A patient with a slowly expanding intramedullary cervical cord lesion develops progressive loss of pain and temperature sensation, yet light touch and pinprick over the perineum are spared until very late. This sacral sparing occurs because sacral fibres within the spinothalamic tract occupy which position?
- A Most medially, adjacent to the grey matter
- B Uniformly scattered throughout the anterolateral column
- C Most laterally, farthest from centrally arising lesions ✓
- D Within the dorsal columns alongside the gracile fasciculus
Correct answer: C. Most laterally, farthest from centrally arising lesions
Explanation
The spinothalamic tract is laminated: sacral fibres lie most lateral and ventral, with lumbar, thoracic and cervical fibres progressively more medial and dorsal. A central intramedullary lesion first destroys the crossing fibres and the medial (cervical, thoracic) fibres, leaving the peripheral sacral fibres intact until late compression. Option D is wrong because dorsal column modalities such as vibration are carried separately and are preserved by design in such lesions.
Reference: Snell's Clinical Neuroanatomy, 8th ed.
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Written and medically reviewed by the StethoPrep medical team.