A clinical examination reveals a patient cannot perceive vibration below the waist but has intact pain, temperature, and motor function. Which tract is primarily affected, and where does this tract first decussate?

• A Posterior (dorsal) columns (fasciculus gracilis); decussation occurs in the medulla at the level of the nucleus gracilis/cuneatus (medial lemniscal decussation) ✓
• B Anterior spinothalamic tract; decussation occurs at the spinal cord level within 1-2 segments
• C Lateral spinothalamic tract; decussation occurs at the anterior white commissure at the entry level
• D Spinocerebellar tract; decussation occurs at the level of the inferior cerebellar peduncle

Explanation

Vibration and proprioception travel in the ipsilateral posterior (dorsal) columns — fasciculus gracilis (lower limb/trunk) and fasciculus cuneatus (upper limb). These fibres ascend ipsilaterally in the dorsal columns and synapse in the nucleus gracilis and nucleus cuneatus in the medulla. Second-order fibres then cross as the internal arcuate fibres (medial lemniscal decussation) in the medulla and ascend as the medial lemniscus to the ventroposterolateral (VPL) thalamus. Therefore, the level of decussation is the MEDULLA, not the spinal cord — distinguishing dorsal column damage from spinothalamic damage clinically.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.