The Cord and Nerve Root: Lesion Patterns and Segmental Anatomy

01Cord level and root level

Cord lesions and root lesions differ by geometry, and the geometry sets the pattern

A cord lesion interrupts fibers in transit, and a root lesion interrupts a segment's exit. The cord runs long tracts down the length of the vertebral canal, so a lesion inside it interrupts fibers that began far above and end far below. The root is a short segmental exit, and a lesion there stops at that segment's borders.

White matter funiculi carry the named tracts. The dorsal columns relay fine touch and proprioception. The lateral corticospinal tract carries voluntary command. The lateral spinothalamic tract conveys pain and temperature, and its fibers cross through the anterior commissure before climbing to the ventral posterior lateral nucleus of the thalamus. Crossed and uncrossed fibers travel side by side, which is what lets one lesion declare a level and a side at the same time. Where those tracts begin is set out in the spinal cord.

Each spinal segment is bounded by the attachment of its dorsal and ventral root filaments, the sensory and motor halves of the Bell-Magendie plan. The dorsal root carries afferent traffic and the ventral root efferent. Together they define one dermatome and one myotome. Neighboring dermatomes overlap, so a sensory level reads more sharply in pain and temperature than in touch, and less sharply in the leg than in the arm.

The conus is where the cord stops and the roots continue

The cord tapers at the conus medullaris near the L1 or L2 vertebral level, and below it the lumbosacral roots descend as the cauda equina. Across 585 upright kinetic MRI studies in adults, the commonest termination was the lower third of L1, at 17.61 percent in neutral posture Liu 2017. The level did not shift with flexion or extension under weight bearing. It did not drift with age. It sat lower in women, especially after 60.

An MRI study of 200 patients put the modal termination at the lower third of L1 in men and at the L1 to L2 disc in women Nasr 2017. In that study's 60 cadaveric specimens, the lower third of L1 held in both sexes. On MRI the dural sac ended at the upper third of S2 in men and the middle third of S2 in women, with no sex difference in the cadaveric specimens. Those two boundaries separate a conus picture from a cauda equina one. The transition point is not identical in every patient, so the same disc level can mean two different syndromes in two different bodies.

02Findings

What the research shows

03Long tract syndromes

Cord syndromes take their pattern from the fibers a lesion crosses, and Brown-Sequard and central cord are the two commonest shapes

A hemisection produces the Brown-Sequard pattern: ipsilateral loss of motor power and dorsal column sensation below the level, with contralateral loss of pain and temperature. The spinothalamic fibers had already crossed. The corticospinal and dorsal column fibers had not.

A regional referral unit managed 51 consecutive patients for spinal stab injury with a retained knife blade. Twenty-one of them (41 percent) had incomplete cord injury, and 17 of those 21 showed features of Brown-Sequard syndrome Enicker 2015. Twelve (24 percent) were complete, and eighteen (35 percent) were neurologically intact with a blade still in the canal. Thirty-eight of the 51 blades sat in the thoracic spine. What the blade crossed on its way in produced the pattern.

A lesion expanding from the central canal strikes the crossing spinothalamic fibers first. That produces a suspended cape of pain and temperature loss across the shoulders while touch and proprioception ride past in the dorsal columns.

Central cord is the most common incomplete cord injury

Acute traumatic central cord syndrome accounts for up to 70 percent of incomplete spinal cord injuries Kumar 2023. Pooling 749 patients from 16 studies, motor recovery reached 76.1 percent after surgery against 66.1 percent with conservative care (p = 0.04). Early and delayed surgery did not differ, at 69.9 percent motor recovery early against 77.2 percent delayed (p = 0.31). The pattern that reads as exotic on a diagram is the ordinary one in a trauma bay.

The Unified Model of Tone takes the timing null as information. Recovery after a central cord injury tracks what the cord carried into the injury: the room in its canal, the spondylosis already there, and how much anterior horn survived. A clock cannot hold any of that. Two cords compressed to the same millimeter arrive at surgery in different states, and the state is what the operation acts on.

