Pneumocephalus

Etiology

Pneumocephalus can arise from various causes, summarized below. The most common cause of this condition is head or facial trauma. Air can enter the cranial cavity through skull fractures. 

Table. Possible Causes of Pneumocephalus

Pneumocephalus can vary in severity, and the symptoms can range from mild headaches to more severe neurological deficits. Treatment depends on the underlying cause and the patient's clinical condition.

Epidemiology

The incidence of pneumocephalus depends on the etiology. The condition is seen in almost all postcraniotomy cases. The incidence of pneumocephalus following head injury varies depending on the study, from 1% to as high as 82%. About 74% of pneumocephalus cases are due to head trauma.[13][14][36]

Pathophysiology

Several theories regarding the etiology of pneumocephalus exist. These theories include the following:

• Air moves unidirectionally from the outside environment into the cranial cavity. This mechanism is responsible for pneumocephalus following positive pressure ventilation. This theory is called the "ball-valve theory of Dandy."
• ICP decreases due to excessive CSF loss, which may occur artificially via lumbar drainage or physiologically during Valsalva maneuvers. A vacuum is created, trapping air inside the cranium. This theory is called the "inverted-soda-bottle effect of Horowitz and Lunsford."
• Blowing the nose, coughing, or sneezing causes a sudden pressure increase in the air sinuses. Air is then drawn into the brain through defects in the skull base.
• The loss-of-resistance technique, used to identify the epidural space, is thought to be responsible for the development of pneumocephalus after epidural injections.
• Air entry through the meninges can be due to accidental injection, inadvertent dural puncture, or pressure gradients between the cranial cavity and atmosphere.
• Nitrous oxide may cause pneumocephalus during anesthetic procedures. The blood–gas partition coefficient of nitrous oxide is 34 times greater than that of nitrogen. Therefore, nitrous oxide can diffuse into the cranial cavity faster than nitrogen or air.
• Boyle's law states that pressure and volume are inversely proportional at constant temperature. Pneumocephalus expands during flights because air volume increases when pressure decreases inside the cranial cavity.[15][16]

Cranial breaks leave the cranial cavity vulnerable to air entry. Large air pockets can compress intracranial structures and increase the ICP, manifesting as focal neurological deficits. Air is also a potential intracranial infection source. Air entering the meninges can likewise irritate the cerebral cortex and produce seizures.

History and Physical

Pneumocephalus is a medical emergency, though it presents variably. A quick primary survey—assessing the airway, breathing circulation, disability, and exposure (ABCDE)—must be performed immediately upon receiving the patient. Unconscious individuals without respirations and pulses must be administered resuscitative measures promptly, regardless of cause. Once the patient's emergent needs are addressed, the secondary survey and diagnostic workup may be initiated.

The patient's history is likely to reveal a recent traumatic incident leading to a skull fracture. Symptoms depend on the pneumocephalus volume and intracranial area affected. Most patients are asymptomatic. However, symptomatic conscious patients may have nausea, vomiting, memory loss, speech and sensory changes, rhinorrhea or otorrhea, seizures, headaches, and motor weakness.

Though rare, patients may describe a splashing sound on head movement, known as "bruit hydroaerique," which is pathognomonic of pneumocephalus. This sign may be confirmed by cranial auscultation.

In severe cases, patients may present with altered sensorium, abnormal breathing patterns, or cardiorespiratory arrest. In individuals without a history of trauma, a recent neurosurgical or spinal procedure, infection symptoms, signs of intracranial mass effects, or a history of ventilator use or congenital anomalies may be elicited.

The head-and-neck examination may reveal scalp or skin lacerations, evidence of a skull fracture, blood and fluid in the nose or external auditory meatus, and tympanic membrane perforation. Evidence of multiple traumatic injuries includes respiratory distress signs, hypotension, tachycardia, abdominal rigidity, blood in the genitalia, bloody urine, and axioappendicular fractures. In patients with barotrauma, unilaterally decreased breath sounds (pneumothorax) or a positive Hamman sign (pneumomediastinum) may be noted on auscultation. Neurologic and musculoskeletal findings vary depending on the site affected.

The following features of the patient's history or examination should make clinicians suspicious that the patient may have pneumocephalus:

• CSF leak from the nose, ear, or surgical site
• Persistent headache after cranial or spinal surgery
• Seizures following surgery
• Postoperative meningitis
• Frontal lobe syndrome
• Flapping scalp sign
• Oculomotor nerve palsy
• Papilledema
• Tinnitus [17]

Posterior fossa involvement may occur in tension pneumocephalus, causing brainstem signs, respiratory irregularities, and cardiac arrest. Even paraplegia and hemiplegia have been reported following tension pneumocephalus.

Evaluation

Monitoring and Laboratory Testing

Individuals who appear unstable require continuous vital signs and cardiac monitoring. A fingerstick glucose test must be performed for all unconscious patients to rule out or address hypoglycemia. Individuals requiring oxygen supplementation must be monitored by pulse oximetry. 

Laboratory testing must be performed immediately for trauma patients, especially if emergency surgery is considered. The initial tests may include a complete blood count, coagulation profile, blood typing, complete metabolic panel, and urinalysis. Arterial blood gases and electrocardiography may be considered if the patient exhibits cardiorespiratory distress. Imaging studies must be immediately scheduled.

Plain Films

Skull x-rays have been used in the past to identify pneumocephalus, but plain films usually miss small quantities of air. Still, x-rays are useful in evaluating patients with multiple injuries.

Head Computed Tomography Scan Without Contrast

Head computed tomography (CT) scanning without contrast is the gold standard imaging modality in diagnosing pneumocephalus. This study can detect even 0.55 ml of intracranial air, while a skull radiograph requires at least 2 ml to be appreciable.[18] Air has a Hounsfield coefficient of -1000.

