A 38-year-old woman presents with progressive loss of pain and temperature sensation in a cape-like distribution over her shoulders and upper arms, while light touch and vibration sensation remain intact. She reports painless burns on her hands from touching a hot stove. MRI of the brain and spine reveals a longitudinal CSF-signal cavity within the spinal cord at the cervical level, with cerebellar tonsillar descent of 7 mm below the foramen magnum (shown as structure **A** in the diagram). Which of the following best explains the dissociated sensory loss pattern observed in this patient?

Explanation

## Why option 1 is right The classic neurophysiologic hallmark of syringomyelia, particularly in Chiari I malformation, is the **cape-like or suspended dissociated sensory loss** — loss of pain and temperature sensation in a shawl distribution while light touch, vibration, and proprioception remain intact. This occurs because the syrinx (the CSF-signal cavity marked **A**) is centrally located and interrupts the **crossing spinothalamic fibers** in the anterior white commissure. These fibers decussate (cross) at the level of entry and ascend contralaterally; a central cavity destroys them bilaterally. In contrast, the **dorsal columns** (fasciculus gracilis and cuneatus) run ipsilaterally and are spared because they occupy the posterior white matter, away from the syrinx. This dissociation is pathognomonic for central cord pathology and is the key diagnostic clue in syringomyelia. The Oldfield theory explains how tonsillar herniation in Chiari I creates a piston-like CSF pulse that drives fluid into the central canal, propagating the syrinx. (Harrison 21e Ch 449) ## Why each distractor is wrong - **Option 2**: Dorsal root ganglia compression would cause **non-dissociated sensory loss** (all modalities affected equally), not the selective loss of pain/temperature with preserved light touch seen here. DRG pathology produces radicular pain and sensory level, not cape-like loss. - **Option 3**: Damage to fasciculus cuneatus and gracilis (dorsal columns) would cause loss of **vibration and proprioception** — the opposite of what this patient has. Her vibration and light touch are intact, ruling out dorsal column involvement. - **Option 4**: Corticospinal tract damage causes **motor signs** (weakness, spasticity, hyperreflexia), not sensory loss. While syringomyelia can compress corticospinal tracts laterally (causing UMN signs in the legs), this does not explain the dissociated sensory pattern. **High-Yield:** Central syrinx → dissociated sensory loss (pain/temp gone, light touch/vibration intact) = spinothalamic crossing fibers interrupted; dorsal columns spared = central cord syndrome. [cite:Harrison 21e Ch 449]