Obstructive sleep apnea syndrome (OSAS) in children has high prevalence and severe complications, and its first line of treatment (adenotonsillectomy) has risk of complications. Even though the use of presurgical CPAP seems logical due to its effects in adults, it must be studied in children due to the different physiopathology and adherence. One non-invasive way of study the effect is via acoustic pharyngometry, which can measure the anatomical site of obstruction. The post-surgical anatomical changes could correlate with a persistent OSAS, which would be helpful in selecting those patients who require a post surgical sleep study. The main goal of the study is to cuantify the changes in the oropharyngeal volume via acoustic pahryngometry after CPAP use, and also the changes after adentonsillectomy in children.

The obstructive sleep apnea syndrome (OSAS) is defined as recurrent events of obstruction of the upper airway (with alteration of normal oxygenation, ventilation or sleep pattern). In children originates complications such as cognitive, cardiovascular and metabolic disorders and growth failure, which make its management imperative. It has a reported prevalence between 1 and 5%, highest between 2 and 8 years old, due to tonsil and adenoid hypertrophy, which causes obstruction of the upper airway, unlike the fat deposits of adults. This physiopathological cause of obstruction makes adenotonsillectomy the first line of management. This is a secure procedure, which still has an incidence of complications of 19%, (9.5% respiratory and 2.4% secondary bleeding). Those with OSAS present complications more frequently than those without7. Even though the surgery is considered succesful, up to 21.6% of those treated present persistence of the disease. Based on these results the American Academy of Pediatrics recommends considering polysomnography after surgery in those patients with high risk of persistence, such as obese children.

The continuous positive airway pressure (CPAP) device provides a continuous pressure through a mask, acting as a pneumatic ferule which keeps the upper airway permeable; with a lower possibility of obstruction. The adherence of pediatric patients to CPAP has been reported between 6 and 65% (which is one of the reasons it's not considered the first line treatment), with an use of 2.1 to 8.2 hours per night, and an AIH (apnea-hypopnea index) between 2 and 6 per hour. In adult patients, a reduction in trans and post-surgical risk has been found, and therefore its pre-surgical use is recommended. In children it is one of the strategies used to reduce surgical risk in OSAS, even though its use is not recommended in any of the main clinical guides of management of children with OSAS. Its use has been described in children with severe illness, and in other studies it was used in 18% of patients, but its real effect in the probable complications has not been prospectively studied.

In adult subjects, inflammation of upper airway has been reported, and it could play a role in the physiopathology of OSAS, with changes in the minimal sectional area and pharyngeal volume measured by MRI after use for 4 to 6 weeks of CPAP use. Albeit the physiopathology is not completely understood, it could be a consequence of the mechanical stress associated to obstruction of airflow, with the repeated trauma related with snore, jointly with the upper airway vibration and the suction from the collapse during the apneas the causes. CPAP eliminates respiratory events, which could cause a reduction in edema and inflammation.

Acoustic pharyngometry is a non-invasive method which uses sound reflection to quickly measure the sectional area of the upper airway in function of the distance from the oral aperture. It can be adapted to its use in children, obtaining highly reproducible results, even though it is limited to those who can follow instructions. In adults a difference was found between the minimum oropharyngeal area and the mean area after 1 week of CPAP use, but not between 1 week and 6 months of use, with a return to basal after a 1 week of nonuse. In our center change in volume and transversal oropharyngeal area have been found with acoustic pharyngometry since the second week of use of CPAP, of 5% at 2 weeks and 6% at 4 weeks. In children changes after surgery have been found, with increase in the minimal sectional area and oropharyngeal volume, even though its correlation with persistent OSAS has not been studied.

In order to study the changes in the upper airway after CPAP, acoustic pharyngometry will be performed before and after CPAP use, and 3 months after adenotonsillectomy to determinate the relation to persistent OSAS. The presurgical CPAP will be randomized in those children with diagnosis of mild to moderate OSAS.

Inclusion Criteria:

• Age between 4 and 14 years old. Both genders. Diagnosis of OSAS based on sleep study (respiratory polygraphy or polysomnography).

Surgical treatment planned by the ORL department of the Institute of Respiratory Diseases of Mexico.

Residents of the Metropolitan Area of Mexico City Parents and child accept to particpate in the study, singning informed consent and assent.

Exclusion Criteria:

• Previous surgery in upper airway. Craneofacial malformation. Previous use of CPAP. Syndromatic patient.

Data from the study could be provided if solicited at the main author's e-mail adress.

The CPAP equipment will be programed by the technician, not knowing the children, the parents or the main researcher whether the pressure is suboptimal or therapeutic.