The Mount Fuji Sign

The Mount Fuji sign is a finding that can be observed on computed tomographic (CT) scans of the brain, in which bilateral subdural hypoattenuating collections cause compression and separation of the frontal lobes. The collapsed frontal lobes and the widening of the interhemispheric space between the tips of the frontal lobes have the appearance of the silhouette of Mount Fuji—hence, the Mount Fuji sign.

A photo of The Mount Fuji.

Unenhanced transverse CT image of the brain demonstrates bilateral subdural areas of hypoattenuation (∗) with compression of the frontal lobes.                       

Unenhanced transverse CT image of the brain obtained caudal to image shown in previous figure. Widening of the interhemispheric space between the tips of the frontal lobes is noted. The medial surface of each frontal lobe is marked (arrows).                       

The hypoattenuating collections are caused by the entry of air into the cranial vault, a condition that can occur in iatrogenic and noniatrogenic disruption of the skull base or calvaria. However, tension pneumocephalus (i.e, subdural air causing mass effect on the brain) requires conditions that lead to increased air pressure within the subdural space. The increased pressure of air is assumed to be due to a ball-valve mechanism. This implies that air enters into the subdural space by means of a dehiscence in the skull base or calvaria and that the egress of air is blocked by an obstruction. An additional mechanism (ie, posterior fossa surgery in the sitting position) has been postulated, but it was later discredited. In these cases, it was believed that nitrous oxide, which was used as an anesthetic, diffused into air-filled spaces and expanded the gaseous volume. Irrespective of the mechanism, the increased pressure may lead to extra-axial mass effect with subsequent compression of the frontal lobes. The presence of air between the frontal tips suggests that the pressure of the air is at least greater than that of the surface tension of cerebrospinal fluid between the frontal lobes.                 

The Mount Fuji sign on CT scans of the brain is useful in discriminating tension pneumocephalus from nontension pneumocephalus. Tension pneumocephalus can be considered a neurosurgical emergency, unlike nontension pneumocephalus. Tension pneumocephalus occurs most commonly after the neurosurgical evacuation of a subdural hematoma.

The Mount Fuji sign on CT scans of the head in trauma patients and in postoperative patients can be a critical finding made by the radiologist. Identification of this sign can have immediate and important clinical implications for patient care and outcome.