DOI: 10.1148/radiol.2322021556
(Radiology 2004;232:449-450.)
© RSNA, 2004
The Mount Fuji Sign1
Steven J. Michel, MD
1 From the Department of Diagnostic Radiology, University of Kentucky, 800 Rose St, Rm HX319, Lexington, KY 40536-0293. Received November 25, 2002; revision requested January 28, 2003; revision received February 6; accepted March 13. Address correspondence to the author (e-mail: s_j_michel@yahoo.com).
Index terms: Brain, CT, 111.1211 • Pneumocephalus, 111.458, 136.455, 136.458 • Signs in Imaging
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APPEARANCE
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The Mount Fuji sign is a finding that can be observed on computed tomographic (CT) scans of the brain (1), 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 (Figs 1, 2)—hence, the Mount Fuji sign.
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Figure 2. Unenhanced transverse CT image of the brain obtained caudal to image shown in Figure 1. Widening of the interhemispheric space between the tips of the frontal lobes is noted. The medial surface of each frontal lobe is marked (arrows).
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EXPLANATION
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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 (ie, 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 (2,3). 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 (4). Irrespective of the mechanism, the increased pressure may lead to extraaxial 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.
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DISCUSSION
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The Mount Fuji sign on CT scans of the brain is useful in discriminating tension pneumocephalus from nontension pneumocephalus. Tension pneumocephalus can be a neurosurgical emergency, unlike nontension pneumocephalus. Tension pneumocephalus occurs most commonly after the neurosurgical evacuation of a subdural hematoma. The prevalence of tension pneumocephalus following the evacuation of chronic subdural hematomas has been reported from 2.5% to 16% (1,5). Tension pneumocephalus can also occur as a result of skull base surgery, paranasal sinus surgery, posterior fossa surgery in the sitting position, or head trauma (4,6–9). To diagnose tension pneumocephalus, the CT findings should correlate with clinical signs of deterioration.
Tension pneumocephalus has been described in the early literature (10). With the advent of CT, radiologists possess the information necessary to assess the effects of pneumocephalus on the brain. In the early 1980s, an attempt was made to explain the mass effect of pneumocephalus based on the volume of gas, with one author using the value of 65 mL of air (11). This method, however, is hindered by the inherent difficulties in volume estimation at CT. In addition, other authors have found no substantial difference between the volume of air in nontension pneumocephalus and that in tension pneumocephalus (1). A "peaking sign" of bilateral compression of the frontal lobes by subdural air collections without the characteristic separation of the frontal lobes has also been linked to tension pneumocephalus (12).
More recently, Ishiwata et al described the appearance of the Mount Fuji sign in four of five patients with surgically confirmed tension pneumocephalus (1). The Mount Fuji sign was not found in any patients with nontension pneumocephalus. These authors noted the peaking sign in patients with nontension pneumocephalus, but not in patients with tension pneumocephalus.
When the clinical and imaging findings are correctly identified, treatment consists of emergent decompression to alleviate pressure on the brain parenchyma. Treatment options for tension pneumocephalus include drilling of burr holes, craniotomy, needle aspiration, ventriculostomy placement, administration of 100% oxygen, and closure of dural defects (3,4). Careful monitoring for clinical signs of deterioration, as well as serial CT scanning of the brain, is recommended. In patients who are treated for tension pneumocephalus, resolution of the subdural air collection is expected (1,3,4).
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.
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FOOTNOTES
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A trainee (resident or fellow) wishing to submit a manuscript for Signs in Imaging should first write to the Editor for approval of the sign to be prepared, to avoid duplicate preparation of the same sign.
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REFERENCES
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- Ishiwata Y, Fujitsu K, Sekino T, et al. Subdural tension pneumocephalus following surgery for chronic subdural hematoma. J Neurosurg 1988; 68:58-61.[Medline]
- Chee NW, Niparko JK. Imaging quiz case 1: otogenic pneumocephalus with temporal bone cerebrospinal fluid (CSF) leak. Arch Otolaryngol Head Neck Surg 2000; 126:1499-1503.[Free Full Text]
- Campanelli J, Odland R. Management of tension pneumocephalus caused by endoscopic sinus surgery. Otolaryngol Head Neck Surg 1997; 116:247-250.[CrossRef][Medline]
- Artru AA. Nitrous oxides play a direct role in the development of tension pneumocephalus intraoperatively. Anesthesiology 1982; 57:59-61.[CrossRef][Medline]
- Bremer AM, Nguyen TQ. Tension pneumocephalus after surgical treatment of chronic subdural hematoma: report of three cases. Neurosurgery 1982; 11:284-287.[Medline]
- Fliss DM, Gil Z, Spektor S, et al. Skull base reconstruction after anterior subcranial tumor resection. Neurosurg Focus [serial online]. May 2002; 12:article 10.
- Toung T, Donham RT, Lehner A, Alano J, Campbell J. Tension pneumocephalus after posterior fossa craniotomy: report of four additional cases and review of post-operative pneumocephalus. Neurosurgery 1983; 12:164-168.[Medline]
- Davis DH, Laws ER, Jr, McDonald TJ, et al. Intraventricular tension pneumocephalus as a complication of paranasal sinus surgery: case report. Neurosurgery 1981; 8:574-576.[Medline]
- Oge K, Akpinar G, Bertan V. Traumatic subdural pneumocephalus causing rise in intracranial pressure in the early phase of head trauma: report of two cases. Acta Neurochir (Wien) 1998; 140:655-658.[CrossRef][Medline]
- Dandy WE. Pneumocephalus (intracranial pneumatocele or aerocele). Arch Surg 1926; 132:949-982.
- Monajati A, Cotanch WV. Subdural tension pneumocephalus following surgery. J Comput Assist Tomogr 1982; 6:902-906.[Medline]
- Pop PM, Thompson JR, Kinke DE, et al. Tension pneumocephalus. J Comput Assist Tomogr 1982; 6:894-901.[Medline]
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