Download The rectum

The rectum
DR. MOHAMMED ABDZAID AGOOL
FIBMS, MRCS, FACS
Surgical anatomy

The rectum begins where the taenia coli of the sigmoid colon join to
form a continuous outer longitudinal muscle layer at the level of the
sacral promontory.

The rectum follows the curve of the sacrum, to end at the anorectal
junction.

The puborectalis muscle encircles the posterior and lateral aspects
of the junction, creating the anorectal angle (normally 120°).

The rectum has three lateral curvatures: the upper and lower are
convex to the right, and the middle is convex to the left. On the
luminal aspect, these three curves are marked by semicircular folds
(Houston’s valves).
That part of the rectum that lies below the middle valve has a much
wider diameter than the upper third and is known as the ampulla of the
rectum.

The adult rectum is approximately 12–18 cm in length and is
conveniently divided into three equal parts:
the upper third, which is mobile and has a peritoneal coat.
the middle third where the peritoneum covers only the anterior and part
of the lateral surfaces.
the lowest third, which lies deep in the pelvis surrounded by fatty
mesorectum and has important relations to fascial layers.

The lower third of the rectum is separated by a fascial condensation–
Denonvilliers’ fascia – from the prostate/vagina in front, and behind
by another fascial layer – Waldeyer’s fascia – from the coccyx and
lower two sacral vertebrae.

Blood supply

The superior rectal artery is the direct continuation of the inferior
mesenteric artery and is the main arterial supply of the rectum.

The arteries and their accompanying lymphatics lie within the loose
fatty tissue of the mesorectum, surrounded by a sheath of connective
tissue (the mesorectal fascia).

The middle rectal artery arises on each side from the internal iliac artery
and passes to the rectum in the lateral ligaments. It is usually small and
breaks up into several terminal branches.

The inferior rectal artery arises on each side from the internal pudendal
artery as it enters Alcock’s canal. It hugs the inferior surface of the
levator ani muscle as it crosses the roof of the ischiorectal fossa to enter
the anal muscles

Venous drainage

The superior haemorrhoidal veins draining the upper half of the anal canal
above the dentate line pass upwards to become the rectal veins: these
unite to form the superior rectal vein, which later becomes the inferior
mesenteric vein. This forms part of the portal venous system and ultimately
drains into the splenic vein.

Middle rectal veins exist but are small, unimportant channels unless the
normal paths are blocked.

Lymphatic drainage

The lymphatics of the mucosal lining of the rectum communicate freely with
those of the muscular layers. The usual drainage flow is upwards, and only
to a limited extent laterally and downwards.

For this reason, surgical ablation of malignant disease concentrates mainly
on achieving wide clearance of proximal lymph nodes. However, if the
usual upwards routes are blocked, flow can reverse, and it is then possible
to find metastatic lymph nodes on the side walls of the pelvis (along the
middle rectal vessels) or even in the inguinal region (along the inferior
rectal artery).

Anorectal Nerve Supply.
Both sympathetic and parasympathetic nerves innervate the
anorectum. Sympathetic nerve fibers are derived from L1-L3 and
join the preaortic plexus.

The preaortic nerve fibers then extend below the aorta to form
the hypogastric plexus, which subsequently joins the
parasympathetic fibers to form the pelvic plexus.
Parasympathetic nerve fibers are known as the nervi erigentes
and originate from S2-S4.

These fibers join the sympathetic fibers to form the pelvic plexus.
Sympathetic and parasympathetic fibers then supply the
anorectum and adjacent urogenital organs.

The internal anal sphincter is innervated by sympathetic and
parasympathetic nerve fibers; both types of fibers inhibit sphincter
contraction. The external anal sphincter and puborectalis muscles
are innervated by the inferior rectal branch of the internal
pudendal nerve.

