Pneumocephalus is the presence of air trapped inside the skull, in spaces normally occupied only by the brain, cerebrospinal fluid, and blood vessels. Head trauma accounts for roughly three-quarters of all cases, but the condition also shows up after brain surgery, spinal procedures, sinus infections, and occasionally with no obvious trigger at all. Most small pockets of intracranial air resolve on their own, yet the condition ranges from an incidental finding on a CT scan to a life-threatening emergency depending on how much air accumulates and whether it builds pressure against the brain.
How Air Gets Inside the Skull
The skull is a sealed compartment under normal circumstances. For air to enter, there has to be a breach somewhere in the bone or the membranes lining the brain, combined with a pressure difference that draws air inward. The most common route is through fractures of the frontal bone or the sinuses, which sit directly behind the forehead and are already air-filled cavities separated from the brain by thin plates of bone. A crack in that barrier lets air travel directly into the cranial cavity.
In a review of traumatic cases, the most frequent injury mechanisms were motor vehicle collisions, assault or blunt trauma to the face, falls, and motorcycle or bicycle crashes. Frontal bone and sinus fractures appeared in nearly half the cases, with ethmoid fractures (the honeycomb-like bone between the eyes and the nose) coming in second. Many patients also had cerebrospinal fluid leaking from the nose, which signals that the membrane surrounding the brain has been torn along with the bone.
1PubMed Central. Traumatic tension pneumocephalus – Two cases and comprehensive review of literatureWhen researchers scanned over 500 patients admitted with acute head injuries, about one in ten turned out to have intracranial air on CT.
2PubMed. Prognosis, incidence and management of acute traumatic intracranial pneumocephalus. A retrospective analysis of 49 casesThat figure captures even tiny amounts of air, which often cause no symptoms and disappear without treatment. Still, it underscores how common the finding is once you look for it with modern imaging.
Pneumocephalus After Brain Surgery
If you or someone you know has had a craniotomy, a post-operative CT showing air inside the skull is practically guaranteed. A retrospective analysis of 240 patients who underwent supratentorial craniotomy found air on every single scan taken in the first two days. About two-thirds of those air collections were moderate or large. The air gradually shrank: by one week after surgery, roughly three-quarters of patients still had visible air; by two weeks, that dropped to about 60 percent; and by three weeks, only about a quarter still showed pneumocephalus.
3PubMed. The incidence of pneumocephalus after supratentorial craniotomy. Observations on the disappearance of intracranial airThe body reabsorbs the trapped air over a couple of weeks in most cases, and the condition is usually benign after routine surgery. But even at the two-week mark, about one in eight patients still had air pockets considered moderate or large, which is why surgeons keep watching.
Endonasal endoscopic skull base surgery, where surgeons access brain tumors through the nose, also introduces air. In one series of 258 patients, about 40 percent had pneumocephalus on post-operative imaging. Certain tumor types and surgical approaches carried a much higher rate. Patients undergoing surgery for craniopharyngiomas had pneumocephalus about 87 percent of the time, while those with pituitary adenomas came in around 21 percent.
4PubMed. Pneumocephalus patterns following endonasal endoscopic skull base surgery as predictors of postoperative CSF leaksResearchers in that study also identified patterns of air distribution that predicted whether a patient would develop a cerebrospinal fluid leak afterward. Air found along the brain’s convexity, between the hemispheres, or around the brainstem correlated with a higher leak risk, while frontal or intraventricular air did not.
Spinal and Epidural Procedures
Most people associate pneumocephalus with head injuries or brain surgery, but it can also follow spinal procedures, including epidural injections used for pain relief or labor analgesia. The connection comes down to how doctors locate the epidural space. One common technique involves advancing a needle while injecting small amounts of air, feeling for the moment resistance suddenly drops, which signals that the needle tip has entered the right layer. If the needle goes slightly too deep and punctures the dura (the membrane around the spinal cord), that injected air can migrate upward through the cerebrospinal fluid and reach the brain.
Even very small quantities of air, less than two milliliters, have been reported to cause pneumocephalus with disabling neurological symptoms when the dura is accidentally punctured.
5PubMed Central. Pneumocephalus Following an Accidental Dural Puncture, Treated Using Hyperbaric Oxygen Therapy. A Case ReportThis has led to a long-running debate in anesthesiology about whether saline should replace air for the loss-of-resistance technique, since saline cannot cause pneumocephalus even if the dura is breached.
6PubMed. Pneumocephalus with headache complicating labour epidural analgesia: should we still be using air?The risk also applies to lumbar epidural steroid injections used for back pain, where inadvertent dural puncture has led to both pneumocephalus and chemical meningitis in documented cases.
