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Pyramidal muscle (pyramidalis muscle)

The pyramidal muscle, also called the pyramidalis, is a small triangular muscle of the anterior abdominal wall. It lies anterior to the lower part of the rectus abdominis within the rectus sheath. Although functionally minor, it serves as an anatomical landmark in abdominal surgery and imaging. Its presence is variable, being absent in up to 20% of individuals, and asymmetrical in some cases.

It plays a supportive role in tensing the linea alba and stabilizing the anterior abdominal wall, and is often encountered during midline laparotomies or imaging of the lower abdomen.

Synonyms

  • Pyramidalis muscle

  • Pyramidal abdominal muscle

  • Small triangular abdominal muscle

Origin, Course, and Insertion

  • Origin:

    • Arises from the anterior surface of the pubis and the pubic symphysis

  • Course:

    • A small, triangular belly that ascends superiorly within the rectus sheath

    • Lies in front of the lower rectus abdominis

    • Enclosed between the anterior and posterior layers of the rectus sheath

  • Insertion:

    • Inserts into the linea alba, midway between the pubis and umbilicus

Relations

  • Anteriorly: Anterior rectus sheath and subcutaneous tissue

  • Posteriorly: Rectus abdominis muscle and posterior sheath

  • Superiorly: Linea alba

  • Inferiorly: Pubic symphysis and pubis

Nerve Supply

  • Subcostal nerve (T12)

Arterial Supply

  • Inferior epigastric artery (main supply)

  • Superior epigastric artery (minor contribution)

Venous Drainage

  • Venous return via the inferior and superior epigastric veins into the external iliac and internal thoracic veins

Function

  • Tenses the linea alba

  • Provides minor reinforcement to the lower anterior abdominal wall

  • Acts as an anatomical landmark in abdominal surgery (often used to locate linea alba midline)

Clinical Significance

  • May be absent unilaterally or bilaterally

  • Acts as a useful surgical landmark for midline incisions

  • Variations in its presence or size are important in imaging interpretation

MRI Appearance

T1-weighted images:

  • Appears as low-to-intermediate signal intensity muscle

  • Fatty streaks may appear as bright areas if degeneration is present

T2-weighted images:

  • Shows low-to-intermediate signal intensity

  • Edema or injury demonstrates bright signal intensity

STIR (Short Tau Inversion Recovery):

  • Normal muscle shows low-to-intermediate signal

  • Strain, inflammation, or edema shows bright signal intensity

Proton Density Fat-Sat (PD FS):

  • Normal muscle appears low-to-intermediate in signal intensity

  • Pathology appears as localized or diffuse bright signal intensity

T1 Fat-Sat Post-Contrast:

  • Normal muscle enhances mildly and evenly

  • Pathological processes demonstrate heterogeneous or rim enhancement

CT Appearance

Non-Contrast CT:

  • Small, homogeneous soft tissue density in the anterior abdominal wall

  • Hematoma appears as localized hyperdense region

  • Fatty change appears as low attenuation

Post-Contrast CT:

  • Mild homogeneous enhancement under normal conditions

  • Inflammation or tumor shows irregular or more intense enhancement

  • Abscess demonstrates central low density with peripheral rim enhancement

MRI image

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