A patient with syringomyelia presents with bilateral loss of pain and temperature sensation in the upper limbs but preserved vibration and proprioception. Which anatomical feature explains this dissociated sensory loss?

• A Syrinx cavitation disrupts the decussating spinothalamic fibers crossing in the anterior white commissure ✓
• B The posterior columns are selectively destroyed by the syrinx
• C The dorsal horn neurons carrying pain are more anteriorly placed
• D Temperature fibers descend in the posterolateral columns before decussating

Explanation

In syringomyelia the expanding fluid cavity first disrupts fibers crossing in the anterior white commissure — these are second-order spinothalamic neurons carrying pain and temperature from both sides at that spinal level, producing a bilateral 'cape-like' dissociated sensory loss. The posterior columns carrying vibration and proprioception travel ipsilaterally and are spared until the syrinx expands posteriorly. Options B and D describe incorrect anatomy.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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