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Pneumocephalus occurs in about 60 per cent of the cases of fracture of the skull involving the frontal sinus. The mechanism of the process whereby air reaches the cerebral ventricles or forms a cyst within the brain substance is generally accepted to be as follows: At the time of the fracture, there occurs, usually, a rupture of the adjacent meninges with secondary adhesions between the pia and the arachnoid. When, subsequently, the patient coughs, sneezes, strains or swallows, increased pressure in the sinus results, and air is forced through the fracture and the torn adjacent meninges into the brain substance, where it is trapped by the valvelike action of the meninges. As more and more air thus accumulates intracerebrally, the cyst gradually approaches the ventricle, into which, finally, the air escapes. In some cases the air is not intraventricular but remains encysted in the brain substance. Another theoretical method by
KESCHNER M, LANDER J. PNEUMOCEPHALUS AND SUBARACHNOID HEMORRHAGE FOLLOWING SKULL FRACTURE. JAMA. 1933;101(1):24–25. doi:10.1001/jama.1933.02740260026008
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