Rectum, hernias, atresias etc Flashcards
What is the rectum?
Most distal component of the large bowel
In the pelvic cavity
What is the anus?
A distensible short section terminating in a valved opening
What is the function of the rectum?
Faecal storage
What is the function of the anus?
Faecal continence
What is the function of the lower bowel?
- Absorbs and stores
- Absorbs water, Na, Cl and VFAs
- Produces faecalith which moves to rectum
What are the landmarks of the rectum?
- Cranially the pelvic inlet
- Caudally the anal canal
Describe the gross anatomy of the rectum compared to the descending colon
There is no significant difference between the two
Describe the histology of the rectum
- Largely similar to rest of GI
- Mucosa, submucosa, Muscularis and serosa
- No villous processes as no absorption is taking place
- Solitary lymph nodules
- Small cratered nodules present
Describe the mucosa of the rectum
- No villi
- Columnar epithelium
- Longer, taller intestinal galnds
- More goblet cells
Describe the submucosa of the rectum
- Lymph nodules
- Nerve plexuses
- Vascular supply
Describe the muscularis of the rectum
- Thicker outer layer (stratum longitudinale)
- Fibres organised dorsocaudally to form rectococcygeus muscle
- Thinner inner layer (stratum circulare)
- Fibres organised caudally to form internal anal sphincter muscle
Describe the serosa of the rectum
- Visceral peritoneum covers most of crnail rectum
- Airtight, watertight seal, prevents bacterial infection
- Caudal rectum (and anal canal) therefore retroperitoneal
- Cranial peritoneum extends from colon, caudal gradully lost and hence retroperitoneal
Descrieb the mesenteric support of the rectum
- Mesorectum
- Extension of mesocolon
- Wider cranially
- Tapers away at coccygeal vertebrae 2 with serosal layer
Describe the structure of the anus
- Fianl section of lower bowel
- Specialised junction between mucosa and integument (mucocutaneous junction)
- Surrounded by smooth and striated muscle sphincters
- Internal and external anal sphincter (muscle rings) present
- Anal sac sits between these sphincters
- Different histological areas
What are the histological regions of the anus?
- Proximal columnar zone (first)
- Short intermediate zone (middle)
- Terminal cutaneous zone (last)
Describe the proximal columnar zone of the anus
- Series of longitudinal ridges (columns)
- Folds which create anal sinuses (pockets)
- Proximal section is anorecta junction
Describe the intermediate zone of the anus
- Narrow mid-section
- 1mm wide
- Ano-cutaneous margin
Describe the cutaneous zone of the anus
- Exernal and internal components
- Anal sac ducts open in this region
Describe the internal anal sphincter
- Smooth muscle
- Autonomic
- Parasympathetic (post-ganglionic) fibres via pelvic and hypogastric plexuses
- Sympathetic is hypogastric via caudal mesenteric ganglion
Descriebt the external anal sphincter
- Striated muscle
- Wider
- Main constrictor muscle of anus
- Laterla: intimate fascial attachment to levator ani mmuscle
- Dorsal attaches to fascia of tail
- In female, ventral part blends with contrictor vulvae muscle
- In male ventral blends with bulbospongiosus muscle
Describe the vascular supply to the rectum
- Extensive
- Caudal, middle and cranial rectal arteries
- Cranial from caudal mesenteric, supplies cranial aspect of rectum
- Caudal from branches of internal pudendal artery
- Anastomoses between middle and caudal rectal
Describe the venous drainage of the rectum
- Cranial rectal artery into caudal mesenteric then portal vein
- Mid and caudal rectal arteries into internal pudendal and then internal iliac vein
Describe the innervation of the rectum
- Sympathetic and parasympathetic
- Sympathetic from many ganglia
- Parasympathetic via pelvic nerves
Describe the blood supply to the anus
- Via anastomoses from the rectum
- Anal extensions of rectal arteries
- Most from caudal rectal, some from middle and less from cranial rectal arteries
Descrbe the inneration of the anus
- Internal sphincter autonomic
- External innervated by anal branch of pudendal nerve
What are the reflexes involved in the control of defaecation?
- Series of other reflexes
- Rectal filling (accomodation)
- Anorectal reflex
- Rectosphincteric reflex
Describe the anorectal reflex
- Rectal fioling
- Rectal stimulation during accomodation increases anal closing pressure
- More stimulation, increase in closure pressure of IAS
- IAS stimulated by SNS innervation via hypogastric nerves, tonically contracted most of the time
- EAS contributes some additional tone
- Increased distension leads to increased stimulation and therefore increased contraction of anal ring
- Movement of faeces from rectum through anus limited to defaecatory episodes i.e. faecal continence
Describe the rectosphincteric reflex
- Distension sufficiently large, leads to afferent stimulation to sacral cord and CNS
- Distension of bowe = conscious perception of needing to defecate
- Parasympathetic system take unconscious control and causes IAS to relax, allowing defecation
What are the events in defaecation?
- Neural initiation
- IAS relaxes
- Rectal peristalsis occurs
- Increased abdominal pressure
- Contraction of pelvic diaphragm muscles
- Once faecolith passed, returns to resting state
What muscles make up the pelvic diaphragm?