04Canal caliber and myelopathy

Cervical spondylotic myelopathy begins where the canal runs out of room, and that shortfall is measurable in millimeters

Measured on radiographs in 997 normal adult Japanese men and women, mean sagittal canal diameter was 15.8 mm at C3 and 15.2 mm at C4, the narrowest level Sasaki 1998. C5 measured 15.3 mm, C6 15.7 mm and C7 15.9 mm. C1 and C2 are the roomiest levels by a wide margin, at 21.0 mm and 18.0 mm. Mean minus two standard deviations reaches 12.2 mm at C4, and myelopathy risk climbs steeply below 12 mm.

Direct caliper measurement of 1,066 cadaveric specimens set congenital cervical stenosis at a sagittal canal diameter under 13 mm with an interpedicular distance under 22.5 mm Bajwa 2012. Those two cutoffs identified congenital stenosis with 88 to 100 percent sensitivity and specificity at every level, at an odds ratio above 18. The 13 mm line sits two standard deviations below the population mean.

The canal is read as a ratio because films are not to scale

Radiographic magnification changes a millimeter. The ratio method divides the sagittal diameter of the canal by the sagittal diameter of its own vertebral body at the same level, which cancels magnification out. It was established on radiographs of 23 patient athletes with cervical spinal neurapraxia Pavlov 1987. Every raw millimeter threshold quoted in a report rests on that correction holding.

Compression after 40 is the rule and myelopathy is the exception

MRI of 183 volunteers over age 40 found cervical cord compression in 108 of them, 59.0 percent Kovalova 2016. Two people, 1.1 percent of the group, had clinical signs of symptomatic myelopathy. Compression rose with age from 31.6 percent in the fifth decade to 66.8 percent in the eighth. A canal diameter under 9.9 mm at disc level carried an odds ratio of 32.5 for compression, which is the neighborhood of the conventional 10 mm line for absolute stenosis. Roughly one compressed cord in fifty was symptomatic, which is why imaging names an anatomy and never names a patient.

Suspicion belongs to anyone over 55 with progressive gait change or loss of fine motor control in the hands de Oliveira Vilaca 2016. Brisk reflexes and spasticity below the level put the lesion in the cord, not at a root. Once symptoms begin, the course tends to progress, and the anatomy that produced them does not reverse on its own.

Snake eyes mark anterior horn loss

Intramedullary high signal appeared in 79 of 144 operated patients with compressive cervical myelopathy, and the snake-eye appearance on axial imaging carried a worse recovery Mizuno 2003. The JOA improvement ratio was 32.2 percent in patients with the snake-eye sign, 47.1 percent in patients whose high signal took another shape, and 50 percent in patients with no intramedullary high signal at all. Autopsy histology in nine patients who died of unrelated causes showed small cystic necrosis in the central gray matter and marked neuronal loss in a flattened anterior horn. The sign is venous infarction and cell death made visible on a film.

05Cervical radiculopathy

Cervical radiculopathy usually comes from a crowded foramen, not from a ruptured disc

In a population-based study of 561 patients in Rochester, Minnesota from 1976 through 1990, a confirmed disc protrusion accounted for 21.9 percent of cervical radiculopathy Radhakrishnan 1994. Spondylosis, disc or both accounted for 68.4 percent. Annual age-adjusted incidence ran 83.2 per 100,000 and peaked at 202.9 per 100,000 between ages 50 and 54. C7 was the most frequent monoradiculopathy, followed by C6.

That distribution matters at the table. A cervical root is usually being crowded by bone and by years, so the relevant quantity is how much room the foramen still holds. Where myelopathy is bilateral and long-tract, radiculopathy is segmental, and the examiner gets one segment to interrogate. The printed dermatome chart is a coarser instrument than it looks, and how far neighboring segments overlap is set out in the peripheral nerve and reflex.

In the lumbar spine the disc-to-root relationship is orderly. A paracentral L4 to L5 disc compromises the traversing L5 root, and an L5 to S1 lesion takes S1. The exiting root escapes at each level because it has already left above the herniation.

Far lateral herniations break the rule

Foraminal and extraforaminal herniations trap the exiting root instead of the traversing one. They were put at 7 to 12 percent of all lumbosacral disc herniations in one report Chun 2016 and at approximately 10 percent in another series Laskay 2024. Of 48 patients operated for far lateral herniation, 43 improved and 27 had complete resolution at a mean 2.6 months Laskay 2024.

