Imaging of perianal inflammatory diseases


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Imaging of perianal inflammatory diseases

  1. 1. M.Tonolini and G. Maconi (eds.), Imaging of Perianal Inflammatory Diseases,DOI: 10.1007/978-88-470-2847-0_1, © Springer-Verlag Italia 20133Perineum: Surgical Anatomyand PhysiologyAndrea Bondurri, Piergiorgio Danelli and Matteo Marone1A. Bondurri ( )General Surgery 1, Luigi Sacco University HospitalMilan, Italye-mail: bondurri.andrea@hsacco.it1.1 IntroductionThe perineum (Fig. 1.1) is defined as the inferior outlet of the pelvis. It is adiamond-shaped area with its major axis between the pubis and the tip of thecoccyx and its minor axis along the ischial tuberosity. The perineum is com-monly divided into two anatomic triangles: the anterior urogenital triangle andthe posterior anal triangle. The deep borders of the perineum are composed ofthe pubic arch, the arcuate ligament of the pubis, and, on either side, the infe-rior portions of the pubis and ischium.The muscle layer comprising the pelvic diaphragm creates the “roof” of theperineum. It is composed of the levator ani and coccygeal muscles and it sep-arates the pelvic cavity from the perineal region. The levator ani consists ofthree muscles: the pubococcygeus, puborectalis, and iliococcygeus. The pub-ococcygeus is the primary muscle; it runs backward from the pubis toward thecoccyx, with some of its fibers inserted into the urethra, prostate, and vagina.The puborectalis runs from the pubis to the rectum, forming a muscular ringhardly separable from the external anal sphincter. The iliococcygeus, whichmakes up the posterior part of the levator ani, is often poorly developed. Thecoccygeal muscle, situated behind the levator ani, is frequently more tendi-nous than muscular. It extends from the ischial spine to the lateral margin ofthe sacrum and coccyx.The base of the perineum is composed of the skin, which is tough and mobilenear the intergluteal line and less mobile near the anal orifice, and the subcuta-neous tissue, which is less represented close to the anal orifice [1].
  2. 2. Dissecting the skin layer reveals the external anal sphincter and between itand the lateral border of the perineum the ischioanal fossa. This fossa has aprismatic shape, with its base directed towards the surface of the perineum andits apex meeting the obturator and anal fasciae. The lateral border is composedof the obturator internus muscle and its fascia. The pudendal vessels andnerves are located in a splitting of this latter fascia. The medial border isformed by the levator ani and the external anal sphincter. The ischioanal fossahas two extensions, an anterior one between the levator ani and perineal fas-cia, and a posterior one, between the levator ani and gluteus major [3].1.2 The Urogenital TriangleThe musculotendinous layer of the urogenital triangle closes the anterior partof the pelvis. In males, it is crossed by the urethra and bulbo-urethral glands,and in females by the urethra, vagina, and vestibular glands. It is composed ofseveral different layers: the deep transverse perineal muscle, between thebones of the ischium; the urethral sphincter surrounding the urethra; theischiocavernosus muscle, extending from the inner surface of the ischialtuberosity to the pubic bones; and a bulbospongiosus muscle arising from thecentral tendinous point of the perineum. In males, it encircles the urethra andin females it covers the vestibular bulb and envelops the vagina [1].4 A.Bondurri et al.Fig.1.1 The perineum
  3. 3. 1.3 The Anal TriangleThe most important structure of the anal triangle is of course the anal canal.Developmentally, it is a region of fusion between endodermal and ectodermaltubes, evident at the dentate line. The distal colon is derived from the hindgutand is thus made up of endodermal tissue. Before the 5th gestational week, theintestinal and urogenital tract flow in the cloaca. During the 6th gestationalweek, the two tracts separate. The anal canal is the terminal portion of thelarge intestine. It forms an angle with the lower part of the rectum, measures2.5–4 cm, and is surrounded by the internal and external anal sphincters. Avariable number of vertical folds, the rectal columns, lie 7–15 mm above theanal orifice and are separated from each other by rectal sinuses, which end inthe anal valves.The