The levator ani receives innervation from both the internal
pudendal nerve and direct branches of S3 to S5. Sensory
innervation to the anal canal is provided by the inferior rectal
branch of the pudendal nerve. While the rectum is relatively
insensate, the anal canal below the dentate line is sensate.
Defecation.
physiology

Defecation is a complex, coordinated mechanism involving colonic
mass movement, increased intra-abdominal and rectal pressure, and
relaxation of the pelvic floor. Distention of the rectum causes a reflex
relaxation of the internal anal sphincter (the rectoanal inhibitory
reflex) that allows the contents to make contact with the anal canal.

This “sampling reflex” allows the sensory epithelium to distinguish solid
stool from liquid stool and gas.

If defecation does not occur, the rectum relaxes and the urge to
defecate passes (accommodation response).

Defecation proceeds by coordination of increasing intra-abdominal
pressure via the Valsalva maneuver, increased rectal contraction,
relaxation of the puborectalis muscle, and opening of the anal canal.

Continence.

The maintenance of fecal continence is at least as complex as the
mechanism of defecation. Continence requires adequate rectal wall
compliance to accommodate the fecal bolus, appropriate neurogenic
control of the pelvic floor and sphincter mechanism, and functional
internal and external sphincter muscles.

At rest, the puborectalis muscle creates a “sling” around the distal
rectum, forming a relatively acute angle that distributes intraabdominal forces onto the pelvic floor.

With defecation, this angle straightens, allowing downward force to be
applied along the axis of the rectum and anal canal.

The internal and external sphincters are tonically active at rest. The
internal sphincter is responsible for most of the resting, involuntary
sphincter tone (resting pressure). The external sphincter is responsible
for most of the voluntary sphincter tone(squeeze pressure).
CLINICAL FEATURES OF RECTAL DISEASE

Symptoms

Rectal diseases are common and serious and can occur at any age. The
symptoms of many of them overlap. In general, the inflammations affect
younger age groups, while the tumours occur in the middleaged and elderly.
But no age is exempt from any of the diseases, however young: ulcerative
colitis has been reported in the newborn and rectal cancer can occur in
young people.
Bleeding

This is often bright red in colour but may be darker, and should be carefully
investigated at any age.
Altered bowel habit

Early-morning stool frequency (‘spurious diarrhoea’) is a symptom of rectal
carcinoma, while blood-stained frequent loose stools characterise the
inflammatory diseases.
Discharge

Mucus and pus are associated with rectal pathology
Tenesmus

Often described by the patient as ‘I feel I want to go but nothing happens’, this
is normally an ominous symptom of rectal cancer but can occur with any rectal
pathology.
Prolapse

This usually indicates either mucosal or full-thickness rectal wall descent.
Loss of weight

This usually indicates serious or advanced disease, e.g. hepatic metastases.

Signs
Because the rectum is accessible via the anus, these can be
elicited by systematic examination. The patient is either positioned
in the left lateral (Sims’) position or examined in the knee–elbow
position.
Inspection
Visual examination of the anus precedes rectal examination to
exclude the presence of anal disease, e.g. fissure or fistula.
Digital examination
The index finger used with gentleness and precision remains
avaluable test for rectal disease .
Tumours in the lower and middle thirds of the rectum can be felt and
assessed; by asking the patient to strain, even some tumours in the
upper third can be ‘tipped’ with the finger. After it is removed, the
finger should be examined for tell-tale traces of mucus, pus or
blood.

Proctoscopy
This procedure can be used to inspect the anus, anorectal junction and
lower rectum (up to 10 cm) . Biopsy can be performed of any suspicious
areas.

Sigmoidoscopy
The sigmoidoscope was in the past a rigid stainless steel instrument of
variable diameter and normally 25 cm in length, The rectum must be
empty for proper inspection with a sigmoidoscope. Gentleness and skill
are required for its use, and perforations can occur if care is not
exercised.

Flexible sigmoidoscope
The ‘flexiscope’ can be used to supplement or replace rigid
sigmoidoscopy. It requires special skill and experience, and the lower
bowel should be cleaned out with preliminary enemas. In addition to the
rectum, the whole sigmoid colon is within visual reach of this instrument.