An intramedullary tumour expanding within the cervical cord compresses the spinothalamic tract from within. Why does such a central lesion tend to spare sacral pain and temperature sensation until late?

• A Sacral fibres do not enter the spinothalamic tract
• B Sacral fibres run deepest within the tract, adjacent to the grey matter ✓
• C Sacral pain fibres ascend in the dorsal columns instead
• D Sacral fibres cross twice and therefore escape compression

Explanation

Within the lateral spinothalamic tract, fibres are laminated so that sacral fibres lie most superficially and posterolaterally, with cervical fibres innermost near the grey matter. An expanding intramedullary lesion compresses from inside outward, sparing the outermost sacral fibres until late, giving the sacral sparing sign. Extramedullary lesions compressing from outside produce the reverse pattern, affecting sacral levels first. Sacral pain fibres never ascend in the dorsal columns.

Reference: Gray's Anatomy, 42nd ed.

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