Orthopedics · Part Four · Seeing and Ruling Out
Lesson 35 / 44
The Slow Cord Compression: How Cervical Spondylotic Myelopathy Announces Itself
Cervical spondylotic myelopathy has a slow, quiet onset. That is exactly why a trained clinician screens for it deliberately rather than waiting for it to announce itself.
Cervical spondylotic myelopathy is slow compression of the spinal cord in the neck, produced by degenerative narrowing of the spinal canal. It arrives quietly, as clumsy hands and an unsteady walk rather than as neck pain, and degenerative forms of cervical myelopathy are the commonest cause of spinal cord impairment in adults. The Unified Model of Tone reads the resulting damage as tensile stress carried into the cord by the dentate ligaments, rather than as squeezing alone.
Prevalence of degenerative cervical myelopathy in North America
at least 605 per million
Silent cord compression to first myelopathic sign, 25th percentile
48.4 months
Hoffmann sign for cervical cord compression
59 percent sensitivity, 49 percent specificity
Denticulate ligament failure force, C1 against C7
1.04 N against 0.55 N
Myelopathy and cervical spondylotic myelopathy
Myelopathy is any disease or dysfunction of the spinal cord, including physical compression of it. In the neck it most often follows degenerative change that narrows the sagittal dimensions of the spinal canal. That presentation is called cervical spondylotic myelopathy, and it is the largest member of a family now grouped as degenerative cervical myelopathy.
How the cord is held inside the canal
The cord does not float. Roughly twenty pairs of dentate ligaments run from the pia mater out to the dura and suspend it within the dural sleeve like a hammock. Those anchors fix the cord’s position and its shape. They also mean that anything displacing the cord within the canal pulls on the tissue holding it, so a bone spur in front changes the tension on ligaments attached at the sides.
01A quiet compression
Cervical spondylotic myelopathy is slow cord compression in an ordinary aging neck
For decades the neck was framed only as a source of pain and stiffness. The modern picture is more precise. Cervical spondylotic myelopathy is the slow, gradual compression of the spinal cord that a clinician learns to suspect long before it becomes obvious. Why does a stiff, aging neck sometimes matter far more than it looks?
Any disease or dysfunction of the cord, including physical compression, is termed myelopathy. It occurs most often in the cervical region, when degenerative changes narrow the sagittal dimensions of the spinal canal. That presentation is cervical spondylotic myelopathy, or CSM. Ossification of the posterior longitudinal ligament does the same work, so the whole family is now named degenerative cervical myelopathy (Nouri 2015).
The numbers behind the screen
Degenerative forms of cervical myelopathy are the commonest cause of spinal cord impairment in adults. Incidence and prevalence in North America are estimated at a minimum of 41 and 605 per million, hospitalizations for CSM run at 4.04 per 100,000 person-years, and surgical rates are rising (Nouri 2015).
Most people with degenerative change are perfectly fine. The point is not alarm. The point is that a large, ordinary section of the older population carries the raw ingredients, so an unhurried one to two minute screen belongs in the routine exam of the right patient.
The natural history says the same thing from the other side. Of 199 people whose scans showed spondylotic cord compression with no clinical myelopathy, 45 developed the first signs of it over 2 to 12 years (Bednarik 2008). Roughly three quarters did not.
02Findings
What the research shows
From an epidemiological review, a natural history cohort, two diagnostic accuracy studies, a mathematical model and a tensile test series.
03Leg stiffness before neck pain
The first signs of cervical myelopathy appear in the legs and the hands
The defining feature of CSM is how gently it arrives. Changes in bowel and bladder function, and the full expression of upper motor neuron signs, are fairly late manifestations. The earliest hints are subtle and easy to attribute to age alone.
A person may simply report stiffness in the legs or feet, or an unsteadiness of gait, with no dramatic neck complaint at all. Buttons and coins become awkward. Nothing in that presentation points at the neck, which is why the neck has to be considered deliberately.
What the cord is doing while this happens
Three mechanisms are described for the injury in cervical myelopathy: static compression, spinal malalignment that alters both cord tension and vascular supply, and dynamic injury during movement (Nouri 2015). Mechanics and blood flow are entangled from the start. The tissue change shows on magnetic resonance imaging as increased signal intensity within the cord, and radiologists call the established form of it myelomalacia.
