The image shown below (Figure 2) depicts a slice from the CT scan of the brain of a 43-year-old female who developed decreased conscious state four days after surgical drainage of a C5-6 epidural abscess.
a) Discuss the CT scan findings. (20% marks)
b) Explain the mechanism by which this has occurred. (20% marks)
c) List your management plan. (20% marks)
CT scan- Axial noncontrast brain CT with pneumocephalus / trapped air in subdural and interhemispheric space bilaterally. Likely tension pneumocephalus based on interhemispheric widening, compression and peaking of the frontal lobes.
Dural tear provides a ball-valve mechanism for the potential route of air entry by the creation of a negative pressure which can draw air into and through the spinal canal, and hence into the cranial cavity through the foramen magnum, but does not allow air to exit. Hence, both a defect in the dura and reduction in intracranial pressure, caused by CSF leakage contribute to pneumocephalus formation.
This is pneumoencephaly, the image of which was stolen shamelessly from Eric Miller's Emergency Medical Minute (Podcast #93). Specifically, it is tension subdural pneumoencephaly, which can be identified by the "Mount Fuji" sign (S.Michel, 2004). Observe how the intracranial air is under pressure: the frontal lobes have been squished and separated, giving a twin peak appearance. The lateral ventricle is somewhat squashed-looking, which also suggests that there is increased intracranial pressure. And yes, that's clearly a contrast-enhanced study, even though the original CICM answer specifies a non-contrast CT. Any imaging for this complication was hellishly difficult to find.
How did this happen? From the history, one would have to think that it is associated with the recent posterio spinal surgery. Indeed, the phenomenon is well known as the "inverted bottle effect". Lundsford et al described this in 1979. In essence, the drainage of CSF from the spine creates a negative intracranial pressure, which entrains air. " The mechanism for entry of air into the intracranial compartment is analogous to the entry of air into an inverted soda-pop bottle", the authors muse. "As the fluid pours out, air bubbles to the top of the container".
This does happen for real. Turgut et al (2007) report one one such case where suction on a lumbar wound resulted in excess entrainment of air into the subarachnoid space and cisterns.
In summary, these are the management options:
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