The rule-breaking shows up at the bedside. A left extraforaminal L5 to S1 herniation with superior migration into the foramen produced foot drop, an L5 deficit at a level that predicts S1 Chun 2016. The same lesions may provoke a reversed Lasegue sign. An examiner who trusts the level over the pattern lands one segment away from the disc.

Cervical roots are tested by the same triad of motor, reflex and sensory examination, sharpened by provocation. The shoulder abduction sign relieves radicular pain when the arm is raised over the head, which unloads the root by shortening the distance it has to travel.

06The Spurling test

The Spurling test has no single accuracy, because the state it interrogates changes

The Spurling maneuver reproduced radicular pain with 95 percent sensitivity in one population and 30 percent in another, and the maneuver never changed. It is extension and lateral rotation toward the symptomatic side under axial load. Sensitivity reached 95 percent and specificity 94 percent in 257 patients with clinical cervical radiculopathy, against CT and MRI as the reference standard Shabat 2012. Among 255 consecutive patients referred for electrodiagnosis, the same maneuver returned a sensitivity of 6 of 20 (30 percent) and a specificity of 160 of 172 (93 percent) Tong 2002.

Specificity held near 93 to 94 percent in both studies. Sensitivity fell by two thirds. The hand doing the test did not change. The reference standard did, and so did the population: imaging gave way to electrodiagnosis, and a cohort of confirmed radiculopathy gave way to everyone who walked into the laboratory. Tong's own conclusion follows the numbers: the maneuver is weak as a screen and strong as a confirmation.

The Unified Model of Tone explains the disagreement without discarding either number. A provocation test delivers a fixed mechanical input and records what the segment does with it, so whether pain appears depends on the room the root has left when extension closes the foramen on it. A negative Spurling leaves a suspected root on the list. A positive one carries 93 percent specificity into the decision.

Where a root refers is not where it maps

Fluoroscopically guided stimulation of 134 cervical nerve roots from C4 to C8 in 87 patients provoked symptoms that resembled the classic dermatomal maps and frequently ran outside them Slipman 1998. The territory a root refers pain into, its dynatome, is wider and less orderly than the territory it maps sensation from.

So a root is named from motor, reflex and sensory findings together, not from where the pain runs. Pain distribution narrows the field to two or three segments. Weakness and a depressed reflex pick one of them. C7 is the most frequently involved cervical root, and it is confirmed by triceps weakness and a depressed triceps reflex, not by the line a patient draws down the forearm Radhakrishnan 1994.

07Conus and cauda equina

Conus and cauda equina lesions share territory and differ in symmetry, tempo and consequence

A conus medullaris lesion strikes the sacral cord directly, which produces early and symmetric sphincter involvement with saddle anesthesia. Cauda equina syndrome compresses the descending lumbosacral roots and tends toward asymmetry. Within each sacral root, the larger posterolateral sensory fibers fail before the anteromedial motor fibers, which is why saddle numbness precedes retention.

Cauda equina syndrome is rare. One year of surveillance across a population of 5.4 million found 149 cases, a crude incidence of 2.7 per 100,000 per year Woodfield 2022. Incidence peaked at 7.2 per 100,000 in women aged 30 to 39. Of 211 patients carrying the diagnostic code, 117 (55 percent) did not have the syndrome on review of the notes. The label is applied about twice as often as the syndrome occurs.

No single finding recognizes the syndrome, and the pattern does. A review of 105 articles identified 17 distinct definitions of cauda equina syndrome Fraser 2009. Bladder disturbance appeared in 74 percent of articles and sensory disturbance in 66 percent. The disc was named as the pathology in 45 percent. The complex runs low back pain with sciatica in one or both legs. Lower limb weakness, saddle sensory change and loss of bladder, bowel or sexual function complete it. In the acute form, back pain mounts suddenly and severely while urinary retention progresses to overflow incontinence. Cauda equina compression is one of the few true spinal surgical emergencies.

Retention at presentation is the line that decides recovery

Between 50 and 70 percent of patients are already in urinary retention when they present, and 30 to 50 percent present with an incomplete syndrome Gardner 2011. Which side of the retention line a patient stands on predicts what comes back. Keeping an incomplete syndrome from crossing that line is the whole point of speed.

