Involuntary action of the external anal sphincter. Manometric and electromyographic studies.

The changes in the rectal neck (anal canal) pressure and electromyography (EMG) of the external anal sphincter and levator ani muscle were studied in 20 dogs before and after internal anal sphincter excision by 2 weeks and monthly up to 10 months. The rectal neck pressure dropped to 41% of the preoperative level. It was then gradually elevated from the beginning of the 1st month following excision till it reached 88% of the level before excision at the 10th month. The external sphincter EMG basal activity after internal sphincterectomy by 2 weeks showed a decrease in the amplitude of the slow and high potential waves. From the 1st to the 7th month after excision, changes in the frequency and amplitude of the waves occurred, and by the 10th month, the external sphincter showed the characteristics of the EMG of both the internal and external anal sphincters. The basal activity of the levator ani muscle increased at the 10th month. As reported previously, the changes in the rectal neck pressure and in the EMG of the external anal sphincter after internal sphincterectomy seem to be due to the histological changes in the muscle and include striated and smooth muscle fiber hypertrophy and proliferation. These changes adapt the external sphincter to serve the function of not only voluntary but also involuntary continence.