Arachnoid cysts (AC) are developmental collections of cerebrospinal fluid surrounded by an arachnoid membrane. They account for about 1% of all intracranial space-occupying lesions. Their manifestation by trigeminal neuralgia (TN) is uncommon [1] [2] [3] [4].

We report a case of TN, secondary to an arachnoid cyst located in the cerebellopontine angle.

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Figure 1. Magnetic resonance imaging

A 56-year-old woman with no medical history presented with lancinating pain on the right hemiface. Examination of the cranial nerves were normal, there was no trigger zone and no evidence of long tract involvement or cerebellar syndrome. Magnetic resonance imaging showed a posterior fossa lesion on the right cerebollopontine angle, the lesion had the same signal as the cerebro-spinal fluid (CSF) hypointense on T1 and FLAIR and hyperintense on T2 without any contrast enhancement (see figure a, b, c). These features support the diagnosis of arachnoid cyst of the posterior fossa. The patient was treated by carbamazepin 1200 mg/day with partial improvement. Surgery was indicated but the patient refused it.

Most AC are non-symptomatic, and frequently expressed in children cases [1] [2] [3] [4] [5]. Even if the posterior fossa (PF) arachnoid cysts are rare, they present the second location after middle cerebral fossa (10-15%) [5] [6] [7] [8] [3]. Their occurrence mechanism remains debated, but several arguments support a congenital origin: prenatal diagnosis of AC, bilaterality and genetic forms [2] [4] [5]. When they are symptomatic the vestibulocochlear nerve involvement is the most frequently reported manifestation [8] [9] [10] [11] [12] [13] [14] [15] [6]. On a serie of 12 arachnoid cysts of the PF, eight were revealed by symptoms related to the eight cranial nerve involvement [8], the trigeminal nerve was involved only in two cases. None of the Five AC of the CPA reported by Jallo and al had symptoms related to trigeminal neuralgia [15] and none of the twelve patients reported by Erdincëler et al [6].

Only 11 cases of TN and AC were reported, three of them occurred after a microvascular decompression of the trigeminal nerve, and one case combined facial hemispasme and trigeminal neuralgia [16] [17] [18] [19] [20] [21] [22] [23]. For our patient a lot of factors could explain neuralgia like distortion of the pons with stretching of the trigeminal nerve or irregular demyelination within the root entry zone [2] [4]. The declaration age of posterior fossa AC is higher than the other location, and there is no sex predominance unlike the other location where males are frequently affected [6] [8]. For AC with trigeminal neuralgia, excluding cases secondary to microvascular decompression and three other for which data were unavailable, five of six cases were female with an average age of 50 years (22-68 years) [16] [17] [18] [19] [20].

The diagnosis is usually easy to find out by the MRI signal of the cyst which is similar to CSF. It can sometimes suggest acoustic shwanoma if the lesion is located at the pontocerebellar angle. But the most important differential diagnosis is between arachnoid cysts and epidermoid cysts. MRI with diffusion-weighted images (DWIs) and FLAIR make differentiating the 2 lesions easier with the similar signal of AC and CSF [1] [2] [5] [6] [13]. It is widely accepted that symptomatic cysts should be treated surgically, and that abstention is appropriate in cases of non-symptomatic cysts [1] [5] [21]. The attitude is more controversial for surgical techniques which include craniotomy, derivation, or endoscopic fenestration. The choice is determined by the size and the accessibility of the lesion [1] [5] [21].