Theme : Dreams

From Snoring to Sleep Apnea in a Singapore Population

Kathiravelu Puvanendran and Kiat Lian Goh
Department of Neurology, Sleep Disorder Unit, Singapore General Hospital, 169608, Singapore
Abstract
We have no information on snoring and obstructive sleep apnea (OSA) in our population, which is predominantly Chinese. Our perception is that sleep apnea syndrome is more common than the 2-4% prevalence (Young et al., 1993) often quoted, judging from the experience in our sleep disorder unit. We studied the snorers in an adult population in Singapore and then went on to see how many snorers suffer pathological apnea and sleep apnea syndrome. Room partners, 220 of them aged 30-60 years, were interviewed for their observation of snoring among each other. 106 consecutive habitual loud snorers of a similar age group in the same population were studied with polysomnography in our sleep laboratory. An apnea index greater than 5 was considered pathological. 24.09% were loud habitual snorers. 87.5% of loud habitual snorers had significant obstructive apneas on the polysomnogram and 72% of these apneics complained of excessive daytime sleepiness (EDS). Given the clinical observation that all apneics snored, by extrapolating these figures, we guess that sleep apnea syndrome affects about 15% of the population. Multiple Sleep Latency Tests validated EDS in our cases with clinical hypersomnia. Hypersomnolence was significantly related to the poor delta wave sleep. Contrary to what was believed, OSA occurred predominantly in stage 1 and 2 non-rapid eye movement (NREM) sleep rather than in REM sleep. The frequent arousals prevented sleep going beyond stage 1 and 2.

Current Claim: The higher-than-expected prevalence of sleep apnea syndrome in our population is probably because they suffer more hypersomnolence which is related to the suppression of delta wave sleep by apnea occurring predominantly in stage 1 and 2 NREM sleep.

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Snoring is a common finding in any population. It is possible that a person can progress from asymptomatic snoring to a full-blown obstructive sleep apnea syndrome (Lugaresi et al., 1983). Asymptomatic snoring itself should be considered as a potential medical problem because it is a risk factor for developing hypertension, ischemic heart disease and stroke (Koskenvuo et al., 1987).

The spectrum of sleep disordered breathing (SDB) ranges from partial airway collapse and increased airway resistance manifesting as snoring to episodes of hypopnea and apnea (OSA) from complete airway collapse (Guilleminault and Stoohs, 1991). Sleep apnea syndrome is characterized by frequent apnea hypopnea, and is associated with excessive daytime sleepiness (International Classification of Sleep Disorders, 1990). It can be life threatening and associated with cardiovascular morbidity and mortality (Koskenvuo et al., 1987). Sleep apnea is a very common disorder and is the most common reason for referral to our sleep disorder unit. It is also the most common diagnosis after an overnight polysomnogram.

Epidemiological studies of SDB are difficult because of the limited resources and facilities to perform polysomnography in a population-based sample (Lavie, 1983; Stradling and Crosby, 1991). Polysomnography is the current standard for evaluation of SDB. It provides data on respiratory effort, airflow, oxygenation, and sleep state among other things. Prevalence varies across populations because of differences in age, gender, varying definition, and whether studies were population-based or in sleep centres.

We initially studied a middle-aged Singapore population to see their snoring patterns. Consecutive loud habitual snorers of similar age and racial mix were studied in our sleep laboratory for apnea, sleep architecture, arousals, and for other possible disorders of sleep. They were also studied for EDS. We performed Multiple Sleep Latency Tests in order to validate the history of EDS. We studied the sleep architecture in OSA to find an explanation for their extreme daytime hypersomnolence.

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Sample

The population was sampled in two stages. Our observation and others' observation (Guilleminault, 1989) was that almost all sleep apnea patients snore. We initially sampled randomly a middle-aged (30-60 years old; average 52 years) otherwise healthy population to study the prevalence of snoring. This was a sample population seen in the premises of the Singapore General Hospital. The subjects came from all parts of the island and were representative of the racial mix in Singapore. Couples in this age group who appeared healthy and had come to visit relatives or friends or accompanied them to hospital were interviewed. All such couples identified were willing responders (response rate 100%). Two-hundred and twenty interviewed were mostly married couples who have been bedmates for many years and were able to describe their spouses' snoring accurately. They were classified as habitual snorers (almost every night) or loud snorers (if snoring disturbs sleep of the bed partner) or as occasional snorers. The patient or partner was asked whether gasping (apnea) was ever associated with snoring. In the second stage, we studied one-hundred and six consecutive cases of loud habitual snorers (age 30-60 years; average 48 years) using polysomnography. Both sample populations were matched ethnic groups. The multiracial population of Singapore is made up of Chinese 77%, Malays 14% and Indians 7% (Yearbook of Statistics, Singapore, 1997). The second pool of patients was referred to our sleep disorder clinic for loud habitual snoring by general practitioners, ear, nose and throat surgeons and respiratory physicians who were alert to the problem of snoring. Patients were routinely asked for a history of loud habitual snoring in the course of history-taking for whatever complaint they consulted. Referral bias is minimized considering that this sole referral centre had patients referred from all parts of this island republic. Both groups were assessed for daytime hypersomnolence by asking whether they often woke up in the morning unrefreshed or felt excessively sleepy in the day during inopportune and bizarre situations, (any one of these for more than 2 days in a week) that interfered with daily routines or exposed them to near accidents while driving or working. Polysomnographic studies were conducted in our sleep laboratory using the Oxford Medilog SAC 847 system, used with the Medilog SAC sleep respiratory interface and video interface processor. Patients were studied for only 1 night.

