Anatomy Atlas

Cauda equina

Cauda equina
Also: Cauda equina syndrome · Conus medullaris syndrome
NerveHigh-yielddraftNeuroMSK

Bundle of lumbar and sacral nerve roots descending in the lumbar cistern below the end of the spinal cord. Compression causes cauda equina syndrome, a surgical emergency.

Core anatomy

Location

Lumbar cistern (subarachnoid space) from the conus medullaris (L1–L2) to the end of the dural sac (S2), and beyond in the sacral canal.

Root values

Dorsal and ventral roots of L2–Co1, descending to their exit foramina.

Origin

Lumbar, sacral and coccygeal segments of the conus medullaris.

Course

The roots descend steeply within the dural sac, each leaving through the intervertebral foramen below its numbered vertebra (the sacral roots through the sacral foramina).

Motor

Lower limb muscles (via the lumbar and sacral plexuses), external anal and urethral sphincters (S2–S4), pelvic floor.

Sensory

Lower limbs, perineum and saddle area (S2–S5), bladder and rectum.

Relations

Floats in CSF, which lets it move aside from an LP needle. Surrounded by the spinal meninges and spaces; the filum terminale runs among the roots.

Mentioned in

Generated automatically from links in other entries.

Lumbar puncture anatomy (sagittal view)The spinal cord ends at the conus medullaris at about L1–L2; the cauda equina continues in the dural sac, which ends at about S2. A needle entering between the L3 and L4 spinous processes passes the ligaments and epidural space into the subarachnoid space below the cord. The intercristal line crosses L4.T12L1L2L3L4L5SacrumSpinal cordConus medullaris (L1–L2)← Cauda equinaDural sac ends ≈ S2Supraspinous / interspinous lig.Ligamentum flavumEpidural space → dura → CSFLP needle (L3–L4)Intercristal (Tuffier) line ≈ L4AnteriorPosterior

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Lumbar puncture anatomy. Sagittal schematic (not to scale): the cord ends at about L1–L2, the dural sac at about S2. The needle enters at L3–L4, just above the intercristal line (≈ L4), well below the conus.
Original schematic, created for this atlas.
Lumbar disc herniation and nerve rootsPosterior view of L4, L5 and S1. Each lumbar root exits below the pedicle of its own vertebra. A posterolateral herniation of the L4–L5 disc compresses the traversing L5 root; a far-lateral herniation at the same level compresses the exiting L4 root.L4L5S1L4–L5 discL5–S1 discDural sac / cauda equinaL4 rootL5 rootS1 root② far-lateral → L4① posterolateral→ L5 (traversing)RootWeaknessReflexSensory lossL4Knee extension, ankle dorsiflexionPatellar ↓Medial leg / ankleL5Great-toe extension, hip abductionNone reliableDorsum of foot, 1st webS1Plantarflexion, eversionAchilles ↓Lateral foot, sole

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Lumbar disc herniation and nerve roots. Posterior view of L4–S1 (right-sided roots shown). Lumbar roots exit below their own pedicle, so a posterolateral L4–L5 herniation (1) compresses the traversing L5 root and a far-lateral herniation (2) compresses the exiting L4 root.
Original schematic, created for this atlas.
Posterior view of the lower spinal canal with the dura opened, showing the conus medullaris, the nerve roots of the cauda equina and the filum terminale
Cauda equina and filum terminale from behind, dura opened.
Henry Vandyke Carter, Gray's Anatomy of the Human Body, 20th ed. (1918), Fig. 662. Source · Public domain (Public domain in the US: published before January 1, 1931 (per Wikimedia Commons))
Sagittal section of the lumbosacral vertebral canal showing the spinal cord ending near L1, the dural sac ending at the second sacral vertebra and the filum terminale
Sagittal section showing the conus medullaris, cauda equina and filum terminale within the dural sac.
Henry Vandyke Carter, Gray's Anatomy of the Human Body, 20th ed. (1918), Fig. 661. Source · Public domain (Public domain in the US: published before January 1, 1931 (per Wikimedia Commons))

Clinical correlations

Cauda equina syndrome PANCE

Mechanism / lesion

Compression of multiple roots, most often by a large central lumbar disc herniation (L4–L5 or L5–S1); also tumor, epidural abscess or hematoma, spinal stenosis, or trauma.

Classic presentation

Low back pain with bilateral sciatica (sometimes unilateral), saddle anesthesia, urinary retention (with overflow incontinence), fecal incontinence, reduced anal tone, sexual dysfunction, and lower motor neuron leg weakness with reduced reflexes. Emergency MRI and surgical decompression: delay worsens bladder and neurologic outcomes.

Conus medullaris syndrome (for comparison)

Mechanism / lesion

Lesion at the conus (around L1–L2), e.g. fracture, tumor or disc.

Classic presentation

More sudden and symmetric, with early bladder, bowel and sexual dysfunction and perianal numbness. Leg findings can be mixed UMN/LMN (e.g. brisk knee jerk with absent ankle jerk).

High-yield PANCE pearls

  • Saddle anesthesia + urinary retention + bilateral sciatica + reduced anal tone → cauda equina syndrome → emergency MRI + decompression.
  • Cauda equina = lower motor neuron signs (reduced reflexes, flaccid weakness).
  • Check post-void residual (retention) and perianal sensation in any back pain with red flags.

Imaging anatomy

MRI

Emergency MRI of the lumbosacral spine is the test of choice; CT myelography if MRI is unavailable or contraindicated.

Ultrasound

Bladder scan measures post-void residual (large residual volumes support retention).

Key views & landmarks
  • Urgent MRI lumbar spine
  • Bladder scan / post-void residual

Physical exam

Surface landmarks

The cauda equina lies deep to the L2–S2 spinous processes.

Strength / function testing

Perianal (saddle) sensation to pinprick, anal tone and voluntary squeeze on digital rectal exam, the anal wink (S2–S4) and bulbocavernosus reflexes, lower limb strength and reflexes, and post-void residual.

Self-quiz

Q1.

A 45-year-old man has severe low back pain with pain radiating down both legs, numbness around the anus and inner thighs, and he hasn't urinated for 12 hours despite the urge. Anal tone is reduced. What is the most appropriate next step?

Show answer

B

Saddle anesthesia, urinary retention, bilateral sciatica and reduced anal tone indicate cauda equina syndrome. Obtain an emergency MRI for urgent surgical decompression.

Q2.

What is the most common cause of cauda equina syndrome?

Show answer

B

A large central disc herniation at L4–L5 or L5–S1 is the most common cause.

Q3.

Cauda equina compression produces which type of motor findings in the legs?

Show answer

B

The cauda equina consists of peripheral nerve roots, so compression produces lower motor neuron signs.

Q4.

In a patient with back pain and suspected cauda equina syndrome, which bedside test helps detect urinary retention?

Show answer

A

Measuring the post-void residual (by bladder scan or catheter) detects urinary retention, a key feature of cauda equina syndrome, along with perianal sensation and anal tone. Suspicion warrants emergency MRI.

Q5.

After an L1 burst fracture, a patient has sudden symmetric bladder and bowel dysfunction, perianal numbness, a brisk knee jerk and absent ankle jerks. Where is the lesion?

Show answer

D

A conus medullaris lesion (around L1–L2) gives early, symmetric bladder, bowel and sexual dysfunction with perianal numbness and mixed UMN/LMN leg findings. Cauda equina lesions produce purely lower motor neuron signs and are often asymmetric.

Flashcards

See also