What Is the Spinal Conus and What Does It Do?

The spinal cord doesn’t run the full length of the spine. It ends in a cone-shaped tip in the lower back called the conus (conus medullaris), and below that tip the spine holds only loose nerve roots. The conus is small, but it holds the cord’s final control centers for the bladder, bowel and sex organs. An injury or growth in this short stretch can cause problems out of proportion to its size.

Where the Conus Sits in Your Spine

Doctors name the spine’s bones by region and number. T12 is the last rib-bearing vertebra of the upper back, and L1 is the first bone of the lower back. The conus is the end of the spinal cord and typically sits at the L1 level in the average adult.¹ A strand of tissue called the filum terminale anchors its tip to the base of the spine.

The cord ends so high because the spinal bones keep growing after the cord stops lengthening. The nerve roots therefore have to travel downward inside the canal to reach their exits lower in the spine. That is why an MRI of the lower back shows cord at the top of the image and only roots below it. Seeing only strands and fluid at the L3 to L5 levels is normal anatomy, not a missing cord.

You can find L1 on your own body. Run a hand down your back to the bottom of the rib cage, and L1 sits roughly level with it. That is higher than most people picture the lower back. The usual spot for a spinal tap, between L3 and L4 or L4 and L5, is well below the conus.

Below the conus, nerve roots run down the canal in a bundle called the cauda equina, Latin for horse’s tail.² These roots carry movement and feeling signals for the legs, bladder, anus and the area between the legs.³

What the Conus Controls

The conus contains the lowest cord segments, the sacral segments S2 to S4, which handle the pelvic organs. Damage here affects bladder, bowel and sexual function.⁴ A small cluster of nerve cells in the conus controls the muscles that hold in urine and stool.⁵ Doctors check this circuit with a reflex: a light touch to the genitals makes the anal sphincter squeeze.⁶

When the bladder fills, stretch signals travel up the sacral nerves into the conus. This helps coordinate the bladder muscle contracting while the sphincter relaxes at the right moment. The same region keeps the anal sphincter closed at rest, so a damaged conus can cause stool leakage without warning. Sexual response, including erection and genital sensation, also depends on these segments.

The conus also contains built-in circuits that can help produce rhythmic leg movements such as stepping.⁷ In some people with complete spinal cord injuries higher up, electrical stimulation of the conus has helped them regain some movement.⁸ The conus sits just beside the cord segments that supply the lower legs and feet, so damage can cause foot weakness as well as pelvic problems.

What Counts as a Normal Conus Level

These percentages show how common each level is, not a cutoff between healthy and unhealthy. Even the rare finding at L2 or beyond came from healthy people, so level alone doesn’t diagnose anything. Radiologists describe the level and then judge it alongside the cord’s shape and the person’s symptoms. A conus below L2 is often called low-lying.

Some studies find no real difference between men and women or with age.¹⁴ Others find a small shift with age and sex.¹⁵ Bending forward or backward doesn’t change the position in any meaningful way.¹⁶ Children’s levels follow the same kind of spread and don’t change much as they grow.¹⁷

The conus reaches its adult level before birth. Early in pregnancy the cord reaches as far down as L4.¹⁸ By about 26 weeks the cord tip has moved up to its normal birth level,¹⁹ and at full term it sits above L2.²⁰ A newborn ultrasound can check the level.

Warning Signs of Conus Damage

Doctors call damage from pressure on the cord between T12 and L2 conus medullaris syndrome, or conus syndrome.²¹ Typical signs are numbness in the saddle area (the buttocks, inner thighs and genitals), bladder, bowel or sexual problems, low back pain, and leg weakness or tingling.²² Up to about 1 in 10 people with conus or cauda equina damage still have normal feeling in the saddle area,²³ so normal saddle feeling doesn’t rule out a problem.

Saddle numbness may show up as not feeling toilet paper when wiping, or as an odd sensation when sitting on a hard seat. Bladder trouble can mean difficulty starting to urinate, a weak stream, a feeling that the bladder isn’t empty, or leaking without noticing the urge. Bowel changes can include constipation or accidental leakage, and a person may not feel stool passing. Sexual changes such as reduced genital sensation or difficulty with erection can be early. They are easy to be embarrassed about, but they count as real warning signs.