Polysomnographic monitoring for sleep stages was mainly by the electroencephalogram (Phillipson and Remmers, 1989). Electrooculogram and chin EMG monitored stage REM sleep. Sleep stages were scored into NREM and REM sleep. Chest and abdominal movements monitored respiratory effort. Airflow was measured by thermistors at the nostrils. Electrocardiogram, leg movement, body position, oximeter, and tracheal microphone inputs (snoring) were the other parameters recorded. Audiovisual monitoring using video was done to see abnormal behavior in sleep and this was time-locked to the various parameters recorded. We used a computer to assist in sleep scoring, recognize and align respiratory events, alpha arousals, muscle arousals, and leg movement, to sleep. Sleep record was scored in epochs of 30 seconds. Sleep was staged as 1 to 4 and rapid eye movement (REM) sleep according to Rechtschaffen and Kales (1968). Complete cessation of airflow lasting 10 seconds or more is defined as apnea. Decrease in airflow of less than 50% of normal level for at least 10 seconds is hypopnea. In obstructive apnea the patients' respiratory effort (chest and abdominal movement) continues while there is cessation of airflow. The number of apneas per hour is the apnea index (AI). We used the conventional cut-off value for AI as 5 (Guilleminault, 1989) to indicate the presence of SDB.

The validity of the symptom of daytime hypersomnolence in the cases of OSA was investigated by performing a Multiple Sleep Latency Test (MSLT) on 27 of the 67 cases of OSA who complained of EDS. This is a well validated measure of daytime sleepiness. This followed the overnight polysomnogram. The patient was given 4 twenty-minute nap opportunities at 2 hour intervals. Normal adult control score is 10-20 min. A mean sleep latency of less than 5 minutes indicates a pathological level of daytime sleepiness associated with impaired performance. A score of 5-10 minutes is a diagnostic grey area (Carskadon et al., 1986). We analyzed the sleep architecture and arousal of the cases diagnosed to have sleep apnea and related these to the symptoms of EDS. In a subset of 43 cases of sleep apnea syndrome we calculated the total OSA events and related them to sleep stages of REM and NREM.

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Figure 1

One-hundred and seventy of the sample population of 220 snored (Table 1). Male to female ratio was 1:23. Only 53 (24.09% of the population) gave a history of loud and habitual snoring. Male to female ratio is 1:65. Gasping or irregular breathing was noticed in 91 cases. Spouses were separated in 5 instances because of the unbearable noise. 63 snorers (37.05%) complained of EDS. Among the 53 loud habitual snorers, 29 (54.7%) complained of EDS.

Polysomnographic studies of the 106 loud habitual snorers were analyzed (Table 2). Male to female ratio was 9:1. They all snored during the recording. AI above 5 indicative of sleep disordered breathing (OSA) occurred in 93 patients (87.5%). The AI varied from 5.1 to 85 (Fig. 1). 67 of the 93 (72%) loud habitual snorers with OSA complained of excessive daytime sleepiness. 27 of these 67 patients who complained of EDS had MSLT studies done on them. All 27 patients had mean latency less than 10 mins. 18 of the 27 patients had mean latency less than 5 mins. In the 67 cases of OSA with EDS, delta wave sleep (Stage 3-4 NREM) was diminished below 5% in 58 cases (87%) and absent in 16 cases (Table 3). Normally delta sleep occupies about 20% of total sleep time (TST). In the 26 cases of OSA without EDS, delta wave sleep was diminished below 5% TST in only 7 cases (25%). This difference is significant (p<0.000001 by Chi-square test). EDS is probably due to diminished delta wave sleep. There were 6 OSA cases with severe hypersomnia who were all driving and had motor accident records (Table 4). Their AI was high and they had absent delta waves and many arousals (above 250 arousals in 3 cases).