7PubMed Central. Pneumocephalus and Chemical Meningitis after Inadvertent Dural Puncture during Lumbar Epidural InjectionLess Common Causes
The ear can serve as an unexpected entry point. Otogenic pneumocephalus occurs when air from the middle ear or the mastoid bone (the spongy, air-filled bone behind the ear) reaches the intracranial space. This typically requires some kind of bony defect connecting the two compartments. Skull fractures are the usual culprit, but in some individuals with heavily pneumatized (air-rich) mastoid bones, the barrier between the ear and the brain can erode on its own.
8PubMed Central. Otogenic pneumocephalus associated with a ventriculoperitoneal shuntSpontaneous otogenic pneumocephalus has been linked to temporal bone defects combined with mastoid hyperpneumatization.
9Brazilian Journal of Otorhinolaryngology. Otogenic Spontaneous pneumocephalus: case reportInfections occasionally produce intracranial gas as well, though through a different mechanism. Gas-forming bacteria, particularly Klebsiella pneumoniae, can create brain abscesses that contain pockets of air visible on imaging. These abscesses tend to occur in people with weakened immune defenses, such as those with poorly controlled diabetes or chronic liver disease.
10PubMed Central. Gas-forming brain abscess caused by Klebsiella pneumoniaeIn a case series examining gas-containing brain abscesses, predisposing factors included blood-borne infection spread, infection extending from nearby sinuses, and abnormal connections left by prior head injuries or sinus surgeries.
11PubMed Central. Gas-containing brain abscess: etiology, clinical characteristics, and outcomeTruly spontaneous, non-traumatic pneumocephalus with no surgical or infectious explanation is rare, representing less than one percent of all cases. It was first described in the medical literature in the 1950s.
12PubMed Central. Pneumocephalus: A Rare Complication of Sinusitis in a Pediatric PatientSymptoms and How It Is Found
The symptoms depend entirely on how much air is present and where it sits. A small, stable air pocket after surgery may produce nothing more than a mild headache or no symptoms at all. Larger collections tend to cause headache, nausea, vomiting, dizziness, and seizures. When the air volume is substantial enough to compress brain tissue, patients can show depressed consciousness, confusion, or focal neurological deficits like weakness on one side of the body.
13PubMed. Pneumocephalus: case illustrations and reviewA non-contrast CT scan of the head is the standard way to identify pneumocephalus. Air shows up as strikingly dark areas on CT, making it straightforward to spot even in small amounts. CT also helps gauge volume and location, both of which guide treatment decisions. MRI is sometimes used as a supplement, particularly when the clinical picture is complicated. It has an edge over CT in certain niche scenarios, such as detecting wooden foreign bodies in orbital injuries, which can look identical to air on CT.
14Survey of Ophthalmology. Orbitocranial wooden foreign body diagnosed by magnetic resonance imaging. Dry wood can be isodense with air and orbital fat by computed tomographyTension Pneumocephalus and the Mount Fuji Sign
The most dangerous form of pneumocephalus is tension pneumocephalus, where air continues to accumulate under pressure and compresses the brain. This is a neurosurgical emergency. The mechanism is often described as a ball-valve effect: air enters the skull through a defect during moments of increased pressure (coughing, sneezing, straining), but the defect seals during lower-pressure moments, trapping each new bolus of air inside.
15PubMed Central. Posttraumatic delayed tension pneumocephalus: Rare case with review of literatureThe trapped air progressively increases intracranial pressure, which can lead to brain herniation if not treated quickly.
CT scans of tension pneumocephalus sometimes show a distinctive pattern called the Mount Fuji sign: the frontal lobes of the brain are compressed and separated by a large collection of subdural air, creating a silhouette that resembles the profile of Japan’s famous volcano. When this sign appears, it confirms that the air is exerting significant mass effect on the brain.
16PubMed Central. The Unspoken Danger of the Mount Fuji Sign Leading to Sudden DeathTreatment typically involves urgent needle aspiration of the air through a burr hole or surgical evacuation to decompress the brain.
17PubMed Central. Tension pneumocephalus as complication of burr-hole drainage of chronic subdural hematoma: A case reportThe Nitrous Oxide Problem
Nitrous oxide, still used as an anesthetic gas in some surgeries, poses a specific risk for pneumocephalus patients. The gas diffuses into air-filled spaces inside the body much faster than nitrogen diffuses out. If a patient already has even a small pocket of intracranial air, breathing nitrous oxide causes that pocket to expand rapidly. In an animal study, researchers found that initiating 75 percent nitrous oxide inhalation when intracranial air was already present caused intracranial pressure to jump from about 12 mmHg to over 22 mmHg almost immediately.
18PubMed. Nitrous oxide withdrawal reduces intracranial pressure in the presence of pneumocephalusThis makes nitrous oxide a known risk factor for converting an otherwise harmless pneumocephalus into a dangerous tension pneumocephalus. Case reports have documented this scenario during neurosurgical procedures, and the general recommendation is to avoid nitrous oxide when preexisting intracranial air is known or suspected.
