Pneumocephalus following insertion of ventriculoperitoneal shunt

56-year-old man with recurrent chondroma of the clivus had underwent several surgeries and radiation therapy, and is currently on systemic therapy with bevacizumab. Due to chronic hydrocephalus, a ventriculoperitoneal (VP) shunt was inserted, and relief of symptoms was reported. He presents two weeks later with sudden headache and stiff neck. No neurological symptoms were reported.

A non-contrast CT (NCCT) was performed one year before the onset of current symptoms. It shows a moderately dilated ventricular system – the beginning stage of hydrocephalus.

An MRI of the brain was performed 3 months prior to insertion of the VP shunt. It shows marked progression of the hydrocephalus. Additionally, growth of the recurrent tumour mass can be seen with the tumour in direct contact with the ventricular system, as well as constriction of the right side of the premedulary cistern.

NCCT was performed on day same day as symptom onset. The inserted VP shunt can be seen in the left frontal horn of the left lateral ventricle, as well as a reduction in volume of the ventricles. A large volume of air can be seen bilaterally anteriorly in the frontal, temporopolar, and suprasellar regions, as well as in the Meckel’s caves and posterior fossa. There is mass effect, but no shift of the central structure can be seen. As the air does not appear to be bound by cranial sutures, it is most likely located in the subdural space. A small amount of blood can be seen in the occipital horns of the lateral ventricles bilaterally – most likely due to insertion of the VP shunt.

Pneumocephalus is a condition characterized by the presence of intracranial gas. Typically arising following trauma or surgical interventions, pneumocephalus can be classified based on its location, encompassing extra-axial spaces such as epidural, subdural, and subarachnoid, as well as intra-axial spaces like parenchymal, intraventricular, and intravascular spaces.

Clinical presentation of pneumocephalus is commonly asymptomatic; however, tension pneumocephalus, where intracranial gas is trapped due to a ball valve effect, can lead to symptoms indicative of increased intracranial pressure, such as headaches [1]. Notably, a minority of patients may present with "bruit hydroaerique" [2], characterized by a splashing noise on head movement, discernible even to the examiner with the aid of a stethoscope.

Pneumocephalus can result from various causes, primarily mechanical trauma and following procedures, including neurosurgery, ventricular drain insertion, sinus surgery, and peridural anaesthesia. Additionally, barotrauma, otogenic pneumocephalus, pneumosinus dilatans, meningitis from gas-forming organisms, and even sneezing can trigger this condition [3–5].

Radiographically, the diagnosis of pneumocephalus is based on CT imaging, where intracranial gas exhibits extremely low density (~ -1000 Hounsfield Units). Careful distinction from fat, which also appears black on brain windows but has higher density (~ -90 Hounsfield Units), is crucial. MRI, although lacking objective density measurements, portrays gas as completely black across sequences, although it might be mistaken for blood products or flow voids based on its location and morphology [6].

Regarding treatment and prognosis, the approach hinges on the underlying cause. In many instances, pneumocephalus resolves spontaneously as the gas is gradually resorbed, particularly in post-operative scenarios, as some degree of pneumocephalus is anticipated in most craniotomy patients [7]. However, tension pneumocephalus might necessitate surgical intervention, such as burr hole procedures, to alleviate pressure [8]. Additionally, when pneumocephalus arises from cerebrospinal fluid leaks, identifying the leak site and subsequent surgical repair are indicated.

In the presented case, the exact cause of the hydrocephalus was not determined. A slightly distended ventricular system, as well as a mild headache, was first noted about two years prior to current symptoms. This roughly coincided with both tumour recurrence and the patient starting treatment with imatinib. It was later noted that the tumour is in direct contact with the premedulary cistern and does appear to exert pressure, albeit unilaterally. CSF analysis was not conducted; thus leptomeningeal spread could not be determined. Side effects of imatinib include headaches and increased intracranial pressure; however, the hydrocephalus and symptoms worsened significantly well after the treatment was terminated.

Pinpointing the precise origin of pneumocephalus proved challenging. Nonetheless, considering the patient's clinical history, it is most likely that a fistula connecting the subdural space with the oropharynx was formed during a previous surgical intervention. This anatomical anomaly likely became patent following the extensive cerebrospinal fluid drainage after placement of the PV shunt.

Written informed patient consent for publication has been obtained.