Pooling 322 patients from 42 studies, decompression within 48 hours carried a significant advantage over decompression after 48 hours, with no additional gain from operating inside 24 hours Ahn 2000. Chronic back pain before surgery predicted worse urinary and rectal recovery. Preoperative rectal dysfunction predicted worse urinary continence. Increasing age predicted worse sexual function. What the root brought to the compression shaped what it took away.

Red flags do not clear a patient. Across seven studies and 569 participants, pooled sensitivity for cauda equina red flags ran from 0.19 to 0.43 against MRI, while pooled specificity ran from 0.62 to 0.88 Dionne 2019. Reduced anal tone, saddle anesthesia, urinary retention and bowel incontinence are far more specific than sensitive. Their presence earns same-day imaging. Their absence rules nothing out. The model expects that asymmetry. The same compressive load meets a different amount of root reserve in every patient, so the load at which a sign appears sits in a different place in each one.

08Localizing a spinal lesion

Localization reads two axes, and the answer separates who stays from who is referred

Transverse localization asks which tracts a lesion has taken, separating a dorsal column pattern from a spinothalamic or corticospinal one. Longitudinal localization asks at what level, read from the highest dermatome of sensory change and from which reflex has dropped out. A bilateral long-tract deficit with upper motor neuron signs points to the cord. A single depressed reflex with dermatomal sensory loss points to the root.

Presentations mislead in named ways. Lumbar stenosis mimics vascular claudication. A spinal dural arteriovenous fistula congests cord venous drainage and produces a slowly progressive myelopathy. Demyelinating disease worsens with heat. Each can pass as an exertional spinal syndrome until the whole pattern is assembled, which is why no single finding settles the question here.

Finding the level is the whole of the work, because the level decides who stays and who is sent on. Four findings end the visit early: progressive gait change after 55, a clumsy hand, saddle sensory change and a new sphincter problem de Oliveira Vilaca 2016 Gardner 2011. Each one sends a patient toward imaging or an emergency department instead of onto a table. Chiropractic care works at a single segment, delivering graded mechanical input to a joint and its afferents. The examination that names the segment is the same examination that rules out a cord.

Most spines are neither pristine nor failing. A canal of 15 mm holding a compressed cord at age 60 with no myelopathy is a segment working inside its range. The reserve still absorbs what decades of loading put on it, and the cord keeps reporting normally through spondylosis a radiologist can list at length. A canal with no surplus left has nothing to absorb with. The same osteophyte that cost the first person nothing takes fine motor control out of the hand and puts a stiff, unsteady gait in place of a normal one. Two people with identical spondylosis land on different amounts of spare capacity, which is why 59.0 percent of volunteers over 40 carry cord compression and 1.1 percent carry myelopathy Kovalova 2016. Spare capacity is what the examination is measuring, and a numb thumb or a leg that will not answer is what running out of it looks like.

09Tone

How this system expresses tone

Room is a number a body was issued, and spondylosis spends it slowly. Tone shows up here as what a segment can still do with the room it has left. That is why one osteophyte ends a career in the hands while the same osteophyte on the next film costs nothing.

Gain: a few degrees of cervical extension under axial load returns a whole limb of pain, which is amplification at the root and not force at the examiner's hand. Input quality: stimulating 134 cervical roots provoked symptoms outside the classic dermatomes, so what a root reports is wider and messier than the map used to read it. Prediction: the shoulder abduction sign catches a patient who has already parked the arm overhead, guarding a root before anyone examined it. Oscillation: a myelopathic cord returns brisk reflexes, spasticity and clonus below the level, a segmental rhythm that surfaces once descending control has been subtracted from the loop. Load: cervical cord compression climbs from 31.6 percent in the fifth decade to 66.8 percent in the eighth, which is decades of loading paid out in millimeters. Set point: retention is the line a cauda equina recovery is measured from, and 50 to 70 percent of patients have already crossed it by the time they present.

10Across the library

How this page relates to the rest of the library

11Frequently asked

Questions about this topic

12The sources

References

Sources: primary literature, linked inline.