Ischioanal fossa

The ischioanal fossa is a fat-filled, wedge-shaped space located on each side of the anal canal, within the perineum. It is an important anatomical feature that allows the expansion of the anal canal during defecation and provides room for the passage of nerves and vessels. The fossa is filled with loose connective tissue and adipose tissue, which cushion and support the surrounding structures.

Location

The ischioanal fossa is located in the perineal region, on either side of the anal canal. It is bordered laterally by the ischial tuberosity and the obturator internus muscle, medially by the levator ani muscle and the external anal sphincter, and posteriorly by the gluteus maximus muscle and the sacrotuberous ligament. Anteriorly, it extends towards the urogenital triangle. The fossa is part of the anal triangle of the perineum, and it plays a key role in accommodating the movements of the pelvic diaphragm and anal canal.

Structure and Anatomy

The ischioanal fossa (formerly known as the ischiorectal fossa) is a fat-filled space located in the perineum, surrounding the anal canal. It is a paired structure, with one fossa on either side of the anal canal. The ischioanal fossa plays an important anatomical role, providing space for the expansion of the anal canal and accommodating structures such as blood vessels and nerves.

Shape and Boundaries

Shape:The ischioanal fossa is a wedge- or pyramid-shaped space with its base positioned inferiorly, near the skin, and its apex directed superiorly, towards the pelvic diaphragm. It is filled with loose connective tissue and fat, which allows for flexibility and expansion during defecation.

Boundaries:

• Lateral Border: The lateral wall of the ischioanal fossa is formed by the ischial tuberosity and the fascia covering the obturator internus muscle. The obturator internus muscle and its covering fascia make up the prominent lateral border of the fossa.
• Medial Border: The medial wall is formed by the levator ani muscle and the external anal sphincter. The levator ani is part of the pelvic diaphragm, which supports the pelvic organs, and it separates the ischioanal fossa from the pelvic cavity above.
• Inferior Border (Base): The base of the fossa is covered by the skin and subcutaneous tissue of the anal region, and it forms the widest part of the fossa. This layer provides protection and flexibility to the underlying structures.
• Superior Border (Apex): The apex of the fossa is located superiorly, where the levator ani muscle meets the obturator internus muscle.
• Posterior Border: The posterior wall is formed by the sacrotuberous ligament and the overlying gluteus maximus muscle. These structures support the posterior aspect of the fossa and separate it from the gluteal region.
• Anterior Border: The anterior part of the fossa extends forward into the urogenital triangle of the perineum and is limited by the posterior margin of the perineal membrane.

Contents of the Ischioanal Fossa

The ischioanal fossa contains several important structures, including fat, connective tissue, nerves, and blood vessels. These contents allow the fossa to accommodate changes in the anal canal during defecation and provide a passageway for important neurovascular elements.

• Fat and Loose Connective Tissue:The fossa is filled with a large amount of fat, which acts as a cushion and provides space for the expansion of the anal canal. This fat is also important for protecting the nerves and vessels that pass through the region.
• Pudendal Canal (Alcock’s Canal):
  • The pudendal canal is a fascial compartment located along the lateral wall of the ischioanal fossa. It runs within the fascia of the obturator internus muscle and contains important neurovascular structures:
    • Pudendal Nerve: This is the main nerve of the perineum and supplies sensation to the external genitalia, as well as motor innervation to the muscles of the pelvic floor, including the external anal sphincter.
    • Internal Pudendal Artery: This artery supplies blood to the perineal region, external genitalia, and the anal canal.
    • Internal Pudendal Vein: This vein drains blood from the perineum and external genitalia into the systemic venous system.
• Inferior Rectal Nerves and Vessels:The inferior rectal nerve and inferior rectal artery are branches of the pudendal nerve and internal pudendal artery, respectively. They supply the external anal sphincter, anal canal, and surrounding skin. These structures travel through the ischioanal fossa and play a crucial role in the motor and sensory control of the anal region.
• Perineal Branches of the Posterior Femoral Cutaneous Nerve:These small nerves supply sensory innervation to the skin over the posterior aspect of the perineum, including the anal region. They emerge from the posterior thigh and pass into the perineal region through the ischioanal fossa.

