Etiology

• In an adult, the spinal cord terminates in a tapering fashion as the conus medullaris (T12-S4) at the L1-L2 vertebral level. The collection of spinal nerves below this point (eg, L3-S4) exit inferiorly through their respective intervertebral foramina and are referred to as the cauda equina (ie, horse's tail).
• Damage to or compression of the cauda equina (nerve fibers L3–S5) located below L2
• Common causes include large posteromedial disk herniation, trauma, and tumors.

Functions

• Innervates the lower limb, perineum, and pelvic organs
• Innervates the internal and external anal sphincter
• Provides parasympathetic innervation to the bladder

Clinical features

• Onset: gradual, typically unilateral
• Pain
  • Lower back pain
  • Severe radicular pain
• Motor symptoms
  • Asymmetric, areflexic, flaccid paresis of the legs
• Sensory symptoms
  • Saddle anesthesia: lack of sensitivity in the dermatomes S3–S5, affecting the areas around the anus, genitalia, and inner thighs (may be asymmetric)
• Urogenital and rectal symptoms
  • Late onset of urinary retention
  • Change in bowel habits due to loss of anal sphincter control
  • Decreased rectal tone or bulbocavernosus reflex
  • Erectile dysfunction