Continuing Education Activity

Pneumocephalus, also known as pneumatocele or intracranial aerocele, is the presence of air in the intracranial space. Lecat first described this condition in 1741, but the term "pneumocephalus" was coined independently by Luckett in 1913 and Wolff in 1914. Pneumocephalus can occur following trauma, cranial surgeries, or spontaneously. It is classified as simple or tension pneumocephalus and can also be classified as acute, or less than 72 hours, delayed, or greater than 72 hours old. This activity reviews the evaluation and management of pneumocephalus and highlights the role of interprofessional team members in collaborating to provide well-coordinated care and enhance patient outcomes.

Objectives:

• Identify the epidemiology of pneumocephalus.
• Describe the evaluation of pneumocephalus.
• Outline the treatment and management options available for pneumocephalus.
• Explain interprofessional team strategies for improving care coordination and communication to advance the management of pneumocephalus and improve outcomes.

Introduction

Pneumocephalus (also known as pneumatocele or intracranial aerocele) is defined as the presence of air in the epidural, subdural, or subarachnoid space within the brain parenchyma or ventricular cavities.[1] Lecat first described this condition in 1741, but the term "pneumocephalus" was coined independently by Luckett in 1913 and Wolff in 1914.[2][3][4] The term "tension pneumocephalus" (TP) was proposed in 1962 by Ectors, Kessler, and Stern.[5]

Pneumocephalus can occur following trauma, cranial surgeries, or spontaneously. It is classified as simple or tension pneumocephalus. It can also be classified as acute (less than 72 hours) or delayed (72 hours or more).[6]

It has to be differentiated from the following terms:

1. Pneumorrhachis denotes intraspinal air.
2. Pneumocele is a focal or diffuse enlargement of any paranasal sinus (usually frontal) associated with thinning of its bony walls and hyperpneumatization
3. Pneumosinus dilatans is the same as pneumocele, but the sinus walls are intact and normal.
4. Pneumoventricle is the presence of intraventricular air

The term tension pneumoventricle is used when there is an intraventricular accumulation of air causing an increase in the intracranial pressure and compression of vital centers.[7]

Etiology

Congenital

• Skull base defects
• Tegmen tympani defect

Traumatic

• Most common cause
• Fractures involving the skull base with breach of the dura
• Fractures of air sinuses
• Penetrating head injuries with a dural laceration
• High-pressure trauma to the conjunctiva[8]
• Subarachnoid-pleural fistula[9]

Infectious

• Meningitis or ventriculitis that is produced by gas-forming organisms[10]
• Chronic otitis media and sinusitis

Neoplastic

• Dermoid cyst rupture
• Tumors eroding the skull or skull base, like osteoma, epidermoid or pituitary adenoma

Iatrogenic[11][12]

• Trans-cranial surgeries (especially posterior fossa surgeries in a prone position and following chronic subdural hematoma evacuation in a supine position)
• Transsphenoidal surgeries
• Following lumbar puncture or spine surgeries
• Ventriculostomies
• Epidural or subarachnoid blocks[13]
• Endoscopic dacryocystorhinostomy or septoplasty
• Secondary to cerebrospinal fluid (CSF) leakage from the surgical site[14]
• Positive pressure ventilation[15]
• Spreading of subcutaneous emphysema through the ventriculoperitoneal shunt track into the brain[16]

Spontaneous

• Following the development of spontaneous cerebrospinal fluid rhinorrhea
• Secondary to CSF leakage from myelomeningocele
• Spontaneous otogenic pneumocephalus occurs when there is an anomalous communication between the intracranial space and a hyperpneumatised temporal bone.

Others

• Barotrauma[17]

Epidemiology

The incidence of pneumocephalus depends on the etiology and is seen in almost all post-craniotomy cases. The incidence following head injury varies depending on the series, from 1% to as high as 82%.[18][19]

Pathophysiology

During head injury or following cranial surgeries, the dura may be opened or torn with or without injury to the arachnoid. In all these cases, air can get inside the cranial cavity.

Mechanism Theories For the Development of Pneumocephalus

Several theories exist for the etiology of pneumocephalus.[5][20] These include:

• Ball valve theory of Dandy: Unidirectional movement of air from the outside environment into the cranial cavity. This is the mechanism behind pneumocephalus following positive pressure ventilation.
• Inverted-soda-bottle effect of Horowitz and Lunsford: Excessive loss of cerebrospinal fluid (CSF) due to drainage in a physiological way during Valsalva or via lumbar drain leads to low intracranial pressure (ICP) and trapping of air in the vacuum created inside the cranium.
• A sudden increase in the pressure in the air sinuses due to blowing the nose, coughing, or sneezing causes the air to be sucked into the brain through the defects in the skull base.
• The development of pneumocephalus during epidural injections can be due to a loss of resistance technique used to identify the epidural space.[21]
• The entry of air through the meninges can be due to accidental injection, inadvertent dural puncture, or the differential pressures between the cranial cavity and atmosphere.[21]
• The cause of pneumocephalus during anesthesia can be attributed to the use of nitrous oxide. The blood–gas partition coefficient of nitrous oxide is 34 times greater than that of nitrogen. This causes nitrous oxide to diffuse into the cranial cavity faster than nitrogen or air.[22]
• Boyle's law states that if the temperature remains fixed in a confined space, the absolute pressure and volume are inversely proportional. Thus, when absolute pressure decreases inside the cranial cavity, the volume of air increases accordingly. This explains the expansion of pneumocephalus during flights.[21]