Communications with Other Spaces

• Anterior Recess:The anterior portion of the ischioanal fossa extends forward into the urogenital triangle, creating a space known as the anterior recess of the ischioanal fossa. This extension lies between the perineal membrane and the levator ani muscle and contains fat and neurovascular structures. The anterior recess allows the fossa to extend into the space near the perineum and external genitalia.
• Posterior Communication:The two ischioanal fossae can communicate with each other through the deep postanal space, located posterior to the anal canal and below the anococcygeal ligament. This communication allows infections or pathological processes in one fossa to potentially spread to the other.

Vascular Supply

• Arterial Supply:
• • The ischioanal fossa is primarily supplied by the inferior rectal artery, which is a branch of the internal pudendal artery. This artery supplies the external anal sphincter, anal canal, and surrounding skin.
  • The internal pudendal artery continues through the pudendal canal and supplies branches to the perineum and external genitalia.
• Venous Drainage:Venous drainage of the ischioanal fossa occurs via the inferior rectal veins, which drain into the internal pudendal vein. These veins collect blood from the anal canal and perineum and drain into the systemic venous circulation.

Nerve Supply

The ischioanal fossa contains several important nerves, which provide both sensory and motor innervation to the perineal and anal regions:

• Pudendal Nerve:The pudendal nerve is the major nerve of the perineum and passes through the pudendal canal along the lateral wall of the ischioanal fossa. It gives off the inferior rectal nerve, which innervates the external anal sphincter and the skin around the anal canal.
• Inferior Rectal Nerve:A branch of the pudendal nerve, the inferior rectal nerve provides motor innervation to the external anal sphincter and sensory innervation to the perianal skin. This nerve is essential for the control of defecation and sensation in the anal region.
• Perineal Branch of the Posterior Femoral Cutaneous Nerve:This nerve provides sensory innervation to the skin of the posterior perineum and the skin overlying the ischioanal fossa. It contributes to the sensory perception of the skin surrounding the anal region.

Lymphatic Drainage

Lymph from the ischioanal fossa drains into the nearby lymph nodes, which are involved in the immune response and help filter fluid from the region:

• Superficial Inguinal Lymph Nodes:Lymphatic drainage from the skin of the perineum and lower anal canal is directed towards the superficial inguinal lymph nodes. These nodes are located in the groin and play an important role in filtering lymph from the perineal region.
• Internal Iliac Lymph Nodes:The deeper structures of the ischioanal fossa, including the anal canal and internal anal sphincter, drain into the internal iliac lymph nodes. These nodes are located along the internal iliac vessels within the pelvis.

Variations in Shape and Size

The size and shape of the ischioanal fossa can vary between individuals, depending on factors such as body composition, muscle mass, and the extent of fat within the fossa. In some individuals, the fossa may extend more deeply into the perineum, while in others, the space may be smaller due to a greater amount of connective tissue.

Function

The ischioanal fossa primarily serves as a support structure for the anal canal and surrounding perineal region. Its unique anatomical location and composition allow for flexibility, accommodation of neurovascular structures, and cushioning during various functions, particularly defecation. Below are the detailed functions of the ischioanal fossa.

Space for Expansion During Defecation

• Accommodation of Anal Canal Movement:The ischioanal fossa plays a crucial role in providing space for the expansion and movement of the anal canal during defecation. As stool passes through the anal canal, the canal must dilate and stretch to accommodate its passage. The loose, fat-filled space of the ischioanal fossa allows the anal canal to expand without resistance, ensuring that the defecation process occurs smoothly and without causing damage to surrounding structures.
• Pressure Buffering:The fat and loose connective tissue within the ischioanal fossa act as a pressure buffer during the passage of stool. This cushioning effect reduces the pressure exerted on nearby structures, including muscles and nerves, thus protecting them from excessive compression during defecation.

Support and Cushioning

• Cushioning for the Perineal Region:The fat within the ischioanal fossa serves as a protective cushion for the perineal region. It helps to absorb mechanical stresses during activities such as sitting, walking, and defecation. By acting as a shock absorber, the fat in the fossa protects the soft tissues, muscles, and nerves in the anal and perineal region from damage due to external forces.
• Support for the Anal Canal and External Anal Sphincter:The ischioanal fossa provides structural support to the anal canal and external anal sphincter. The surrounding fat and connective tissue help to maintain the position of the anal canal while still allowing flexibility for its movement. This support is essential for maintaining the integrity of the anal canal and ensuring the proper function of the sphincters involved in continence.