anorectal blood supply comes from the superior and inferior hemor-rhoidal arteries, which in turn derive, respectively, from the superior mesen-teric and pudendal arteries. The veins of the rectum and anal canal converge ina plexus that surrounds the canal and contains small saccular dilatations justwithin the margin of the anus. This plexus typically gives off six vessels ofconsiderable size that penetrate the muscular coat and converge in a singletrunk, the superior hemorrhoidal vein. This so-called hemorrhoidal plexuscommunicates with the tributaries of the middle and inferior hemorrhoidalveins and establishes a communication between the systemic and portal circu-lations.The muscular structure of the anal canal is composed of the external andinternal sphincter ani. The external sphincter measures 8–10 cm in length andabout 2.5 cm in width. Its elliptical muscular fibers are intimately adherent tothe margin of the anus. Of the two layers that comprise the external analsphincter, the lateral layer, formed by fibers from the levator ani muscle, is themain one. The deep layer forms a complete sphincter surrounding the analcanal and it is closely applied to the internal anal sphincter. It is always in astate of tonic contraction and has no antagonistic muscle, which allows it tokeep the anal canal and orifice closed. According to will, it can be put in a stateof more firm contraction to occlude anal opening.The internal anal sphincter is a muscular ring that surrounds the anal canal.It is about 5 mm thick and is formed by an aggregation of involuntary circularmuscular fibers. Between the two sphincters is the intersphincteric space,which is important in the pathogenesis of perianal abscesses because it con-tains most of the anal glands.Motor innervation of the rectum is provided by sympathetic and parasym-pathetic nerves. The external anal sphincter is innervated by the inferior rectalbranch of the pudendal nerve (S3 S4) and by the perineal branch of S4.Sensory innervation is provided by free nerve terminations as well as organ-ized nerve endings and derives from the inferior rectal branch of the pudendalnerve [1-3].1 Perineum: Surgical Anatomy and Physiology 5
  4. 4. 1.4 DefecationThe anorectum (Fig. 1.2) acts as a reservoir, with a capacity of about 0.5 l, andas a “pump” for the evacuation of feces. Defecation involves the semi-volun-tary emptying of the rectum and requires a sequence of events that integratessmooth and striated muscle and the central, somatic, autonomic, and entericnervous systems.Defecation begins with rectal distension caused by caudally migrating con-tractions that originate in the colon and drive feces from there to the rectum.When a characteristic threshold is reached, an awareness of rectal fillingbegins. The intensity of this sensation increases as the rectum continues to filluntil it becomes an urge to defecate. Distension of the rectum causes variousreflexes that end in the contraction of the rectum itself, relaxation of the inter-nal anal sphincter, and contraction of the external anal sphincter. During defe-cation, the anal canal must be open and the intrarectal pressure must exceedthe anal canal pressure. Straightening of the anorectal angle and, subsequent-ly, opening of the rectum require that contraction of the pelvic floor is inhib-ited. The pelvic floor descends and muscle contraction shortens the anal canal.In a Valsalva maneuver, intrarectal pressure increases, assisting defecation.Defecation terminates when contraction of the puborectalis and external anal6 A.Bondurri et al.Fig.1.2 Transverse view of the anorectum
  5. 5. sphincter restores the anorectal angle, the internal anal sphincter recovers itstone, and the anal canal closes [4].References1. Balboni G (2007) Trattato di anatomia umana, 4th edn. Edi Ermes, Milan, pp. 122-134, 467-4722. Towsend CBR, Evers B (2008) Sabiston text book of surgery: the biological basis of modernsurgical practice, 18th edn. Saunders Elsevier, Philadelphia, pp. 1447–14493. Corman ML (2005) Colon& rectal surgery, 5th edn. Lippincot Williams & Wilkins, Philadel-phia, pp. 7-27, 279-3324. Brookes, SJ, Dinning PG, Gladman MA (2009) Neuroanatomy and physiology of colorectalfunction and defaecation: from basic science to human clinical studies. NeurogastroenterolMotil. 21(2):9-191 Perineum: Surgical Anatomy and Physiology 7