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Clinical Note |

Arachnoid Cyst Middle Ear Mass Diagnosis to Consider

Matthew S. Clary, MD; Ryan Walker, MD; Brian D. Nicholas, MD; Joseph F. Goodman, MD; Girish Kori, MD; Gregory J. Artz, MD
Arch Otolaryngol Head Neck Surg. 2011;137(4):398-400. doi:10.1001/archoto.2011.31.
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The differential diagnosis of middle ear masses includes a wide array of typically benign diseases, with cholesteatoma as the leading entity. Other lesions frequently considered include glomus tumors, schwannomas, choristomas, and other vascular variants. Arachnoid cysts, despite being present in 4% of the population, are seldom considered.1

Arachnoid cysts are cerebrospinal fluid (CSF) filled spaces that usually form within the arachnoid membrane covering the brain and spinal cord. Most commonly, arachnoid cysts present in the middle cranial fossa.1 Their presentation in the middle ear space has been described as typically due to erosion through the tegmen tympani. We present the case of a patient with a middle ear mass that ultimately was found to be an arachnoid cyst isolated to the tympanic segment of the fallopian canal. We use this previously undescribed (to our knowledge) presentation to discuss the clinical and radiologic characteristics of this entity, as well as its treatment. Institutional review board approval was not required for this report.

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Figure 1.

Right temporal bone axial computed tomographic image (bone window) demonstrates smooth, margined bone scalloping, and expansion of the geniculate fossa, along the expected location of the tympanic segment of the facial nerve. There is opacity of the normally air-filled medial and lateral epitympanic space. The arrow points to the arachnoid cyst.

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Figure 2.

Direct acquisition, coronal right temporal bone computed tomographic (bone window) images demonstrate opacity of the epitympanic recess, lateral epitympanic space (Prussak space), and mesotympanum. There is associated smooth, margined bone scalloping of the tegmen tympani, an indication of chronic pressure erosion often seen with arachnoid cysts (arrow).

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Figure 3.

Axial T2-weighted magnetic resonance image through the level of the right internal auditory canal/porus acusticus demonstrates T2 hyperintense signal just anterior to the limb of horizontal semicircular canal in the expected location of the geniculate ganglion. The arrow points to the arachnoid cyst.

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Figure 4.

Axial fluid attenuation inversion recovery (FLAIR) sequence at the identical level shown in Figure 3 demonstrates nulling of the hyperintense T2 signal, confirming intrinsic fluid content of the arachnoid cyst (arrow).

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Neuroepithelial cysts of the middle ear. Ann Otol Rhinol Laryngol 2003;112(4):356-60.
Arachnoid cyst: middle ear mass diagnosis to consider. Arch Otolaryngol Head Neck Surg 2011;137(4):398-400.
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