Conus Medullaris and Cauda Equina Anatomy

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

The conus medullaris is the tapered, cone-shaped termination of the spinal cord, where the solid cord ends and only nerve roots continue caudally. The cauda equina ("horse's tail") is the bundle of lumbosacral and coccygeal nerve roots that streams through the vertebral canal below the conus, and the filum terminale is the thin connective-tissue strand that anchors the cord's tip to the caudal dura and coccyx.

These three structures sit at the center of some of the most common and most serious conditions in small animal neurology. Degenerative lumbosacral stenosis, disc extrusion, lumbosacral transitional vertebrae, and synovial cysts all compress the cauda equina in dogs [1][2][3]. In people, the same anatomy separates conus medullaris syndrome from cauda equina syndrome, two conditions with different signs, different urgency, and different outcomes [4][5]. In every species, the level at which the cord ends determines where a needle can safely enter the spinal canal.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

Why the Termination Level Matters

The spinal cord does not fill the vertebral canal all the way to the tail. During development, the vertebral column elongates faster than the cord, so the cord's caudal end appears to "rise" relative to the vertebrae. In adult humans the conus medullaris typically ends near the L1-L2 disc space, with the most common single location at the lower third of the L1 vertebral body [6]. In dogs the conus ends much farther caudal, most often at L6, and in cats around L4. In sheep the conus terminates at S1 or S2, well caudal to the lumbosacral space [7].

That species difference is not trivia. It decides whether a needle placed at the lumbosacral junction will meet nerve roots only (dog, cat, sheep) or could meet the solid cord itself (human). It also explains why myelography, which relies on contrast outlining the cord, is a poor test for cauda equina compression in dogs: the dural sac is elevated from the vertebral canal floor and frequently ends before the lumbosacral junction, so the compressive lesion sits in a region the contrast column never reaches [8].

Anatomy of the Conus Medullaris

Shape and internal structure

The conus medullaris is the terminal segment of the cord. It contains the sacral and coccygeal spinal cord segments, so its internal gray matter holds the lower motor neurons for the bladder, bowel, tail, and perineum. The cord narrows here because the surrounding white matter tracts have already distributed most of their fibers to more cranial segments.

The conus has been classified by shape on MRI into three types (A, B, and C) in human studies, and shape is believed to relate to neurological conditions, though no correlation between shape and termination level has been found [6]. In dogs, the equivalent structure is simply the tapered end of the cord, and its termination level varies by breed and body size [9].

The terminal ventricle

At the very tip of the conus lies the terminal ventricle (ventriculus terminalis), a small dilation of the central canal. It is a normal developmental remnant of the caudal neural tube, not a cyst or a tumor. In imaging studies it can be mistaken for a syrinx or a cystic lesion, so recognizing it as a normal structure prevents unnecessary workup. The terminal ventricle communicates with the central canal cranially and is enclosed by the same ependymal lining as the rest of the ventricular system.

The filum terminale

The filum terminale is a slender band of pia mater and connective tissue that extends from the tip of the conus to the caudal end of the dural sac and then to the coccyx. It has two parts: the filum terminale internum, inside the dural sac, and the filum terminale externum, which continues beyond the dura to attach to the coccyx.

Filum terminale internum length (FTIL) has been measured in dogs by MRI. Heavier dogs have a more cranial conus termination and a longer filum, and FTIL correlates with age [9]. The filum's clinical importance is mainly mechanical: it tethers the cord caudally, and abnormal thickening or shortening is the basis of tethered cord syndrome in humans. In dogs, the filum is a normal structure and is not a routine target of surgery.

Anatomy of the Cauda Equina

What forms the cauda equina

Below the conus, the vertebral canal contains no solid cord, only nerve roots. These roots are the ventral and dorsal roots of the lumbosacral and coccygeal spinal nerves, plus the filum terminale. Together they form the cauda equina, named for its resemblance to a horse's tail.

Each root in the bundle carries specific functions. The L7 and S1 roots supply much of the hind limb. The S1-S3 roots carry parasympathetic fibers to the bladder and bowel and somatic fibers to the perineum. The coccygeal roots supply the tail. Because the roots are arranged in a compact bundle, a single compressive lesion can affect several of them at once, producing a mixture of hind limb weakness, pain, and sphincter dysfunction.

Why the roots are vulnerable

The cauda equina roots lack the protective cushioning that the cord has more cranially. They sit in the epidural space surrounded by epidural adipose tissue (EAT) and a venous plexus. When the lumbosacral canal narrows, the roots and the EAT are compressed together.

That compression is not purely mechanical. Epidural adipose tissue from dogs with cauda equina syndrome shows upregulation of tumor necrosis factor alpha (TNFα) and interleukin-10 (IL-10), and reduced expression of the adipokine leptin, compared with control dogs [10]. Inflammatory stimulation of EAT explants increased cytokine release [10]. This suggests that compressed epidural fat actively contributes to pain amplification rather than simply transmitting pressure.

Chronic compression also changes the nerve roots themselves. In one dog with long-standing L7-S1 disc extrusion, histopathology of the L7 nerve root showed chronic neural fibrosis with atrophy [3]. That is a structural change, not just a functional one, and it helps explain why some dogs have residual deficits after decompression.

