Conus medullaris and cauda equina

Why it matters

Below the end of the cord, the canal holds nerve roots rather than cord. That is why a lumbar puncture can be done in the lower lumbar region without reaching the spinal cord ^[1].

What it is

The conus medullaris is the tapered lower end of the spinal cord ^[2]. Below it:

• the filum terminale, a fibrous continuation of the cord, anchors the conus to the coccyx ^[2];
• the lower nerve roots run down in a long bundle that resembles a horse's tail, the cauda equina ^[3];
• the dural sac continues below the conus; on MRI its lower end was most often at S2 ^[4].

Why it is built this way

The cord does not grow significantly longer after the first or second year, but the skeleton keeps growing ^[3]. The nerves grow with the column, so the lower roots must run down the canal before they leave through their own foramina ^{[3, 5]}.

Normal behaviour

The level of the conus varies between people. In MRI of 629 healthy Japanese adults, L1 was the most common level ^[6]:

• opposite L1 in 45.8 %^[6];
• opposite the T12–L1 disc in 32.4 %^[6];
• opposite the L1–L2 disc in 12.6 %^[6].

Pelvic incidence and height were significant factors in the level, and the authors concluded that the length of the cord varies little between people while differences in the skeleton move the level ^[6]. In a cadaver series of 30, the conus most often ended at the lower third of L1, ranging from the middle of T12 to the L2–L3 disc ^[2].

When it does not behave

Variation in the level of the conus and in the shape of the filum terminale matters in the evaluation of tethered cord syndrome, a condition of pathological traction on the spinal cord ^[2].

Common misconceptions

Check yourself

Why does the canal below L1 hold nerve roots rather than cord?

The cord stops lengthening after the first year or two while the column keeps growing, so the lower roots run down the canal to reach their exits ^[3].

What anchors the conus to the coccyx?

The filum terminale ^[2].

Read next

• Conus medullaris, with the variation chart.
• Filum terminale.
• Vertebral and spinal cord levels.