Change in stool shape/colour

Much is made in the lay press and Internet about the importance of a change in stool width (caliber), shape, length, etc.  The fact is that these changes are almost never of any consequence.

Colon cancers almost never result in a change of shape of the stool.  They are simply too high up.  The stool will go around the cancer and reform normally below it so that when it is expelled it is not necessarily different from usual.  There is one exception to this – advanced colon cancers that are causing a significant obstruction of the colon can result in diarrhea since only the fluid in the stool can pass through the narrowed colon.  But these patients are uncommon and they usually have other symptoms that point to something serious.

Rectal cancer (cancer in the last foot of the bowel) can lead to stool shape and size changes but only if the cancer is very low down, and usually only when they are quite advanced (large).  Such cases can often be identified by a digital rectal examination by a physician.

So, in general, colon and rectal cancers usually do not effect stool shape substantially.  In particular, long narrow ‘pencil-thin’ stools are almost always benign.  Stories about the seriousness of long narrow stools or stools with a ‘groove’ in them are propagated by well-meaning individuals who have no real experience in this area.  Long thin stools are almost always benign, caused by a length of muscle spasm in the bowel.  They are a common accompaniment of Irritable Bowel Syndrome.  There are some changes in overall bowel habits that can be very important and these are discussed under ‘Change in Bowel Habit’.

© Pezim Clinic, Vancouver, British Columbia, Canada