Anal sac disease is the most common disease entity of the anal region in dogs. Small breeds are predisposed; large or giant breeds are rarely affected. In cats, the most common form of anal sac disease is impaction.
Etiology and Pathogenesis
Anal sacs may become impacted, infected, abscessed, or neoplastic. Failure of the sacs to express during defecation, poor muscle tone in obese dogs, and generalized seborrhea (which produces glandular hypersecretion) lead to retention of sac contents. Such retention may predispose to bacterial overgrowth, infection, and inflammation.
Clinical Findings and Lesions
Signs are related to pain and discomfort associated with sitting. Scooting, licking, biting at the anal area, and painful defecation with tenesmus may be noted. Induration, abscesses, and fistulous tracts are common. In impaction, hard masses are palpable in the area of the sacs; the sacs are packed with a thick, pasty, brown secretion, which can be expressed as a thin ribbon only with a large amount of pressure. When the sacs are infected or abscessed, severe pain and often discoloration of the area are present. Fistulous tracts lead from abscessed sacs and rupture through the skin; these must be differentiated from perianal fistulas. Anal sac neoplasms are usually nonpainful and are associated with perineal edema, erythema, induration, or fistula formation. Apocrine gland adenocarcinomas of the anal sac are typically seen in older female dogs. These dogs may be presented for signs secondary to hypercalcemia, such as polyuria and polydipsia, or for problems related to the perineal mass.
Diagnosis of impaction, infection, or abscessation is confirmed by digital rectal examination, at which time the sacs can be expressed. Microscopic examination of the contents from infected sacs reveals large numbers of polymorphonuclear leukocytes and bacteria. A tumor should be suspected (anal sac apocrine adenocarcinoma) in anal sacs that are firm, enlarged, and nonexpressible even with irrigation. Ultrasonographic examination may be useful to determine whether a firm, nonexpressible anal sac is due to infection/abscessation or neoplastic disease. In the case of a suspected tumor, the diagnosis should be confirmed by biopsy. Regional and systemic metastasis should be evaluated, and serum calcium measured.
Impacted anal sacs should be gently, manually expressed. A softening or ceruminolytic agent or saline can be infused into the sac if the contents are too dry to express effectively. Infected sacs should be cleaned with antiseptic, followed by local and systemic antibiotic therapy. Hot compresses, applied every 8–12 hr for 15–20 min each, are beneficial for abscesses. Repeated weekly flushings combined with infusion of a steroid-antibiotic ointment may be needed. Adding supplemental fiber to the diet may increase fecal bulk, facilitating anal sac compression and emptying. If medical treatment is ineffective, or if neoplasia is present, surgical excision of the sac is indicated. The closed technique for excision is preferred and has the lowest complication rate. However, fecal incontinence, a common complication of anal sac surgery, may result from damage to the caudal rectal branch of the pudendal nerve and may be complete if damage is bilateral. Chronic fistula formation may be seen when sac removal is incomplete or when the sac ruptures. Scar formation in the external anal sphincter may result from surgical trauma and result in tenesmus. (Also see Apocrine Gland Tumors of Anal Sac Origin.)