Termination Levels by Species

The table below summarizes the typical termination levels of the conus medullaris and dural sac across species, with the clinical implication for needle placement and imaging.

The sheep data come from CT myelography in 40 adult Merino-mixed sheep. The conus was visible in 28 animals and terminated at S1 in 14 and S2 in 14. The dural sac was evaluable in 33 animals and ended caudal to the sacrococcygeal junction in 31 [7]. No termination cranial to the lumbosacral space was observed, which is why careful site selection matters for neuraxial anesthesia in that species [7].

Conus Medullaris Syndrome vs Cauda Equina Syndrome

These two syndromes are the classic clinical pairing in human neurology, and the distinction is directly relevant to veterinary students because the same anatomical logic applies to animals.

Conus medullaris syndrome

Conus medullaris syndrome results from a lesion at the cord's tapered end. Because the conus contains the sacral cord segments, the signs are symmetric and sphincter involvement appears early. Patients describe saddle anesthesia, meaning loss of sensation in the perineal and perianal region. Motor signs are typically mild and symmetric. Pain is less prominent than in cauda equina syndrome because the lesion is in the cord, not in the compressed roots.

Cauda equina syndrome

Cauda equina syndrome results from compression of the nerve roots below the conus. The signs are asymmetric because individual roots are affected to different degrees. Radicular pain, meaning pain radiating along the distribution of a compressed root, is a hallmark. Sphincter signs appear later than in conus syndrome, and lower limb weakness is often asymmetric.

In dogs, cauda equina syndrome presents with pain on rising, reluctance to jump, hind limb lameness, and mild ataxia [12]. A 7-year-old castrated male Rottweiler with an 18-month history of bilateral hind limb lameness and pain on rising was found to have a vacuum disk and vacuum facet phenomenon on CT [12]. The vacuum disk phenomenon, a gas-filled lumbosacral disk space, indicates degenerative disk disease and is more commonly seen on CT than on plain radiography [12].

How to tell them apart

The practical distinction rests on symmetry, pain, and timing of sphincter involvement. A symmetric deficit with early bladder dysfunction points to the conus. An asymmetric deficit with radicular pain and later sphincter signs points to the cauda equina. In dogs, the clinician relies on the neurological examination, imaging, and electrodiagnostics rather than on patient-reported sensory symptoms.

How These Structures Are Tested and Imaged

Imaging modalities

Many modalities are available for the canine lumbosacral region: conventional radiography, stress radiography, myelography, epidurography, transosseous and intravenous venography, discography, linear tomography, CT, and MRI [8]. Myelography, epidurography, and discography are commonly used but often lack sensitivity. Myelography is of little value for the cauda equina because the dural sac is elevated from the canal floor and frequently ends before the lumbosacral junction. Epidurography identifies ventrally located compressive lesions, and discography can delineate the dorsal extent of a diseased disc, but both are sometimes difficult to interpret. More than one technique is often needed to make a diagnosis [8].

CT and MRI have become the most valuable tools. Both are sensitive and specific for determining cauda equina compression in humans and dogs [8]. MRI reveals soft tissue such as the cauda equina, epidural fat, and intervertebral disc at the lumbosacral region clearly without contrast medium [13]. This is a major advantage because it avoids the risks and interpretive limits of contrast studies.

Electrodiagnostics

Electromyography (EMG) and spinal evoked potentials have been studied in dogs with cauda equina compression. After stimulation of the tibial, peroneal, pudendal, and coccygeal nerves and recording at the lumbosacral junction, latencies and nerve conduction velocities were largely normal, but amplitudes were decreased and potential morphology was altered depending on compression severity [14]. EMG showed increased spontaneous activity with fibrillations and positive sharp waves in the myotomes of the lumbosacral nerves. The number of these findings increased with compression severity and followed a centrifugal distribution [14]. The authors suggested performing EMG before myelography [14].

Radiographic and CT findings

Plain radiography can show dorsal dislocation of L7, spondylosis deformans, a sloped craniodorsal contour of S1, sclerosis of the cranial plate of S1, and narrowing or increased density of the L7/S1 intervertebral foramen [15]. In 15 of 227 large-breed dogs examined for lumbosacral disease, dorsal dislocation of L7 by 1 to 8 mm was found, and an extended position was more successful than a flexed one for demonstrating it [15]. A definite correlation between spondylolisthesis of L7/S1 and compression of the cauda equina could not be found on plain radiography alone [15].

Clinical Relevance, Limitations and Common Mistakes

Lumbosacral transitional vertebrae

A lumbosacral transitional vertebra (LTV) is a vertebra at the junction of the lumbar and sacral spine that takes on intermediate characteristics. In a study of 4000 control dogs without signs of cauda equina syndrome, 3.5% had an LTV, while 16.3% of 92 dogs with cauda equina syndrome had one [1]. The lesion causing cauda equina syndrome always occurred between the last true lumbar vertebra and the LTV. Dogs with an LTV were eight times more likely to develop cauda equina syndrome than dogs without one, and German Shepherd Dogs were eight times more likely to develop it than other breeds. Male dogs were twice as likely as females, and dogs with an LTV developed the syndrome 1 to 2 years earlier than dogs without one [1].