The presence of air is a source of infection, which can lead to the development of meningitis. Also, it can cause seizures by irritating the cerebral cortex.

History and Physical

The following features of the patient's history and/or examination should make clinicians suspicious that the patient has pneumocephalus:

1. CSF leak from the nose, ear, or surgical site
2. Persistent headache after cranial or spinal surgery[23]
3. Seizures following surgery[24][25]
4. Postoperative meningitis
5. Frontal lobe syndrome
6. Flapping scalp sign[26]
7. Oculomotor nerve palsy[27]
8. Tinnitus[28]

Physical Examination

Pneumocephalus is a clinically difficult diagnosis. Rarely, some patients may describe a splashing sound on head movement (known as bruit hydro-aerique), which can be auscultated as well. TP can lead to deterioration in sensorium and papilledema. The same features in the posterior fossa may cause brainstem signs, respiratory irregularities, and cardiac arrest. Even paraplegia and hemiplegia have been reported following TP.[5]

Evaluation

Skull X-ray

X-rays have been used in the past to identify pneumocephalus, but they will miss small quantities of air.

Plain Computed Tomography (CT) Scan of the Head

This imaging modality is the gold standard investigation in the diagnosis of pneumocephalus. It can detect even 0.55 ml of intracranial air, whereas a skull radiograph requires at least 2 ml.[29] Air has a Hounsfield coefficient of -1000. There are two signs which were identified as characteristic of TP by Ishiwata et al.[30]

1. "Mount Fuji sign" (named after Mount Fuji, the highest volcano mountain in Japan) formed by the accumulation of air in the frontal region, with separation of tips of two frontal lobes, in a patient in the supine position is diagnostic of tension pneumocephalus.
2. "Air bubble sign" denotes the presence of multiple air bubbles scattered in several cisterns.

"Peaking sign" denotes bilateral compression of frontal lobes without separation of the tips. It shows a less severe condition compared to the Mount Fuji sign.

Calculation of the Volume of Post-operative Pneumocephalus

• Bed-side technique - ABC/2 method)[31]

      Steps:

1. Select a representative slide near the center of the pneumocephalus.
2. "A" = longest longitudinal length in millimeters (mm) of the pneumocephalus in the axial plane
3. "B" = maximum width (in mm) of the pneumocephalus (measured from the inner table of the skull) perpendicular to "A" on the same slice
4. "C" = height of the air in the coronal plane = Number of CT scan axial slides showing the pneumocephalus multiplied by the slice thickness.
5. Estimated volume (in ml) = A x B x C / 2

• Computer-assisted volumetric measurement

Magnetic Resonance Imaging (MRI) Scan of the Brain

MRI may also be useful but not as sensitive as a CT scan in diagnosing pneumocephalus. Moreover, air may be mistaken for flow voids or blood products, appearing dark in almost all sequences.

Treatment / Management

Initial treatment of any head injury should follow the Advanced Trauma Life Support (ATLS) protocol.

Simple Pneumocephalus

Usually conservative. It involves the following steps:

1. Bed rest
2. Placing the patient in 30 degrees Fowler position
3. Avoiding Valsalva maneuvers like nose-blowing, coughing, and sneezing
4. Analgesics and antipyretics
5. Osmotic diuretics, if indicated
6. High-flow oxygen therapy should be given (5 L per minute for five days at least) via a face tent or 100% non-re-breather mask with absolute avoidance of positive pressure. The air is composed of 78% nitrogen and 21% oxygen. The rate of nitrogen absorption from pneumocephalus depends on the partial pressure of nitrogen in the blood, which is inversely proportional to the FiO2. When clinicians supplement oxygen, the nitrogen concentration in blood and brain tissue is reduced, increasing the nitrogen concentration gradient for absorption between the air collection and surrounding brain tissue. Slowly pneumocephalus will be replaced by oxygen, which has got high solubility within brain tissue and blood, which, in turn, facilitates its absorption leading to the final resorption of pneumocephalus.[32]
7. Hyperbaric oxygen therapy[33]

Surgical

1. Symptomatic pneumocephalus
2. TP
3. Recurrent pneumocephalus
4. Persistent traumatic pneumocephalus lasting more than one week
5. Tension pneumoventricle

TP following cranial surgery can be treated by introducing a needle through the bur hole of the previous craniotomy and aspirating the air with a syringe.

