Disorders of the perineum and anus

Anatomy and physiology


The perineum is made up of the structures which make up the boundary of the pelvic outlet. It extends externally from the dorsal aspect of the scrotum or vulva to the base of the tail; its lateral margins extend to the skin covering the tubers ischii and superficial gluteal muscles. The deep portion is delineated by the ischial arch ventrally, the third coccygeal vertebra dorsally and, in the dog, by the sacrotuberous ligament laterally. Due to the absence of a sacrotuberous ligament in the cat the lateral margins are less well defined. The perineum essentially surrounds the anal and urogenital canals. Within the perineum the most important structures are those which make up the pelvic diaphragm; namely levator ani and coccygeal muscles. These muscles act as a division between the pelvic canal and the wedge shaped ischiorectal fossa, which is bound laterally by the caudal portion of the superficial gluteal muscle, medially by the external anal sphincter levator ani and coccygeus muscles, and ventrally by the internal obturator muscle ().

The muscles of the pelvic diaphragm are crucial in supporting the rectum and act not only as a physical partition but are essential in counteracting the effects of raised intra-abdominal pressure. Failure of these muscles has the potential to allow abdominal viscera to herniate through the pelvic canal, as happens in a perineal hernia ().

Within the ischiorectal fossa lies the internal pudendal artery and vein and the pudendal nerve, together with fat. The neurovascu far structures course caudomedially from the ventrolateral aspect of the coccygeus, over the internal obturator muscle towards the clitoris or root of the penis. Lymphatic drainage of the perineum is via the internal (medial) iliac lymph nodes, while the cutaneous anal area drains via the superficial inguinal lymph nodes.


The anus, more correctly termed the anal canal, is approximately 1 cm in length and comprises three zones:

  • Cutaneous zone
  • Intermediate zone
  • Columnar zone.

The most caudal zone, the cutaneous zone, is divided into internal and external regions. The external region is the keratinized hairless area and contains the circumanal glands (hepatoid or perianal glands). The ‘true’ anal glands together with the anal sacs are to be found in the internal region. The anal sacs () are invaginations of the internal region and lie between the internal and external anal sphincter muscles. The intermediate zone forms a scalloped fold, which is termed the anocutaneous line and is composed of 8 stratified squamous epithelium. The cutaneous zone together with the anal glands are also lined with stratified squamous epithelium. The inner columnar zone, which is lined by mucosa, compfises longitudinal ridges that fold to create a number of pockets or anal s›nuses ().

The internal anal sphincter muscle, unlike the external anal sphincter muscle, is composed of smooth muscle anJ is a continuation and thickening of the rectal muscularis. It is innervated by autonomic fibres from the pelvic plexus. Sympathetic innervation is derived from the hypogastric nerve and parasympathetic innervation from the pelvic nerves. The external anal sphincter is composed of striated muscle, which encompasses the anus and is essential for faecal continence. The caudal rectal branch of the pudendal nerve pfovides motor innervation to the external anal sphincter muscle () and the perineal branch provides sensory input; smaller branches of the pudendal nerve supply the anal canal.

The sympathetic nerve supply to the internal anal sphincter muscle is excitatory whilst the parasympathetic supply is inhibitory. This is in contrast to the rectum where the situation is reversed, i.e. sympathetic stimulation leads to relaxation of the rectum and contraction of the internal anal sphincter.

The perineal reflex is elicited via stimulation of the perianal area, the response being contraction of the anus and flexion of the tail. This assesses the pudendal nerve, which arises from spinal cord segmentS S1 — S3. Loss of the perineal reflex suggests damage to the spinal cord at, or caudal to, the level of lumbosacral vertebra 5 or 6.

‘True’ anal glands

These are modified tubular sweat glands, which lie craniolateral to the circumanal glands and open into the intermediate zone (). The secretion is fatty and their true function is unknown.

Circumanal (hepatoid or perianal) glands Circumanal glands are androgen-sensitive glands found subcutaneously up to 4 cm from the anal canal. The glands have a deep non-secretory, non-sebaceous portion and a superficial sebaceous portion. Under the influence of androgens the circumanal glands continue to grow throughout a male dog’s life.

Anal sacs (paranal sinuses)

Anal sacs are paired structures, which are invaginations of the inner area of the cutaneous zone. The sacs are positioned at 4 and 8 o’clock relative to the anus; the diameter of the fundus is usually 8 — 10 mm in dogs, with the ducts being 1 — 2 mm wide. In the dog, the duct is predominantly lined by sebaceous glands whilst in the fundus apocrine glands are more prevalent. The combined secretion from these glands, together with bacteria and desquamated epithelial cells, creates a malodorous oily/pasty fluid. In cats there are few apocrine glands, with sebaceous glands being found in both duct and fundus.