Ishiwata et al identified 2 signs as characteristic of tension pneumocephalus. The first is the Mount Fuji sign, named after Mount Fuji, the highest volcano in Japan. This sign is diagnostic of tension pneumocephalus and is formed by the accumulation of air in the frontal region, separating the frontal lobes' tips in a patient in the supine position (see Image. Mount Fuji Sign in Tension Pneumocephalus). The second is the "air bubble sign," denoting the presence of multiple air bubbles scattered in several cisterns (see Image. Air Bubble Sign in Tension Pneumocephalus).[19]

On the other hand, the "peaking sign" denotes bilateral compression of frontal lobes without separation of the tips. This sign indicates a less severe condition compared to the Mount Fuji sign.

Calculation of the Volume of Postoperative Pneumocephalus

The volume of intracranial air may be determined in 2 ways. The first is by computer-assisted volumetric measurement, which requires special software. The second is by using the ABC/2 formula at the bedside, which has the following steps:

1. Select a representative slide near the center of the pneumocephalus.
2. Measure "A" or the longest longitudinal length of the pneumocephalus in millimeters in the axial plane.
3. Measure "B" or the maximum width in millimeters of the pneumocephalus from the skull's inner table perpendicular to "A" on the same slice.
4. Measure "C" or the height of the air in the coronal plane, which is the same as the number of CT scan axial slides showing the pneumocephalus multiplied by the slice thickness.
5. Calculate the estimated volume in milliliters using the formula = A x B x C / 2.[20]

The ABC/2 formula correlates well with computer-derived values.[37]

Brain Magnetic Resonance Imaging

Brain magnetic resonance imaging (MRI) may also be useful but is less sensitive than CT in diagnosing pneumocephalus. Air may be mistaken for flow voids or blood products on MRI, appearing dark in almost all sequences.

Treatment / Management

The Advanced Trauma Life Support (ATLS) protocol is a widely recognized, systematic approach to the care of trauma patients that focuses on critical interventions during initial patient assessment and resuscitation. The initial management of any head injury should follow this protocol. Once stabilized and diagnostics have been obtained, the pneumocephalus must be classified to determine the appropriate treatment plan.

Most patients will present with simple pneumocephalus, lacking signs of increased ICP and are asymptomatic. The head CT scan will show intracranial air without tension pneumocephalus indicators like the Mount Fuji sign. Intracranial air disappears over time due to absorption in the tissues. Management is conservative and involves the following measures:

1. Bed rest
2. Placing the patient in a 30° 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 given at 5 liters per minute for at least 5 days via a face tent or 100% nonrebreather mask with absolute avoidance of positive pressure
7. Hyperbaric oxygen therapy.[21]
(B3)

Atmospheric air is composed of 78% nitrogen and 21% oxygen. The rate of nitrogen absorption from intracranial air depends on nitrogen's partial pressure in the blood, which is inversely proportional to the fraction of inspired oxygen.

Oxygen supplementation reduces blood and brain nitrogen concentration, increasing the nitrogen gradient between the pneumocephalus air collection and surrounding brain tissue. Over time, oxygen replaces intracranial nitrogen—which is well-absorbed in brain tissue and blood—and facilitates nitrogen absorption and eventual pneumocephalus resolution.[22] Serial CT scans may be obtained to monitor the condition.

Although pneumocephalus poses an increased intracranial infection risk, prophylactic antibiotics have not been proven to have a protective advantage in patients with traumatic pneumocephalus.[38]

Surgery is indicated in the following conditions:

• Symptomatic pneumocephalus
• Tension pneumocephalus
• Recurrent pneumocephalus
• Persistent traumatic pneumocephalus lasting more than 1 week
• Tension pneumoventricle

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

Surgical decompression options include the following:

• Needle aspiration, either blind or under radiological guidance through an existing burr hole or craniotomy [23]
• Controlled decompression via a subdural drain connected to an underwater seal followed by dural defect closure [24]
• Ventriculostomy for pneumoventricle
• Emergency decompression by the creation of fresh cranial burr holes
• Decompressive craniectomy [25]
• Saline-primed Camino bolt insertion
• Subdural evacuating port system (SEPS)™ insertion [26]
(B3)

Endoscopic endonasal eustachian tube obliteration has also been tried successfully as a treatment for tension pneumocephalus following lateral skull base surgery.[27]

Differential Diagnosis

The differential diagnosis of pneumocephalus is broad but includes mostly neurologic conditions such as the following:

• Brain concussion
• Primary headache syndromes
• Stroke
• Metabolic encephalopathy
• Intracranial hematoma
• Diffuse axonal injury
• Arteriovenous malformations
• Intracranial neoplasm
• Intracranial infection

On imaging, pneumocephalus may be mistaken for other conditions. For example, intracranial fat can appear hypodense on CT scans and be mistaken for pneumocephalus, though it has a much higher density (-90 HU) than air (-1000 HU). On MRI, pneumocephalus may be mistaken for blood products or flow voids.

Prognosis

Simple pneumocephalus usually resolves spontaneously with conservative therapy. However, it can occasionally produce seizures and meningitis. Prognosis is generally good even with tension pneumocephalus, provided timely treatment is given.

The presence of pneumocephalus might act as a predictor of chemical meningitis following corpus callosotomy.[28] The postsurgical appearance of pneumocephalus is also a predictor of chronic subdural hematoma recurrence after evacuation. Thus, pneumocephalus is a component of the Puerto Rico Recurrence Scale.[29]

Complications

The potential complications of pneumocephalus include meningitis, seizures, brain abscess, brain herniation secondary to tension pneumothorax, and death. Early intervention can prevent these complications and improve patient outcomes.