Central sleep apnea in hypothyroidism

Respiratory manifestations in endocrine diseases

The control mechanisms of respiration as a vital function are complex: voluntary - cortical, and involuntary - metabolic, neural, emotional and endocrine. Hormones and hypothalamic neuropeptides (that act as neurotrasmitters and neuromodulators in the central nervous system) play a role in the regulation of respiration and in bronchopulmonary morphology. This article presents respiratory manifestations in adult endocrine diseases that evolve with hormone deficit or hypersecretion. In hyperthyroidism, patients develop ventilation disorders, obstructive and central sleep apnea, and pleural collection. The respiratory abnormalities in hyperthyroidism as a result of the hypermetabolic action of thyroid hormones are hyperventilation, myopathy and cardiovascular involvement; recent studies have reported pulmonary arterial hypertension in Graves' disease, as a result of the association of several mechanisms. Thyroid hypertrophy can induce through compression of the upper airways dyspnea, stridor, wheezing and cough. The respiratory disorders in acromegaly are ventilatory dysfunction and sleep apnea, which contribute to an unfavorable evolution of the disease. Respiratory changes in parathyroid, adrenal and reproductive system diseases have been described. Respiratory disorders should be recognized, investigated and monitored by medical practitioners of various specialties (family physicians, internists, endocrinologists, pneumologists, cardiologists). They are frequently severe, causing an unfavorable evolution of the associated endocrine and respiratory disease.

Contemporary insights and novel treatment approaches to central sleep apnea syndrome in heart failure

OPINION STATEMENT

Central sleep apnea (CSA) is a common and under-diagnosed condition commonly associated with Cheyne-Stokes respiration. It is particularly prevalent in the heart failure population affecting up to 40 % of all patients with heart failure. The pathophysiology associated with CSA is based on the underlying effects of hypoventilation and hyperventilation, with neurologic dysregulation of respiratory control as the primary defect. However, therapeutic options are limited because of the prevailing perception that CSA is a consequence, rather than cause of morbidity and mortality. At present, the main focus remains treating the underlying problem (ie, intensifying heart failure therapeutics, decongestion), whereas additional suggestions of using acetazolamide, progesterone, nocturnal oxygen, and theophylline have not been validated with contemporary clinical trials. Positive pressure ventilation is currently the primary recommendation for all patients with sleep-disordered breathing (CSA included), and in some patients may effectively reduce the apnea-hypopnea index. However, significant research is ongoing to determine how to treat this complex patient population.

Central sleep apnea

Neurophysiologically, central apnea is due to a temporary failure in the pontomedullary pacemaker generating breathing rhythm. As a polysomnographic finding, central apneas occur in many pathophysiological conditions. Depending on the cause or mechanism, central apneas may not be clinically significant, for example, those that occur normally at sleep onset. In contrast, central apneas occur in a number of disorders and result in pathophysiological consequences. Central apneas occur commonly in high-altitude sojourn, disrupt sleep, and cause desaturation. Central sleep apnea also occurs in number of disorders across all age groups and both genders. Common causes of central sleep apnea in adults are congestive heart failure and chronic use of opioids to treat pain. Under such circumstances, diagnosis and treatment of central sleep apnea may improve quality of life, morbidity, and perhaps mortality. The mechanisms of central sleep apnea have been best studied in congestive heart failure and hypoxic conditions when there is increased CO2 sensitivity below eupnea resulting in lowering eupneic PCO2 below apneic threshold causing cessation of breathing until the PCO2 rises above the apneic threshold when breathing resumes. In many other disorders, the mechanism of central sleep apnea (CSA) remains to be investigated.

Inactivity-induced respiratory plasticity: protecting the drive to breathe in disorders that reduce respiratory neural activity

Multiple forms of plasticity are activated following reduced respiratory neural activity. For example, in ventilated rats, a central neural apnea elicits a rebound increase in phrenic and hypoglossal burst amplitude upon resumption of respiratory neural activity, forms of plasticity called inactivity-induced phrenic and hypoglossal motor facilitation (iPMF and iHMF), respectively. Here, we provide a conceptual framework for plasticity following reduced respiratory neural activity to guide future investigations. We review mechanisms giving rise to iPMF and iHMF, present new data suggesting that inactivity-induced plasticity is observed in inspiratory intercostals (iIMF) and point out gaps in our knowledge. We then survey conditions relevant to human health characterized by reduced respiratory neural activity and discuss evidence that inactivity-induced plasticity is elicited during these conditions. Understanding the physiological impact and circumstances in which inactivity-induced respiratory plasticity is elicited may yield novel insights into the treatment of disorders characterized by reductions in respiratory neural activity.

