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. Lecat first described this condition in 1741, but the term "pneumocephalus" was coined independently by Luckett in 1913 and Wolff in 1914. The term "tension pneumocephalus" (TP) was proposed in 1962 by Ectors, Kessler, and Stern.
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).
It has to be differentiated from the following terms:
During head injury or following cranial surgeries, dura may be opened or torn with or without injury to arachnoid. In all these cases, air can get inside the cranial cavity. There are 2 theories about the mechanism for the development of pneumocephalus:
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.
The following features of the patient's history should make clinicians suspicious that the patient has pneumocephalus:
Pneumocephalus is a difficult diagnosis clinically. 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.
X-rays have been used in the past to identify the pneumocephalus, but it will miss small quantities of air.
Head Plain Computed Tomography
This is the gold standard investigation in the diagnosis of pneumocephalus. It can detect even 0.55 ml of intracranial air, whereas skull radiograph requires at least 2 ml. Air has a Hounsfield coefficient of -1000. There are 2 signs which were identified as characteristic of TP by Ishiwata et al.
"Peaking sign" denotes bilateral compression of frontal lobes without separation of the tips. It shows a less severe condition compared to Mount Fuji sign.
Brain Magnetic Resonance Imaging
MRI may also be useful, but not as sensitive as CT scan in the diagnosis of pneumocephalus. Moreover, air may be mistaken for flow voids or blood products, and it appears dark in almost all sequences.
Initial treatment of any head injury should follow the Advanced Trauma Life Support (ATLS) protocol.
Treatment of simple pneumocephalus:
Usually conservative. It involves the following steps:
Indications for surgical intervention:
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. Other cases of TP may require a fresh frontal bur hole and aspiration or insertion of a subdural drain connected to an underwater seal followed by closure of the dural defect, or insertion of saline primed Camino bolt.
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.
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.
Prevention of Pneumocephalus
The following methods can be done to prevent the development of pneumocephalus after neurosurgical procedures:
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 7 days before taking a flight as the cabin pressure changes can introduce air inside the skull.
There is no proper evidence to support the prophylactic administration of ceftriaxone for preventing meningitis in patients with traumatic pneumocephalus.
All patients with head injuries and post-craniotomy status should be strictly monitored for the development of pneumocephalus. Nurses should monitor the sensorium and should be careful regarding the positioning of patients and give instructions to avoid a Valsalva maneuver. 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.
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