Where the adult spinal cord ends and the cauda equina
In adults the spinal cord ends at about L1 to L2, and below it a bundle of nerve roots (fibres leaving the cord) forms the cauda equina.
Remember
In adults the spinal cord ends at about L1 to L2, so a lumbar puncture (a needle test to sample cerebrospinal fluid) below that level meets only the cauda equina nerve roots and the fluid. Compressing those roots causes cauda equina syndrome.
Memory aid
like the strands of a mop head
Picture the roots below the end of the cord as loose mop strands, not as a solid cord.
The spinal cord does not run the whole length of the vertebral canal, which forms when the holes (foramina) in the vertebrae line up and contains the spinal cord and the cauda equina. In an adult the cord usually ends between the L1 and L2 vertebral levels, where it tapers into the conus medullaris, the cone-shaped lower end of the cord.
Below the conus, the canal is occupied by the cauda equina (Latin for "horse's tail"). It is a bundle of lumbar (lower back), sacral (to do with the sacrum) and coccygeal (to do with the coccyx) nerve roots. The cord stops growing in infancy while the bony spine keeps growing, so the lower roots have to travel a long way down the canal before they reach their own exit holes.
These roots lie in the subarachnoid space, the space around the cord that is filled with cerebrospinal fluid (CSF). Below the conus this space widens into the lumbar cistern, a pool of CSF that can be sampled. A needle placed between L3 and L4, or between L4 and L5, therefore passes into the CSF without crossing the cord in a typical adult.
A fracture at L3 threatens these roots rather than the spinal cord itself. If the roots are compressed, the result is cauda equina syndrome, and the main signs are saddle-area anaesthesia (numbness in the area that would touch a saddle), bladder and bowel disturbance, reduced anal tone and leg weakness. Suspected cases need an urgent MRI scan. Damage to the tapered tip of the cord gives a very similar picture, so the level of the damage matters.
Key facts
- Where the adult cord ends
- usually between L1 and L2, at the conus medullaris, its tapered lower tip.
- Below the cord
- the canal holds the cauda equina, a bundle of lumbar, sacral and coccygeal nerve roots, not cord tissue.
- Lumbar puncture level
- the usual site for the needle is between L3 and L4 or between L4 and L5, below the end of the cord.
- Cord segment and vertebral level
- cord segments lie higher than the vertebrae of the same number, because the cord ends at about L1 to L2.
- Filum terminale
- a thin strand of fibrous tissue that anchors the cord and its coverings to the coccyx; it contains no nerve roots.
- Cauda equina syndrome
- compression of these roots gives saddle anaesthesia, bladder and bowel disturbance, reduced anal tone and weakness, and suspected cases need urgent MRI.
Exam traps
- A vertebral fracture does not lie beside the cord segment with the same number, because cord segments lie higher than the vertebrae of the same number. The cord ends at about L1 to L2, so at L3 the canal holds nerve roots, not cord.
- The conus medullaris is not at risk from a fracture at L3, because it lies at about L1 to L2 in adults. It would be the structure at risk if the fracture were at about L1 or L2.
- The filum terminale is not the bundle of nerve roots below the end of the cord, because it is a thin fibrous strand that anchors the cord and its coverings to the coccyx and contains no nerve roots.
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