Correlation of clinical profiles with obstructive sleep apnea and metabolic syndrome

BACKGROUND

Obstructive sleep apnea (OSA) is a common condition being increasingly recognized and is associated with many diseases. Few data are available for Indian patients and public health hazard of the condition continues to be seriously underestimated in this country. The objective of this study was to evaluate the prevalence of obesity, hypertension, diabetes mellitus, and hyperlipidemia (HLP) in a South Indian population with proven OSA.

METHODS

A retrospective analysis of data accrued in patients undergoing polysomnography (PSG) was undertaken. Two hundred thirty-four patients of either sex in the age of 54 ± 11 years who tested positive for OSA were compared with control group. An apnea-hypopnea index (AHI) of >5 in the presence of snoring and daytime somnolence was taken to define sleep apnea.

RESULTS

The mean AHI among the study group with OSA was 31.3 ± 18.6 as compared with 1.2 ± 0.5 in the control group. The BMI among patients with OSA was 36 ± 6 as compared with 29 ± 4 in comparison group (p < 0.001). Among the patients with OSA, 31% were having HLP, 59% were diabetics, and 86% were hypertensive. Subjects with PSG-proven OSA had significant levels of day time sleepiness, lack of concentration, changes in mood, morning headache, and dry mouth. Nonrestorative sleep, awakening with choking, nocturnal dyspnea, insomnia with frequent awakenings, nocturia, and diaphoresis were observed in (>80%) patients. A significant difference in the sleep efficiency (16%), sleep onset (14%), and mean saturation (9%) was observed in the study group as compared to the control group.

CONCLUSIONS

The risk of obesity, hypertension, diabetes mellitus, and HLP in OSA syndrome patients was almost three times more than that of the comparing group.

Central sleep apnea

Central apnea is caused by temporary failure in the pontomedullary pacemaker generating breathing rhythm, which results in the loss of ventilatory effort, and if it lasts 10 seconds or more it is defined as central apnea. This article reviews current knowledge on central sleep apnea.

Reversibility of fibrotic appearance of lungs with thyroxine replacement therapy in patients with severe hypothyroidism

OBJECTIVE

To present 2 cases of hypothyroidism with hypoxia associated with computed tomographic (CT) features suggestive of pulmonary fibrosis that resolved with correction of the hypothyroidism.

METHODS

Clinical case histories are described, comparative radiologic pulmonary images before and after treatment are provided, and the pertinent literature regarding possible pathologic mechanisms is reviewed.

RESULTS

Our first patient, a 68-year-old woman, presented with symptomatic severe hypothyroidism associated with respiratory failure. A CT scan of her lungs showed appearances suggestive of pulmonary fibrosis. Replacement therapy with levothyroxine led to correction of hypoxia and radiologic abnormalities. Our second patient, a 26-year-old man, presented with symptoms suggestive of obstructive sleep apnea that persisted despite use of positive pressure ventilation. Biochemical evaluation revealed severe hypothyroidism, and a CT scan disclosed pulmonary appearances consistent with fibrosis. His symptoms and radiologic abnormalities also improved after correction of hypothyroidism with levothyroxine therapy.

CONCLUSION

Radiologic pulmonary abnormalities suggestive of fibrotic disease are associated with severe hypothyroidism. Invasive investigations such as lung biopsy should be deferred until the clinical and radiologic responses to thyroxine replacement therapy have been assessed.

Endocrine aspects of obstructive sleep apnea

CONTEXT

Some endocrine and metabolic disorders are associated with a high frequency of obstructive sleep apnea (OSA), and treatment of the underlying endocrine disorder can improve and occasionally cure OSA. On the other hand, epidemiological and interventional studies suggest that OSA increases the cardiovascular risk, and a link between OSA and glucose metabolism has been suggested, via reduced sleep duration and/or quality.

EVIDENCE ACQUISITION

We reviewed the medical literature for key articles through June 2009.

EVIDENCE SYNTHESIS

Some endocrine and metabolic conditions (obesity, acromegaly, hypothyroidism, polycystic ovary disease, etc.) can be associated with OSA. The pathophysiological mechanisms of OSA in these cases are reviewed. In rare instances, OSA may be improved or even cured by treatment of underlying endocrine disorders: this is the case of hypothyroidism and acromegaly, situations in which OSA is mainly related to upper airways narrowing due to reversible thickening of the pharyngeal walls. However, when irreversible skeletal defects and/or obesity are present, OSA may persist despite treatment of endocrine disorders and may thus require complementary therapy. This is also frequently the case in patients with obesity, even after substantial weight reduction.

