Constipation can be characterised by speed of transit of stools through the colon. Three patterns or phenotypes have emerged :
- Normal transit
- Slow transit constipation (STC)
- Rectal evacuation disorder (RED)
The simplest way to measure transit colonic transit is to see how long an ingested marker takes to pass out into stool. Indeed, the earliest transit studies were designed in this way.
In paediatric practice, colonic transit marker studies (CTMS) are done using a multiple-marker single xray to minimise the X-Ray exposure for children. Multiple radio-opaque markers. An X-Ray is taken after a few days. The bowel is divided into sectors using imaginary lines. The pattern of constipation is defined by the number of markers in each sector.
Various protocols have been described for these ‘sectoral’ methods, varying in the number of markers, types of markers and X-Ray schedule.
Another method, used primarily in research, is the scintigraphy transit study. This is a ‘density’ method and is considered the most sensitive method. A meal containing a radioactive isotope is ingested. After a period, the progress of transit through the bowel is checked with a gamma camera. For the purpose oif analysis, the bowel is divided into sectors. A mathematical formula is used to sensitively calculate isotope density in different sectors, leading to characterisation of normal, STC or RED patterns.
What ‘sectoral’ and ‘density’ methods have in common is a means to visualise transit and established methods of intepreting the pattern or marker retention.
Transit index combines ‘sectoral’ and ‘density’ methodology. This method enables :
- visualization of retained markers
- single X-ray
- avoidance of radio-isotope ingestion
- mathematical description of retained markers