A 43-year-old man presents to the emergency department after falling down a flight of stairs and landing on his head. He did not lose consciousness. He complains of severe headache, marked decreased acuity in hearing in the left ear, and a "runny nose" since the fall. On physical examination, he is found to have a left sided Battle's sign (an ecchymosis in the area of the left mastoid process) and hemotympanum. He has a constant dripping of a clear, watery fluid through his nose. Findings on his neurologic examination, other than the hearing loss, are completely normal. X-ray studies will reveal which of the following?

The presence of a Battle's sign and hemotympanum is highly suggestive of the possibility of a left temporal bone fracture. When this occurs, it is common for the dura mater at this site to be torn. This leads to leakage of CSF into the mastoid air cells and middle ear. CSF is subsequently able to reach the nasopharynx via the eustachian tube, a phenomenon called paradoxical rhinorrhea, which is a serious but usually self-limiting condition. Most cases of traumatic CSF leaks heal spontaneously within approximately 1 week. Patients require close in-hospital observation, however, because bacterial meningitis readily occurs in the presence of CSF leakage to the outside.