The passage of atmospheric air into the tissues of the cranium and its contents through trauma, infection, tumor growth or congenital defect produces a variety of clinical syndromes. For the purpose of discussion, these disturbances arising from such an invasion may be grouped under the following headings.
1. Emphysema capitis
2. Pneumocephalus internus
3. Pneumatocele frontalis externa, or pneumatocele syncipitalis
4. Pneumatocele supramastoidea
5. Pneumatocele occipitalis
Emphysema capitis represents the diffusion of air between the pericranium and the galea aponeurotica, with or without infiltration of the entire thickness of the scalp. Most commonly it follows operation on the frontal sinus, but it may be recognized as a complication of trauma to the frontal sinus or of operation on the mastoid and must be differentiated from gas-forming infections of the scalp. From a didactic point of view, it must be differentiated as well from the emphysema involving the orbits, facial tissues