CONCLUSIONS

Given the potential neurocognitive consequences and increased cardiovascular risk associated with OSA, specific therapy such as continuous positive airway pressure is recommended if OSA persists despite effective treatment of its potential endocrine and metabolic causes. "Apropos of sleep, that sinister adventure of all our nights, we might say that men go to bed daily with an audacity that would be incomprehensible if we did not know that it is the result of ignorance of the danger." Charles Baudelaire, in "Fusées, IX"

Thyroid status affects 5-HT2A receptor modulation of breathing before, during, and following exposure of hamsters to acute intermittent hypoxia

The BIO 14.6 hamster (dystrophic), animal model of limb girdle muscular dystrophy, exhibits low plasma triiodothyronine levels, muscle weakness, and decreased breathing. After exposure to acute intermittent bouts of hypoxia, dystrophic hamsters depress ventilation relative to baseline resulting in ventilatory long-term depression (LTD). Control hamsters may increase ventilation relative to baseline resulting in ventilatory long-term facilitation (LTF). Serotonin (5-HT) receptors, especially the 5-HT(2A) subtype, are involved in the development of LTF. The purpose of this study was to evaluate the role of 5-HT(2A) receptors in ventilatory and metabolic responses before, during, and following intermittent hypoxia in eleven euthyroid, nine dystrophic, and eleven propylthiouracil (PTU)-induced hypothyroid male hamsters. Animals received subcutaneous injections of vehicle or 0.5 mg/kg MDL (5-HT(2A) receptor antagonist). Plethysmography was used to evaluate ventilatory responses of the three groups to air, five bouts of 5 min of 10% oxygen, each interspersed with 5 min of air, followed by 60 min of exposure to air. CO(2) production was measured using the flow-through method. Vehicle-treated dystrophic and PTU-treated hamsters exhibited LTD. MDL decreased body temperature in all groups. After MDL treatment, the euthyroid group exhibited LTD. MDL treatment in the dystrophic, but not in the PTU-treated hamsters, maintained tidal volume, but did not reverse LTD. CO(2) production was increased in the euthyroid group with MDL treatment. Thus, 5-HT(2A) receptors affect body temperature, ventilation, and metabolism in hamsters. The differential responses noted in this study may be in part dependent on thyroid hormone status.

Perioperative management of patients with hypothyroidism

Hypothyroidism is a common disorder affecting the cardiovascular, respiratory, hematopoietic, and renal organ systems--each of which is particularly germane in the management of the surgical patient. In general, treatment of recognized hypothyroidism is recommended before any surgical procedure whenever possible and euthyroidism should be documented by measurement of serum TSH as part of the preoperative evaluation. Such a strategy is likely to result in better surgical outcomes with improved morbidity and mortality. One exception to treating first with thyroid hormone is the patient with angina or coronary artery disease requiring bypass grafting, angioplasty or stenting. In this setting, preoperative thyroid hormone therapy could tax the ischemic myocardium. The coronary blood flow should be addressed first, and thyroid hormone therapy initiated afterwards. The authors have emphasized the need for caution in the interpretation of low serum thyroid hormones in sick or surgical patients because of the importance of distinguishing between hypothyroidism and the "euthyroid sick syndrome." There is no clear evidence at this point to support thyroid hormone replacement in the latter patients, and it may be potentially harmful. Rather, we hold that T3 treatment of various surgical and other patients with nonthyroidal illness should be deferred until proof of its therapeutic efficacy is demonstrated.

Management of myxedematous respiratory failure: review of ventilation and weaning principles

Respiratory failure in myxedema is a complex medical emergency and may require prolonged ventilatory assistance. Appropriate management of this medical problem requires an understanding of its effects on the central nervous system and peripheral neuromusculoskeletal system. Weaning of these patients can be very protracted and requires a diligent multidisciplinary approach. Because of its infrequency, ventilatory management of severe hypothyroidism has not been studied in a controlled fashion. The first part of this review discusses the mechanisms of respiratory failure in myxedema. The second part explores strategies in mechanical ventilation and weaning of myxedematous patients.