Diagnostic approach


Diseases of the perineum and anus are likely to present with clinical signs that may be easily confused with colorectal disease, such as haematochezia, tenesmus or dyschezia. It is therefore essential that a good history is taken and that a thorough physical examination is carried out. Haematochezia is often seen with anal turunculosis (perianal fistula) or ulcerated circumanal gland tumours in which blood is added to faeces after it has been passed. Dyschezia and tenesmus are very common in conditions such as anal sacculitis, perineal herniation, anal furunculosis and anal tumours. With some perineal disease there may be constipation or obstipation due to an obstruction, as is seen with a/resia ani, intrapelvic or perineal masses, diffuse anal or perianal tumours, and extensive anal furunculosis. In cases of perineal hernia where there has been herniation and retroflexion of the bladder or where a perineal mass may compress the urethra, dysuria or stranguria may be present.

The age and sex of the patient are also helpful in formulating a differential diagnosis arresia ani will be prevalent in the very young; an older entire male dog with a perineal swelling may have a perineal hernia; and a bitch over 10 years old with a perianal swelling may have an anal sac adenocarcinoma.

Physical examination

Together with the history, a physical examination is perhaps the most important diagnostic step in many cases. It is also important to remember that many perineal and anal diseases are very painful and that a thorough physical examination of the area may have to be carried out with the patient either sedated or anaesthetised. In addition to examining the perineal region a thorough general physical examination is also required.

Palpation and digital rectal examination Palpation of the area is essential and may give some indication as to whether the swelling has been caused by a hernia or a space-occupying mass. External palpation should be combined with a digital rectal examination (); it is extremely helpful in diagnosing both hernias and intrapelvic masses extending into the perineum. If an intrapelvic mass is suspected then caudal abdominal palpation is also useful. With perineal hernias, digital rectal examination will usually reveal that there is sacculation and a lack of support for the rectum (). The anal sacs should be palpated per rectum to assess size and for evidence of any mass, or to express them ().

If an anal or perianal mass is palpated, it may be possible in small dogs to palpate the medial iliac lymph node; the superficial inguinal lymph nodes should also be palpated.

Diagnostic imaging


Radiography can be used to assess the extent of local lesions and importantly to check, when tumours are suspected or diagnosed, for evidence of distant spread. Both lateral and ventrodorsal plain radiographs of the caudal pelvic area should be taken; these may show the extent of a mass lesion, evidence of constipation or obstipation, or a herniated bladder. The clinician should look for evidence of medial ili8c lymph node enlargement () und take let and right lateral project ons of the thorax for evidence of possible lung metastases. Positive and negative contrast studies may be helpful in delineating the urinafy tract (pneumocystography, retrograde urethrography) or to assess the extent of an anal lesion (barium enema).


Where a perineal mass is present ultrasonography is invaluable in determining whether it is solid or fluidfilled (). Both a retroflexed bladder and paraprostatic cysts are not uncommon in the ischiorectal fossa. Ultrasonography oan also be used to evaluate the prostate in cases where a paraprostatic cyst is suspected. Ultrasonography also aids in allowing accurate fine needle aspirates to be carried out on the mass or fluid-filled structures


Proctoscopy is of limited value in assessing perineal and anal disease, though it is useful to evaluate possible concurrent colitis in patients with anal furunculosis.

Clinical disorders of the perineum

Atresia ani

Congenital abnormalities of the anal region are to thought to be rare but the recorded incidence may not be a true incidence as many are euthanized due to the poor prognosis associated with these conditions. Classically atresia ani is classified into four types:

l. Congenital stenosis of the anus

II. Persistent anal membrane with a blind-ending rectal pouch just cranial to the anus

III. Closed anus with the rectum ending more cranially within the pelvic canal ()

IV. Anus and distal rectum are normal; proximal rectum ends as a pouch within the pelvic canal.

These cases usually present in the first few weeks of life with a history of tenesmus and constipation. On examination they usually have an imperforate anus (apart from those patients with Type IV atresia ani) and are thin, in poor condition but are very pot bellied. Surgical correction may be attempted but is often unsuccessful and may require repeated procedures. Complications of surgery include recto-anal stricture formation, faecal incontinence, or constipation due to irreversible megacolon.

