A 21-year-old woman incurs a blow to her head from a fall while mountain biking. She then has loss of consciousness for 5 minutes. On examination her deep tendon reflexes are diminished. A head CT scan 6 hours later shows no abnormalities. She recovers over the next week, with no neurologic deficits, but cannot remember this event. During the next year she has irritability, headache, difficulty sleeping, trouble concentrating, and fatigue. Which of the following is the most likely consequence from her injury?

Concussion leads to altered consciousness, starting with the instantaneous onset of transient neurologic dysfunction with head trauma, amnesia for the event, then neurologic recovery or post-concussion syndrome with disabling neuropsychiatric manifestations, worsening with repeated concussions. There are no radiologic or pathologic findings. Arteriolosclerosis is typically seen with hypertension. Hemorrhage following the traumatic event could organize to obstruct CSF flow or absorption, but this would have been identified with the original CT scan. The term leukoencephalopathy applies to degenerative white matter changes, typically with metabolic and infectious insults. Post-traumatic dementia from repeated blows to the head often has pathologic findings of neurofibrillary tangles and neuritic plaques similar to Alzheimer's disease.