Ischiorectal fossa

Hello everyone! This is Nicole from Kenhub, and in this tutorial, we're going to be looking at the fat-filled space of the perineum known as the ischiorectal fossa. The ischiorectal fossa which we can see here in this lovely mid-coronal plane of the pelvis at the level of the rectum with our rectum cut in cross-section here is also known as the ischioanal fossa. And the ischiorectal fossa – well, actually there are two so really they are the ischiorectal fossae – are two triangular-shaped spaces situated bilateral to the inferior aspect of the rectum and the anus. And we can see them highlighted in green on our slide here as we mentioned on either side of the anus. And while we're here, let's have a brief chat about the structure of the ischiorectal fossae. So, as the ischiorectal fossae are shaped somewhat like prisms, they, therefore, have a few aspects to them which are rather prismatic namely the apex, the roof, the lateral walls, the medial walls, and the floor.

So, taking this image of a prism, let's start by talking about the apex. And the apex of the space is directed anteromedially towards the pubic symphysis which we can't see on this slide but if you can imagine it coming out of your computer, then you should be able to visualize it coming together at a point. And this apex or point is where the levator ani and the obturator internus muscles intersect. The roof of one of the ischiorectal fossae is formed by the obliquely coursing levator ani muscle and the inferior fascia of the pelvic diaphragm which we can see in this image. So, the levator ani muscle is just here creating the roof and I suppose this little line coursing just beneath the levator ani could be considered the inferior fascia of the pelvic diaphragm.

The lateral walls of the ischiorectal fossae are formed superiorly by the obturator internus muscle and its fascia and inferiorly it's formed by the medial aspect of the ischial tuberosity. And we can see the obturator internus muscle just here and also the ischial tuberosity although that's not quite right as this image is a little further forward than where the ischial tuberosity is but the walls are indeed around here. And, of course, the roof and the lateral walls of the ischiorectal fossa meet at the apex where the levator ani and the obturator internus muscles intersect.

The medial walls of this space are bordered by the medial part of the levator ani muscle superiorly and by the external anal sphincter muscles inferiorly. And the levator ani muscle is here, of course, while the external anal sphincter muscle is just here. And just for your interest, over here is our internal anal sphincter muscle.

The floor of the ischiorectal fossa is formed by the perineal skin while it is bordered at its base by the gluteus maximus muscle and the sacrotuberous ligament. And let's just point out our perineal skin which is along the bottom here and the gluteus maximus and the sacrotuberous ligament, you won't be able to see because they are not shown in this image.

The ischiorectal fossae are separated from one another by the anococcygeal body, however, they still communicates superior to this and posterior to the anal canal. The ischiorectal fossae contained adipose lobules sometimes referred to as the ischiorectal or ischioanal fat pad. And in addition to that, it also houses several neurovascular and connective tissue structures including the inferior rectal branches of the pudendal nerve, the posterior scrotal nerves and vessels in the male pelvis, the posterior labral nerves and vessels in the female pelvis, the perforating cutaneous nerve, the perineal branch of the fourth sacral nerve S4, and the lymphatic trunks.

The internal pudendal vessels, the pudendal nerve and the perineal nerve course through the ischiorectal fossae to their points of supply through a tunnel formed in the fascia of the obturator internus muscle known as Alcock's or the pudendal canal.

So let's just have a brief chat about the clinical significance of the ischiorectal fossa. So, because communication between the two fossae is limited in certain points, if an infected abscess were to occur at or near the midline of one of the fossa, the contamination would not spread to the other fossa but having said all that, the cross-contamination of the ischiorectal fossae is possible via the retrosphincteric space of Courtney which is also called the deep postanal space and is located above the anococcygeal ligament but below the levator ani and this is particularly important to note as if infection does spread there, a so-called horseshoe abscess can occur.

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