Down Syndrome (DS) is the most common cause of mental retardation with incidence of 1 in 848 (Lin, Hu et al. 1991). Although prenatal Down syndrome and Amniocentesis had been applied for years, in the survey of 2005, current birth incidence of DS is 1.6 in 10,000 live birth, meaning a 30-50 new cases per year (Jou, Kuo et al. 2005). Patients with DS will have multisystemic manifestations including short stature, mental retardation, dysmorphism, congenital heart disease, congenital anomaly of gastrointestinal and genitourinary tract, abnormal endocrine function, leukemia and leukemoid reaction. Beside mental retardation, other anomalies could be treated or controlled by current medical care. The IQ of DS is around 20-80 with significant cognitive, language, and behavior problems (Dierssen, Ortiz-Abalia et al. 2006).

In addition, obstructive sleep apnea syndrome (OSAS) had been observed in DS people with prevalence about 45-79% in the literature (de Miguel-Diez, Villa-Asensi et al. 2003; Dyken, Lin-Dyken et al. 2003; Shott, Amin et al. 2006; Fitzgerald, Paul et al. 2007), which is much higher than the 1-3% prevalence rate in general population (Holmes 1993; Gislason and Benediktsdottir 1995). The reasons of DS people prone to have OSAS are due to the combination of anatomical and physiological factors. In DS people, facial dysmorphism (midfacial hypoplasia, mandibular hypoplasia), macroglossia, small hypopharynx, pharyngeal hypotonia, tonsil and adenoid enlargement, obesity, laryngomalacia, and tracheomalacia contributed to upper airway obstructions in DS people (Trois, Capone et al. 2009; Pandit and Fitzgerald 2012). In addition, DS people has increased incidence of lower respiratory tract diseases including gastroesophageal reflux, immunological dysfunction, tracheal bronchus, airway malacia, congenital heart disease, and pulmonary hypoplasia, which will predispose to OSAS. While growing up, DS people still have generalized hypotonia with increasing risk of developing hypothyroidism and obesity, which are also risk factors for OSAS (Trois, Capone et al. 2009).

It has been noted that sleep disordered breathing is associated with neurocognitive deficit, particularly of memory, learning, attention, hyperactivity, executive functioning, cognitive capacity, and poor school performance (Beebe 2006; Pandit and Fitzgerald 2012). And a number of studies have reported improved attention, executive functioning, analytical thinking, verbal functioning, memory and academic progress at 6-12 months post- adenotonsillectomy (Chervin, Ruzicka et al. 2006). In DS, study demonstrated that a higher number of apneic episodes on polysomnography was correlated to the decreased visuoperceptual skill in DS (Andreou, Galanopoulou et al. 2002). Similarly, presence of snoring in DS was associated with a much higher rate of disruptive school behavior than without snore (Carskadon, Pueschel et al. 1993). Although learning disability and memory defect had been globally known in DS, the behavior, cognitive, and developmental impairment caused by OSAS is especially concerning because it might adversely affect their ability, even the social adaptation (Rihtman, Tekuzener et al. 2010) . Therefore, investigators would like to know the correlation between severity of OSAS and Neurocognitive and behavior in DS people in Taiwan. Also, investigators would like to follow the Neurocognitive and behavior changes in those who had been treated for OSAS, including tonsillectomy or Bilevel Positive Airway Pressure.