Syringomyelia

Syringomyelia is a fluid cavity, a syrinx, within the spinal cord, and the classic exam cause of dissociated sensory loss over a cape distribution. It is also a silent driver of two orthopaedic problems, Charcot shoulder and progressive scoliosis, and a known cause of catastrophic neurological injury during scoliosis correction if it is not recognised before the operation. The orthopaedic surgeon may be the first to suspect it, faced with a painless destroyed shoulder or an atypical curve.

Who. The prevalence is estimated at 8-9 per 100,000 in the general population. Chiari-associated disease peaks at 20-50 years, and post-traumatic disease has a bimodal age distribution. There is a slight male predominance overall, and a female predominance where the cause is Chiari I.

Onset. Chiari-associated syringomyelia comes on insidiously. A post-traumatic syrinx classically presents months to years after the index injury, often in a patient whose neurology had appeared stable.

The anatomical bottleneck. Crossing spinothalamic fibres ascend 1-2 segments before they decussate through the anterior white commissure, which lies immediately anterior to the central canal. A central syrinx expands from the canal outward and strikes these crossing fibres first, so pain and temperature are lost while the dorsal columns (proprioception, vibration, fine touch) are spared until late disease. Hence the term dissociated sensory loss.

Why a cape. The C2-T1 crossing fibres are the longest, travel through the longest vertical segment of the cord and have the most extensive central representation, so C2-T1 is the classic zone of loss. The result is a bilateral, symmetrical, suspended segmental loss over the shoulders, upper arms and upper chest. It reflects damage to the crossing fibres at the level of the syrinx, and it follows no peripheral nerve territory.

How the syrinx forms. Four theories of formation and propagation are quoted:

A syrinx is classified by its cause, by its anatomy (Milhorat) and by the cord level it occupies.

History. The sensory story is of painless burns or injuries to the hands and an inability to feel hot water. Motor symptoms are progressive upper-limb weakness, gait disturbance and falls, and the pain is deep, poorly localised and often in the cape distribution. Ask about autonomic change (hyperhidrosis or anhidrosis, bladder symptoms, Horner syndrome) and about the trigger: antecedent trauma, often years earlier, or new shoulder swelling.

Examination. The pattern is lower motor neurone signs at the level of the syrinx and upper motor neurone signs below it. At the level, the small muscles of the hand waste and the biceps and triceps reflexes are lost; below it, the knee and ankle reflexes are brisk and the Babinski sign is present. Look at the shoulder for painless swelling, instability and crepitus, the Charcot sign, and at the spine for scoliosis, abdominal reflex asymmetry and a stiff gait.

Testing for dissociation. Test pin-prick (pain) and cold (temperature) in the upper limbs, trunk and lower limbs, then vibration with a 128 Hz tuning fork and joint position sense (toe up or down). In early syringomyelia pin-prick is lost over a cape while vibration is preserved, and a sensory level on the trunk implies thoracic extension. Always compare both sides, and document the upper limit of the sensory loss: it corresponds to the rostral end of the syrinx.

The syringomyelic hand. Wasted small muscles (LMN at T1) sit beside preserved forearm muscle bulk, because the syrinx is sparing the anterior horn cells at C5-C7. The result is the main en griffe, or intrinsic-minus, pattern, with trophic skin changes, recurrent burns and Charcot joints alongside. Recognising this combination on inspection is a viva favourite.

When to suspect it. The presentations that should send a patient for MRI:

MRI, of the whole neuraxis. MRI is the only definitive investigation. Plain films are normal, and CT is unhelpful except to define associated bone abnormalities. Request the brain and the whole spine, with a CSF flow study, because an associated Chiari I, hydrocephalus or dysraphism will change management.

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The sequences:

• Sagittal T2 of the entire spine
• Axial T2 at the level of the syrinx
• T1 before and after gadolinium
• Cine phase-contrast MRI, which confirms obstruction at the foramen magnum in Chiari-associated cases

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What to look for. The position and length of the cavity, nodularity of its wall (which suggests tumour), a Chiari I and loss of the craniocervical CSF flow void. Chiari I is taken as tonsillar descent greater than 5 mm, although in Milhorat's series of symptomatic patients (see Evidence) descent of less than 5 mm did not exclude the diagnosis.

When a tumour is suspected. Focal cord expansion, a mural nodule or a syrinx centred on an enhancing mass calls for MRI with contrast and delayed imaging. Each tumour has its signature:

• Ependymoma: classically central, with a cap sign (haemosiderin)
• Astrocytoma: eccentric, long segment
• Haemangioblastoma: cyst with a mural nodule; spinal angiography is reserved for its pre-operative planning

Surveillance. A confirmed syrinx under surveillance, or after treatment, is followed with serial MRI at 6 months and then annually if stable, sooner if the neurology changes. The threshold for intervention is an increase in length of more than one level, or in diameter of more than 2-3 mm per year, with symptoms.