Thyroid Testing and Thyroid Hormone Replacement in Patients with Sleep Disordered Breathing

The current literature recommends that patients who have symptoms of sleep disordered breathing should be evaluated for hypothyroidism. Thyroid hormone replacement therapy has been reported by some authors to be effective in treating obstructive sleep apnea in hypothyroid patients. The present study prospectively evaluated the prevalence of hypothyroidism in 1,000 consecutively presenting patients who came to the office for evaluation of snoring or obstructive sleep apnea syndrome. The authors also examined the efficacy of treatment for hypothyroidism on sleep apnea in patients with both disorders.

Of the 1,000 patients, routine thyroid testing was performed on 834; only 10 of these patients (1.2%) were discovered to have previously undiagnosed clinical hypothyroidism. Four of the 10 patients with newly diagnosed clinical hypothyroidism had obstructive sleep apnea syndrome, and they received thyroid hormone replacement therapy. Once these four patients achieved a euthyroid state, repeat polysomnography showed that there was no significant difference between their pre- and posttreatment respiratoiy disturbance index.

Based on the results of our study, we conclude the following: 1) The prevalence of hypothyroidism in patients who are evaluated for sleep disordered breathing is no greater than that of the general population. 2) Thyroid replacement therapy results in little or no improvement in sleep apnea in patients with clinical hypothyroidism. 3) Routine thyroid function screening is not indicated for patients who are being evaluated for sleep disordered breathing.

Screening for hypothyroidism in sleep apnea

Primary sleep apnea-hypopnea syndrome (obstructive sleep apnea [OSA]) and hypothyroidism have many signs and symptoms in common. The overlap in clinical presentation, and the sleep-disordered breathing that can accompany hypothyroidism, create a significant risk of misdiagnosis of sleep apnea among patients referred to sleep clinic who have undiagnosed hypothyroidism. We determined the point prevalence of hypothyroidism in our sleep clinic patients with newly diagnosed sleep-disordered breathing. Of 290 sequential patients referred to sleep clinic, 200 (69%) patients judged at high risk for OSA underwent polysomnography (PSG) and biochemical screening for hypothyroidism. Of these, 124 (62%) were judged to have sleep apnea. This included three patients (1.5% of patients undergoing PSG or 2. 4% of those judged to have OSA) who were also discovered to have previously undiagnosed hypothyroidism. These three patients with "secondary" sleep apnea were treated with thyroxine therapy alone, and followed with sequential sleep studies and serum thyroid hormone assays; symptoms, sleep-disordered breathing, nocturnal hypoxia, and thyroid deficiency resolved simultaneously. We conclude that biochemical screening for hypothyroidism is required to prevent inadvertent misdiagnosis of hypothyroid sleep-disordered breathing as primary sleep apnea, and that it is a cost-effective component of the investigation of sleep apnea.

Sleep development in children with congenital and acquired hypothyroidism

To determine how thyroid hormone deficiency before birth influences early development of the human central nervous system, we examined sleep development in children with congenital hypothyroidism using all-night polysomnography. Although the sleep-wakefulness circadian rhythm was not disturbed, two patients under 1 year of age showed mild decreases of REM sleep. Rapid eye movements during REM sleep were significantly decreased in one patient in whom thyroxine replacement was delayed. Regarding phasic body movements during sleep, generalized gross movements were more frequently disturbed than localized muscle twitches, and even after L-thyroxine replacement, they were significantly reduced when compared with those in controls. Children with acquired hypothyroidism and neonatal transient hyperthyrotropinemia showed no severe abnormalities of the sleep parameters examined here. Sleep development in patients with congenital hypothyroidism is believed to be useful for regional evaluation of the brain impairment in congenital hypothyroidism.

Upper airway obstruction in hypothyroidism

OBJECTIVES

To determine the occurrence and frequency of nocturnal upper airway obstruction in hypothyroidism.

DESIGN

A case-control study of patients with newly diagnosed hypothyreosis and euthyroid subjects who had been selected from the population register. The subjects underwent sleep recordings with a static-charge-sensitive bed (SCSB).

SETTING

Turku University Hospital and Research and Development Unit, Social Insurance Institution, Turku, Finland.

SUBJECTS

Twenty six consecutive patients underwent sleep recordings with SCSB and a pulse oximeter. One hundred and eighty-eight euthyroid subjects who were previously studied with the SCSB were used as controls.

MAIN OUTCOME MEASURES

In a multivariate analysis, hypothyroidism, gender, age, and body mass index (BMI) were considered as predictors for the occurrence of nocturnal breathing abnormalities.