Decompression Options

1. Needle aspiration - either blind or under radiological guidance through an existing bur hole or craniotomy[34]
2. Controlled decompression via a subdural drain connected to an underwater seal followed by the closure of the dural defect[35]
3. Ventriculostomy for pneumoventricle[36]
4. Emergency decompression by the creation of fresh cranial bur holes[36]
5. Decompressive craniectomy[36]
6. Insertion of saline-primed Camino bolt
7. Insertion of the subdural evacuating port system (SEPS)™[37]

Endoscopic endonasal eustachian tube obliteration has also been tried as a treatment for TP following lateral skull base surgery.[38]

Differential Diagnosis

Intracranial fat, although having a much higher density (-90 HU) compared to air (-1000 HU), can appear hypodense on CT scans and can be mistaken for pneumocephalus. 

In MRI, pneumocephalus may be mistaken for blood products or flow voids.

Prognosis

Simple pneumocephalus is a condition that usually resolves by itself with conservative therapy. Sometimes it can produce seizures and meningitis. Prognosis is usually good even with tension pneumocephalus, provided timely treatment is given.

Following corpus callosotomy, the presence of pneumocephalus might act as a predictor of chemical meningitis.[39]

The presence of pneumocephalus after a chronic subdural hematoma evacuation has been linked to its recurrence and is one of the components of the Puerto Rico Recurrence Scale.[40]

Complications

The following complications are likely to occur in a patient with pneumocephalus:

1. Meningitis
2. Seizures
3. Brain abscess
4. Brain herniation secondary to TP

Deterrence and Patient Education

Prevention of Pneumocephalus

The following methods can help prevent the development of pneumocephalus during or after surgical procedures:

• Filling of the surgical site with saline at the time of closure of the dura
• Administering the Valsalva maneuver before taking the last bite of a suture through the dura during its closure to allow air to escape outside
• A smaller gauge spinal needle makes a smaller dural perforation and prevents cerebrospinal fluid leakage. Hence, such needles should be used while doing the lumbar puncture.
• Keeping the patient in the supine position with no head end of bed elevation following chronic subdural hematoma evacuation
• Positioning the head during dural closure in such a way that the last part of the dural defect becomes the highest point to facilitate the escape of residual air while filling the subdural space with saline
• Nitrous oxide (N2O), an anesthetic agent, had been previously proposed to cause pneumocephalus unless it is discontinued before the time of dural closure. But based on a randomized control trial, such an adverse effect of N2O on intracranial pressure was not noticed.[41]
• The use of saline rather than air to identify the epidural space during epidural injections.
• Avoiding high airway pressure and hyperventilation during invasive ventilation.

Neurosurgical procedures can result in residual intracranial air and can also result in a continuous entry of air into the cranial cavity. Hence the patient is advised to wait for at least seven days before taking a flight as the cabin pressure changes can introduce air inside the skull.[42]

There is no proper evidence to support the prophylactic administration of ceftriaxone for preventing meningitis in patients with traumatic pneumocephalus.[43]

Air travel is not advisable for patients with an intracranial air volume of more than 30 ml. Intracranial air volume of 20 ml and initial intracranial pressure of 15 mmHg was identified as conservative thresholds for safe air travel among such patients.[44]

The following methods can be adopted to avoid complications from pneumocephalus during a flight:

• A low-level flight or by maintaining ground-level cabin pressure during the flight
• Reduced rates of change in cabin pressure
• Positioning to reduce the CSF leak
• Administering supplementary oxygen
• Pre-flight decongestants
• Avoidance of Valsalva maneuvers
• If the patient is ventilated, hypoventilation and carbon dioxide should be prevented.
• Performing recent pre-flight imaging to assess the volume of pneumocephalus and to rule out an extracranial-intracranial fistulous process.
• Develop an understanding of the patient-specific pathophysiology and the time course of pneumocephalus.
• Reduce the stresses induced by accelerations (including changing g-forces), noises, or hypoxemia.[45]

It is usually recommended to avoid air travel for 2 to 8 weeks after intracranial surgery.[46]

Enhancing Healthcare Team Outcomes

All patients with head injuries and post-craniotomy status should be strictly monitored for the development of pneumocephalus. Care of these patients requires the efforts of an interprofessional team that includes clinicians, surgeons, and nursing staff. Nurses should monitor the sensorium, be careful regarding patients' positioning, and give instructions to avoid a Valsalva maneuver. They should contact a clinician if there is a serious change. [Level 5] If the patient develops TP, which leads to a drop in sensorium, basic supportive care, including maintenance of airway, breathing, and circulation, followed by definitive management, should be provided.