That signal is also a timing marker. In the presymptomatic cohort, magnetic resonance hyperintensity predicted progression to symptomatic myelopathy beyond twelve months, while radiculopathy and abnormal evoked potentials predicted the early cases (Bednarik 2008).
The diagnosis is confirmed or refuted by what happens to the neural contents of the canal, not by the width of the canal on its own. A narrow canal with a quiet examination is a measurement. A narrow canal with brisk reflexes and a changed gait is a finding.
04What a clinician checks
The screen for cervical myelopathy centers on upper motor neuron signs
Because damage to the corticospinal tract tends to produce the earliest findings in cervical spondylotic myelopathy, the screen centers on upper motor neuron, or pyramidal, signs. A clinician looks for hyperreflexia, a Babinski response, ankle clonus, the Hoffmann sign, and everyday clues such as reduced hand dexterity or a change in gait. Each is a small window onto the long motor tracts passing through the cervical cord.
The techniques are precise. Hyperreflexia is read best through asymmetry between sides. The Babinski response is fanning of the toes and extension of the hallux to a firm plantar scratch. Clonus is a repetitive beating of the foot, greater than three beats, after a rapid dorsiflexion stretch. The Hoffmann sign is adduction of the thumb and flexion of the fingers when the examiner flicks the nail of the partially extended middle finger.
How often each sign is actually present
Chikuda and colleagues reviewed 275 surgical cases of cervical myelopathy and examined the 120 patients with increased T2 signal in the cord. Hyperreflexia appeared in 94 percent, the Hoffmann reflex in 81 percent, the Babinski sign in 53 percent and ankle clonus in 35 percent (Chikuda 2010).
The ranking carries information beyond frequency. The Babinski sign, ankle clonus and the Hoffmann reflex were each associated with a lower motor score for the legs, and none with the arm score. The rarest signs appear in the people who are worst affected.
Reduced dexterity and altered gait round out the pattern, and together they help a trained examiner localize the concern to the cervical cord. The Neurological Exam covers the wider testing sequence.
When the neck is aging and the legs feel subtly stiff, the value of a trained examiner is not fear. It is the calm, deliberate screen that catches the rare cord that needs another set of eyes, early enough to make a difference.
05Sensitivity and specificity
Pyramidal signs are strong evidence when present and weak evidence when absent
A confident clinician also knows what these signs can and cannot do. They are specific pieces of evidence rather than a sensitive net. Cook and colleagues examined seven frequently used tests for myelopathy, and none of them, alone or clustered, produced a low negative likelihood ratio (Cook 2009).
The Babinski sign performed best. Its positive likelihood ratio was 4.0, with a confidence interval of 1.1 to 16.6, moving a pretest probability of 40 percent to a posttest probability of 73 percent. Its negative likelihood ratio was 0.7. Clusters of tests did not beat single findings.
What the Hoffmann sign is worth
The Hoffmann sign has been checked directly against imaging. Among 91 patients with a positive sign, 32 had severe cervical cord compression or myelomalacia. Among 80 with a negative sign, 21 did. That yields 59 percent sensitivity, 49 percent specificity and a negative predictive value of 72 percent (Grijalva 2015).
Chikuda and colleagues reached the same verdict from the other direction. Low sensitivity in patients with mild disability limits the utility of pyramidal signs in the early diagnosis of cervical myelopathy (Chikuda 2010). A sign appearing usually means meaningful cord involvement is already present.
That honesty cuts in a reassuring direction. A clear screen is welcome and common, and an absence of pyramidal signs alone is not treated as proof that CSM has been excluded. A positive finding raises the threshold for further evaluation rather than for reassurance.
The finding is framed as the rare exception an examiner catches, never a checklist a person runs on themselves. Eliciting a Babinski response is a trained motor skill, and interpreting one is a judgment about a whole examination. Recognizing myelopathy is expert work.
06Tension against compression
Modeling of cervical cord pathology favors dentate-mediated tensile stress over compression alone
The clinical literature on cervical spondylotic myelopathy is a compression literature. Canals are measured, cords are decompressed, and decompression helps. A separate line of mechanical work reaches a different conclusion about what damages the tissue, and the two have never been reconciled.
