Chiari Malformation (Orthopaedic Relevance)

A Chiari malformation is a structural problem at the junction of the skull and the spine, where part of the cerebellum - the cerebellar tonsils - sits too low and herniates down through the foramen magnum (the large hole at the base of the skull where the brainstem becomes the spinal cord). The type the orthopaedic surgeon meets is Chiari I, defined on MRI as tonsils lying 5mm or more below the foramen magnum.

On its own, a low tonsil might seem like a neurosurgical curiosity. The reason it lands on the orthopaedic radar is what it does downstream. The crowded foramen magnum disturbs the normal pulsatile flow of cerebrospinal fluid (CSF) around the cord, and over time this drives the formation of a fluid-filled cavity inside the spinal cord itself - a syrinx, the condition called syringomyelia. That syrinx is the real troublemaker. It produces two classic orthopaedic problems: an atypical scoliosis and a neuropathic (Charcot) joint in the upper limb.

For the exam, the whole topic reduces to a simple chain of reasoning: Chiari low tonsils, then blocked CSF flow, then a syrinx in the cord, then scoliosis and/or neuropathic joints. Your job is to recognise when a "routine" scoliosis is hiding this chain, to image the right thing (the whole neuraxis), and to know the order of operations - decompress the Chiari before you fuse the spine.

The anatomy. The cerebellum sits in the posterior fossa, the lowest compartment of the skull, just above the foramen magnum. Its lowest parts, the tonsils, normally rest at or above the level of the foramen magnum. In Chiari I the posterior fossa is often small or shallow, so the tonsils are pushed down and herniate below the foramen magnum, crowding the cervicomedullary junction where the brainstem becomes the spinal cord.

How the syrinx forms. CSF normally pulses freely up and down across the foramen magnum with each heartbeat and breath. The herniated tonsils act like a plug, obstructing CSF flow and creating abnormal pressure waves at the craniocervical junction. The leading explanation is that these disturbed pressure dynamics drive fluid into and along the spinal cord, hollowing out a central cavity over months to years - the syrinx. The exact mechanism is debated, but the practical point is fixed: relieving the obstruction (decompression) is what allows the syrinx to shrink.

Why the syrinx causes scoliosis. A syrinx expands within the central cord and damages the crossing fibres and motor pathways asymmetrically. This is thought to cause uneven activity in the paraspinal muscles on the two sides of the spine, pulling the spine into a curve. Because the cause is neurological and often asymmetric, the resulting scoliosis behaves differently from ordinary idiopathic scoliosis - the curve is frequently left-sided in the thoracic spine and may progress fast.

Why the syrinx causes neuropathic joints. The fibres carrying pain and temperature sensation cross in the very centre of the cord, right where the syrinx expands. A cervical syrinx therefore knocks out pain and temperature sensation in the arms while light touch and position sense are relatively preserved - a "dissociated" sensory loss in a cape distribution across the shoulders and arms. Without protective pain sensation, repeated micro-trauma destroys the joint painlessly: a neuropathic (Charcot) joint, classically of the shoulder or elbow.

The Chiari types are a favourite quick-fire question. For orthopaedics, you mostly care about type I, but you should be able to separate it cleanly from type II.

Chiari I sits at the craniovertebral junction (CVJ), and the orthopaedic surgeon must remember it frequently coexists with bony CVJ anomalies that can cause instability and change the operation from a simple decompression to an occipitocervical fusion.

Anomalies to look for on MRI/CT:

• Basilar invagination - the odontoid peg projects up through the foramen magnum into the posterior fossa, further crowding the cervicomedullary junction; a reason a simple posterior decompression alone may fail.
• Atlas (C1) assimilation / occipitalisation - congenital fusion of C1 to the occiput, which reduces space at the foramen magnum and shifts stress to C1-2.
• Atlanto-axial instability - may need reduction and occipitocervical or C1-2 fusion rather than decompression alone.
• Klippel-Feil syndrome (congenitally fused cervical vertebrae, short webbed neck, low hairline) and Sprengel deformity are recognised associations to screen for.
• Platybasia (flattening of the skull base) often accompanies these.

The classic CVJ measurement lines (high-yield):

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Chiari I reaches the orthopaedic clinic in a few recognisable ways. Most often it is discovered when an MRI is done for scoliosis; sometimes it presents through the neurological features of the malformation or the syrinx, and occasionally through a dramatic neuropathic joint.

Examination clues that change your plan

• Abdominal reflexes: asymmetric or absent superficial abdominal reflexes are a well-known soft sign of an underlying cord problem in a child with scoliosis - check them.
• Upper motor neurone signs in the legs (brisk reflexes, clonus, upgoing plantars) with lower motor neurone signs in the hands (wasting) suggest a cervical cord lesion such as a syrinx.
• Dissociated sensory loss - test pain/temperature versus light touch separately; loss of one with preservation of the other points squarely at a central cord cavity.