RESULTS

Nocturnal breathing abnormalities were frequent in both groups. Fifty per cent of the hypothyroid patients and 29.3% of the control subjects had at least some episodes of partial or complete upper airway obstruction (P = 0.04). Severe obstruction with episodes of repetitive apnoea was present in 7.7% of the patients and in 1.5% of the controls. The multivariate analysis revealed that the presence of hypothyroidism did not significantly (P = 0.06), and independently of age, BMI or gender, predict nocturnal breathing abnormalities, whereas obesity (P < 0.0001) and male gender (P = 0.0001) were independent and significant predictors.

CONCLUSIONS

The incidence of nocturnal upper airway obstruction is increased in hypothyroidism, but is related to obesity and male gender rather than to hypothyroidism per se.

Control of breathing in patients with severe hypothyroidism

PURPOSE

Hypothyroid patients have been reported to have a blunted ventilatory response to carbon dioxide stimulation. However, previous data did not clarify the localization of abnormalities responsible for that disorder. The present investigation was aimed at evaluating to what extent central (neural) and/or peripheral (muscular) factors are involved in the abnormalities of the ventilatory control system in hypothyroid patients.

PATIENTS AND METHODS

We studied 13 patients with severe hypothyroidism before and after 6 to 9 months of replacement therapy; 7 age- and sex-matched normal subjects were also studied as a control. In each subject, we assessed (1) inspiratory muscle strength by measuring maximal inspiratory pressure (MIP), and (2) respiratory control system during a carbon dioxide rebreathing test by measuring minute ventilation (VE), tidal volume (VT), mean inspiratory flow (VT/TI), and electromyographic (EMG) activity of the diaphragm (Edi) and intercostal (Eint) muscles.

RESULTS

Compared with the normal control group (Group C), patients exhibited similar MIP, and similar VE and EMG response slopes to carbon dioxide. However, evaluating individual VE response slopes, we were able to identify two subsets of patients: Group A (six patients) with low VE response (less than mean -SD.1.65 of Group C) and Group B (seven patients) with normal VE response. Compared with both Groups B and C, Group A exhibited significantly lower VT/TI, Edi, and Eint response slopes; the difference between Groups B and C was not significant. Six patients (two from Group A and four from Group B) exhibited low MIP values compared with that in Group C. After replacement therapy, (1) VE, VT/TI, and Edi response slopes increased significantly in Group A; and (2) MIP increased, but not significantly in patients with low MIP.

CONCLUSIONS

We conclude that: (1) In patients with severe hypothyroidism, the ventilatory control system may be altered at the neural level, as indicated by a blunted chemosensitivity; (2) Impaired respiratory muscle function does not seem to play a major role in the decreased ventilatory response to carbon dioxide stimulation; (3) Replacement therapy appears to normalize the response to hypercapnic stimulation, but not respiratory muscle strength.

Mechanisms and management of central sleep apnea

Central sleep apnea is not a single disease but represents the final pathway in a large group of heterogeneous disorders. Control of normal breathing during sleep relies upon finely coordinated anatomical and physiological mechanisms and their destabilization leads to central apnea. The causes of central sleep apnea can be classified into 4 groups: neurologic disorders, periodic breathing, upper airway abnormalities, and idiopathic syndromes. Clinical features result from the interaction between the underlying disorder and control of respiration. Two different prototypes emerge: patients who are hypercapnic (central hypoventilation and/or impaired respiratory mechanics) and those who are eucapnic or hypocapnic (periodic breathing and idiopathic hyperventilation). The causes and severity of apnea can be determined by clinical assessment, pulmonary function testing, and overnight polysomnography. Further management involves specific treatment of the underlying condition and reducing the sequelae of recurrent apneas during sleep, namely cardiorespiratory dysfunction and sleep disruption. This review outlines an approach to the management of central sleep apnea based upon an understanding of its pathophysiology.