Alf Breig opened it. Working on fresh cadaver material, he studied the effects of mechanical stresses on the cord in cervical spondylosis (Breig 1966). That work established the cord as a tensioned structure held in shape by the dentate ligaments anchoring it inside the canal. A suspended object is loaded in tension.
The model that tested both theories
Levine put the two accounts against each other and let the pathology decide. The compression theory holds that the cord is squeezed between a spondylotic bar in front and the ligamenta flava behind. The dentate tension theory holds that the cord is pulled sideways by the dentate ligaments, tensed by that same bar (Levine 1997).
He modeled the cord cross section at the level of the bar as a circular disc loaded around its circumference, then computed shear stress across the interior. Each theory predicts a different pattern. Only one matches the topography of damage that reports of myelopathic pathology describe.
The dentate tension pattern matched. The compression pattern did not. Levine concluded that the results strongly favor the theory that cervical spondylotic myelopathy is caused by tensile stresses transmitted to the cord from the dura through the dentate ligaments. A bar in front displaces the cord backward, the dural attachments move less, and the ligaments take up the difference.
What the mechanical measurements show
The suspension is stronger where the neck moves more. Across 98 denticulate ligament samples from seven porcine cervical cords, failure force fell in the caudal direction, from 1.04 N at C1 to 0.55 N at C7 (Polak 2014). Mean ultimate force was 0.88 N and mean Young modulus 2.06 MPa.
Levine states the limits of his own work plainly. The generality of the result is tempered by the simplifying assumptions a mathematical model requires. The tensile measurements are porcine, and the epidemiological reviews still list static compression first. Both readings sit on the table.
Two of Levine’s consequences are specific and testable. Neck flexion raises dural tension and belongs on the avoid list during conservative management. Both anterior and posterior extradural operations reduce dentate tension, which offers one account of why two very different surgeries help the same condition. The mainstream review names cord tension itself as one of three injury mechanisms (Nouri 2015).
This page claims the cord-specific half of that argument. The same tension runs outward along the nerve roots, and Adverse Neural Tension carries the neurodynamic reading and the limb tests that follow from it.
07Two years to diagnosis
Delay is the variable that costs function in cervical myelopathy
Early recognition is the entire reason this screen exists, and the delay is measurable. Among 42 patients eventually operated on for CSM, the mean time from first symptom to diagnosis was 2.2 years. Ninety percent first presented to a family practitioner or an orthopedic surgeon (Behrbalk 2013).
That delay has a price. A systematic review of 91 graded studies found longer symptom duration associated with poorer outcome on both the modified Japanese Orthopaedic Association scale and the Nurick score. Duration and baseline severity were the two strongest predictors of surgical outcome, and age was not (Tetreault 2015). Catching the pattern early is what preserves options.
What surgery does, and what it does not settle
Decompression works. Fehlings and colleagues enrolled 278 patients at twelve North American centers, graded at baseline as mild at an mJOA of 15 or above, moderate at 12 to 14, or severe below 12. At one year the mJOA, the Nurick grade, the Neck Disability Index and the SF-36v2 composites had all improved significantly (Fehlings 2013). Fifty-two patients experienced a complication, a prevalence of 18.7 percent.
For mild and moderate disease the picture is genuinely unsettled. A ten-year randomized trial assigned 64 patients with mild or moderate, slowly progressive CSM to conservative or surgical treatment. Seventeen died of unrelated causes, leaving 25 and 22 for final evaluation. No significant difference emerged on the mJOA, on blinded video review of daily activities or on the timed 10-meter walk (Kadanka 2011).
The authors state the honest reading themselves. In both groups patients get better and worse, and the small numbers leave the comparison underpowered. Severe myelopathy is a surgical question. Mild myelopathy is a watching question, and watching requires someone who knows what to watch for.
Referring out is the strength
An examiner who asks about leg stiffness, notes a subtle gait change and runs a two minute pyramidal screen is practicing the vigilance that serves an older patient well. When suspicion is genuine, the confident move is a prompt referral for advanced imaging and specialist evaluation.
This practice does not operate imaging equipment, which is the ordinary arrangement in a portal-of-entry profession. The skill is knowing which patient needs a scan, ordering it through the right channel and reading the result in context. What MRI Is Really For sets out when a scan changes management.
It is a pattern a trained clinician recognizes, and recognizing it early is one of the quiet ways good musculoskeletal care protects the people who trust it.
