Pneumoencephalography (sometimes abbreviated PEG; also referred to as an "air study") was a common medical procedure in which most of the cerebrospinal fluid (CSF) is drained from around the brain by means of a lumbar puncture and replaced with air, oxygen, or helium to allow the structure of the brain to show up more clearly on an X-ray image. It was derived from ventriculography, an earlier and more primitive method where the air is injected through holes drilled in the skull.
Pneumoencephalography was performed extensively throughout the early through mid-20th century. Though it was the single most important way of localizing brain lesions of its time, it was nevertheless extremely painful and was generally not well tolerated by patients. Headaches and severe vomiting were common side effects. During the procedure, the patient's entire body would be spun into different positions in order to allow air to displace the CSF in different areas of the ventricular system and around the brain. This further added to the patient's already heightened level of discomfort. Following the exam, replacement of the drained CSF occurs by slow natural production, and therefore required recovery for as long as 2–3 months before normal fluid volumes were restored. Video of the procedure is documented in a BBC documentary of an early EMI CT installation. . A related procedure is pneumomyelography, where gas is used similarly to investigate the spinal canal.
Pneumoencephalography makes use of plain X-ray images. Unfortunately, these are very poor at resolving soft tissues, such as the brain. Moreover, all the structures captured in the image are superimposed on top of each other, which makes it difficult to pick out individual items of interest (unlike modern scanners which are able to produce fine virtual slices of the body, including of soft tissues). Therefore, pneumoencephalography did not usually image abnormalities directly, rather their secondary effects. The overall structure of the brain contains crevices and cavities which are filled by the CSF. Unfortunately, both the brain and the CSF produce the same signal on an X-ray image. However, draining the CSF allows for greater contrast between the brain matter and the (now drained) crevices in and around it, which then show up as as dark shadows on the X-ray image. The aim of pneumoencephalography is to outline these shadow-forming air-filled structures so that their shape and anatomical location can be examined. Following the procedure, an experienced radiologist reviews the X-ray films to see if the shape or location of these structures have been distorted or shifted by the presence of certain kinds of lesions. Unfortunately, this also means that in order to show up on the images, lesions have to either be located right on the edge of the structures or if located elsewhere in the brain, be large enough to push on surrounding healthy tissues to an extent necessary to cause a distortion in the shape of the more distant air-filled cavities (and hence distal tumors detected this way tended to be fairly large).
Despite its overall usefulness, there were major portions of the brain and other structures of the head that pneumoencephalography was unable to image. This was partially compensated by increased use of angiography as a complementary diagnostic tool, often in an attempt to infer the condition of non-neurovascular pathology from its secondary vascular characteristics. This additional testing was not without risk though, particularly due to the rudimentary catheterization techniques and deleterious contrast agents of the day. Another drawback of pneumoencephalography was that the risk and discomfort it carried meant that repeat studies were generally avoided, thus making it difficult to assess disease progression over time.
Modern imaging techniques such as MRI and CT have rendered pneumoencephalography obsolete. Widespread clinical use of diagnostic tools using these newer technologies began in the mid-to-late 1970s. These revolutionized the field of neuroimaging by not only being able to non-invasively examine all parts of the brain and its surrounding tissues, but also by doing so in much greater detail than previously available with plain X-rays, thus making it possible to directly visualize and precisely localize soft-tissue abnormalities. This led to significantly improved patient outcomes while reducing discomfort. Today, pneumoencephalography is limited to the research field and is used under rare circumstances.
Pneumoencephalography appears in popular culture in the movie The Exorcist (1973), when Linda Blair's Regan MacNeil character undergoes the procedure. It is also referred to in Episode 7, Season 7 of House M.D. as an example of a dangerous procedure.
- "Walter Dandy". Walter Dandy. Society of Neurological Surgeons. Retrieved 2011-04-28.
- Greenberg, Mark (2010). Handbook of Neurosurgery. Thieme.