Muscle thickness and neuron density in the caecum of horses with chronic recurrent caecal impaction

In this study, the hypothesis that caecal smooth muscle layers would be thinner and the linear neuron density of myenteric plexus greater was tested in normal horses compared to those with chronic recurrent caecal impaction. Four normal horses and 18 horses with chronic recurrent caecal impaction were subjected to euthanasia and 7 tissue samples were collected from each horse at different regions of the caecum (apex, dorsal body, cranial base, dorsal base, caudal base, caudal body, ventral body). Twelve horses with chronic recurrent caecal impaction were treated surgically. Only one tissue sample of the cranial part of the caecal base close to the caecocolic orifice was taken during surgery. The thickness of the circular muscle layer of all caecal regions measured in killed horses with chronic recurrent caecal impaction was significantly increased compared to the equivalent caecal region of normal horses. On the other hand, the longitudinal muscle layer was significantly thicker only in the cranial and caudal caecal base and in the dorsal region of the caecal body. The linear neuron densities of all caecal base areas and 2 caecal body regions, the caudal body region and of the apex, of killed horses with chronic recurrent caecal impaction were significantly lower compared with those in clinically normal horses. The circular muscle layer of all caecal regions was thickened (hypertrophied) probably as a consequence of chronic uncoordinated hypercontractility due to neuron deficit in the myenteric plexus of the caecal base.