Patient Assessment of Upper Gastrointestinal Disorders - Quality of Life questionnaire: The score ranges from 0 to 5 for each symptom (30 questions) and these are averaged into subscores and into a total score (also ranging from 0 to 5). Higher scores indicate lower quality fo life.

Gastric scintigraphy protocol endorsed by both American Neurogastroenterology and Motility Society and American Nuclear Medicine Society will be used to assess gastric emptying with a standard 200 kcal, 2% fat egg-substitute test meal. Gamma camera images will be obtained immediately after meal ingestion and then at 1, 2, 3 and 4 hours. Diagnostic criterion for delayed gastric emptying is defined as the percentage of gastric retention >60% at 2 h or ≥ 10% at 4 h or both. Half-time (T1/2) emptying time will also be calculated. At least 72 hours before gastric emptying test, narcotics and other medications that can interfere with gastric emptying will be discontinued.

Gastric emptying assessed using 13C breath test with octanoic acid. This measurement is an alternative to the measurement of gastric emptying by scintigraphy. The choice depends on the practice in a particular center.

Surface electrogastrography (EGG) is a non-invasive method for the evaluation of myoelectric activity of the stomach. Gastric pacemaker generates electrical phenomenon termed "gastric slow waves", which originate in a "pacemaker region" located on the greater curvature near the junction of the fundus and gastric body. EGG recordings will be accomplished by means of EGG Stand (MMS, Enschede, the Netherlands) with a MMS software. Six active self-adhesive electrodes will be placed on the upper part of the abdomen, the 7th electrode (basal) will be placed in the left supraclavicular area. A special abdominal belt (respiratory sensor) will be used to identify possible artefacts due to breathing and body movements . All EGGs will be performed at a lying position in the morning after an overnight fasting. Basal EGG (under fasting condition) will be recorded for 20 minutes. A standard test breakfast will be served afterwards. Postprandial EGG will be recorded for next 40 min.

Autonomic function testing will be performed by the golden standard battery of tests introduced by Ewing. The test involves four phases: (1) resting phase (5 min), (2) deep breathing test (1 min of 0,1 Hz breathing rate), (3) short Valsalva maneuvers (1 min), and (4) standing Tilt test (5 min). An ECG will be recorded during all phases. The Heart rate variability triangular index will be assessed.