Abnormal Caudal Fixation of the Spinal Cord
- Caudal fixation of the cord: a low conus (below L2) with a thickened, often fatty filum, producing progressive symptoms
- The orthopaedic surgeon is often first to suspect it: cavovarus foot, asymmetric leg, scoliosis
- Cutaneous stigmata over the lumbosacral midline should raise suspicion and prompt imaging
- Three domains: neurological, urological/bowel and pain
- MRI confirms the low conus, thickened filum and any dysraphic lesion; ultrasound screens infants
- Untether to halt progression; established deficits may not fully reverse
- “Tethered cord = low conus (below L2) + thickened/fatty filum causing traction/ischaemia and progressive neuro/uro/pain symptoms.
- “Orthopaedic clues: unilateral cavovarus foot, asymmetric leg atrophy/leg-length difference, scoliosis (atypical/neurological). Plus cutaneous stigmata (dimple, hairy patch, lipoma).
- “MRI confirms (low conus, thick filum); treat by surgical untethering (filum section). Halts progression - established deficits may not fully reverse.
Definition and Presentation
Definition. Tethered cord syndrome is the clinical syndrome produced by abnormal caudal fixation of the spinal cord. Classically the conus medullaris lies low, ending below the normal L1-L2 level, and is held there by a thickened and often fatty filum terminale, or by another dysraphic lesion. The fixed, stretched distal cord suffers traction and impaired perfusion, and the symptoms that follow are progressive.
Why the orthopaedic surgeon sees it first. The presenting clues are frequently musculoskeletal, so the orthopaedic surgeon is often the first to suspect the diagnosis. Each of these findings should prompt a search for an underlying tethered cord:
- A unilateral or progressive cavovarus foot
- Asymmetric calf or leg atrophy, or a leg-length discrepancy
- Scoliosis, especially an atypical or left thoracic curve, or any curve with a neurological deficit
The skin. Cutaneous stigmata over the lumbosacral midline are an important external marker of underlying spinal dysraphism and tethering, and their presence should raise suspicion and prompt imaging. They are a sacral dimple (especially a large or high one), a hairy patch (hypertrichosis), a subcutaneous lipoma, a haemangioma or capillary malformation, and a dermal sinus.
The symptoms. They run across three domains and progress, the deterioration often precipitated or worsened by growth in children, or by flexion and activity.
- Neurological: lower-limb weakness, sensory change, reflex changes
- Urological and bowel: neurogenic bladder, incontinence, recurrent urinary infection, which is a key reason for urological assessment
- Pain: back and leg pain
What Tethers the Cord: Classifying the Lesions
The tethering lesions are best organised into a causal spectrum of primary and secondary tethering, because the cause determines the operation and the risk of re-tethering. Different lesions, one syndrome.
Primary tethering is congenital or occult, and takes these forms:
- Tight or thickened filum terminale, often a fatty filum or fibrolipoma: the simplest, sectioned at surgery
- Spinal lipomas: lipomyelomeningocoele and intradural, dorsal or terminal lipomas
- Dermal sinus tract, with or without a dermoid or epidermoid
- Split-cord malformation (diastematomyelia)
- Others: terminal myelocystocele, meningocoele manque, caudal regression
Secondary (acquired) tethering. Scar tethers the cord after previous surgery, most importantly as re-tethering after a myelomeningocoele repair or other spinal surgery. It is a common cause of late deterioration in repaired spina bifida: symptomatic tethering developed in 23 percent of one long-term cohort of children who had neonatal closure.
The open dysraphisms and the individual tethering lesions are developed separately in myelomeningocele, spina bifida, split-cord malformation and caudal regression syndrome.
Pathophysiology: Why Traction Damages the Cord (and Why Untethering Helps)
From an elastic filum to a stretched conus. A normal filum terminale is elastic and lets the cord glide with flexion, extension and growth. A tight, thickened, often fatty filum loses that elasticity and transmits tension to the conus instead.
A metabolic injury. Experimental work by Yamada showed that traction reduces mitochondrial oxidative metabolism (cytochrome-oxidase redox changes) and blood flow in the lumbosacral cord. Tethered cord syndrome is a reversible metabolic and ischaemic dysfunction of the stretched conus, not a simple mechanical transection.
Why it progresses, and why surgery helps. Symptoms worsen with anything that increases tension: growth spurts, flexion, activity. Early untethering can reverse the metabolic dysfunction and recover recent deficits, whereas long-standing structural damage may be fixed. That is the rationale for timely surgery.
Investigations
MRI. MRI is the key investigation. It shows the low-lying conus (below L2) and the thickened or fatty filum, and it defines any associated dysraphic lesion: lipomyelomeningocoele, dermal sinus, split-cord malformation or diastematomyelia. A filum thickness of around 1.3 mm and over has been associated with tethering and filum lipoma in studies. Look on the same MRI for an associated syringomyelia.
Prone imaging. Turned prone, a normal filum falls anteriorly with gravity. A thickened filum that stays against the dorsal wall has lost that movement, which is imaging evidence of tension, and prone imaging settles the borderline case.