Anogenital clefts

With anogenital cletts there is a failure of the embryological cloaca to separate so that there is direct communication between the anus and vagina or urethra (). In males the condition is often associated with hypospadias (incomplete formation of the urethra ventrally); both faecal and urinary continence is usually maintained. In females faecal incontinence is common and more importantly there is often severe faecal contamination of the urinary tract, which may result in pyelonephritis.

Surgical correction () may be attempted in both males and females and can be successful, though wound dehiscence and infection are commonly encountered. In the male as well as reconstruction of the ventral anus, a scrotal urethrostomy may also be required in cases of hypospadias as there is often insufficient tissue present to allow reconstruction.


This is a condition, usually of long-haired animals, where perineal hair and faeces is matted over the anus, resulting in a physical obstruction of the anus. The condition tends to occur in poorly groomed individuals, especially following diarrhoea. Treatment simply involves clipping the hair away and removing any anorectal faecoliths.

Anal sac disease

Anal sac problems account for approximately 120 » of canine clinical presentations, though cats are less commonly presented for these problems. The classical presenting signs for any anal sac problem are:

• Chewing or licking excessively over the tail base or anus

• Reluctance or discomfort on sitting

• ‘Scooting’ — dragging the anus along the ground with the hind limbs extended

• Dyschezia may be noted in extreme cases

• Draining tracks in ruptured abscesses.

It is thought that most disease is related to some degree of obstruction of the anal sac duct, though the precise aetiology is not fully understood. Factors that are believed to initiate and/or perpetuate anal sac problems include:

• Stool consistency

• Diet

• Lack of anal sphincter muscle tone

• Inactivity

• Obesity

• Dogs with generalized seborrhoea

• Recent oestrus

• Anal furunculosis and/or perianal fistulae

• Breed predisposition:

— Small toy breeds have small ducts

German Shepherd Dogs have very deep anal sacs which lie along the rectal wall.

Anal sac impaction

Anal sac impaction is the most common presentation and is straightforward to treat by digital expression of the anal sacs situated at 4 and 8 o’clock. Internal (per rectum) compression is preferable and more effective than external compression. External compression is reserved for only the smallest of dogs or cats where digital rectal examination is not feasible. The anal sac secretion is viscous, often grey in colour and putty-like in consistency.

In some cases digital expression may be required at regufar intervals and in ch ronic cases anal sacculectomy may be considered.

Anal sacculitis

In cases of anal sacculitis the anal sacs are often very painful and either sedation or general anaesthesia may be required in order to express them. Following expression of infected anal sacs, they should be cannulated and flushed. The glands can be flushed with either sterile isotonic solution, such as lactated Ringer’s solution, or a mild antiseptic solution (e.g. 0.05% chlorhexidine). There is some debate as to the effectiveness of instilling an antibiotic, an antibiotic plus a corticosteroid, or a corticosteroid alone into the sacs after flushing. If there is an infection present, a suitably chosen systemic antibiotic is likely to prove more effective because the tissue concentration of the antibiotic can be maintained for longer than with a single application of a topical antibiotic. The effectiveness, or otherwise, of instilling corticosteroids has not be proven.

Anal sac flushing may be required at intervals of 10 — 14 days until the problem resolves. In a few refractory cases anal sacculectomy can be considered (see BSA VA Manual of Canine and Feline Abdominal Surgery).

Anal sac abscessation

As with all abscesses it is essential to incise and drain the infected sacs, it is also beneficial to lavage with an isotonic solution or a dilute 0.05% solution of chlorhexidine (dilute povidone iodine is of little value as it is inactivated by organic matter). The incised sacs should be left open and the animal given a systemic broad spectrum antibiotic, pending the results of culture and sensitivity. The most frequently cultured organisms are Escherichia coli Streptococcus faecalis and Proteus spp.

Recurrent anal sac abscessation is best managed by anal sacculectomy. The risks associated with this procedure include faecal incontinence and recurrent draining sinuses; the latter being associated with incomplete excision of the sac lining.

Anal sac adenocarcinoma

Anal sac (apocrine) gland adenocarcinoma predominantly affects older bitches (over 90% of cases), is highly malignant and tends to metastasize readily to the medial iliac lymph nodes. The presenting clinical signs are those typically seen with anal disease and include dyschezia, tenesmus, flattened (ribbonlike) stools and a perineal swelling. Very occasionally a perianal mass is noted during a routine clinical examination.