08Claims removed
Three claims from the earlier version were removed
The earlier text reported cervical spondylosis in up to 90 percent of people by the seventh decade. No source could be found for that figure, so it came off, and the epidemiological estimates above replaced it. The claim that a shift in signal intensity on magnetic resonance imaging is the most reliable diagnostic criterion for CSM also came off. Increased T2 signal is a real marker, and the studies here use it to define cord involvement and to predict later progression. None of them rank it against every other criterion.
The statement that sustained compression provokes a persistent neuroinflammatory reaction came off as well, because the reviews cited here describe altered vascular supply and altered cord tension instead. The claim that CSM is the most common form of cord dysfunction specifically after age 55 was replaced with the sourced version. Degenerative cervical myelopathy is the commonest cause of spinal cord impairment in adults.
09The model on the compressed cord
What the Unified Model of Tone claims about cervical myelopathy
Everything above is established science, including the ten-year trial that found no difference between surgery and conservative care in mild disease. What follows is this model’s reading, stated as ours rather than drawn from the papers cited.
Tone is the integrated organization through which the body’s interacting processes relate to one another at a given moment, and mechanical tension is one of the things it gathers. Our model holds that the mechanical environment of the cord is a regulated variable rather than a fixed geometry. The dura is innervated by autonomic branches, and its tension changes with position and with muscular tone.
That is why the same canal diameter is a different mechanical problem in two different people. It is also a different problem in the same person on two different days.
Why the tensile reading matters clinically
Read as compression, myelopathy is a geometry problem, and the number that matters is the width of the canal. Read as dentate-mediated tensile stress, myelopathy becomes a loading problem. What matters then is how far the cord is displaced, how much the ligaments take up, and how the neck is used across a day. Levine’s model chose the second and matched the observed topography of damage (Levine 1997).
Our model takes that further. Tension is distributed along the whole neural container, so a spondylotic bar at one level changes the load at levels it never touches. This is the argument Beyond the Single Joint makes about the spine as a whole, applied to the tissue inside the canal.
The prediction
From that follows a claim the myelopathy literature does not make. Our model predicts that among people matched for anteroposterior canal diameter and cord cross-sectional area, the ones whose cord tension rises most with neck flexion will show the greatest functional deficit. Canal width alone will predict function less well than canal width combined with a flexion loading measure.
Four measures recorded together in the same people will share one underlying factor rather than varying independently. They are stride-time variability during walking, resting heart rate variability, reflex responsiveness at the patellar tendon, and time for hand dexterity to return to baseline after a sustained neck flexion challenge.
This is a claim about how cord dysfunction is organized rather than a claim about what any treatment does. If stride-time variability, resting heart rate variability, patellar reflex responsiveness and recovery time for hand dexterity after a flexion challenge are shown to move together, the unification claim is confirmed.
10The tone reading
How cervical myelopathy expresses tone
Every topic in this library expresses all of tone. In cervical spondylotic myelopathy three aspects carry the signature, because the damage tracks how the cord is loaded rather than how wide the canal is.
Constraint
The canal narrows and the dentate ligaments hold the cord against the encroachment. Available movement collapses first, and the tissue absorbs the load that movement used to spread.
Load
Failure force of the denticulate ligament falls from 1.04 N at C1 to 0.55 N at C7. The suspension is strongest exactly where the neck moves through the widest arc.
Time course
Of 199 silent cord compressions, 45 became symptomatic, with a 25th percentile time of 48.4 months. The disease is measured in years, which is why delay decides outcome.
The remaining foundations run through cervical myelopathy as well. Input quality: a cord carrying degraded proprioceptive traffic gives the brain a poorer estimate of where the legs are. Gain: hyperreflexia is amplification released from descending control, which is why brisk reflexes appear before weakness. Coupling: hand dexterity and gait degrade together because both read the same tracts. Set point: resting dural tension sets the level from which every neck movement departs. Oscillation: clonus is a regulator that has fallen into a rhythm rather than a muscle that has failed. These are readings of one organization rather than separate systems, which is the core claim of the Unified Model of Tone.
11Across the library
How this page relates to the rest of the library
Myelopathy sits where the orthopedic examination hands the problem to neurology.
The other compression where time is the variable, below the cord rather than through it.
