Dealing with AF, thrombosed hemorrhoid, and now tight sphincter after BM this morning. . . I can tell muscle spasms are next.

What can I do to prevent this? Add heat? Expand manually?

Hot sitz baths straight after BM then nitro cream if you can get it. You can ask your doc to prescribe anti spasmodically but they never worked for me. Once the spasm starts there's not overly much you can do. Heat is the best thing