a difference in sleep efficiency or by the duration of rapid eye movement sleep as they were similar. We suspect this inconsistency to be due to the following factors: (1) the inaccuracy of the detection of flow, as the sensitivity of the thermistor for detecting hypopnea in children was low compared to the nasal cannula10; (2) intrascorer or interscorer variabilities; and (3) the presence of a confounding factor such as upper respiratory tract infection that was associated with a deterioration in the conditions of patients with OSAS or allergic rhinitis, which might improve in the allergen-free hospital environment.
Guilleminault and Lee2 commented in their editorial that "we have never seen a child with only primary snoring." We agree partially with this statement as we demonstrated that primary "habitual" snorers had higher daytime systemic BP and reduced arterial distensibility compared to healthy subjects.11 However, our unpublished data showed that only a small proportion of nonhabitual primary snorers (defined as an AHI 
1.5) who were of normal weight (3 of 49 patients) were hypertensive. Hence, it is possible that "benign" primary snorer exists if they fulfill all of the following criteria: (1) no paradoxical breathing while asleep in children > 3 years of age; (2) normal results of 24-h ambulatory BP monitoring; (3) no daytime symptoms of SDB (eg, attention deficit, excessive daytime sleepiness and the like); (4) snoring does not occur more than every other day; (5) AHI < 1.5; and (6) end-tidal CO2 level of > 45 mm Hg for < 60% of sleep time.
Guilleminault and Lee2 have suggested that polysomnography performed at home could be used to diagnose OSAS. We urge caution in applying this to patients who have been referred to a sleep center for suspected OSAS. Our experience in pediatric sleep studies suggests the importance of performing an attended polysomnography. Nearly all children who have undergone overnight polysomnography in our sleep laboratory required a few adjustments of the end-tidal CO2 sensors because of blockage or displacement. Hence, an unattended home study with a positive result would be diagnostic of OSAS, but a negative finding should be followed by an attended study with end-tidal CO2 level measurement to diagnose obstructive hypoventilation.
References
Hong Kong
Correspondence to: Yun Kwok Wing, MRCPsych (UK), Seventh Floor, Sleep Assessment Unit, The Chinese University of Hong Kong, Hong Kong; e-mail: ykwing{at}cuhk.edu.hk
To the Editor:
We thank Dr. Ng and his colleagues for their valuable comments on our article.1 Our study aimed to answer two questions. First, does a first-night effect exist in childhood sleep; and second, is single-night polysomnography adequate in assessing children with sleep-related disordered breathing? We agree that our sample population was biased toward obese children, and we fully acknowledged that limitation in our discussion. This limitation may have some bearing to the second question our study tried to answer, but we do not think the presence of the first-night effect would have been affected by the body mass index of the subjects.
The diagnostic cutoff for childhood obstructive sleep apnoea (OSA) is still very much a controversial issue. Dr. Ng and colleagues argued that the apnea-hypopnea index (AHI) would have been a better diagnostic parameter for diagnosing OSA. They suggested that we use a cutoff of AHI > 1.5, and they based their suggestion on a letter to the editor by Witmen et al,2 who reviewed the original overnight polysomnographic data of 41 children in the study by Marcus and colleagues3 for obstructive hypopneas. Witmen et al2 concluded that obstructive hypopneas are uncommon, but their data are limited in that they are based on older technology. Some authorities including reviewers of our article would recommend the use of AHI cutoffs of 2, 3, and 5. Until we have evidence-based science to support that an AHI > 1.5 corresponds to significant long-term morbidity or even mortality, using a more conventional diagnostic cutoff (obstructive apnea index > 1) is still acceptable. We agree with Dr. Ng and colleagues that if resources allow, attended polysomnography remains the "gold standard."
References
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