Coccygeus, levator ani and anal sphincter muscles
What is the function of the pelvic diaphragm?
- Supports rectum laterally
- Used in defaecation
What is the rectal function during defaecation dependent on?
The capacity of the pelvic diaphragm muscles to compress the pelvic contents
Describe the levator ani (origin, insertion, innervation)
- Origin: medial ilium, pelvic symphysis
- Insertion: tendon to 7th coccygeal vertebrae
- Innervation: ventral branches of S3 and Co1 nerve
What is the function of the levator ani?
- Medial compression bilaterally of rectum during defaecation
- Presses tail against anorectal region = increases pressure on rectum, expel faeces
Describe the coccygeus muscle (origin, insertion, innervation)
- Origin: tendon on ischiatic spine, cranial to internal obturator muscle
- 2nd-5th CO vertebrae
- Innervation: ventral branches of S3
What is the function of the coccygeus muscle?
Compresses during defaecation, presses tail ventral
Describe the relationship between the anal sphincter, levator ani and coccygeus muscle in the pelvic diaphragm
- Fibrous unrion betweel EAS, levator ani, coccygeus
- No gap
- Pelvic diaphragm is intact structure
Describe the retrococcygeus muscle
- Pararectal muscle
- Origin; dorsolateral surface of rectum
- Insertion: fused below 5th-6th Co vertebrae
- Runs up to tail base
- Stabilises anal canal during defaecation
What are the perianal structures?
- Anal sacs
- Circumanal glands
- Anal glands
Describe the anal sacs
- Not glands, glands within walls of sacs
- Paired, pea sized
- Either side of anal opening
- 20-to-4 position
- Embedded between IAS and EAS muscles
- Short duts open to anal area
- Coiled, apocrine tubules
- Cornified, stratified epithelium
- Secrete foul smelling fluid
Describe the circumanal glands
- Around anus in subcut layer
- Sebaceous
- Not in cats
- Often referred to as hepatoid glands
Describe the anal glands
- Cranial to circumanal glands
- Secrete fatty substance
Where do the lymphatics of the anus drain to?
Drain to sacral hypogastric and internal iliac nodes
List some of he clinical conditions commonly affecting the rectum and anus
- Perineal hernias
- Anal sac impaction/abscess
- Anal furunculosis
- Tumours
Describe perineal herniation
- degeneration of pelvic diaphragm
- Separation of anal sphincter and levator muscle
- Less commonly coccygeus and levator
- Rectal enlargement seen
- Faecal accumulation beyond pelvic brim, sacculation, deviation of rectum into hernial sac, unable to defecate as pelvic diaphragm not in tact
What is unilateral perineal swelling in a perineal hernia caused by?
Sacculation
What is bilateral perineal swelling in a perineal hernia caused by?
Dilatation
Describe some of the pathophysiology of perineal herniation
- Failure of pelvic diaphragm allows movement of abdominal contents into perineal region
- Fat, bladder, prostate gland
- Bladder can retroflex caudally due to straining to defecate pushing bladder caudally
- Prevents urination due to kink in urethra
Describe anal sac impaction/abscessation
- Inflammatin of anal sac ducts
- Impaction of anal sac secretion due to failure to empty, fluid thickens
- Secondary infection
- Abscess ruptures to skin surface
Describe anal furunculoses
- Immune mediated fistula
- Breakdown at mucocutaneous junction
- Immune suppressive therapy needed
Desribe anal tumours
- Benign tumours (adenomas) common in older male entire dogs
- growth hormonally mediated
- Castrate, remove hormonal drive, tumour regresses
What is an anal atresia?
Where the anal opening has failed to develop properly
What are the types of anal atresia?
- 4 types
- I, II, III, IV
Describe a type I anal atresia
Congenital stenosis of anus
Describe a type II anal atresia
Persistent anal membrane with blin ending rectal pouch just cranial to anus
Describe a type III anal atresia
Closed anus with rectum ending in pelvic canal
Describe a type IV anal atresia
Anus and distal rectum normal, proximal rectum ends as pouch in pelvic canal
What are the clinical signs of an anal atresia
- Tenesmus and constipation
- Thin
- Pot bellied
What is tenesmus?
Continual or recurrent inclination to evacuate the bowels, caused by disorder of rectum or other illness
What is a faecalith?
- Akafaecaloma, coprolith
- Stone made of faeces
- Hardening of faeces into lumps of varying size inside the colon which may appear whenever chronic obstruction of transit occurs e.g. megacolon and chronic constipation
How could you repair an anal atresia?
- Depending on type can treat surgically
- Can treat type I and type II
- Make hole in membrane, will not damage the anal sphincters
Describe umbilical hernias
- Intestines protruding through intestinal wall
- Can be classified as indirect or direct
- Can occur in most species
- Large ernias require surgery to repair deficit
Describe scrotal/inguinal hernias
- Intestines in inguinal canal
- Everything may continue to function normally so animal may not show signs of discomfort
- More common in males than females
Describe a bilateral perineal hernia
- Pelvic diaphragm degenerated
- Bladder may be retroflexed by straining to defecate
- Large swelling around anus, stranguria occurs
