The Mount Fuji sign is a radiological sign caused by a bilateral intracranial air collection and is indicative of tension pneumocephalus.
Naming
The name Mount Fuji sign is derived from Japan’s Mount Fuji, as the sign resembles the appearance of the mountain on CT imaging.
Pathophysiology
The Mount Fuji sign is caused by an intracranial air collection. This may be iatrogenic or non-iatrogenic in origin. During CT examination in the supine position, the air rises anteriorly, resulting in compression of the frontal lobe and the characteristic Mount Fuji sign on CT imaging. Typically, a one-way valve mechanism is required, in which air enters the intracranial space without being able to escape, in order to reach a sufficiently high pressure to cause compression of the frontal lobe.
Etiology
One possible cause of the bilateral air collection is an intracranial procedure, such as a burr-hole craniostomy following evacuation of a chronic subdural hematoma, in which an incorrectly placed drain creates a one-way valve mechanism, allowing air to enter the intracranial space without being able to escape.
Imaging
The Mount Fuji sign is classically diagnosed on CT imaging, but it can also be diagnosed on MRI.
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