Vertebral and spinal cord levels

Why it matters

A level on a scan names a bone; a level in a neurological examination names a piece of spinal cord. The two systems share letters and numbers but do not line up, so the same label can point to different places ^{[1, 2]}. Every label on this site says which system it uses.

What it is

Vertebral levels count the bones. The adult column has 24 vertebrae, plus the sacrum and coccyx: seven cervical, twelve thoracic and five lumbar ^[3].

Segment levels count the pieces of cord that give off each pair of spinal nerves. There are 31 pairs: eight cervical, twelve thoracic, five lumbar, five sacral and one coccygeal ^[1].

• There are eight cervical nerves but seven cervical vertebrae: C1 emerges between the skull and the first vertebra, and C8 between the seventh cervical and the first thoracic vertebra ^[1].
• Each thoracic and lumbar nerve emerges between the vertebra with the same name and the next one down ^[1].
• The sacral nerves leave through the sacral foramina ^[1].

How it works

The nerves are named for the level of the cord they come from, and each leaves the column through the intervertebral foramen at its level ^[1]. The cord, however, ends high in the canal. As the vertebral column grows, the lower nerves grow with it and form a long bundle in the canal that resembles a horse's tail, the cauda equina ^[4].

So in the neck a segment lies close to the vertebra with the same name, while low in the back the segments sit higher than the vertebrae their nerves leave by, and the nerves run down the canal to reach their exits ^{[1, 4]}. You can see this in the viewer by switching between the two level systems.

Normal behaviour

Where the cord ends varies between people. In MRI of 629 healthy Japanese adult volunteers, the end of the cord was most often opposite L1, followed by the T12–L1 disc ^[2]:

• opposite L1 in 45.8 %^[2] of people;
• opposite the T12–L1 disc in 32.4 %^[2];
• opposite the L1–L2 disc in 12.6 %^[2].

In that study, pelvic incidence and height were significant factors in where the cord ended, and the authors concluded that cord length varies little between people while differences in the skeleton move the level ^[2].

In theatre and clinic

A lumbar puncture is done low in the back to avoid the central tissue of the spinal cord, which ends above that level ^[8]. This is a learning page: it does not tell anyone where to place a needle.

Common misconceptions

Check yourself

Between which bones does the L4 nerve leave the canal?

Between the L4 and L5 vertebrae: thoracic and lumbar nerves emerge between the vertebra of the same name and the next one ^[1].

Why is there a cauda equina?

The vertebral column grows longer than the cord, and the lower nerves grow with it, forming a long bundle in the canal below the end of the cord ^[4].

Read next

• Conus medullaris, with the variation chart.
• Cauda equina.
• The L4–L5 motion segment.