A 34-year-old woman presents with a 6-month history of progressive occipital headaches triggered by coughing and Valsalva maneuver, along with bilateral upper extremity pain and temperature loss in a cape-like distribution with preserved proprioception. MRI brain shows that the structure marked **A** extends 7 mm below the foramen magnum. Whole-spine MRI reveals a cervical syringomyelia. Which of the following best describes the pathophysiological mechanism linking the structural abnormality marked **A** to the sensory deficit pattern observed?

Explanation

## Why option 1 is right Chiari I malformation is defined by cerebellar tonsillar herniation >5 mm below the foramen magnum. The herniated tonsils obstruct normal CSF circulation at the foramen magnum, leading to increased pressure and syrinx formation within the spinal cord. The syrinx expands centrally, disrupting the crossing spinothalamic fibers in the anterior white commissure, which carry pain and temperature sensation. This produces the pathognomonic "cape-like" or "shawl" distribution of dissociated sensory loss (pain and temperature loss with preserved proprioception and vibration sense from intact dorsal columns). This mechanism is the hallmark of syringomyelia from Chiari I and is the direct consequence of the structural abnormality marked **A** (Gray's Anatomy 42e Ch 22; Harrison 21e Ch 446). ## Why each distractor is wrong - **Option 2**: While dorsal column compression does occur in some myelopathies, it would cause loss of proprioception and vibration with preserved pain—the opposite of what this patient exhibits. The dissociated sensory loss pattern specifically indicates anterior commissure involvement, not dorsal column pathology. - **Option 3**: Direct compression of corticospinal tracts by cerebellar tissue would produce upper motor neuron signs (hyperreflexia, spasticity), but the patient's primary deficit is sensory, not motor. Corticospinal involvement is a late finding in advanced Chiari, not the initial mechanism. - **Option 4**: Anterior spinal artery ischemia causes a different pattern—loss of pain, temperature, AND motor function (anterior cord syndrome) with preserved proprioception. The patient's selective pain/temperature loss with normal motor function at presentation excludes this mechanism. **High-Yield:** Chiari I = tonsillar herniation >5 mm below foramen magnum → CSF obstruction → syrinx → anterior commissure disruption → dissociated sensory loss (cape-like pain/temperature loss + preserved proprioception). [cite: Gray's Anatomy 42e Ch 22; Harrison 21e Ch 446]