Correct answer:

Fractures extending through any of the paranasal air sinuses (frontal, ethmoidal or sphenoid) communicate with the outside and are therefore compound (open) fractures, as the overlying dura is usually breached. This external communication may manifest as a runny nose, the clear cerebrospinal fluid (CSF) being rich in glucose and low in mucin content (and positive for tau protein), compared with the normal nasal secretion, which contains no sugar and is rich in mucin. Such a connection may also be indicated by intracranial air (aerocele), or fluid in one of the sinuses on computed tomography scan or skull X-ray. Anosmia may occur if the fracture crosses the cribriform plate. Such patients are at risk of meningitis. Some CSF leaks heal spontaneously, particularly those involving the temporal lobe, but a persistent leak will require dural repair.