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)
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.
- ABC stabilization
- Neurosurgical review and consider aspiration of air with insertion of drain to drain the pneumocephalus
- High flow oxygen (nitrogen wash-out)
- Lie flat
- Fluid replacement
- May need surgical repair of CSF leak
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 venticle is somewhat squashed-looking, which also suggests that there is increased intracranial pressure.
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:
- Do nothing. The vast majority (85%) are reabsorbed spontaneously, without any intervention and with few clinical manifestations. Karavelioglu (2014) suggest that patience is required, as the process of passive reabsorption may take a couple of weeks.
- Conservative management: This consists of putting the patient head-up (30°) and avoiding Valsalva maneuvers (coughing, sneezing, straining to open bowels). As an aside, this preventative strategy also includes avoiding aeromedical retrieval. According to Donovan et al (2008), the intracranial gas pocket predictably expands in a depressurised cabin.
- Isobaric oxygen: after 24 hours of 100% oxygen by mask, Gore et al (2008) found that the mean volume of their patients' pneumocephalus decreased more than in those patients who only had room air.
- Hyperbaric oxygen seems to have a good effect: Paiva et al (2014) found that 1-hour sessions at 2.5 atmospheres of O2 resolved the pneumocephalus much faster than standard 5L/min nasal prong oxygen.
- Surgical management is mainly indicated in the context of symptomatic or tensioning pneumocephalus. "Drilling of burr holes, needle aspiration, and closure of the dural defect" is advocated.
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