Normal Axial MRI Brain Medulla

A 35-year-old woman presents to the neurology clinic with a 2-year history of progressive occipital headaches that worsen significantly with coughing and Valsalva maneuver. On examination, she has mild gait ataxia and reports recent onset of dissociated sensory loss in a cape distribution over her shoulders and arms. MRI brain shows the structure marked **B** herniated more than 5 mm below the foramen magnum. Subsequent MRI of the entire spine reveals a fluid-filled cavity within the spinal cord. Which of the following best describes the pathophysiological consequence of the structural abnormality shown at **B**?