Cauda equina

At a glance

English

Cauda equina

Latin (TA2)

Cauda equina

System

Nerve

Region

Trunk

Side

Single, on the midline

About the cauda equina

The cauda equina, Latin for horse's tail, is the bundle of nerve roots filling the lower part of the vertebral canal. It is made of the anterior and posterior roots of the second to fifth lumbar nerves, the five sacral nerves and the coccygeal nerve, all leaving the lower end of the cord at the lumbar enlargement and the tapering conus medullaris. The roots float in cerebrospinal fluid in the lumbar cistern, the widened part of the subarachnoid space below the cord, with the thin filum terminale running down among them.

Its shape comes from unequal growth. In early development the cord fills the whole canal, but the vertebral column then grows faster. At birth the cord ends at about L3, and by roughly the end of the first year it has reached its adult level near L1 or L2, though it may lie anywhere from T12 to L3. Each root stays attached to its own cord segment while still having to leave through its own foramen, so the lower roots are drawn out into long, nearly vertical strands. The bundle thins from above down as pairs peel off at each level.

Through these roots pass the motor and sensory fibres for the lower limbs, covering the hip, knee, ankle and foot, along with sensation from the perineum and saddle area and supply to the anal sphincters. The sacral roots S2 to S4 also carry parasympathetic fibres for bladder and bowel emptying.

Because the cord has ended above, a needle can be passed between the L3 and L4 or L4 and L5 laminae into the lumbar cistern to sample fluid or give spinal anaesthesia; the free-floating roots tend to move aside. Compression of the cauda equina, most often by a large central lumbar disc prolapse but also by tumour, infection, bleeding or narrowing of the canal, produces cauda equina syndrome: back and leg pain, saddle numbness, leg weakness and loss of bladder and bowel control. It is a surgical emergency, and even prompt decompression may not restore full function.

Course and supply

Arises from

Roots of spinal nerves L2 to S5 and the coccygeal nerve, leaving the lumbar enlargement and conus medullaris at about L1 to L2.

Ends in

Each root pair leaves the canal through its own intervertebral or sacral foramen, the last at the sacral hiatus.

Supplies

Carries motor and sensory fibres to the lower limbs, perineum and anal sphincters, and parasympathetic fibres to the bladder and lower bowel.

Blood supply

Supplied by radicular arteries that run along the roots, and by branches of the anterior and posterior spinal arteries near the conus.