Causes include fractures, tumors, abnormal blood vessels, inflammation and slipped discs.²⁴ When cancer spreads to the conus, people most often notice numbness (3 in 4), bladder problems (6 in 10) and back pain (half).²⁵ The conus’s blood supply has few backup routes, so a drop in blood flow can injure it.²⁶ Its symptoms can also be mistaken for other conditions, which can delay diagnosis.²⁷²⁸

Most back pain is muscular and has nothing to do with the conus. The warning signs are the pelvic and saddle symptoms, especially with weakness in both legs. They matter most when they are new or appear together, such as back pain plus a change in bladder function. A single mild symptom doesn’t prove conus damage, but pelvic changes with leg weakness deserve prompt assessment.

Go to the emergency room the same day for new numbness between the legs, new trouble starting or holding urine, or loss of bowel control. An MRI is the test that shows the problem.

How Conus Syndrome Differs From Cauda Equina Syndrome

Cauda equina syndrome is pressure on the nerve roots below the cord, most often at L3 to L5.²⁹ Injuries to the bones from T12 to L2 tend to damage the conus itself.³⁰ Because the conus level varies from person to person, the exact injury site isn’t always clear from the bone level alone.³¹

The symptoms tend to differ. Conus damage often brings bladder and saddle problems early, sometimes with less pain. Root damage more often causes sharp leg pain and one-sided weakness. Both are emergencies.

The conus is part of the central nervous system, while the cauda equina is made of peripheral nerve roots, and the two react to injury differently. Conus damage can produce a mix of signs, with some reflexes lost and others preserved, depending on which segments are hurt. Root damage tends to follow individual nerve paths, so symptoms may be patchy and affect one leg more than the other.

Tethered Cord and Other Conditions at the Conus

In tethered cord syndrome, the end of the cord is anchored by stiff tissue and gets stretched.³² The stretching cuts blood flow, which leads to leg weakness, numbness and bladder problems.³³ The stretched tip is pulled further whenever the spine bends or a child grows, so symptoms can slowly worsen. In children it may show up as changes in walking, foot deformity, back or leg pain, or new bladder accidents after toilet training. Adults may notice back or leg pain, numbness, or bladder changes, which are easy to blame on other causes.

Tethered cord is often found in children, sometimes with a dimple, hair tuft or mark on the low back. Adults can develop symptoms after years of growth or bending. MRI shows a low-lying conus and sometimes a thickened filum or other cause of the tether. Surgery releases the tight tissue so the cord can move freely, and the aim is often to stop symptoms from worsening.

Abnormal tangles of blood vessels can also form at the conus.³⁴ A blood-filled vessel cluster there has been mistaken for an inflammation of the cord.³⁵

Recovery After a Conus Injury

After surgery to relieve pressure from a conus injury, nerve-root function improved in about 8 in 10 people.³⁶ Bladder function improved in about 6 in 10 and cord function in about 4 in 10.³⁷ But in one small group with fractures, 3 of 8 still needed a catheter to empty the bladder.³⁸ Half reported unsatisfying sexual function afterward, more often men.³⁹

Improvement can continue over months. Rehab often includes bladder retraining, pelvic floor physiotherapy and sexual health care. Retraining includes timed voiding and learning cues for when to try to empty the bladder. Some people learn clean intermittent self-catheterization, where a thin tube is passed several times a day to empty the bladder fully. Regular emptying protects the kidneys and avoids infections, which is why it is taken seriously.

Implanted conus stimulators have let some paralyzed people empty their bladder on demand.⁴⁰ Overall, though, stimulating the conus to improve bladder control has given mixed results.⁴¹ During surgery near the conus, teams test its reflexes to protect bladder function.⁴²⁴³

