Conus Medullaris and Cauda Equina¶
Definition¶
The conus medullaris is the tapered terminal end of the spinal cord, normally located at the level of L1–L2 in adults. Below the conus, the cauda equina ("horse's tail") is a bundle of lumbar, sacral, and coccygeal nerve roots that descend through the lumbar cistern within the thecal sac before exiting at their respective neural foramina. The filum terminale is a thin filament of pia mater extending from the conus tip to the coccyx, anchoring the cord inferiorly.
Anatomy¶
Conus Medullaris¶
- Cone-shaped termination of the spinal cord
- Normal position: L1–L2 level in adults; may be as low as L2–L3 in neonates
- A conus terminating below L2–L3 in adults or below L3 in neonates is considered abnormally low and raises concern for tethered cord
- Contains the sacral spinal cord segments (S3–S5) and the coccygeal segment
- The epiconus (L4–S1 segments) is located just above the conus
Cauda Equina¶
- Composed of the L2–S5 nerve roots plus the coccygeal nerve
- Nerve roots are peripheral nerves (lower motor neurons), not central nervous system tissue
- Float freely within the CSF-filled thecal sac (lumbar cistern)
- The thecal sac terminates at approximately S2
- Cauda equina nerve roots are more resistant to compression than the spinal cord itself, but acute compression constitutes a surgical emergency
Filum Terminale¶
| Component | Description |
|---|---|
| Filum terminale internum | Extends from the conus to the end of the thecal sac (S2); composed of pia mater; normally ≤ 2 mm thick |
| Filum terminale externum (coccygeal ligament) | Extends from S2 to the coccyx; composed of dura and pia |
Clinical Pearl
A filum terminale thicker than 2 mm on axial MRI, or a filum that enhances with contrast or contains fat (bright on T1), suggests a thickened filum or fatty filum — findings associated with tethered cord syndrome even if the conus level is normal. See Tethered Cord Syndrome.
Imaging Findings¶
MRI¶
MRI is the definitive modality for evaluating the conus and cauda equina:
| Structure | T1 Signal | T2 Signal | Key Features |
|---|---|---|---|
| Normal conus | Intermediate | Intermediate | Smooth taper, terminates at L1–L2 |
| Cauda equina roots | Intermediate | Low (within bright CSF) | Appear as linear filling defects in the thecal sac |
| Filum terminale | Low (thin) | Low (thin) | Should be ≤ 2 mm; no fat or enhancement |
| CSF (lumbar cistern) | Dark | Bright | Surrounds and outlines the cauda equina |
Conus pathology on MRI:
| Finding | Appearance | Differential |
|---|---|---|
| Low-lying conus (below L2–L3) | Cord extends too far caudally | Tethered cord syndrome |
| Conus expansion | Enlarged conus on sagittal/axial | Ependymoma, astrocytoma, conus infarction |
| Conus T2 hyperintensity | High signal within conus | Infarction, myelitis, demyelination, tumor |
| Clumped cauda equina roots | Roots adherent, not floating freely | Arachnoiditis, carcinomatous meningitis |
| Enhancing cauda equina roots | Post-contrast enhancement | Leptomeningeal disease, Guillain-Barré, infection |
CT Myelography¶
When MRI is contraindicated, CT myelography demonstrates:
- Conus level and morphology
- Nerve root cutoff or compression, Cauda Equina syndrome
- Intradural filling defects (tumors, disc fragments)
- Evaluation for CSF leaks
- Evaluation for arachnoiditis or clumping of nerve roots
- CSF venous fistula evaluation
Clinical Syndromes¶
Conus Medullaris Syndrome¶
- Caused by lesions affecting the conus (typically at T12–L2)
- Findings: bilateral, symmetric; early bladder/bowel dysfunction; saddle anesthesia; relatively preserved lower extremity strength; absent bulbocavernosus reflex
- Common causes: disc herniation, fracture, tumor, infarction
Cauda Equina Syndrome¶
- Caused by compression of the nerve roots below the conus
- Findings: asymmetric radiculopathy; progressive leg weakness; late bladder/bowel dysfunction; saddle anesthesia; diminished reflexes
- Surgical emergency — requires urgent decompression to prevent permanent neurological deficit
Emergency
Cauda equina syndrome with progressive neurological deficit requires emergent MRI and surgical consultation. Delay in decompression is associated with permanent bladder, bowel, and sexual dysfunction. See Cauda Equina Syndrome — Imaging.
Key Points¶
- The conus medullaris normally terminates at L1–L2; a low conus suggests tethered cord
- The cauda equina consists of L2–S5 nerve roots floating within the lumbar cistern
- The filum terminale should be ≤ 2 mm; thickening or fat content suggests tethered cord
- Conus medullaris syndrome and cauda equina syndrome have distinct clinical presentations
- Cauda equina syndrome is a surgical emergency
- MRI is the imaging standard for both the conus and cauda equina
References¶
- Saifuddin A, Burnett SJ, White J. The variation of position of the conus medullaris in an adult population. A magnetic resonance imaging study. Spine (Phila Pa 1976). 1998;23(13):1452-6. Available from: https://pubmed.ncbi.nlm.nih.gov/9670396/
- Nene Y, Jilani TN. Neuroanatomy, Conus Medullaris. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2023. Available from: https://www.ncbi.nlm.nih.gov/books/NBK545227/
- Berg EJ, Ashurst JV. Anatomy, Back, Cauda Equina. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2023. Available from: https://www.ncbi.nlm.nih.gov/books/NBK513251/
- Moon MS, Jeong JH, Kim SJ, Kim MS, Choi WR. Magnetic Resonance Imaging Observations of the Conus Medullaris in a Korean Population. Asian Spine J. 2019;13(2):313-7. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC6454280/
- Otto N, Kucera J, Hayes LL, Chandra T. Tethered Cord Syndrome: Role of Imaging Findings in Surgical Decision-Making. Cureus. 2023;15(9):e44854. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC10560071/
- Saker E, Henry BM, Tomaszewski KA, Loukas M, Iwanaga J, Oskouian RJ, Tubbs RS. The filum terminale internum and externum: A comprehensive review. J Clin Neurosci. 2017;40:6-13. Available from: https://pubmed.ncbi.nlm.nih.gov/28087185/
- Conus medullaris. Radiopaedia.org. Available from: https://radiopaedia.org/articles/conus-medullaris