Perineal hernia is a hernia involving the perineum. The hernia may contain fluid, fat, any part of the intestine, the rectum, or the bladder. It is known to occur in humans, dogs, and other mammals, and often appears as a sudden swelling to one side (sometimes both sides) of the anus. A common cause of perineal hernia is surgery involving the perineum. Perineal hernia can be caused also by excessive straining to defecate. Other causes include prostate or urinary disease, constipation, anal sac disease (in dogs), and diarrhea. Atrophy of the levator ani muscle and disease of the pudendal nerve may also contribute to a perineal hernia.