The investigation that matters is MRI, and the exam point is what to scan and when.

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A whole-neuraxis MRI for atypical scoliosis is not just "looking for a Chiari." It screens for the whole family of intraspinal causes of a non-idiopathic curve, and a syrinx itself has causes other than Chiari.

Other causes of syringomyelia (a syrinx is not always Chiari):

• Post-traumatic - a delayed, ascending syrinx after spinal cord injury causing late neurological decline.
• Post-infective / post-inflammatory arachnoiditis (e.g. after meningitis or tuberculosis) tethering the cord and blocking CSF flow.
• Intramedullary spinal cord tumour (e.g. ependymoma, haemangioblastoma) with an associated tumoural syrinx - a key one not to miss.
• Tethered cord and other closed dysraphism.

Other intraspinal causes of atypical / non-idiopathic scoliosis the MRI must exclude:

• Tethered cord syndrome (low conus, thickened filum) - look for cutaneous stigmata (hairy patch, dimple, lipoma).
• Diastematomyelia / split cord malformation - a bony or fibrous septum splitting the cord, which must be addressed before deformity correction.
• Intraspinal / intramedullary tumour.
• Neurofibromatosis type 1 (short sharp dystrophic curves, café-au-lait spots) and other syndromic causes.

Management is multidisciplinary - the neurosurgeon deals with the Chiari and syrinx, the orthopaedic spine surgeon deals with the curve - and the sequence is the high-yield point.

Why decompress before fusing?

Two reasons, both examinable. First, the curve may respond - relieving the obstruction and shrinking the syrinx can stop a young curve from worsening, so you may avoid or delay a big fusion. Second, safety: correcting a deformity stretches and manipulates the cord. A cord that contains a syrinx and sits under an obstructed craniocervical junction is more vulnerable to a neurological injury, so it is safer to decompress and let the syrinx settle before applying corrective forces.

Neuropathic (Charcot) joint - the upper-limb classic

The destroyed shoulder of syringomyelia is a favourite exam image. Because the cervical syrinx abolishes pain and temperature sensation in the arms, the joint is repeatedly injured without the patient noticing, leading to a grossly swollen, unstable, painless joint with massive bony destruction and debris on radiographs. The crucial lessons are: (1) a painless destroyed upper-limb joint is a syrinx until proven otherwise - image the neuraxis; and (2) operating on a neuropathic joint is hazardous - arthroplasty and fusion fail at high rates because protective sensation is gone, so management is usually supportive plus treatment of the underlying syrinx.

For the orthopaedic exam, Chiari I is really a scoliosis-screening and spinal-cord-safety topic dressed up as a brain malformation. It appears in paediatric and adult spine vivas (atypical scoliosis, "when do you MRI the neuraxis?", "what order do you operate in?"), in upper-limb vivas (the painless destroyed shoulder), and in basic-science discussions of CSF dynamics and syrinx formation. The whole topic hangs on one chain - low tonsils, then obstructed CSF, then a syrinx, then scoliosis and neuropathic joints - and on two practical rules: image the whole neuraxis in atypical scoliosis, and decompress the Chiari before you fuse the spine. Get those across and you have the marks.

• Neuraxis imaging in atypical scoliosis is a globally consistent message: scoliosis with a left thoracic apex, pain, neurological signs, abnormal abdominal reflexes, rapid progression, or young onset should prompt a whole-spine MRI including the craniocervical junction rather than being treated as idiopathic. This recommendation is echoed across scoliosis research society teaching and national paediatric spine guidance worldwide.
• Decompress before fusing is the widely accepted sequence for a symptomatic Chiari with a syrinx and scoliosis. The supporting evidence is largely multicentre registry and meta-analysis level (for example the Park-Reeves Syringomyelia Research Consortium and a 2025 meta-analysis of 380 children) rather than randomised trials, so practice is guided by large observational series.
• Best candidates for a decompression-first strategy are younger children (under about 10) with smaller curves (35 to 40 degrees or less), who are most likely to stabilise or regress; older children and larger curves more often progress and need fusion regardless.
• It is the syrinx, not the tonsil descent, that predicts the spine problem - a point reinforced by data showing Chiari without a syrinx behaves largely like ordinary scoliosis. This shapes counselling and the threshold for intervention across centres globally.
• Neuropathic joints are managed cautiously everywhere: diagnose and treat the underlying syrinx, and avoid arthroplasty or arthrodesis where possible because of the high failure rate in an insensate joint.

According to PubMed, the modern literature on Chiari and the spine is dominated by large paediatric series and a recent meta-analysis. The recurring themes are that the syrinx drives scoliosis, that younger age and smaller curves do best after decompression, and that Chiari without a syrinx behaves much more benignly.