An intramedullary tumour expands within the cervical spinal cord over several segments. Pain and temperature sensation is lost in the arms and trunk, yet the saddle area (perineum) remains spared. What anatomical arrangement of the spinothalamic tract explains the preserved perineal sensation?

• A Sacral fibres travel in the fasciculus cuneatus
• B Sacral fibres occupy the most lateral portion of the tract
• C Sacral fibres cross twice within the cord
• D Sacral fibres occupy the most medial portion of the tract ✓

Explanation

Within the lateral spinothalamic tract, fibres are laminated so that sacral fibres lie most superficially and laterally... correction: sacral fibres lie outermost laterally only in the dorsal columns; in the spinothalamic tract sacral fibres are most medial and cervical fibres most lateral. An expanding intramedullary lesion damages the lateral cervical fibres first, sparing medial sacral fibres and preserving perineal sensation, called sacral sparing. Extramedullary compression produces the reverse pattern, which is the discriminating point.

Reference: Snell's Clinical Neuroanatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.