Mechanism of action and toxicities of purgatives used for colonoscopy preparation

Abstract

Importance of the field: In developed countries, colonoscopy volume has increased dramatically over the past 15 years and is the principle method used to screen for colon cancer. Preparations used for colon cleaning have evolved over the past 30 years. Some preparations have been shown to be unsafe and are now used on a limited basis. There has been progress on limiting the volume required and on taste improvement.

Areas covered in this review: This review provides an account of preparations used from 1980 when PEG-based preparations became widely available, until the present day. The review highlights their mechanism of action and principle toxicities. The handling of solutes and solute-free fluid by the colon is also reviewed.

What the reader will gain: The reader will gain a perspective on the factors considered in developing colonic purgatives and the rationale for choosing selected preparations based on patient factors such as age, co-morbidities and concomitant medications.

Take home message: Although generally safe and effective, colonic purgatives have both acute and permanent toxicities. The safest preparations utilize PEG combined with a balanced electrolyte solution. Limitations of this preparation center on the volume required and poor taste. Alternative formulations are now available; however, those using sodium phosphate have fallen out of favor due to a risk of renal toxicity.

Figures

Figure 1

Figure 1a. Photograph of the transverse colon demonstrating an excellent preparation. Figure 1b. Photograph of the colon in another patient who only drank approximately 2 liters of the recommended 4 liters of polyethylene glycol preparation. Note the film of stool precluding optimal mucosal evaluation. Figures courtesy of Dr. Friedenberg.

Figure 1

Figure 1a. Photograph of the transverse colon demonstrating an excellent preparation. Figure 1b. Photograph of the colon in another patient who only drank approximately 2 liters of the recommended 4 liters of polyethylene glycol preparation. Note the film of stool precluding optimal mucosal evaluation. Figures courtesy of Dr. Friedenberg.

Figure 2

Models for electrolyte transport in proximal and distal colonic epithelium and expression of different ion transporters along the crypt axis. Electroneutral NaCl absorption [parallel Na1/H1 and Cl2/HCO3 2 exchange] dominates in the surface epithelium and is also present in the crypts. Electrogenic Na1 absorption via the epithelial Na1 channel [ENaC] takes place in the surface epithelium and upper crypts of the distal colon. The cystic fibrosis transmembrane conductance regulator [CFTR] is expressed throughout the colonic epithelium and dominates in the crypts. Kunzelmann and Mall, Physiol Rev, 2002, Am Physiol Soc, used with permission.

Figure 3

Timeline of colonoscopy preparations.

Figure 4

Renal biopsy specimen demonstrating precipitates of calcium phosphate [arrow] in a patient who received sodium phosphate for colon preparation and developed progressive deterioration of renal function. H+E stain at magnification 40X. Image courtesy of Dr. Nathan Hellman.