💧 Syringomyelia
The fluid-filled central cord cyst that destroys crossing pain fibres from the inside out — producing the classic dissociated sensory loss in a cape distribution.
The Anatomy & Mechanism
Syringomyelia is only confusing if you try to memorise it as a list. Understand two anatomical facts — where the spinothalamic fibres cross and where the central canal sits — and the entire syndrome falls out logically.
The Crossing Wires
When a pain or temperature signal enters the spinal cord via the dorsal root, it does not immediately ascend. Instead:
Signal enters dorsal horn
The first-order neurone enters the cord at its level and ascends briefly (1–2 segments) in Lissauer’s tract (dorsolateral fasciculus) before synapsing.
Synapse in the dorsal horn → second-order neurone
The second-order neurone arises in the substantia gelatinosa (laminae I–II) of the dorsal horn.
The second-order neurone CROSSES the midline
This is the critical step. The second-order spinothalamic fibre travels horizontally, passing immediately anterior to (and through) the central grey matter — right through the area surrounding the central canal — to reach the opposite anterolateral cord. This crossing is the “X” at the centre of the spinal cord.
Ascends contralaterally to the thalamus
The crossed fibre ascends in the contralateral anterolateral cord all the way to the thalamus. This is why a right-sided cord lesion causes left-sided pain/temperature loss.
The Internal Balloon
A syrinx (from Greek: “pipe”) is a fluid-filled cyst that forms within the substance of the spinal cord, centred on the central canal. As the cyst slowly expands, it acts like an internal balloon being inflated inside the cord.
Why Pain and Temperature Are ALWAYS First
The crossing spinothalamic fibres are the first structures the expanding syrinx encounters as it grows outward from the central canal. They pass directly through the pericentral grey matter — right in the bull’s eye. No other tract fibres cross at this level:
🔴 Spinothalamic fibres (pain/temp) — cross near the central canal → destroyed first as syrinx expands
✅ Dorsal columns (proprioception/vibration) — sit in the posterior cord far from the canal → not touched until very late
✅ Corticospinal tract (motor) — sits in lateral cord → not touched until late expansion
This spatial selectivity is why the initial deficit is purely pain and temperature loss — the other modalities are anatomically out of reach of an early central syrinx.