A basal fracture through the sphenoid bone may lacerate the internal carotid artery or one of its intracavernous branches where it lies in the cavernous sinus. Within hours or a day or two, a disfiguring pulsating exophthalmos develops as arterial blood enters the sinus and distends the superior and inferior ophthalmic veins that empty into the sinus. The orbit feels tight and painful, and the eye may become partially or completely immobile because of pressure on the ocular nerves traversing the sinus (see Fig. 13-5). The sixth nerve is affected most often, and the third and fourth nerves less often. Also, there may be a loss of vision as a result of ischemia of the optic nerve and retina, although the mechanism has not been entirely clear; congestion of the retinal veins and glaucoma are potential factors in the visual failure. There is usually an easily audible bruit over the eye. Some 5 to 10 percent of fistulas resolve spontaneously, but the remainder must be obliterated by interventional radiologic means, usually a detachable balloon, or by a direct surgical repair of the fistula (see Stern).
Not all carotid–cavernous fistulas are traumatic. They may occasionally occur with rupture of an intracavernous saccular aneurysm or in Ehlers-Danlos disease, where the connective tissue is defective; or the cause may be unexplained. Occasionally, a dural-based arteriovenous fistula opens in the region of the cavernous sinus after an injury and they cause fewer signs such as orbital swelling than a carotid–cavernous fistula.
Pneumocephalus, Aerocele, and Rhinorrhea (Cerebrospinal Fluid Leak)
If the skin over a skull fracture is lacerated and the underlying meninges are torn, or if the fracture passes through the inner wall of a paranasal sinus, bacteria may enter the cranial cavity, with resulting meningitis or abscess formation. CSF that leaks into the sinus presents as a watery discharge from the nose (CSF rhinorrhea). The nasal discharge can be identified as CSF by testing it for glucose with diabetic test tape (mucus has no glucose) or by the presence of fluorescein or radionuclide-labeled dye that is injected into the lumbar subarachnoid space and then absorbed by pledgets placed in the nasal cavity. Mucus, when absorbed onto a handkerchief and allowed to dry, will leave the material stiff, whereas CSF, will not. A more elaborate test is to detect tau protein in the discharge; it is present only in CSF and not in mucus or blood. Most cases of acute CSF rhinorrhea heal by themselves. An indwelling lumbar drain for a few days may aid the process but this approach has only been tested in small trials such as the one by Albu and colleagues, in which the leak persisted for 2 fewer days but meningitis was not avoided. If the condition is persistent or is complicated by an episode of meningitis, surgical repair of the torn dura is indicated and this is sometimes possible through endoscopic methods. The prophylactic use of antibiotics to prevent meningitis in cases of nasal CSF leak is controversial, but many neurosurgeons continue this practice, particularly in children.
A collection of air in the cranial cavity (aerocele) is a common occurrence following skull fracture or any extended neurosurgical procedure. The pocket of air is apparent by CT scan in the epidural or subdural space over the cerebral convexities or between the hemispheres, and serves only to warn of a potential route for the entry of bacteria into the cranium. Small collections of air are usually absorbed without incident, but a large volume may act as a mass and cause clinical deterioration after injury (tension pneumocranium; Fig. 34-3). Inhalation of 100 percent oxygen has a slight salutary effect, but aspiration of the air is required if the collection is causing clinical signs.