The perineum is an anatomical region located in the pelvis. It is the most inferior part of the pelvic outlet, located between the thighs. It is separated from the pelvic cavity superiorly by the pelvic floor.
Functionally, the perineum contains structures that support thmar urinary, genital and gastrointestinal viscera. They play a vital role in micturition, defecation, sexual intercourse and childbirth.
In this article, we shall look at boundaries, contents and clinical correlations of the perineum.
The perineum is a diamond shaped structure. There are two ways in which the boundaries of the perineum can be described. The anatomical borders refer to its exact bony margins. The surface borders describe the surface anatomy which marks the borders of the perineum.
- Anterior – Pubic symphysis.
- Posterior– The tip of the coccyx.
- Laterally – Inferior pubic rami and inferior ischial rami, and the sacrotuberous ligament.
- Roof – The pelvic floor.
- Base – Skin and fascia.
The perineum can be subdivided by a theoretical line drawn transversely between the ischial tuberosities. This split forms the anterior urogenital and posterior anal triangles. These triangles are associated with different components of the perineum.
For clinical purposes, it is important to be aware of the surface anatomy which marks the boundaries of the perineum. These boundaries are best shown when the lower limbs are abducted, and a diamond shape is depicted. The skin boundaries of the perineum are:
- Anteriorly: Mons pubis in females, base of the penis in males.
- Laterally: Medial surfaces of the thighs.
- Posteriorly: Superior end of the intergluteal cleft.
The perineum can be subdivided by a theoretical line drawn transversely between the ischial tuberosities. This split forms the anterior urogenital and posterior anal triangles. These triangles are associated with different components of the perineum – which we shall now look at in more detail.
The anal triangle is the posterior half of the perineum. It is bounded by the coccyx, sacrotuberous ligaments and an imaginary line between the ischial tuberosities. The components of the anal triangle are:
- Anal aperture – the opening of the anus.
- External anal sphincter muscle – voluntary muscle responsible for opening and closing the anus.
- Two ischioanal fossae – these are spaces located laterally to the anus.
The anal aperture is located centrally in the triangle with the ischioanal fossae either side. These fossae contain fat and connective tissue, which aid expansion of the anal canal during defecation. They extend from the skin of the anal region inferiorly to the pelvic diaphragm superiorly.
The urogenital triangle comprises of the anterior half of the perineum. It is bounded by the pubic symphysis, ischiopubic rami, and an imaginary line between the two ischial tuberosities. The triangle is associated with the structures of the urogenital system – the external genitalia and urethra.
Structurally, the urogenital triangle is complex, with a number of fascial layers and pouches. Unlike the anal triangle, the urogenital triangle has an additional layer of strong deep fascia, known as the perineal membrane. This membrane has pouches on its superior and inferior surfaces.
We shall explore each layer of the urogenital triangle in more detail (deep to superficial):
- Deep perineal pouch – A potential space between the pelvic floor superiorly, and the perineal membrane inferiorly. It contains part of the urethra and the external urethral sphincter. In males, it also contains the bulbourethral glands and the deep transverse perineal muscles.
- Perineal membrane – A layer of tough fascia, which is perforated by the urethra and vagina. The role of the membrane is to provide attachment for the muscles of the superficial external genitalia, and to help support the pelvic viscera.
- Superficial perineal pouch – A potential space between the perineal membrane superiorly, and the perineal fascia inferiorly. It contains the erectile tissues that form the penis and clitoris, and three muscles – the ischiocavernosus, bulbospongiosus and superficial transverse perineal muscles. The greater vestibular glands (Bartholin’s glands) are also located in the superficial perineal pouch.
- Deep perineal fascia – Fascia covering the superficial perineal muscles.
- Superficial perineal fascia – It is continuous with the superficial fascia of the abdominal wall. The superficial fascia itself can be divided into superficial and deep layers. The superficial layer is fatty in structure, forming the labia majora and mons pubis in women.
- Skin – The urethral and vaginal orifices open out onto the skin.
The Perineal Body
The perineal body is an irregular and fibromuscular mass. It is located at the junction of the urogenital and anal triangles – the central point of the perineum. This structure contains skeletal muscle, smooth muscle and collagenous and elastic fibres.
Anatomically, the perineal body lies just deep to the skin. It functions as a point of attachment for muscle fibres from the pelvic floor and the perineum itself:
- Levator ani (part of the pelvic floor).
- Bulbospongiosus muscle.
- Superficial and deep transverse perineal muscles.
- External anal sphincter muscle.
- External urethral sphincter muscle fibres.
In women, it acts as a tear resistant body between the vagina and the external anal sphincter, supporting the posterior part of the vaginal wall against prolapse. In men it lies between the bulb of penis and the anus.
Nerve and Arterial Supply
The major neurovasculature supply to the perineum is from the pudendal nerve (S2 to S4) and the internal pudendal artery.
Clinical Relevance: Disorders of the Perineum
Damage to the Perineal Body
The perineal body is a central attachment for perineal muscles, and functions to support the pelvic floor.
Childbirth can lead to damage (stretching/tearing) of the perineal body, thus leading to possible prolapse of pelvic viscera. This may be avoided by an episiotomy (a surgical cut in the perineum). This inevitably causes damage to the vaginal mucosa, but prevents uncontrolled tearing of the perineal body.
Other causes of perineal body damage include inflammatory disease, infection or trauma.
Bartholin’s Gland Cysts
The bartholin’s glands are located within the superficial perineal pouch of the urogenital triangle.
The role of these glands is to make a small amount of mucus-like fluid, which maintains a moist vagina. Normally these are not detected on examination, however if the duct becomes blocked then these glands swell, forming fluid-filled cysts. These cysts can become infected and inflamed, a condition known as bartholinitis. The most common causes of infection are from the bacteria which cause skin and uterine infections, such as Staphylococcus spp. and Escherichia coli.