Passageway for Neurovascular Structures

Conduit for Nerves and Blood Vessels:

• The ischioanal fossa serves as an important passageway for nerves, arteries, and veins that supply the anal region and perineum. Key structures such as the pudendal nerve, internal pudendal artery, and internal pudendal vein travel through the fossa within the pudendal (Alcock’s) canal.
• The inferior rectal nerve and inferior rectal artery pass through the ischioanal fossa to reach the external anal sphincter and perianal skin. These structures are critical for providing motor control to the sphincter and sensory feedback from the perianal region, which are essential for defecation and maintaining continence.

Protection of Nerves and Blood Vessels:The fat and loose connective tissue within the fossa also protect the neurovascular structures from compression or damage during body movements. This protective role ensures that nerves, such as the pudendal nerve, and blood vessels maintain their function, allowing for proper innervation and blood supply to the perineal region.

Facilitation of Movement in the Perineal Region

• Flexibility During Body Movements:The loose connective tissue and fat in the ischioanal fossa allow for flexibility and mobility of the surrounding structures. This flexibility is particularly important during activities such as walking, sitting, or squatting, where changes in position may cause shifts in pressure and movement within the perineal region. The ischioanal fossa accommodates these movements without causing strain on the anal canal or surrounding muscles.
• Expansion and Contraction of the Pelvic Floor:The ischioanal fossa plays a role in allowing the pelvic floor muscles, particularly the levator ani, to contract and relax during various activities, such as defecation and urination. By providing space for these muscles to move, the ischioanal fossa contributes to the normal function of the pelvic floor and its role in supporting the pelvic organs.

Communication Between Adjacent Spaces

• Link Between Perineal and Pelvic Regions:The ischioanal fossa acts as a connecting space between the perineal region and the pelvic cavity. It allows for the passage of neurovascular structures from the pelvic region into the perineum and external genitalia. This communication ensures that the perineal and pelvic structures receive adequate innervation and blood supply, which are essential for normal function.
• Facilitating Spread of Fluids:The fat and loose connective tissue of the ischioanal fossa also facilitate the movement of fluids, such as blood or lymph, within the region. For example, in cases of infection or injury, the fossa can act as a pathway for the spread of pus or inflammatory fluid, which may extend into neighboring spaces, such as the urogenital triangle or the opposite ischioanal fossa. The anatomical connections between these spaces allow for the free movement of fluids within the perineal region.

Role in Maintaining Continence

• Support for the External Anal Sphincter:The ischioanal fossa supports the external anal sphincter, a muscle responsible for maintaining voluntary control of defecation. The neurovascular structures that pass through the fossa, including the inferior rectal nerve, provide motor control to the sphincter, allowing it to contract and relax as needed to retain or release stool. By supporting the structures involved in continence, the ischioanal fossa plays a role in preventing involuntary leakage of stool.
• Sensory Innervation for Defecation:The inferior rectal nerve provides sensory innervation to the perianal skin, allowing for the detection of stool in the anal canal and the sensation of the need to defecate. This sensory feedback is critical for triggering the appropriate motor responses needed to control the external anal sphincter and initiate defecation. The passage of this nerve through the ischioanal fossa ensures that the sensory function of the perineum remains intact.

Clinical Significance

The ischioanal fossa is clinically significant due to its involvement in various conditions, particularly infections and abscess formation. Ischioanal abscesses are common in this region due to its fat-filled nature and proximity to the anal canal, which can allow infections from the anal glands to spread into the fossa. These abscesses cause pain, swelling, and fever, and may require drainage to prevent complications.

The fossa also has clinical relevance in cases of trauma, where injury to the pudendal nerve or inferior rectal nerve could lead to loss of sensation or impaired control of the external anal sphincter, affecting continence. Additionally, the fossa serves as a pathway for the spread of infections or malignancies, particularly those originating from the anal canal or pelvic region.