A separate study of 161 German Shepherd Dogs found an association between transitional vertebral segments and cauda equina syndrome, and between degenerative disk disease and the syndrome. The proportion of affected dogs was higher when both findings were present. Because transitional vertebrae are probably inherited, the authors suggested considering the lesion in breeding selection [16].

Other compressive causes

Synovial cysts at the lumbosacral joint can compress the cauda equina. In two German Shepherd Dogs, cysts detected by MRI were removed via dorsal laminectomy, and both dogs were free of clinical signs 6 and 8 months after surgery [2]. Synovial cysts should be considered in the differential diagnosis when lumbosacral degenerative joint disease is present [2].

Disc extrusion can also cause severe compression. One German Shorthaired Pointer with a chronic history of progressive paraparesis, limp tail, and spinal pain had marked bilateral L7 nerve root enlargement and L7-S1 disc extrusion on MRI, with chronic neural fibrosis and atrophy on nerve biopsy [3].

Common mistakes

The most common mistake is assuming that the conus medullaris ends at the same level in every species. A needle placed at the lumbosacral junction is safe in dogs and cats because the cord ends cranially, but the same logic does not transfer to humans, where the cord ends near L1-L2 [6]. In sheep, the conus ends at S1 or S2, so lumbosacral and sacrococcygeal epidural injections carry a risk of iatrogenic cord injury if the site is selected carelessly [7].

A second mistake is relying on myelography alone for suspected cauda equina compression in dogs. The dural sac often ends before the lumbosacral junction, so the contrast column may not reach the lesion [8]. CT or MRI is needed.

A third mistake is treating cauda equina syndrome as a purely mechanical problem. The epidural adipose tissue in affected dogs shows an inflammatory cytokine profile, with elevated TNFα and IL-10 and reduced leptin [10]. Pain in these patients has an inflammatory component that mechanical decompression alone may not fully address.

A fourth mistake is confusing the conus medullaris with the cauda equina in clinical descriptions. The terms are sometimes used interchangeably in casual conversation, but they describe different structures with different lesion syndromes. Symmetric early sphincter signs point to the conus. Asymmetric radicular pain points to the cauda equina.

A fifth mistake is overlooking the filum terminale and terminal ventricle as normal structures. Both are developmental remnants, and mistaking them for pathology leads to unnecessary testing.

Quick Review

1. The conus medullaris is the tapered end of the spinal cord. The cauda equina is the bundle of nerve roots below it. The filum terminale anchors the cord tip caudally.
2. The conus ends at approximately L1-L2 in adult humans, L6-L7 in dogs, around L4 in cats, and S1-S2 in sheep [7][9][6].
3. Conus medullaris syndrome is symmetric with early sphincter involvement and saddle anesthesia. Cauda equina syndrome is asymmetric with radicular pain and later sphincter signs.
4. Myelography is of limited value for the canine cauda equina because the dural sac often ends before the lumbosacral junction [8]. CT and MRI are more sensitive and specific.
5. Lumbosacral transitional vertebrae increase the risk of cauda equina syndrome eightfold in dogs and are probably inherited [1][16].
6. Compressed epidural adipose tissue releases inflammatory cytokines that amplify pain [10].
7. The terminal ventricle is a normal dilation of the central canal at the conus tip, not a cyst.

Frequently Asked Questions

What is the difference between the conus medullaris and the cauda equina?

The conus medullaris is the solid, tapered end of the spinal cord itself. The cauda equina is the collection of nerve roots that continues below it. The conus contains cord tissue, while the cauda equina contains only roots and the filum terminale.

Where does the spinal cord end in dogs?

In most dogs the conus medullaris ends at L6, though this varies by breed and body size. Cavalier King Charles Spaniels and Corgis may reach the sacrum, while Boston Terriers tend to end more cranially at L5 or L6 [9].

Why is myelography less useful for cauda equina lesions in dogs?

The dural sac in dogs is elevated from the vertebral canal floor and frequently ends before the lumbosacral junction, so contrast injected for myelography may not reach the compressive lesion [8]. CT and MRI are preferred for this region.

What is the filum terminale?

The filum terminale is a thin band of pia mater and connective tissue extending from the conus tip to the coccyx. It helps anchor the cord caudally. Its length varies with body weight and age in dogs [9].

Can a dog recover from cauda equina syndrome?

Many dogs improve after appropriate treatment. In one surgical series, 96.5% of 86 dogs treated with partial dorsal laminectomy had relief of dorsal pressure and rapid regression of clinical symptoms [17]. Outcomes depend on the cause, duration, and severity of compression.

Is cauda equina syndrome an emergency in animals?

Cauda equina syndrome in dogs is usually a chronic, progressive condition rather than an acute surgical emergency like it is in humans [5]. Dogs typically present with pain and mild neurological deficits that worsen over months [12][3]. Any dog with sudden inability to urinate, severe pain, or rapid loss of hind limb function needs immediate veterinary assessment.

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