Anal sac adenocarcinoma may also present initially with polydipsia and polyuria secondary to paraneoplastic hypercalcaemia related to a parathyroid hormonerelated peptide produced by the mass.

In all cases of a suspected anal sac mass it is important to obtain a full biochemistry profile to assess the concentrations of calcium and phosphate; it is also essential to assess renal function. In cases with hypercalcaemia, high rates of physiological (0.9%) saline should be administered to diurese calcium. Once normocalcaemic, furosemide may be administered (2 mg/kg intravenously) to prevent calcium resorption.

Since metastasis is reportedly seen in over 50% of cases at presentation, it is essential to assess the draining lymph nodes and to check for thoracic metastases. The medial iliac lymph nodes are occasionally palpable per rectum in small dogs or if the node is massive. However, it is better to take caudal abdominal radiographs or to use abdominal ultrasonography.

If there are no detectable metastases and the patient is normocalcaemic, the treatment of choice is surgical excision of the mass. The success in removal will depend on the size of the tumour, which can be between 1 and 10 cm. Potential postoperative sequetae include wound dehiscence, infection, faecal incontinence and local recurrence (25% of cases). There is no reported survival benefit in excising the draining lymph nodes. Recurrence of hypercalcaemia without local tumour recurrence is reported in 35-50% of cases due to the paraneoplastic effects from metastatic tumour deposits. It is therefore important to assess blood calcium levels regularly as well as to check for local regrowth in these patients. Post-surgical survival ranges from 2 to 39 months, with the average survival time being around 8 months.

Anal furunculosis and/or perianal fistulae

This is an extremely frustrating disease encountered commonly in German Shepherd Dogs and Border Collies, and is characterized by chronic infection and ulceration of the tissues around the anus (). There are often deeply infiltrating sinus tracts which can form true fistulae with the rectum. The underlying cause of this disease remains elusive. There is some evidence that there is an association with inflammatory bowel disease in both humans and dogs.

The range of clinical signs seen with anal furunculosis vary immensely, from those patients which show few signs apart from licking the perianal region, to advanced cases where damage and scarring of the anal sphincter and rectum prevents normal dilatation of the anus. These dogs can show severe faecal tenesmus, dyschezia and pain. Conversely, some dogs with severe anal furunculosis lose the ability to close the anal sphincter and become faecally incontinent.

Successful long-term management is still elusive but recent use of immunosuppressive agents holds out the best hope. A number of agents have been tried including prednisolone, azathioprine and ciclosporin. Of these drugs ciclosporin is the most effective, but a major disadvantage of this drug is cost; concurrent administration of ketoconazole helps to reduce the overall dose required. Ketoconazole and ciclosporin compete for the cytochrome P-450 enzyme system and this helps to maintain higher circulating levels of ciclosporin. Though ciclosporin is very effective, usually a 6-8 week course is required to reduce and eliminate the lesions; recurrence once the drug has been stopped is still a problem. A long-term low dose maintenance level of ciclosporin has yet to be established. In cases where cost is an issue then immunosuppressive doses of prednisolone can be tried, initially at 2 mg/kg orally for two weeks then reducing the dose over 4 — 6 weeks, although historically success has been rare.

Where there is recurrence post-immunosuppresion or where the lesions are reduced but not resolved with drug therapy, then surgical excision should be considered ().

Anal strictures

Strictures occur secondary to trauma, anal furunculosis, neoplasia or surgery. The clinical signs are faecal tenesmus, dyschezia and haematochezia with flattened stools. Confirmatory diagnosis is by digital rectal examination but many patients require anaesthesia for this to be carried out due to the painful nature of the condition.

Treatment can involve bougienage to dilate the stricture, though this may need to be carried out at regular intervals. In severe cases surgical excision can be attempted, though strictures may recur at the surgical site. In extreme cases, anal resection and a rectal pull-through procedure can be considered. Though a radical procedure this can greatly improve the patient’s quality of life and the degree of faecal incontinence is usually mild.

Anal prolapse

Anal prolapse presents with oedematous, red anal mucosa pfotruding through the anal orifice at the end of defecation (). The cause is usually associated with faecal tenesmus and it is essential to determine and treat the underlying cause of the tenesmus as well as managing the prolapse.

I n most cases the prolapse can be reduced digitally, following lubrication. For recurrent cases a temporary purse string suture can be placed in the anus for 48 hours; some authors suggest that the purse string is tied loosely to allow faecal passage but prevent mucosal prolapse. I n severe cases it may be necessary to carry out mucosal resection if the tissue is devitalized.


