1.Anal canal disorders under surgery.pptx

Anal canal disorders

1. The anal canal commences at the level where the rectum passes through the pelvic diaphragm and ends at the anal verge. The muscular junction between the rectum and anal canal can be felt with the finger as a thickened ridge – the anorectal ‘bundle’ or ‘ring ’.

2. The anorectal ring marks the junction between the rectum and the anal canal. It is formed by the joining of the puborectalis muscle , the deep external sphincter, conjoined longitudinal muscle and the highest part of the internal sphincter. .

3. The Puborectalis, part of the funnel-shaped muscular pelvic diaphragm, maintains the angle between the anal canal and rectum and hence is an important component in the continence mechanism .

4. The muscle derives its nerve supply from the sacral somatic nerves, and is functionally indistinct from the external anal sphincter.

5. The position and length of the anal canal, as well as the angle of the anorectal junction, depend to a major extent on the integrity and strength of the puborectalis muscle sling. It gives off fibres that contribute to the longitudinal muscle layer.

6. The external sphincter forms the bulk of the anal sphincter complex and, although traditionally it has been subdivided into deep, superficial and subcutaneous portions, it is a single muscle, which is variably divided by lateral extensions from the longitudinal muscle layer.

7. Some of its fibres are attached posteriorly to the coccyx, whereas anteriorly they fuse with the perineal muscles. Being a somatic voluntary muscle, the external sphincter is red in colour and is innervated by the pudendal nerve.

8. The intersphincteric plane between the external sphincter muscle laterally and the longitudinal muscle medially exists a potential space, the intersphincteric plane. This plane is important as it contains intersphincteric anal glands

9. This plane is important as it contains intersphincteric anal glands is a route for the spread of pus, which occurs along the extensions from the longitudinal muscle layer. The plane can be opened up surgically to provide access for operations on the sphincter muscles.

10. The longitudinal muscle is a direct continuation of the smooth muscle of the outer muscle coat of the rectum, augmented in its upper part by striated muscle fibres originating from the medial components of the pelvic floor.

11. Most of the muscle continues caudally before splitting into multiple terminal septa that surround the muscle bundles of the subcutaneous portion of the external sphincter to insert into the skin of the lowermost part of the anal canal and adjacent perianal skin.

12. Milligan and Morgan named the most medial of these septa, passing around the inferior border of the internal sphincter, the ‘anal intermuscular septum’.

13. As it descends, however, it gives off fibres that pass medially across the internal sphincter to reach the submucosal space, and laterally across the external sphincter and ischiorectal space to reach the fascia of the pelvic side walls.

14. As well as providing a supportive mesh for the anal canal and other muscular components, its ramifications provide potential pathways for the spread of infection.

15. During defaecation, its contraction widens the anal lumen, flattens the anal cushions, shortens the anal canal and everts the anal margin; subsequent relaxation allows the anal cushions to distend and thus contribute to an airtight seal.

16. The internal sphincter The internal sphincter is the thickened (2–5mm) distal continuation of the circular muscle coat of the rectum, which has developed special properties and which is in a tonic state of contraction.

17. This internal sphincter an involuntary muscle commences where the rectum passes through the pelvic diaphragm and ends above the anal orifice, its lower border palpable at the intersphincteric groove.

18. It is pearly-white in colour and it has circumferentially placed fibres can be seen clearly. It is innervated by the autonomic nervous system, it receives intrinsic non-adrenergic and non-cholinergic (NANC) fibres, stimulation of which causes release of the neurotransmitter nitric oxide, which induces internal sphincter relaxation.

19. The pink columnar epithelium lining the rectum extends through the anorectal ring into the surgical anal canal. Passing downwards the mucous membrane becomes cuboidal and redder in colour whereas above the anal valves it is plum coloured.

20. Just below the level of the anal valves there is an abrupt, albeit wavy, transition to stratified squamous epithelium, which is parchment coloured. This wavy junction constitutes the dentate line.

21. The dentate line is a most important landmark both morphologically and surgically, representing the site of fusion of the proctodaeum and post-allantoic gut, and being the site of the crypts of Morgagni

22. The latter are small pockets between the inferior extremities of the columns of Morgagni through which anal ducts that communicate with deeper placed anal glands open into the anal lumen

23. The squamous epithelium lining the lower anal canal is thin and shiny and is known as the anoderm; it differs from the true skin in that it has no epidermal appendages, i.e. hair and sweat glands.

24. At the dentate line, the anoderm is attached more firmly to deeper structures. The mucosa and submucosa above the dentate line is uneven and thrown into folds, the so called anal cushions.

25. There are variations in the numbers and positions of these cushions but there are usually three, corresponding to those seen in later life.

26. These are described classically as occupying the left lateral, right posterior and right anterior positions, and they continue proximally as the primary rectal foldings.

27. Secondary foldings (the rectal columns of Morgagni) lie both over and between the primary folds.

28. This area is the caudal limit of the so called epithelial transitional zone, below which the stratified squamous epithelium is richly innervated by sensory nerve endings serving several modalities including touch, pain and temperature.

29. The bulk of the anal cushions themselves, situated in the upper part of the anal canal, receive only visceral afferent innervation and, although there is perception of stretching, sensitivity to noxious stimuli is much more blunted than distally.

30. Between the epithelial layer and the internal sphincter lies the submucosa, consisting of vascular, muscular and connective tissue supportive elements.

31. From the longitudinal muscle, medial extensions cross the internal anal sphincter and form part of the supporting meshwork of the submucosa, blending with the true submucosal smooth muscle layer and thereby supporting the mucosa itself.

32. Parks described the increased density of fibres that insert into the mucosa of the anal crypts at the level of the dentate line, termed the ‘mucosal suspensory ligament’.

33. One feature of this structure is that it separates the superior (portal) and inferior (systemic) haemorrhoidal plexuses, another is that the mucosa is more firmly tethered to underlying tissues at this level than above.

34. With age, the smooth muscle component of this mesh is reduced and muscle fibres are gradually replaced with fibroelastic connective tissue, which in turn becomes fragmented.