Ultrasound in infants. Spinal ultrasound can screen infants before the posterior elements ossify: the unossified posterior arch gives an acoustic window, and MRI later confirms.
Management
Assess all three domains. Once MRI has confirmed and characterised the lesion, assess the neurological examination, the bladder (urological assessment and urodynamics) and the orthopaedic deformities of the foot, leg and spine. Manage the patient within a multidisciplinary team of neurosurgery, urology and orthopaedics.
The operation. Surgical untethering releases the tethering element, most simply by sectioning a tight or thickened filum terminale, or by addressing a more complex lesion. The aim is to halt progression and, where possible, to improve symptoms.
Timing. Established neurological and urological deficits may not fully reverse, so timely surgery in a symptomatic or deteriorating patient matters.
A unilateral cavovarus foot, asymmetric leg atrophy or an atypical or neurological scoliosis is frequently the first sign of an underlying tethered cord. With lumbosacral cutaneous stigmata or any bladder symptoms especially, it should prompt a neuro-axis MRI rather than treatment of the foot or curve in isolation. Operating on the foot or spine without recognising and addressing the tethered cord risks ongoing progression and poorer outcomes, so timely untethering in a symptomatic or deteriorating patient, coordinated with neurosurgery and urology, is the priority. Recognising the cord does not replace managing the foot, spine and bladder in their own right.
Who Actually Needs Untethering - and What It Delivers
The trigger is deterioration, not the scan. Release is offered for documented clinical or urodynamic deterioration: new or progressive lower-limb weakness, increasing spasticity or contracture, gait change, a progressing or new scoliosis, back or leg pain, or a deteriorating bladder.
After myelomeningocoele repair. The cord is radiologically tethered in almost everyone, because the placode is scarred to the dural repair by definition. A low conus on MRI is therefore an expected postoperative finding, not an indication, and operating on the scan alone commits a child with a normal trajectory to a re-operation through scar.
The incidental fatty filum. In the closed dysraphisms, a fatty filum found on imaging performed for another reason, in a child with no symptoms, no urodynamic abnormality and a normal examination, is watched rather than sectioned. Surveillance covers the examination, growth, the curve and the bladder.
A distinct group has the clinical and urodynamic picture of tethering but a conus in a normal position on MRI: occult tethered cord syndrome.
Uncontrolled series are enthusiastic. In 60 children (aged 3-18, mean follow-up 13.9 months) selected on strict criteria, urinary incontinence or retention resolved completely in 52 percent and markedly improved in a further 35 percent, faecal incontinence resolved in 56 percent, and weakness, sensory change and pain improved or resolved in all.
The only randomised comparison, filum section plus medical treatment versus medical treatment alone in children refractory to a year of conservative management with a normal conus and abnormal urodynamics, found no significant difference in urodynamic score, bowel-and-bladder dysfunction score or quality of life at one year. That trial accrued only 21 patients over 8 years, and its own interim analysis calculated that more than 700 per arm would be needed to show a urodynamic difference of the size observed, so it is a genuinely underpowered null rather than proof that the operation fails.
The defensible examination answer is that the entity exists as a clinical description but its surgical indication is unproven, and the null result is what a candidate should be able to quote, not the uncontrolled improvement rates alone.
What untethering delivers. Long-term single-institution follow-up of the myelomeningocoele population (mean 12 years) ranks the response by symptom, so counsel by domain rather than with a single global figure:
- Pain responds best, improved in 100 percent
- Lower-limb strength improved in 70 percent on three-month manual muscle testing
- Bladder function improved in 64 percent
- Spasticity improved in about two-thirds
- Contractures were, as expected, merely stable (78 percent) rather than improved
What it does not deliver: the scoliosis. In the same cohort scoliosis still progressed after untethering in 52 percent, and 28 percent went on to need spinal fusion. Untethering reliably relieves pain, but it does not guarantee that a neuromuscular curve will stabilise. Promising a family that releasing the cord will settle the spine over-reads the evidence; the curve continues to be managed in its own right.
Re-tethering is common enough to be part of the consent, not a footnote. Of 114 children who developed symptomatic tethering after myelomeningocoele closure, 33 (29 percent) required more than one untethering, for 163 procedures across 114 patients - so recurrence should be quoted as roughly one in three needing further surgery over a decade of follow-up, and lifelong surveillance is the corollary.
Mnemonics & Memory Aids
TETHER
Hook:TETHER: Thick filum/low conus, External skin stigmata, Toes (cavovarus), Hold of bladder lost, Examine/MRI, Release (untether).
Clinical Decision Scenarios
Practise clinical reasoning and management decisions out loud
“A child has a unilateral cavovarus foot and a sacral dimple. What underlying diagnosis must you consider and how do you investigate?”
“How is tethered cord syndrome managed, and what do you tell the family about outcome?”