Treat the cause: Chiari decompression, tumour resection or arachnoidolysis. Direct drainage of the syrinx is the rescue option once the cause has been addressed.

The Charcot shoulder. Neuropathic arthropathy of the shoulder is the classic orthopaedic presentation, and it is uncommon: the entire world literature pooled in Wawrzyniak's 2023 systematic review amounted to 65 patients and 71 shoulders drawn from 39 case reports and small series, which is not the profile of a complication affecting half of all syrinx patients. The high percentage runs the other way. Syringomyelia accounts for the great majority of upper-limb Charcot joints, so an unexplained destructive shoulder should prompt MRI of the cord. The elbow is the second most common upper-limb neuropathic joint.

Why the joint fails. Three losses combine:

• Loss of proprioception, so microtrauma goes unperceived
• Loss of pain, so injury is repeated without protective guarding
• Loss of sympathetic tone, with vasodilation, hyperaemia and osteoclastic activation

The result is joint destruction, often with a massive effusion. The shoulder is the most common upper-limb site because the cord level matches.

The resorptive pattern. The 6 D's describe the hypertrophic (productive) neuropathic joint, with the exuberant sclerosis, osteophyte and debris typical of the diabetic foot and tabetic knee. The upper-limb Charcot of syringomyelia is classically the atrophic (resorptive) form, and recognising it is a distinct radiographic exam point.

• Atrophic, resorptive: gross osteolysis and resorption of the bone ends with little or no new bone. The humeral head can simply melt away, giving the tapered "licked candy stick" or "sucked candy" appearance, a surgical-looking "resorptive amputation" of the proximal humerus, scant debris and gross instability or pseudarthrosis. More common in non-weight-bearing, proximal upper-limb joints.
• Hypertrophic, productive: the 6 D's with abundant osteophyte and fragmentation. More common in weight-bearing lower-limb joints.

Why the pattern matters. The massive osteolysis of the resorptive shoulder is easily mistaken for an aggressive tumour or infection; the painless presentation, the dissociated sensory loss and a cervical syrinx on MRI make the diagnosis. It also explains why reconstruction is so hard, with little bone stock left to fuse or implant into.

Treating the Charcot shoulder. Bracing and activity modification are first line. Treating the syrinx does not reverse established Charcot change, though it may halt progression, and the syrinx is always addressed before reconstructive surgery. For the end-stage shoulder, arthrodesis is the most reliable procedure despite the destruction; reverse total shoulder arthroplasty is controversial, with high failure from persistent denervation, and is used selectively in stable neurology.

Scoliosis: get the direction right. The two conditionals differ by an order of magnitude. Scoliosis is common among syrinx patients: it was present in 42% of 364 symptomatic Chiari I patients in Milhorat's series, and that is the direction the commonly repeated "25-50% of idiopathic scoliosis" belongs to. The yield of screening a presumed idiopathic scoliosis is small. A systematic review of 51 studies and 8,622 patients found a syrinx in 3.7%, a Chiari in 3.0% and all neural axis anomalies together in 11.4%, rising only to 14.2% in patients preselected for risk factors, not to anything like 90%. In infantile curves the figure for all anomalies is higher, around 13-22%, but still nowhere near a majority.

Who to image. The low yield sharpens the imaging argument. The recognised risk factors are early-onset curves, male sex, atypical or left-thoracic curves (right thoracic is the usual idiopathic pattern), increased thoracic kyphosis and any abnormal neurological finding, including reflex or sensory change. The other red flags:

• Painful scoliosis in a child or adolescent
• Rapid progression of the curve despite bracing
• Asymmetric reflexes, abdominal reflex loss or a change in gait
• Skin stigmata of dysraphism: a dimple, hair patch, lipoma or sinus
• Foot deformities: cavus, claw toes, leg length discrepancy

The sequence of care.

1. MRI of the whole neuraxis to confirm the syrinx and identify its cause
2. Referral to spinal neurosurgery to address the cause
3. A brace for curves under 25-30 degrees while the syrinx is treated
4. Fusion only after the syrinx has stabilised or resolved
5. Follow-up for neurological change and curve progression

Who does well. The best prognosis belongs to a Chiari I-associated syrinx with a short duration of symptoms, no fixed neurological deficit, an intact gait and normal hand function.

Who does not. A long-standing dense motor deficit, established Charcot joints, a post-traumatic aetiology, post-arachnoiditic scarring and a syrinx recurring after multiple shunts all carry a poor prognosis.

What improves. Pain is the most responsive symptom. Fixed sensory and motor deficits are stabilised more often than reversed.