Breig’s tension argument carried outward along the nerve roots, with the limb tests it produces.
What a screening question is actually worth when the condition it hunts is uncommon.
The decision rules that separate a neck needing a film from a neck that does not.
The thresholds at which escalating to a larger intervention becomes the correct dose.
The tract-by-tract patterns that explain why hands and legs fail in a particular order.
The anatomy of the tissue this page is about, from the dorsal horn to the descending tracts.
12Frequently asked
Questions patients ask about cervical myelopathy
What is cervical myelopathy?
Myelopathy is any dysfunction of the spinal cord, and cervical myelopathy is that dysfunction in the neck. It usually follows degenerative change that narrows the spinal canal, a picture called cervical spondylotic myelopathy. Ossification of the posterior longitudinal ligament and degenerative disk disease produce the same result, so the group now travels as degenerative cervical myelopathy. Taken together these are the commonest cause of spinal cord impairment in adults, with prevalence in North America estimated at a minimum of 605 per million.
What are the signs of spinal cord compression in the neck?
The earliest ones are unglamorous. Clumsy hands, difficulty with buttons and coins, an unsteady walk and stiff legs come before anything dramatic, and neck pain is often absent. A clinician then checks upper motor neuron signs, meaning hyperreflexia, the Hoffmann reflex, the Babinski sign and ankle clonus. Among 120 patients with confirmed cord signal change, hyperreflexia appeared in 94 percent and ankle clonus in 35 percent. Bowel and bladder changes are late findings rather than early ones, which is why waiting for them is the wrong strategy.
Why does early recognition matter?
Because time is the variable that predicts how well someone does. A systematic review of 91 studies found that longer symptom duration was associated with worse outcome after surgery on both the modified Japanese Orthopaedic Association scale and the Nurick score, alongside baseline severity. Diagnosis is often slow: among 42 patients later operated on, the mean delay from first symptom was 2.2 years. Recognizing the pattern early is exactly what a careful neurological examination in a first-contact setting is for.
Does everyone with cord compression on a scan develop myelopathy?
No, and most do not. In a prospective cohort of 199 people whose magnetic resonance imaging showed spondylotic cord compression without clinical myelopathy, 45 developed the first signs during a follow-up of two to twelve years. The 25th percentile time to those signs was 48.4 months. Increased cord signal on imaging predicted later conversion, while coexisting radiculopathy and abnormal evoked potentials predicted conversion within the first year. Compression on a film is a reason to follow someone, not a verdict.
Is surgery always needed for cervical myelopathy?
No. Severity drives the decision, rather than the presence of compression on a scan. In 278 patients graded by the modified Japanese Orthopaedic Association scale, decompression produced significant improvement in function, disability and quality of life at one year, with complications in 18.7 percent. For mild and moderate disease the evidence is genuinely split. A ten-year randomized trial of 64 patients found no significant difference between conservative and surgical management on function, walking speed or blinded video review of daily activities.
Can the neck feel fine while the cord is involved?
Yes, and that is the central difficulty. Cervical spondylotic myelopathy is often painless in the neck itself, because the cord and the joints report through different channels. People describe stiffness in the legs, a changed walk or hands that drop things. Examination findings run the same way. The Hoffmann sign carries only 59 percent sensitivity and 49 percent specificity against imaging, so neither a quiet neck nor a quiet reflex screen settles the question by itself. The history and the examination are read together.
What does the Unified Model of Tone say about cervical myelopathy?
That the mechanical environment of the cord is a regulated variable rather than a fixed geometry. The cord hangs from dentate ligaments, the dura is innervated and its tension changes with position and muscular tone, so identical canal diameters load two cords differently. Modeling of cervical cord pathology already favors dentate-mediated tensile stress over compression alone. From that the model predicts that stride-time variability, heart rate variability, reflex responsiveness and recovery of hand dexterity after a flexion challenge share one underlying factor.
13The sources
References
12 primary sources, each linked to its record. Figures quoted on this page were checked against the published abstract.
Related evidence
Neurology
Spinal Cord Syndromes
Neurology
The Spinal Cord
Lesson 34
Cauda Equina and the Spinal Emergencies
Lesson 36
Cervical Trauma and the Systematic Read
Lesson 37
Adverse Neural Tension
Lesson 33
The Red Flags Clinicians Screen For