Are metabolic syndrome, obstructive sleep apnoea & syndrome Z sequential?--a hypothesis

The metabolic syndrome (MS) is a risk factor for development of cardiovascular disease and is closely associated with obstructive sleep apnoea (OSA). Co-occurrence of both OSA and MS is called syndrome Z. It has been hypothesized that the OSA may be a manifestation of MS. We collected data on polysomnography (PSG) and biochemical investigations on middle aged urban Indians during a community based study in South Delhi while studying prevalence of obstructive sleep apnoea and analysed to find out the ages at which the OSA, MS and syndrome Z exist in these subjects.

A 2-stage, cross-sectional, population-based study in subjects of either gender between 30-65 yr of age in 4 different socio-economic zones of the South Delhi, India, was performed earlier (from April 2005 through June 2007). In-hospital, supervised PSG studies were performed and biochemical investigations for the MS using National Cholesterol Education Programmme Adult Treatment Panel (NCEP ATP) III criteria were carried out. In this communication, the data were further analysed to estimate the prevalences of MS alone, OSA alone and syndrome Z and average ages of 3 conditions.

Three hundred and fifty one subjects had satisfactory PSG studies. The MS alone was present in 105 [29.9%; (95% CI 25.1-34.7)] while OSA alone was present in 24 [6.8%; (95% CI 4.2-9.5)] subjects and the syndrome Z was present in 70 [19.9%; (95% CI 15.8-24.1)] subjects. Median ages of normal subjects, and subjects with MS, OSA and syndrome Z were 40, 43, 43 and 47 yr respectively. Minimum ages of normal subjects, and subjects with MS, OSA and syndrome Z were 30, 30, 32 and 32 yr respectively.

When body mass index (BMI) was normal, the increasing median ages of these conditions indicated that the MS may be the first event followed by OSA and eventually syndrome Z develops. With BMI >25 or >30 no clear-cut difference was noted, indicating that the BMI itself could have an independent role in MS, OSA and syndrome Z.