A patient with slowly progressive extrinsic compression of the cervical spinal cord retains pinprick sensation over the perineum despite dense loss below the limbs. This 'sacral sparing' is explained by which arrangement within the spinothalamic tract?
- A Sacral fibres lie most laterally and are compressed last ✓
- B Sacral fibres lie most medially near the midline
- C Sacral pain fibres travel in the dorsal columns
- D Sacral segments have doubled representation in the tract
Correct answer: A. Sacral fibres lie most laterally and are compressed last
Explanation
The spinothalamic tract is laminated so that sacral fibres are most superficial and lateral, with cervical fibres deepest and medial. An expanding extrinsic lesion compresses from outside inward, sparing the laterally placed sacral fibres until late, giving sacral sparing. A central intramedullary lesion such as syringomyelia does the opposite, destroying crossing fibres first and sparing sacral sensation. Sacral pain fibres do not run in the dorsal columns.
Reference: Gray's Anatomy, 42nd ed.
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Written and medically reviewed by the StethoPrep medical team.