Anorectal 3D manometry (3D HRAM) is the most advanced version of manometric equipment that measures pressures along the anal canal in a very detailed manner. It provides complete data about pressure profile of anorectum and may indicate impaired defecation dynamics. Resent studies suggest that the position in which 3D HRAM is performed should be changed. So far, no scientific research has been performed in children that directly compares both positions. The reference values of registered pressures during 3D HRAM in people without complaints were not specified, which would allow for a more precise diagnosis of patients with an incorrect defecation model and precise selection of patients who could benefit from biofeedback therapy.

Patients with the diagnosis of constipation will be enrolled in the study. Each patient will be investigated by anorectal manometry in both- sitted and lying position.

During anorectal manometry conventional manometric parameters will be recorded, such as resting pressure, squeeze pressure, bear down manoeuver, thresholds of sensation and threshold of recto anal inhibitory reflex. 3D picture of anal canal will be recorded.

Inclusion criteria - the study group:

• age from 5 to 18 years old
• functional constipation diagnosed on the basis of Rome IV criteria
• parental or guardian written consent to the participation of the child in the study; in the case of a child ≥16 years old also the child's written consent

Inclusion criteria - the control group:

• age from 5 to 18 years old
• no lower gastrointestinal symptoms
• parental or guardian written consent to the participation of the child in the study; in the case of a child ≥16 years old also the child's written consent

Exclusion criteria - the study group and the control group:

• undergone surgeries due to congenital anomalies in the lower gastrointestinal tract
• inflammatory bowel diseases or inflammation of the large intestine with a different aetiology
• anal fissure, inflammation of the anal area or other conditions which, in the opinion of the researcher, could affect the functioning of the distal gastrointestinal tract

The sample size was calculated to detect a 45% difference in the percentage of patients with a negative rectoanal gradient, assuming equal groups, alpha = 0.05, beta = 0.1, and a 30% drop out percentage. There should be 30 people in the constipation group and 30 healthy volunteers.

Due to possible differences depending on age, it was decided to conduct the study in two separate age groups (I. younger, i.e. 5-10 years old and II. older, 11-18 years old). The total number of patients in the study will therefore be 120 people.