Pneumocephalus: Pathophysiology, Imaging, and Clinical Insights from a Rare Case

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Abstract

Pneumocephalus refers to the presence of air within the cranial cavity, most often in the subdural, epidural, subarachnoid, or intraventricular compartments. 

Although typically associated with head trauma, skull base fractures, or postoperative neurosurgical complications, spontaneous cases without preceding injury are increasingly recognized. 

This article reviews the pathophysiology, epidemiology, clinical manifestations, imaging features, differential diagnoses, and management strategies of pneumocephalus, based on an instructive case of a 54-year-old woman presenting with auditory disturbances, aphasia, and visual impairment.

Case Presentation

A 55-year-old woman presented with progressive abnormal auditory sensation, aphasia, and visual field defect. She denied any history of head trauma, infection, or prior surgery.
Initial skull radiography revealed intracranial air in the left temporal region without evidence of fracture.

[Figure 1] Skull A-P radiograph showing localized air pocket in the left temporal region without skull fracture.

Subsequent non-contrast CT of the brain demonstrated a 4 × 3 × 5 cm pocket of air compressing adjacent gyri, sulci, and the left lateral ventricle, causing mild midline shift.

[Figure 2] Axial non-contrast CT scan showing large air collection in left temporal lobe producing mass effect and midline shift.

The most likely diagnosis was Pneumocephalus.

Pathophysiology

Pneumocephalus occurs when air enters the intracranial space through a breach in the skull or dural barrier.
The underlying mechanisms include:

  1. Ball-valve mechanism: A one-way air entry through dural defects due to pressure gradients.

  2. Inverted soda bottle effect: Loss of cerebrospinal fluid (CSF) creates negative intracranial pressure, drawing air inward.

  3. Gas-forming infections: Rare anaerobic bacterial infections may generate intracranial gas.

In tension pneumocephalus, trapped air behaves as a space-occupying lesion, raising intracranial pressure and causing brain compression — a neurosurgical emergency requiring decompression.

Epidemiology

Pneumocephalus is relatively rare, accounting for <1% of intracranial pathologies.

Most cases arise post-trauma (75%), followed by postoperative neurosurgical causes (15–20%), and a smaller fraction due to barotrauma or infection.

Spontaneous pneumocephalus, as seen in this case, is extremely uncommon and often idiopathic or secondary to occult dural defects.

Recent studies report an increasing incidence due to improved CT sensitivity, detecting even minimal intracranial air volumes (<0.5 mL).

Clinical Presentation

Symptoms depend on the volume and location of intracranial air.
Common manifestations include:

  • Headache and nausea due to meningeal irritation.

  • Altered mental status or aphasia, from temporal or frontal compression.

  • Visual disturbances when the occipital or temporal regions are involved.

  • “Succussion splash” or “bruit hydroaerique” — a characteristic audible splash on head movement.

In tension pneumocephalus, patients may present with acute deterioration, seizures, or coma due to mass effect.

Imaging Features

CT scanning is the diagnostic gold standard.

Findings include air-density regions (−1000 HU) within the cranial cavity, often outlining sulci or ventricles.

Typical CT patterns:

  • Mount Fuji sign: Bilateral subdural air separating and compressing frontal lobes.

  • Air bubbles along cisterns or ventricles.

  • Midline shift in large collections.

MRI may demonstrate signal voids in air-filled regions but is less sensitive than CT.

[Figure 3] Preoperative MRI showing tumor mass adjacent to normal paranasal sinus before craniotomy.

[Figure 4] Postoperative MRI showing venous sinus thrombosis associated with tension pneumocephalus.

Differential Diagnosis

When intracranial air is detected, differentials include:

ConditionDistinguishing Features
Epidural hematoma   Hyperdense crescent on CT, no air density
Subdural empyema   Fluid collection with rim enhancement
Brain abscess (gas-forming)   Central air-fluid level, restricted diffusion
Postoperative change   Known surgical site, expected distribution
Tumor necrosis     Mixed density, irregular margins

Diagnosis

Diagnosis is established radiographically via CT brain demonstrating air within cranial compartments.
Further evaluation includes:

  • CSF leak testing (β-2 transferrin assay).

  • MRI with T2 and FLAIR for secondary edema or venous sinus involvement.

  • ENT and neurosurgical assessment for skull base integrity.

Treatment and Management

Management depends on volume, cause, and symptoms:

  1. Conservative therapy for small, asymptomatic cases:

    • Head elevation (30° Fowler’s position).

    • Avoidance of Valsalva or positive pressure.

    • High-flow oxygen therapy (FiO₂ 100%) to accelerate nitrogen absorption.

  2. Surgical decompression for tension or symptomatic pneumocephalus:

    • Needle aspiration, burr-hole drainage, or craniotomy.

    • Repair of dural or skull base defects to prevent recurrence.

  3. Management of complications, such as cerebral venous sinus thrombosis, requires anticoagulation and close monitoring.

In the cited case, a 38-year-old man developed tension pneumocephalus complicated by cerebral venous sinus thrombosis following tumor resection and CSF drainage — emphasizing the importance of vigilant postoperative monitoring.

Prognosis

With prompt diagnosis and management, most cases resolve within 2–3 weeks.
However, delayed recognition of tension pneumocephalus can result in brain herniation or death.
Prognosis depends on etiology, volume of air, and timeliness of intervention.

Quiz

Q1. Which imaging sign is most characteristic of tension pneumocephalus on CT?
A. Cotton wool sign
B. Mount Fuji sign
C. Delta sign
D. Empty sella sign

Q2. Which of the following mechanisms best explains spontaneous pneumocephalus without trauma?
A. Ball-valve effect
B. Hematogenous spread
C. Osmotic gradient
D. Barotrauma

Q3. In managing pneumocephalus, which intervention accelerates air resorption?
A. High-flow oxygen therapy
B. Hypertonic saline infusion
C. Mannitol administration
D. Lumbar drain placement

Answer & Explanation

1. Answer: B. Explanation: The Mount Fuji sign — bilateral subdural air separating the frontal lobes — is diagnostic of tension pneumocephalus.

2. Answer: A. Explanation: The ball-valve mechanism allows air to enter but prevents its exit, leading to intracranial accumulation.

3. Answer: A. Explanation: High-flow oxygen increases nitrogen gradient, facilitating rapid absorption of intracranial air.

References

[1] M. M. Azher et al., “Tension pneumocephalus: Clinical review and management update,” World Neurosurg., vol. 168, pp. 140–148, 2023.
[2] S. J. Chang and Y. H. Kim, “Spontaneous pneumocephalus associated with sinus pathology: A case-based review,” J. Clin. Neurosci., vol. 104, pp. 62–69, 2022.
[3] P. Das et al., “Postoperative tension pneumocephalus following craniotomy: Prevention and early detection,” Neurosurg. Rev., vol. 47, no. 1, pp. 71–80, 2024.
[4] A. K. Sharma et al., “Intracranial air: Etiopathogenesis and imaging spectrum,” Clin. Radiol., vol. 78, no. 9, pp. 713–722, 2023.
[5] N. Shaikh et al., “Tension pneumocephalus causing cerebral venous sinus thrombosis,” Neurosurg. Cases Rev., vol. 2, no. 4, 2020.
[6] R. Patel et al., “Management of pneumocephalus after neurosurgical interventions,” Front. Neurol., vol. 12, 2021.
[7] A. J. Lee and M. R. Cho, “Emergent decompression in tension pneumocephalus: Review of outcomes,” Eur. J. Trauma Emerg. Surg., vol. 51, no. 2, pp. 345–354, 2025.

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