Definition
- Abnormal caudal fixation of the cord (traction/ischaemia)
- Low-lying conus (below L2) + thickened/fatty filum terminale
- Progressive; often worsened by growth
Clues
- Orthopaedic: unilateral cavovarus foot, asymmetric leg atrophy/leg-length, scoliosis
- Cutaneous: sacral dimple, hairy patch, lipoma, haemangioma, dermal sinus
- Urological: neurogenic bladder, incontinence, recurrent UTI
Diagnosis
- MRI: low conus, thickened/fatty filum, associated dysraphic lesion
- Spinal ultrasound screens infants (before ossification)
- Urodynamics for bladder assessment
Management
- Surgical untethering (section the filum / release the lesion)
- Indication is documented clinical/urodynamic deterioration, NOT a low conus on MRI
- Halts progression; established deficits may not fully reverse - act timely
- Outcomes by domain: pain 100%, strength 70%, bladder 64%; scoliosis still progressed in 52% (28% fused)
- Re-tethering in about 29% (more than one operation); lifelong surveillance
- Occult TCS (normal conus): the one RCT was null and underpowered - indication unproven
- Multidisciplinary; manage foot/spine/bladder in their own right
Evidence & Key Studies
Cutaneous markers and filum terminale thickness in identifying tethered cord (children with sacral dimples)
- In children with sacral dimples, tethered cord syndrome requiring surgery was identified in 4.6%, supporting the link between lumbosacral cutaneous markers and underlying tethering.
- A dimple long diameter of 5 mm or more was significantly associated with tethered cord syndrome.
- A filum terminale thickness cutoff of 1.3 mm or more predicted filum terminale lipoma with about 93% sensitivity and 80% specificity, and spinal ultrasound can screen for it.
Surgical management of tethered spinal cord through a minimally invasive approach (filum release)
- Tethered cord release can be performed by hemilaminectomy, durotomy and careful separation/sectioning of the filum terminale, here via a minimally invasive approach.
- The case (a young adult with chronic back pain and thigh numbness) had a midline fusion defect and abnormal conus medullaris termination, illustrating the typical pathology.
- Postoperative recovery was rapid without complications, illustrating the principle of untethering to relieve traction on the cord.
Tethered cord release: long-term outcomes, by symptom domain, in 114 patients
- Single-institution cohort of 502 children who had myelomeningocoele closure between 1975 and 2008: symptomatic tethering developed in 114 (23 percent); mean follow-up 12 years (range 1 month to 23.3 years).
- Shunt malfunction is the commonest cause of decline in these children; symptomatic tethering is second - so a new deficit is not automatically the cord.
- Response ranked by domain: pain improved in 100 percent, lower-limb strength in 70 percent at 3 months, bladder function in 64 percent, spasticity in about two-thirds; contractures were stable (78 percent) rather than improved.
- Scoliosis progressed after untethering in 52 percent, with 28 percent requiring spinal fusion - untethering does not guarantee curve stabilisation.
- 33 of the 114 (29 percent) needed more than one untethering, for 163 operations in total: re-tethering is common.
- Retrospective, single centre, no control group - the authors state this explicitly.
Filum section for occult tethered cord syndrome: the only randomised comparison was null (and underpowered)
- Randomised controlled pilot: children with urinary incontinence refractory to a year or more of medical management, a NORMAL conus position on MRI and abnormal urodynamics, randomised to filum section plus medical treatment versus medical treatment alone.
- At one year the bowel-and-bladder dysfunction score improved in both arms (20 percent surgical, 24 percent medical) and the urodynamic score barely moved (6 percent and 4 percent); quality-of-life scales improved modestly in both. No difference reached significance.
- Accrual was only 21 patients over 8 years; interim analysis indicated more than 700 patients per arm would be needed to demonstrate a urodynamic difference of the size seen - so this is an underpowered null, not a demonstration of no effect.
- The authors conclude the data challenge the concept of occult tethered cord syndrome as a surgically treatable entity.
The association of lumbosacral cutaneous markers (sacral dimple size) with tethered cord and the filum terminale thickness threshold (around 1.3 mm) with spinal ultrasound screening come from the cited Watanabe study; the surgical principle of untethering by sectioning/separating the filum terminale, and the typical pathology (abnormal conus termination with a dysraphic defect), from the cited Ozer report. The low-conus/thickened-filum definition, the orthopaedic clues (cavovarus foot, asymmetric leg, scoliosis), the neuro/urological/pain symptom triad and the multidisciplinary, progression-halting principle are standard, well-established teaching. The domain-by-domain outcome figures, the scoliosis-progression and fusion rates and the re-tethering rate come from the cited Bowman cohort, which is retrospective, single-centre and uncontrolled and drawn entirely from a repaired-myelomeningocoele population - so those numbers should not be transferred without qualification to a child having a simple fatty filum sectioned. The occult tethered cord randomised result and its power calculation come from the cited Steinbok trial; the uncontrolled improvement rates quoted alongside it are from Wehby and colleagues (60 children, mean follow-up 13.9 months, PMID 15292632). No agreed diagnostic criteria for occult tethered cord syndrome exist, and no threshold of conus level or filum thickness has been shown to identify which asymptomatic child will later deteriorate, so none is offered here. See also spina bifida, myelomeningocele, split-cord malformation, caudal regression syndrome, syringomyelia, cavovarus foot and paediatric cavus foot.