1
PubMed. Cauda Equina and Conus Medullaris Syndromes ↩︎
2
PubMed. Cauda Equina and Conus Medullaris Syndromes ↩︎
3
PubMed. Cauda Equina and Conus Medullaris Syndromes ↩︎
4
PubMed. Cauda Equina and Conus Medullaris Syndromes ↩︎
5
PubMed Central. Bulbocavernosus Reflex Monitoring During Intramedullary Conus Tumor Surgery ↩︎
6
PubMed Central. Bulbocavernosus Reflex Monitoring During Intramedullary Conus Tumor Surgery ↩︎
7
PubMed Central. Spinal cord stimulation and rehabilitation in an individual with chronic complete L1 paraplegia due to a conus medullaris injury: motor and functional outcomes at 18 months ↩︎
8
PubMed Central. Spinal cord stimulation and rehabilitation in an individual with chronic complete L1 paraplegia due to a conus medullaris injury: motor and functional outcomes at 18 months ↩︎
9
PubMed Central. The Level of Conus Medullaris in 629 Healthy Japanese Individuals ↩︎
10
PubMed Central. The Level of Conus Medullaris in 629 Healthy Japanese Individuals ↩︎
11
PubMed Central. The Level of Conus Medullaris in 629 Healthy Japanese Individuals ↩︎
12
PubMed. Magnetic resonance imaging study of the level of termination of the conus medullaris and the thecal sac: influence of age and gender ↩︎
13
PubMed. The variation of position of the conus medullaris in an adult population. A magnetic resonance imaging study ↩︎
14
PubMed. The variation of position of the conus medullaris in an adult population. A magnetic resonance imaging study ↩︎
15
PubMed. Magnetic resonance imaging study of the level of termination of the conus medullaris and the thecal sac: influence of age and gender ↩︎
16
PubMed. Level of conus medullaris termination in adult population analyzed by kinetic magnetic resonance imaging ↩︎
17
PubMed Central. Assessment of the levels of termination of the conus medullaris and thecal sac in the pediatric population ↩︎
18
PubMed. Development of the fetal spinal cord: time of ascendance of the normal conus medullaris as detected by sonography ↩︎
19
PubMed Central. Ascent of the conus medullaris in human foetuses: a systematic review and meta-analysis ↩︎
20
PubMed. Development of the fetal spinal cord: time of ascendance of the normal conus medullaris as detected by sonography ↩︎
21
PubMed. Cauda Equina and Conus Medullaris Syndromes ↩︎
22
PubMed Central. Pathology of the conus medullaris and cauda equina. Beyond the usual suspects ↩︎
23
PubMed. Saddle sensation is preserved in a few patients with cauda equina or conus medullaris lesions ↩︎
24
PubMed Central. Pathology of the conus medullaris and cauda equina. Beyond the usual suspects ↩︎
25
PubMed. Understanding metastatic involvement of the conus medullaris: a systematic review of clinical presentations, diagnostic approaches, treatment options, and patient outcomes ↩︎
26
PubMed Central. Artery of Adamkiewicz: a meta-analysis of anatomical characteristics ↩︎
27
PubMed Central. Conus medullaris tumor co-existed with chronic low back pain: a case report and literature review ↩︎
28
PubMed. Cavernoma of the conus medullaris mimicking transverse myelitis ↩︎
29
PubMed. Cauda Equina and Conus Medullaris Syndromes ↩︎
30
Spinal Cord. Definitions of traumatic conus medullaris and cauda equina syndrome: a systematic literature review ↩︎
31
PubMed. Factors affecting neurological outcome in traumatic conus medullaris and cauda equina injuries ↩︎
32
Journal of Taibah University Medical Sciences. Tethered cord syndrome: A study of 35 patients ↩︎
33
PubMed. Spinal cord traction, vascular compromise, hypoxia, and metabolic derangements in the pathophysiology of tethered cord syndrome ↩︎
34
PubMed Central. Concomitant Conus Medullaris Arteriovenous Malformation and Sacral Dural Arteriovenous Fistula of the Filum Terminale ↩︎
35
PubMed. Cavernoma of the conus medullaris mimicking transverse myelitis ↩︎
36
PubMed Central. Efficacy of Surgical Decompression in Regard to Motor Recovery in the Setting of Conus Medullaris Injury ↩︎
37
PubMed Central. Efficacy of Surgical Decompression in Regard to Motor Recovery in the Setting of Conus Medullaris Injury ↩︎
38
PubMed Central. Surgical outcomes in thoracolumbar fractures with pure conus medullaris syndrome ↩︎
39
PubMed Central. Surgical outcomes in thoracolumbar fractures with pure conus medullaris syndrome ↩︎
40
Applied Neurophysiology. Electrical Stimulation of the Conus medullaris in the Paraplegic: A 5-Year Review ↩︎
41
PubMed Central. Rehabilitation of Spinal Cord Injury: WFNS Spine Committee Recommendations ↩︎
42
PubMed Central. Bulbocavernosus Reflex Monitoring During Intramedullary Conus Tumor Surgery ↩︎
43
PubMed. The predictive value of intraoperative bulbocavernosus reflex monitoring for postoperative voiding function in patients with conus medullaris and cauda equina tumors: a retrospective single center study ↩︎