Syringomyelia

A 35-year-old woman presents with a 2-year history of progressive painless burns on her shoulders and upper back after touching hot objects unknowingly. MRI of the cervical spine shows a T2-weighted sagittal image with a central hyperintense cavity within the spinal cord at C3–C5 level (marked **A**), and sagittal brain imaging reveals cerebellar tonsillar herniation 7 mm below the foramen magnum. On examination, she has loss of pain and temperature sensation in a cape-like distribution across the shoulders and upper limbs, but light touch and vibration sense are preserved. Which of the following best explains the neuroanatomical basis for the dissociated sensory loss pattern in this patient?