19PubMed Central. Intraoperative Development of Tension Pneumocephalus in a Patient Undergoing Repair of a Cranial-Dural Defect Under Nitrous Oxide AnesthesiaTreatment for Uncomplicated Cases
When pneumocephalus is not causing significant symptoms or compressing the brain, treatment is conservative. That generally means keeping the patient lying flat, administering high-concentration supplemental oxygen, and monitoring closely for any neurological changes that might signal worsening.
20Acute and Critical Care. What should an intensivist know about pneumocephalus and tension pneumocephalus?The oxygen therapy piece is more than just supportive. Breathing high concentrations of oxygen works by lowering the amount of nitrogen in the lungs and blood. Since intracranial air is mostly nitrogen, this creates a gradient that draws nitrogen out of the trapped air pocket, through the bloodstream, and into the lungs, where it is exhaled. The air pocket gradually shrinks as nitrogen is replaced by oxygen, which the body absorbs much more readily.
21PubMed. Oxygen Therapy with High-Flow Nasal Cannula as an Effective Treatment for Perioperative Pneumocephalus: Case Illustrations and Pathophysiological ReviewHigh-flow nasal cannula oxygen delivery has emerged as an effective approach for this purpose.
22Journal of Neurocritical Care. Oxygen supplementation via high-flow nasal cannula is an effective treatment for pneumocephalusIn severe or refractory cases, hyperbaric oxygen therapy takes the same principle further. Inside a pressurized chamber, the increased atmospheric pressure directly compresses the gas pocket (following Boyle’s Law), while 100 percent oxygen flooding the bloodstream accelerates nitrogen washout. This is not a first-line treatment, but case reports describe near-complete resorption of intracranial gas after a single hyperbaric session, with rapid neurological improvement.
23PubMed Central. Hyperbaric oxygen therapy in the treatment of pneumocephalus associated with epidural block: case reportFlying with Air in Your Head
Air travel after pneumocephalus is a real concern, and one that comes up often enough that researchers have studied it experimentally. The issue is straightforward physics: airplane cabins are pressurized to the equivalent of roughly 6,000 to 8,000 feet of altitude, meaning the ambient pressure is lower than at sea level. Any gas trapped inside the skull will expand as cabin pressure drops during ascent. If you already have a pocket of intracranial air, that expansion translates directly into increased pressure on the brain.
Laboratory experiments using a model skull with simulated brain tissue and an air pocket demonstrated this effect quantitatively. Researchers found that the rise in intracranial pressure during simulated flight depended on both the volume of trapped air and how quickly cabin pressure dropped. An intracranial air volume of 20 milliliters and an initial intracranial pressure of 15 mmHg were proposed as conservative thresholds below which air travel could be considered safe.
24PubMed Central. Air travel with pneumocephalus: a systematic reviewSimulation studies suggested that even volumes above 11 milliliters could produce intracranial hypertension during the cabin pressure drop, and that faster pressure changes worsened the effect.
A complementary experimental investigation confirmed that larger air volumes and faster rates of cabin pressure change led to greater spikes in intracranial pressure. The largest pressure increase observed in the lab was 5 mmHg, measured with 20 milliliters of air and a rapid simulated climb.
25PubMed Central. Pneumocephalus and air travel: an experimental investigation on the effects of aircraft cabin pressure on intracranial pressureWhile 5 mmHg may not sound dramatic, in a patient whose intracranial pressure is already elevated, that additional push can tip the balance toward dangerous territory. The practical upshot is that patients with known pneumocephalus are typically advised to wait until imaging confirms the air has resolved, or at least fallen below a safe threshold, before boarding a flight.
When Imaging Findings Can Be Misleading
One subtlety worth knowing is that not all dark spots on a CT scan of the head are air. Fat, certain foreign materials, and motion artifacts can mimic the appearance of pneumocephalus. Dry wood, for instance, has a density on CT that is nearly identical to air. A case report involving a wooden foreign body lodged near the orbit showed that CT could not distinguish it from an air pocket, while MRI was able to identify the object clearly.
26Survey of Ophthalmology. Orbitocranial wooden foreign body diagnosed by magnetic resonance imaging. Dry wood can be isodense with air and orbital fat by computed tomographyThis is an edge case, but it illustrates why clinicians sometimes use more than one imaging modality when the clinical picture does not quite match the scan. Context matters: a dark spot on CT after craniotomy is almost certainly air, but a similar finding after a penetrating injury near the eyes or sinuses deserves a second look.
In the context of postoperative skull base surgery, the location of air on imaging carries clinical significance beyond just confirming the diagnosis. As the endoscopic surgery study showed, frontal and intraventricular air after endonasal surgery was generally benign, while air in more unusual locations like the convexity or around the brainstem raised the odds of a cerebrospinal fluid leak threefold.
27PubMed. Pneumocephalus patterns following endonasal endoscopic skull base surgery as predictors of postoperative CSF leaksKnowing which patterns are “benign” and which are “suspicious” helps surgical teams decide who needs additional intervention versus who can simply be monitored as the air reabsorbs naturally over the following weeks.