Overview

Roots containedL2–Co1 (dorsal + ventral)

Lumbar puncture siteL3–L4 or L4–L5

Lumbar cisternL2 to S2

The vertebral column grows faster than the spinal cord, so the lower cord segments come to lie well above the intervertebral foramina through which their nerves exit. The dorsal and ventral roots of segments L2 to Co1 therefore run nearly vertically downward through the subarachnoid space (the lumbar cistern) before leaving at their own foramina. This sheaf of roots, grouped around the filum terminale, is the cauda equina.

Because the roots float freely in CSF, a needle introduced in the midline at L3–L4 or L4–L5 usually pushes them aside rather than piercing them. This is the anatomical basis of lumbar puncture and of spinal (intrathecal) anaesthesia. The roots are peripheral nervous tissue (Schwann cell myelin), so lesions produce lower motor neuron signs.

Location: Within the dural sac from the conus medullaris (L1–L2) to the end of the sac (S2); individual roots then leave through the lumbar and sacral foramina.

Anatomy

Nerve roots: Dorsal and ventral roots of L2–L5, S1–S5 and Co1.
Relations: It lies within the lumbar cistern of the subarachnoid space, posterior to the lumbar vertebral bodies and intervertebral discs and anterior to the laminae and ligamenta flava. The filum terminale runs in its centre.

Function

Carries motor fibres to the lower limbs, pelvic floor and sphincters.
Carries sensory fibres from the lower limbs, perineum (saddle area), bladder and rectum.
Carries parasympathetic (S2–S4) outflow to the bladder, rectum and genitals.

Clinical significance

Cauda equina syndrome

Compression of several lumbosacral roots, most often by a large central L4–L5 or L5–S1 disc herniation, but also by tumour, abscess, haematoma or spinal stenosis. Features: bilateral or alternating sciatica; saddle anaesthesia (S3–S5); urinary retention with overflow (late, painless); faecal incontinence and reduced anal tone; sexual dysfunction; asymmetric LMN leg weakness with reduced ankle reflexes. It is a surgical emergency: decompress as soon as possible, ideally within 48 h, to preserve sphincter function.

Lumbar puncture

Performed at L3–L4 or L4–L5. The supracristal (Tuffier) line joining the iliac crests crosses L4 or the L4–L5 interspace. The needle passes through skin, supraspinous ligament, interspinous ligament, ligamentum flavum ("pop"), epidural space, dura and arachnoid to reach the lumbar cistern. Look for papilloedema or focal signs first: an LP with a mass lesion risks herniation.

Cauda equina versus conus lesions

A cauda equina lesion is LMN only, often asymmetric, gradual and painful (radicular pain), and sphincter involvement comes late. A conus lesion is symmetric and sudden, with early sphincter loss, less pain and mixed UMN and LMN signs.

Test yourself

A patient with sciatica develops saddle anaesthesia, painless urinary retention and bilateral weak ankle plantarflexion. What is the most appropriate interpretation?

Simple L5 radiculopathy; analgesia
Cauda equina syndrome; emergency MRI and decompression
Peripheral neuropathy; nerve conduction studies
Piriformis syndrome; physiotherapy

Answer: Cauda equina syndrome; emergency MRI and decompression

Compression of multiple sacral roots (S2–S5) below the conus causes perineal anaesthesia and bladder and bowel dysfunction. Delayed decompression risks permanent incontinence.