A 71-year-old woman with history of hemicranial headache treated with venlafaxine presented with 3 months of eating difficulties and left deviation of the tongue. Physical examination revealed left hypoglossal paresis with tongue hemiatrophy (Figure 1). Magnetic resonance resonance (MRI) and computed tomography (CT) scan showed bilateral arachnoid cysts in the cerebellopontine angles (CPA), which had intraosseous extension into the left hypoglossal canal. Left side of tongue demonstrated T2-weighted hyperintensity suggesting subacute denervation (Figure 2).

Figure 1: (A–D) 71-year-old woman with left hypoglossal paresis and tongue hemiatrophy on physical examination.

Figure 2: Axial T2-weighted imaging (A–C) and coronal FIESTA MRI (D) show bilateral arachnoid cysts in cerebellopontine angles (arrowheads) and left intradiploic meningocele (short arrow). Note the T2 hyperintensity of left tongue (long arrow) suggesting subacute denervation. Axial (E) and coronal (F) non-contrast CT scan show intradiploic extension along hypoglossal canal (red arrows).

Arachnoid cysts are benign lesions usually located within the subarachnoid space and filled with cerebrospinal fluid. ^{Reference Sharma, Sharma, Mittal and Gandhi1} Its etiology remains controversial, but they are thought to be congenital or secondary to infection, trauma, or changes in cerebrospinal fluid (CSF) pressure. ^{Reference Sharma, Sharma, Mittal and Gandhi1}

They are most frequently located in the middle cranial fossa. Only 11% of arachnoid cysts are located in CPA. Bilateral arachnoid cysts in this location are even more uncommon, with just four cases reported in the literature. ^{Reference Gelabert González, Arán Echabe and Pita Buezas2}

The differential diagnosis of CPA arachnoid cysts includes epidermoid and neuroenteric cysts and cystic acoustic schwannomas. ^{Reference Sharma, Sharma, Mittal and Gandhi1} On MRI, arachnoid cysts show CSF signal characteristics in all MR sequences, unlike epidermoid cysts that restrict (show high signal intensity) on diffusion-weighted images. ^{Reference Osborn and Preece3} Therefore, for proper diagnosis and management of this type of lesions, we recommend a careful radiological evaluation, including MRI with diffusion-weighted images and high-resolution bone CT scan.

Most arachnoid cysts are asymptomatic. However, up to 5% produce symptoms due to direct compression. In CPA, the most common symptoms include facial spasm, trigeminal neuralgia, hearing loss, and vertigo ^{Reference Gelabert González, Arán Echabe and Pita Buezas2} . Paresis of twelfth cranial nerve due to an arachnoid cyst has only been described four times in the literature to the best of our knowledge (Table 1). ^{Reference Ryu, Makino and Hinokuma4,Reference Burkhardt, Bal, Neidert and Bozinov5} In our case, the patient presented with hypoglossal palsy only.

Table 1: Previous published cases of arachnoid cysts causing hypoglossal palsy

The management of CPA arachnoid cysts is still controversial. It is generally accepted that asymptomatic patients do not require treatment and should undergo radiological follow-up. However, symptomatic arachnoid cysts that affect quality of life should be treated surgically, either by fenestration or CSF shunting. ^{Reference Sharma, Sharma, Mittal and Gandhi1,Reference Gelabert González, Arán Echabe and Pita Buezas2}

Nevertheless, it is still difficult to establish which is the best treatment option. Fenestration by craniotomy allows a greater resection of the membranes and a wide communication with subarachnoid cisterns. In fact, it was the treatment option in most of cases of bilateral CPA cysts and in all cases of arachnoid cysts invading the hypoglossal canal. ^{Reference Gelabert González, Arán Echabe and Pita Buezas2,Reference Ryu, Makino and Hinokuma4,Reference Burkhardt, Bal, Neidert and Bozinov5}

Shunt placement is a less aggressive option; however, it presents a high rate of complications, such as valve dependence, dysfunction, and infection.

Endoscopic fenestration is proposed today as the least aggressive option with fewer complications; however, more studies must be carried out to establish the advantage of this technique over the previous ones. ^{Reference Gelabert González, Arán Echabe and Pita Buezas2}

In our case, patient chose not to be treated surgically since the symptoms that she presented did not significantly affect her quality of life.

In conclusion, this is a unique case of a bilateral posterior cerebral artery arachnoid cyst affecting left hypoglossal canal resulting on a twelfth cranial nerve paresis. Both conditions, bilateral CPA arachnoid cysts and arachnoid cysts affecting hypoglossal canal, have never been reported on the same patient before. ^{Reference Gelabert González, Arán Echabe and Pita Buezas2,Reference Burkhardt, Bal, Neidert and Bozinov5}