Description

A 47-year-old woman presented with an episode of left-sided Bell's palsy. The patient was treated with corticosteroids with complete recovery in 3 months. However, coincidental with the onset of the Bell's episode, she began experiencing headaches in the occipital region. Clinical examination revealed only lymphadenopathy in the left posterior cervical chain. Secondary causes of Bell's palsy were investigated with MRI of the brain, which demonstrated a large well-circumscribed mass in the posterior fossa (figure 1). MRI showed marked hyperintensity on axial T2-weighted images (T2WIs; figure 1A–C) and hypointensity on sagittal T1WI (figure 1D) and coronal T1WI (figure 1E, F). A radiological diagnosis of an arachnoid cyst was made.

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Figure 1

Axial T2-weighted (A–C), saggital T1-weighted (D) and coronal T1-weighted (E and F) images showing a large well-circumscribed arachnoid cyst in the posterior fossa.

Arachnoid cysts are benign cerebrospinal fluid-filled divisions of the arachnoid layer. They are predominantly primary in nature, but may also be secondary due to the existence of prior pathological causes such as infection and head injury.1 Arachnoid cysts are known to develop de novo, and increase or decrease in size.2 Growth or development of the cyst can cause headaches and other neurological symptoms, and may require neurosurgical treatment.2 The association of occipital headaches in this case is a known symptom of arachnoid cyst irritability, and cranial nerve (CN) palsies have previously been identified as an uncommon sequela.3 Growth of the lesion may have resulted in an increase in the posterior fossa pressure with associated mass effect, visible as compression of the facial nerve (CN VII). In this case, after resolution of the Bell's palsy, discussions with the neurosurgical team advised that surgical intervention for the cyst was unnecessary.