UGRAexp

1 – Indications of proximal sciatic nerve blocks

Proximal sciatic nerve block is indicated:

– for surgical anaesthesia with peripheral nerve blocks that includes surgery of the thigh or knee

– for surgical anaesthesia with peripheral nerve blocks that includes a thigh tourniquet

– as an alternative to popliteal sciatic nerve block when that is not feasible

Proximal sciatic nerve blocks are:

– the subgluteal approach

– the anterior approach combined with a saphenous nerve block (SPEDI)

– the parasacral parallel shift approach to block the sacral plexus (as an alternative to subgluteal approach or
SPEDI – single penetration dual injection)

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14 – Alcocks canal

Alcocks canal (synonym: pudendal canal) is a fascial compartment on the lateral wall of the ischioanal fossa that contains the pudendal nerve, the internal pudendal artery and the internal pudendal vein.

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The yellow Alcocks canal (cyan arrows) contains the pudendal nerve (magenta arrows). Inferior rectal nerve (green arrow), dorsal nerve of the penis (black arrow), deep and superficial (blue and grey arrows) perineal nerves, sacrospinous (magenta asterix) and sacrotuberous (green asterix) ligaments, spinal nerve roots (red arrows).

16 – Position of patient and probe

Place the patient supine with the pelvis elevated by a pillow. Hips and knees are flexed and the thighs are abducted with external rotation.

Place the linear high-frequency probe on a line between the scrotum/mons pubis and the lateral border of the anus on top of the ischiopubic ramus. Visualize the ischiocavernosus muscle on top of the ischiopubic bone.

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Ischiocavernosus muscle (ic), ischiopubic bone (ipb).

12 – Pudendal nerve below the levator ani

In the ilioanal fossa below the levator ani, the pudendal nerve has three branches: the inferior rectal branch, the perineal branch, and the dorsal nerve of the penis (blue arrow).

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The entry of the pudendal nerve in the ilioanal fossa (red arrow) deep to the sacrotuberous ligament (red asterix). The inferior rectal branch (yellow arrow), the perineal branch (cyan arrow), and the dorsal nerve of the penis.

13 – The internal pudendal artery

The pudendal nerve runs alongside the internal pudendal artery in the Alcocks canal along the medial margin of the inferior pubic ramus deep to the muscles of the superficial pelvic floor.

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The pudendal nerve (red arrow) runs with the internal pudendal artery (blue arrow) in the Alcocks canal. Branches from the internal pudendal artery are the inferior rectal artery (magenta arrow), the posterior scrotal artery (black arrow), and the bulbourethral artery (yellow arrow).

8 – The ischioanal fossa

The anatomical space called the ischioanal fossa (synonym: the ischiorectal fossa) is the space inferior and lateral to the levator ani, lateral to the anal canal, medial to the internal obturator muscle, and superior to the urogenital diaphragm (syn: the superficial pelvic floor).

The ischioanal fossa mainly contains fat but also the pudendal canal (synonym: the Alcock’s canal) on the lateral wall of the ischioanal fossa. The pudendal canal contains the pudendal nerve and the internal pudendal blood vessels.

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The picture shows the boundaries of the ischioanal fossa: The superficial pelvic floor inferiorly, the levator ani superiorly and medially, the anal canal medially and the internal obturator muscle laterally. The internal obturator muscle is only shown on the left side of the image.

9 – External anal sphincter

The external anal sphincter origins from the perineal body and inserts on the anococcygeal ligament. It is innervated by the inferior rectal branch of the pudendal nerve.

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The external anal sphincter is depicted with yellow and orange.

10 – The pudendal nerve

The pudendal nerve exits the pelvis through the greater sciatic foramen, curls around the sacrospinous ligament and enters the ischioanal fossa through the lesser sciatic foramen.

The lesser sciatic foramen is formed by the sacrospinous ligament and the sacrotuberous ligament.

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The pudendal nerve (red arrow) enters the ischioanal fossa via the lesser sciatic foramen formed by the sacrospinous ligament (green asterix) and the sacrotuberous ligament (magenta asterix). The sacrospinous ligament inserts on the ischial spine (green arrow). The internal pudendal artery is depicted with cyan arrow.

11 – Pudendal nerve above the levator ani

The pudendal nerve origins from the spinal nerves S2-S4 and whirls around the ischial spine.

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The pudendal nerve depicted above the levator ani. The cyan arrow points at the pudendal nerve where it whirls around the ischial spine. Coccygeus muscle (yellow asterix), puborectal muscle (red asterix), pubococcygeal muscle (white asterix), iliococcygeal muscle (green asterix), internal obturator muscle (blue asterix).

5 – The deep pelvic floor – levator ani

The muscles of the deep pelvic floor are known as the levator ani muscles. The deep pelvic floor has two components: The pubovisceral and the iliococygeal muscles. The pubovisceral muscles are the pubococcygeus and the puborectalis muscles.

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The levator ani muscles are: The iliococcygeal muscle (green asterix), the pubococcygeus muscle (cyan asterix) and the puborectalis muscle (yellow asterix).