Syrinx (Syringomyelia)

A syrinx is a fluid-filled cavity within the spinal cord; when it produces symptoms the condition is termed syringomyelia. It most often arises in association with disturbed cerebrospinal fluid dynamics at the craniocervical junction, classically a Chiari I malformation, and as the cavity enlarges it disrupts the surrounding neural tissue, producing a characteristic dissociated sensory loss.

Definition

A syrinx is a longitudinal fluid-filled cavity within the substance of the spinal cord, and syringomyelia is the clinical disorder produced when such a cavity damages surrounding cord tissue.

Scope

The topic covers what a syrinx is, the leading hypotheses for how cerebrospinal fluid dynamics drive cavity formation and expansion, the typical clinical syndrome, and the principle that treatment is usually directed at the underlying flow disturbance rather than at the cavity alone. The entry is a reference orientation and does not specify operative indications or techniques.

Core questions

• What distinguishes a syrinx from the normal central canal and from hydromyelia?
• How do disturbances of cerebrospinal fluid dynamics drive syrinx formation?
• Why does syringomyelia classically produce dissociated, cape-like sensory loss?
• Why is treatment usually directed at the underlying cause rather than the cavity itself?

Key concepts

• Fluid-filled intramedullary cavity
• Chiari I malformation
• Cerebrospinal fluid flow obstruction
• Dissociated sensory loss
• Cape-like distribution
• Treatment of the underlying cause
• Central cord involvement

Key theories

Craniospinal pressure-dissociation mechanism

Obstruction of cerebrospinal fluid flow at the foramen magnum, as in Chiari I malformation, transmits abnormal pressure waves that drive fluid into and along the cord, accounting for syrinx formation and its response to restoring flow at the craniocervical junction.

Mechanisms

A syrinx is thought to form when cerebrospinal fluid dynamics are disturbed, most often by obstruction at the craniocervical junction such as the descended cerebellar tonsils of a Chiari I malformation. Abnormal pressure relationships then drive fluid accumulation within the cord and propagate the cavity longitudinally (Oldfield, 1994). Because the expanding cavity tends to begin centrally, it first interrupts the decussating spinothalamic fibres carrying pain and temperature while initially sparing the dorsal columns, producing a dissociated, often cape-like sensory loss, and with further enlargement it affects motor and other long tracts. This mechanistic understanding is why interventions classically aim to relieve the underlying flow obstruction rather than simply drain the cavity.

Clinical relevance

Syringomyelia is a recognised cause of progressive, sometimes painless neurological deficit, and its mechanism explains why imaging of the whole neuraxis and attention to the craniocervical junction are central to its study. This entry is a reference description of the disorder and does not provide diagnostic criteria or treatment recommendations, which rest with treating clinicians.

Epidemiology

Syringomyelia is uncommon and is frequently associated with Chiari I malformation; it can also follow trauma, arachnoiditis, or tumour, reflecting diverse causes of disturbed cerebrospinal fluid flow (Oldfield, 1994; Milhorat, 1999). Detailed incidence figures vary with the underlying cause and are not summarised here.

History

Syringomyelia was recognised clinically and pathologically in the nineteenth century, but its mechanism remained debated until imaging and physiological study clarified the role of cerebrospinal fluid dynamics at the craniocervical junction. Oldfield's 1994 work articulating a pressure-driven mechanism in Chiari-associated syringomyelia was influential in reframing treatment around restoring fluid flow.

Debates

What is the precise mechanism of syrinx fluid accumulation?

Several hypotheses have been proposed for how fluid enters and propagates within the cord, and while the link to craniocervical cerebrospinal fluid obstruction is well supported, the exact physical mechanism of fluid movement remains a subject of continuing study.

Key figures

• Edward Oldfield
• Thomas Milhorat
• Hans Chiari

Seminal works

• oldfield-1994

Frequently asked questions

What is the difference between a syrinx and syringomyelia?

A syrinx is the fluid-filled cavity within the spinal cord itself, while syringomyelia is the clinical condition that results when such a cavity damages the surrounding cord and produces symptoms.

Why is syringomyelia often treated by addressing the craniocervical junction?

Because many syrinxes arise from obstruction of cerebrospinal fluid flow at the craniocervical junction, such as a Chiari I malformation, relieving that obstruction targets the underlying driver rather than only the cavity; the mechanistic basis for this approach was articulated by Oldfield and colleagues.