Imaging case of the month: Cystic lesions of the petrous apex: identification based on magnetic resonance imaging characteristics.

A 25-year-old woman presented to our clinic with leftsided facial numbness, headaches, and left-sided sensorineural hearing loss. The hearing loss had occurred suddenly at age 13; a magnetic resonance imaging (MRI) obtained at that time noted only mild fluid in the left mastoid air cells, and her hearing never recovered. Her medical history also was significant for enucleation of a left expansile jaw cyst at age 12 and left-sided migraine headaches, which she began to experience at age 15. Before her presentation in our clinic, she developed a sinonasal infection followed by a loss of sensation throughout the left side of her face. Physical examination was notable for left-sided facial numbness in the distributions of all 3 trigeminal nerve branches. Audiometry demonstrated a 35-dB speech reception threshold and a 28% speech discrimination score on the left, with normal hearing sensitivity on the right. Computed tomography and MRI scans were obtained and revealed a 1.0 2.6 1.4Ycm well-defined lytic lesion within the left petrous apex, which was hypointense on T1 and hyperintense on T2, and did not enhance with gadolinium or on diffusion-weighted images (DWI) (Fig. 1). Continuity between the petrous apex lesion and Meckel’s cave on heavily-weighted T2 MRI supported the diagnosis of a cephalocele (Fig. 2) (2). Petrous apex cephaloceles are a group of rare acquired lesions characterized by herniation of intracranial contents (e.g., the Gasserian ganglion in Meckel’s cave) into the petrous apex. They have been reported to present with symptoms including hearing loss, vertigo, diplopia, headaches, trigeminal and abducens nerve palsies, and cerebrospinal fluid (CSF) leak, although a causal association has never been established (3). An association with elevated intracranial pressure has been postulated in certain cases (4). Subtypes of petrous apex cephaloceles include meningocele, or herniation of the meninges (including arachnoid and dura mater), and encephalocele, or herniation of brain parenchyma along with the investing meninges. The petrous apex cephaloceles have been termed arachnoid cysts, although this nomenclature is probably erroneous as arachnoid cysts are congenital lesions that can be separated from the rest of the CSF space by a surrounding arachnoid membrane. The petrous apex lesion in this case was acquired and was not completely surrounded by an arachnoid membrane as demonstrated by high-resolution heavily T2-weighted images, distinguishing it from an arachnoid cyst. The lesion also was filled with CSF (without filling defects suggestive of cortical herniation), characteristic of a meningocele. The differential diagnosis of lesions of the petrous apex includes inflammatory lesions such as cholesterol granuloma, cholesteatoma and mucocele, neoplastic lesions that could represent primary tumors or metastases, vascular malformations or aneurysms, or CSF-filled cephaloceles. Absence of contrast enhancement eliminates tumors and vascular malformations from the differential diagnosis. Cholesteatomas can potentially show similar signal characteristics, although they are hyperintense on DWI (1). The distinction between mucoceles and cephaloceles can be difficult to make on conventional imaging: mucoceles are always separate from the CSF space even if any intervening bone is completely eroded. The natural history and the role of surgery for petrous apex cephaloceles (including meningoceles and encephaloceles) is not well characterized (5). Surgery may be necessary for certain clinical presentations of a petrous apex cephalocele, such as CSF otorhinorrhea or a history of meningitis. We elected to observe our patient’s presumed petrous apex meningocele with serial MRI scanning. MRI scans obtained 6 then 12 months later showed stability of the lesion. The patient has not experienced any change in her symptoms over this 18-month interval. This case suggests that precise subtypes of petrous apex cystic lesionsVin this case, a meningoceleVcan be identified based on distinctive MRI characteristics particularly on high-resolution heavily T2-weighted sequences, with implications for management. Address correspondence and reprint requests to Yuri Agrawal, M.D., Department of OtolaryngologyYHead and Neck Surgery, The Johns Hopkins University School of Medicine, 601 N. Caroline Street, Baltimore, MD 21287; E-mail: yagrawa1@jhmi.edu The authors disclose no conflicts of interest. Otology & Neurotology 33:e75Ye76 2012, Otology & Neurotology, Inc.