Control of breathing in patients with short-term primary hypothyroidism

In 8 patients (3 men and 5 women) with short-term primary hypothyroidism before and during replacement therapy, and in an age-matched control group (9 men and 7 women), we assessed maximal inspiratory muscle force (Pimax) and the ventilatory control system at neural (EMG), muscular (P0.1), and ventilatory (VE and Vt/Ti) levels. While hypothyroid, patients exhibited a significantly lower Pimax. During a CO2 rebreathing test, hypothyroid patients exhibited similar diaphragmatic electromyographic (EMGd) and ventilatory (VE) response slopes to increasing end-tidal CO2 tension (delta EMGd/delta Pet CO2 and delta VE/delta PetCO2), but significantly less delta P0.1/delta PetCO2 (p less than 0.05) and delta(Vt/Ti)/delta PetCO2 (p less than 0.05) response slopes. During replacement therapy with L-triiodothyronine (L-T3), Pimax (p less than 0.05), delta P0.1/delta PetCO2, and delta(Vt/Ti)/delta PetCO2 response slopes were found to be significantly increased (p less than 0.05 for both) while neither EMGd nor VE response changed significantly. We concluded that short-term hypothyroidism does not seem to be associated with blunted neural inspiratory output (EMGd), the respiratory control system seems to be affected mostly at a peripheral (muscular) level, and L-T3 increases both force (P0.1 and Pimax) and velocity (Vt/Ti) of inspiratory muscle contraction.

Sleep apnea, sleep disorders, and hypothyroidism

The symptoms of hypothyroidism are protean and include apathy, somnolence, lethargy, personality change, and intellectual deterioration. Many of these symptoms may be related to hypothyroid-induced sleep disorders. Hypothyroidism is associated with abnormal ventilatory drive, abnormal sleep architecture, and sleep apnea. Central, obstructive, and mixed patterns of sleep apnea are commonly observed in hypothyroidism. A case of severe sleep apnea in a grossly myxedematous patient who improved dramatically following thyroid replacement alone is presented. Myxedema is a reversible cause of sleep apnea, and thyroid function testing should be considered in its diagnostic work-up.

Sleep apnea and hypothyroidism: mechanisms and management

PURPOSE

There is a high incidence of sleep apnea in patients with untreated hypothyroidism. Thyroxine treatment is said to significantly reduce the apnea index and length and sleep apnea symptoms. We undertook a review of 10 consecutive hypothyroid patients with sleep apnea to investigate mechanisms and management of these two disorders.

PATIENTS AND METHODS

Polysomnograms were obtained in 10 consecutive hypothyroid patients referred to our sleep disorders unit. All patients were studied while hypothyroid. Eight patients were restudied later when euthyroid. Lung function, blood gas values, and awake supraglottic resistance were also assessed in each patient.

RESULTS

All 10 patients had sleep apnea and were treated with thyroxine. In one patient, hypothyroid myopathy involving the upper airway was demonstrated to be a potential mechanism of sleep apnea in hypothyroidism. Nocturnal angina and ventricular arrhythmias developed in two patients, despite the use of low thyroxine doses. Nasal continuous positive airways pressure (CPAP) was begun in eight patients. Initiation of CPAP prevented further angina or arrythmia in the patients with these cardiac complications. Six of the eight patients who were available for follow-up studies had persistent sleep apnea despite an euthyroid status (apnea index before thyroxine, 51 +/- 6; apnea index after thyroxine, 45 +/- 8), and CPAP therapy was continued in these patients.

CONCLUSION

Our experience suggests that the apnea index does not decrease significantly in all patients with hypothyroidism and sleep apnea when euthyroidism is achieved. Treatment of hypothyroidism in the presence of sleep apnea is potentially hazardous and may lead to cardiovascular complications. Management by a combination of CPAP and low-dose thyroxine is helpful in this situation.

Reversal of central sleep apnea using nasal CPAP

Based on the theory that obstructive (OSA) and central (CSA) sleep apneas share common pathophysiologic mechanisms, we attempted to treat eight patients with predominantly CSA by continuous positive airway pressure (CPAP). All patients exhibited repetitive episodes of CSA and mixed sleep apneas (MSA) in the supine position with a mean duration of 23.7 +/- 0.7 s and 34.5 +/- 1.3 s, respectively. The pattern of apnea changed when the subject lay in the lateral position. Five patients were observed to develop OSA in the lateral position with a mean duration of 27.2 +/- 1.5 s, while the other three patients snored continuously. High levels of CPAP (range 9.0 to 16.5 cm H2O) prevented all CSA and MSA and resulted in quiet breathing in all eight patients. Intermediate levels of CPAP produced firstly MSA, then purely OSA and/or continuous snoring. Low levels of nasal CPAP also prevented OSA and snoring occurring in the lateral posture in all subjects (range 2.0 to 8.3 cm H2O). Three patients are currently on home CPAP therapy for a range of four to 36 months. We conclude that upper airway collapse in the supine posture has a key role in the induction of CSA. We suggest that a reflex inhibition of respiration through activation of supraglottic mucosal receptors during passive oropharyngeal airway closure caused CSA in these patients.