We analyzed sleep architecture in a random sample of 43 cases of OSA (Table 5). In 95% of cases, OSA predominates in stage 1 and 2. In the 43 cases studied, there was a total of 7,712 OSA attacks (Table 6). OSA occurred in REM in only 452 attacks (5.86%) and in NREM sleep in 7,260 (94.14%). Contrary to what is believed (Sackner et al., 1975), OSA occurred predominantly in NREM sleep rather than in REM sleep.

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This study highlights how common snoring and sleep apnea syndrome are in our population in Singapore. Snoring is to be taken as a serious public health hazard. Its greater morbidity is through its association with OSA. 24.09% of our people suffer from loud habitual snoring affecting men and women. In a survey of married couples (Norton and Dunn, 1985) 86% of husbands and 57% of wives were habitual snorers. The most pragmatic way to obtain snoring history is to interview the subject's cohabiting relatives or friends rather than by questionnaire (Palomaki et al., 1992).

87.5% of loud habitual snorers in our population have an AI more than 5 and 72% of these cases complained of EDS, which is part of the OSA syndrome. As all apneics are known to snore, by extrapolating these results we suspect that 15% of our population (.2409 x .875 x .72 x 100 = 15.17%) could have sleep apnea syndrome, more males than females having snoring and OSA. Our population's male to female ratio of 8 to 9 is close to the widely cited clinic-based ratio (Guilleminault et al., 1988) and is probably due to self-selection because men are more likely to complain of symptoms. A high incidence of OSA in a Singapore population has been highlighted before (Tan and Koh, 1991). It was found that 75.6% of snorers had OSA and 85.7% of the OSA patients complained of EDS clinically. Population studies using the polysomnogram in the USA have reported a prevalence of SDB from 62% of an elderly population in San Diego (Ancoli-Israel et al., 1991) to 9% women and 24% men in a population in Wisconsin (Young et al., 1993). The latter defined SDB as an apnea-hypopnea score of 5 or higher. They estimated an incidence of sleep apnea syndrome (SDB and hypersomnolence) in 4% of men and 2% of women of middle-aged population. Their estimate of SDB among men and women (16.5%) is only slightly lower than our estimate (.2409 x .875 x 100=20.8%). However, their estimate of hypersomnia in 19% of their healthy and employed population with SDB is much lower than the 72% in our study, contributing to the great discrepancy in prevalence. Their insistence on all three stringent indications of hypersomnolence on the questionnaire compared to any one indicator in our study could have made the difference. Clinical evaluation usually underestimates sleepiness (Thorpy, 1992). Our cases of hypersomnolence on history are validated by MSLT, which is abnormal in all patients (below 10 min). The majority (66%) of them had latency less than 5 min, indicating a pathological level of daytime sleepiness which is associated with impaired performance. The normal range starts at 10 min and the interval that lies between 6-9 min is a diagnostic grey area (Carskadon et al., 1986) which is interpreted on a case-by-case basis according to the symptoms. In sleep apnea syndrome the MSLT typically shows the onset of sleep on an average latency of 10 min or less (Culebras, 1996). The EDS is related to the diminished delta wave sleep and this significant relationship is emphasized by the 6 patients who had motor car accidents while driving. They all had loss of delta wave sleep and many arousals. According to Weitzman (1981) the explanation for EDS lies in the loss of stage 3-4 NREM sleep and the frequent brief arousals. The apneas are so frequent and associated with so many arousals that sleep hardly went beyond stages 1 and 2. Thus, pathological apneas have hardly a chance to occur in REM, unlike the physiological apneas.

Although the AI is widely used to define OSA, it is not a good predictor of disease severity by itself. It is often poorly correlated with subjective symptoms as for example in upper airway resistance syndrome, where the patient complains of EDS. A universally accepted threshold level of significant SDB for defining a clinically significant sleep apnea syndrome does not exist. In the literature, the most commonly used level is an AI above 5. It is generally accepted that SDB is considered significant in the presence of symptoms, especially hypersomnolence. Although the primary event is the apnea, the immediate consequences are more severe in patients in sleep clinics, even at the same apnea score, compared to the same asymptomatic adult population. Thus, in most sleep centres the prevalence figures for OSA are higher (45-55%) because of the referral (Flemons and McNicholas, 1997). Most of the published data on OSA is from the Caucasian population. The prevalence of SDB increases with age, being higher in males. It is higher in African-Americans (Ancoli-Israel et al., 1995) and in the United States in Hispanics and minorities (Kripke et al., 1997) than in Caucasians. The degree to which an increase in SDB relates to the clinical diagnosis of the OSA syndrome is controversial. Perhaps the higher prevalence among the predominantly Chinese population needs further study of anthropomorphic variables such as body mass index, waist and neck circumference and cephalometrics.

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This study was made possible by a donation from The Shaw Foundation, Singapore.

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