Perianal and perineal trauma may be the result of projectile injuries, bite wounds or severe road traffic accidents. The management of such wounds may be problematic as either faecal incontinence or stricture formation may result (].

Circumanal gland adenomas

The circumanal gland, hepatoid gland or perianal adenoma is the commonest anal tumour of the dog, with some 85% being reported in the older intact mate; it is rarely reported in cats. Circumanal gland adenocarcinomas are very rare. Anatomically the adenoma are found in the external region of the outer cutaneous zone, they may be single or multiple on presentation. They may be seen and palpated as discrete swellings often bluish in colour, but can also be large and ulcerated (). Many owners do not realize there is a problem until there is either fresh blood seen on the stools or there is frank haemorrhage from the ulcerated tumour.

As these tumours are known to respond to androgenic stimulation, castration of the entire male is the treatment of choice. After castration the tumour, even if ulcerated, will regress. If the tumour is large then surgical excision, in addition to castration, should be carried out.

Perineal hernias

Perineal hernias or ruptures are mostly seen in the entire male dog, though they are reported in bitches and in the cat. The aetiology remains unclear but is associated with the degenerative changes in the muscles of the pelvic diaphragm. Many factors () have been implicated in the aetiopathogenesis but they are difficult to substantiate.

The hernia occurs between the levator ani and coccygeus muscles and the external anal sphincter muscle. With the loss of lateral support there is progressive rectal enlargement. Unilaterally this is termed sacculation whilst the bilateral disease is termed dilatation. True rectal diverticula with rectal mucosa protruding through the rectal musculature are extremely rare. Hernias may be complicated by inclusion of pelvic and peritoneal fat, loops of small intestine and, in severe cases, by retroflexion of the bladder with or without the prostate gland.

The commonest clinical sign is perineal swelling either bilaterally () or unilaterally, reportedly more common on the right. Defecatory tenesmus is seen in 75 — 80% of cases. Dysuria and stranguria are seen with bladder herniation (seen in up to 20% of cases) indicating urethral obstruction. Diagnosis can be confirmed by rectal examination where sacculation and lack of lateral rectal wall support is noted (). Radiography or ultrasonography are of use to confirm a retroflexed bladder.

To avoid future risk of bladder involvement, all perineal hernias should be managed surgically by reconstruction of the pelvic diaphragm. In those cases with bladder retroflexion, Renal and electrolyte status should be assessed and c frected by fluid therapy, if required, before undertaking surgery. If urinary catheterization is not possible, relief can be gained by perineal cystocentesis. Where there is preoperative bladder retroflexion, replacement into the abdomen and herniorrhaphy is usually adequate and should preclude the need for cystopexy or vas deferensopexy. Castration is carried out routinely in the management of perineal hernia as there is evidence of reduced incidence of recurrence. Potential complications seen following repair include wound dehiscence, rectal eversion and prolapse, sciatic paralysis (very rare) and recurrence.

Perineal swelling (intrapelvic masses, paraprostatic cysts)

Perineal swellings are present as discrete unilateral bulges in a patient with a history of tenesmus and passing ribbon-like faeces. On examination the perineal swelling is firm and non-reducible and is palpable per rectum. It is important to rule out the possibility of a retroflexed bladder associated with a perineal hernia at an early stage (see above).

Ultrasonography is invaluable in determining whether the mass is soft tissue or fluid-filled. The latter may occur either with› a bladder or if a paraprostatic cyst has herniated through the pelvic diaphragm. A fluidfilled cyst can also be detected on ultrasound examination adjacent to the prostate in the caudal abdomen or pelvic inlet.

Intrapelvic masses may or may not extend into the abdominal cavity and caudal abdominal and pelvic radiographs are valuable in determining the extent of a lesion (). Intrapelvic masses tend to be of connective tissue origin and range from benign leiomyomas to fibrosarcomas; in view of this a minimum database for an intrapelvic mass should include incisional biopsy, abdominal radiography and ultrasonography to assess draining lymph nodes and thoracic radiography.

Paraprostatic cysts are managed by surgical exploration (), drainage and repair of the pelvic diaphragm; castration should also be carried out and in some cases it is necessary to omentalize the intraabdominal portion of the cyst.

Benign intrapelvic masses may be excised from either a perineal approach, or a combined caudal abdominal and pelvic osteotomy approach. If a sarcoma is diagnosed radical surgery, such as hemipelvectomy, can be considered in order to achieve surgical margins.