Anal sinuses

Anal sinuses are located at the base of the longitudinal folds called anal columns, forming small pockets or depressions between adjacent columns. They are typically found within the upper part of the anal canal, just distal to the anorectal junction and above the anocutaneous (pectinate) line.

Each sinus opens to the lumen of the anal canal and in some species, especially dogs, receives the ducts of anal glands (paranal sinuses). These glands produce secretions that may play a role in lubrication and scent marking.

Structure and histology

• Epithelium The sinuses are lined by simple columnar epithelium, similar to that of the anal columns. Goblet cells are common and contribute mucus.

• Glandular openings In species like the dog and cat, ducts from anal sacs open into these sinuses. These ducts pass through the muscular layers and empty into the sinuses at the mucosal surface.

• Vascular supply The region around the sinuses may contain a rich vascular plexus. In some animals, this vascularization helps form cushions that assist with anal canal closure.

Species-specific considerations

Dog and cat The anal sinuses are well developed and clinically significant. In dogs, the paranal sinuses (anal sacs) are paired structures located on either side of the anal canal. Each opens via a duct into an anal sinus at approximately the 4 and 8 o'clock positions. Problems such as impaction, infection, or abscessation of these sacs are common and may require medical or surgical treatment.

Horse Anal sinuses are poorly developed or absent. The transition between rectum and anal canal is relatively smooth, and there is minimal mucosal folding or glandular involvement.

Cattle, sheep, and goats Anal sinuses are generally absent. There is a direct transition from rectal mucosa to squamous epithelium without significant folds or gland openings in the anal canal.

Pig Anal sinuses may be present but are not prominent. The glandular structures are less developed than in carnivores.

Functional significance

• Lubrication: Mucus secreted from goblet cells and glands opening into the sinuses helps lubricate the anal canal during defecation.

• Scent marking: In species like dogs, secretions from anal sacs released through the sinuses carry unique odors used for social and territorial communication.

• Anatomical landmarks: The sinuses serve as landmarks for the ducts of anal sacs and are important in veterinary surgical procedures.

Clinical relevance

• Anal sac impaction: Ducts opening into the sinuses may become blocked, leading to distension of the sacs.

• Infection and abscess formation: Bacterial colonization of blocked ducts can lead to painful infections or rupture.

• Surgical access: Understanding the location of the sinuses is important in anal sacculectomy procedures.

• Perianal fistulae: Inflammatory tracts in chronic perianal disease may involve or pass through the anal sinuses.