Teaching Case

A 13-year-old boy presented with a history of headache and nausea. His neurological examination revealed papilledema and features of raised intracranial pressure. He reported a trauma of being hit by a ball on the head about two weeks back.

Computed tomography (CT) scan of head shows an extra-axial lesion of fluid density filling the middle cranial fossa and extending along the left Sylvian fissure. Displacement of temporal and frontal lobes with gaping of the Sylvian fissure is seen associated with thinning of the adjacent bones suggesting a long-standing mass effect. A crescent-shaped subdural fluid collection is present in left hemisphere. There is effacement of lateral ventricles and midline shift to the contralateral side. No fracture of the bony calvaria is evident on the CT scan.

Magnetic resonance imaging (MRI) of the brain confirms the extra-axial location and cystic nature of the middle cranial fossa lesion. It follows the CSF signal on all pulse sequences. Additionally, on heavily T2 weighted sequence, a focal cyst wall defect is visualized on the lateral aspect of the cyst where it is communicating with the subdural fluid collection. No area of blooming on susceptibility-weighted imaging that would suggest of intracranial haemorrhage is seen.

Spontaneous rupture of a middle cranial fossa arachnoid cyst is a rare complication with approximately 74 cases reported in the literature [1], mostly in children and adolescents. It is more frequent in male patients (M: F= 3:1) and the average age of presentation ranges from 8.3 years to 16.7 years.[1]

Rupture of an arachnoid cyst may occur following head trauma, or it might happen spontaneously following a transient increase of the intracranial pressure like during the Valsalva manoeuvre.  Cysts larger than 5cm in maximum dimension have higher propensity to rupture[2]. The opinion is split regarding the mechanism of the rupture of an arachnoid cyst where one of the theory attributes the contact between the cyst wall and the sharp ragged edges of the inner table of the bony calvaria (like the sphenoid wing and the tentorial incisures) as the causal factor. [3]. Another opinion states that trauma may lead to the passage of CSF from the subarachnoid space into the cyst, thus increasing intra-cystic pressure. This leads to tear in the cyst wall [4].

Subdural haemorrhage is another complication of ruptured arachnoid cyst due to laceration of weak leptomeningeal and bridging veins.  MR findings are variable depending on the different stages of blood.

Headache is the most common presenting feature in these patients followed by nausea and vomiting. Papilloedema is frequent on fundoscopy examination. Focal neurological deficit at presentation is uncommon [1].

The imaging appearance of an arachnoid cyst is classically described as a well-circumscribed, extra-axial, cystic lesion that follows CSF signal on all pulse sequences. It is non-calcified, do not show restricted diffusion and are non-enhancing. Spontaneous rupture of an arachnoid cyst with subdural hygroma or intracystic haemorrhage is a rare complication but is readily diagnosed on MR imaging.

In clinically stable patients, conservative treatment with follow up alone or with acetazolamide can be helpful. The most recommended technique is cyst fenestration creating a connection between the cyst and the normal CSF-circulation. Surgical option includes cyst-peritoneal shunting, which reduces the risk of sudden decompression, but has higher risks of obstruction and infection [4,5].

Rupture of arachnoid cyst either spontaneously or after trauma is a relatively rare entity leading to a number of neurological symptoms and complications. Asymptomatic patients with arachnoid cysts should be informed about the very rare possibility of cyst rupture along with the predisposing factors and clinical presentations. This can warrant prompt management in case of future complications.