Common Benign Conditions of the Petrous Apex in the Pediatric Population.
The petrous apex is the pyramidal shape medial projection of the temporal bone anteromedial to the inner ear and lateral to the petro-occipital fissure. It is subdivided by the internal auditory canal into larger anterior and smaller posterior segments (1). Petrous apex lesions are rare; however, a wide number of entities may be seen during radiological studies of this region incidentally for an unrelated disease. In addition, pseudolesions involving the petrous apex may make the diagnosis difficult and cause unnecessary treatment. Owing to its complex anatomical localization imaging, plain radiography, high-resolution computed tomography (HRCT), and magnetic resonance (MR) imaging with diffusion-weighted (DW) sequences play an important role in the assessment of petrous apex (2, 3). The aims of this pictorial essay were to describe the pseudolesions involving the petrous apex, to summarize the benign pathologies, and to recognize the patterns and imaging features of diseases that may involve the petrous apex.
Normal Anatomic Variants of the Petrous Apex
Mostly, the petrous apex is non-aerated and composed of dense bone and bone marrow (Figure 1). In up to 30% of the cases, variable degree of aeration of the petrous apex occurs. Approximately 4%-7% of the pneumatized petrous bones are asymmetric (4). When the pneumatization is not symmetric, the non-pneumatized petrous apex may be misinterpreted as pathologic (Figure 2). Absence of mass effect and close observation of the signal changes with fat-suppressed sequences prevent misinterpretation as cholesteatoma, effusion, or cholesterol granuloma. In addition, CT can easily visualize normal cortex or bony trabecula without mass effect. Asymmetric aeration is associated with asymmetric fatty marrow in the petrous apex. Typically, signal intensity of fatty marrow follows the signal of retro-orbital fat in all MR sequences. Fat-suppressed MR sequences or CT can confirm the normal fatty marrow (2, 3, 5).
By definition, petrous apex cephalocele is the cystic expansion and herniation of the posterolateral portion of Meckel's cave into the superomedial portion of petrous apex. They are cerebrospinal fluid (CSF)-filled cystic structures. Except for fibers of the fifth cranial nerve, petrous apex cephalocele does not contain other neuronal tissues including the brain parenchyma. This entity is usually regarded as an incidental finding and believed to occur when CSF pressure caused over petrous apex roof dehiscence. On imaging, cephaloceles follow the CSF density or intensity on CT and MR imaging, respectively (Figure 3) (1, 3, 6).
Effusion and Petrous Apicitis
Petrous apex cells have communication with the middle ear; therefore, they are involved with similar disorders including inflammation and obstruction. Following a middle ear infection, a sterile fluid accumulation can be seen in petrous apex air cells; however, in the antibiotic era, extension of middle ear infection into the pneumatized petrous apex is rare. In addition to secondary involvement, isolated/primary mucosal inflammation and fluid retention can also occur due to viral agents and allergens. Isolated petrous apex effusion is a relatively frequent incidental imaging finding, and usually no follow-up or treatment is necessary (4). On imaging, fluid accumulation follows the CSF density and intensity on CT and MR imaging, respectively, with the preservation of internal septations (Figure 4). However, T1 signal can be intermediate or high depending on the protein content of the fluid, which can cause confusion with cholesterol granuloma and mucocele. Lack of contrast enhancement and trabecular disruption can be helpful in differentiation from these lesions (5).
Mucocele is a rare inflammation that is believed to result from air cell obstruction. Accumulation of mucoid material within the obstructed air cells causes expansion, remodeling, and destruction of the bony septations. This disorder rarely primarily occurs in the petrous apex; instead, it usually extends from the adjacent structures. CT of the temporal bone reveals opacification of the air cells that is indistinguishable from cholesterol granuloma. Mucoceles have high T2 signal intensity with variable T1 signal on MR imaging (3, 7, 8). It can also show peripheral enhancement without restriction in diffusion (Figure 5) (5).
Cholesterol granuloma is the most frequent adult petrous apex disorder that may occur in the mastoid, middle ear, and petrous apex; however, it is an uncommon petrous apex lesion in children. Its cause is unknown and believed to occur as an air cell obstruction, followed by mucosal edema, hemorrhage, inflammatory reaction, and bony expansion. Incidental small petrous apex cholesterol granulomas can be found on imaging studies that were performed for other symptoms. These asymptomatic lesions do not need any intervention until they progressively enlarge and cause symptoms. CT demonstrates an expansile, well-defined mass within the petrous apex with cortical thinning. Cholesterol granulomas classically have high signal intensity on both T1- and T2-weighted MR sequences due to accumulation of blood products and proteinaceous debris (Figure 6). They also stay hyperintense after fat-suppressed MR sequences (8).
Cholesteatoma or epidermoid cyst is a cystic mass-like lesion originating from the aberrant ectoderm trapped within the petrous apex. It can be seen at any part of the temporal bone including petrous apex. On imaging, they are expansile lesions with variable bone destruction. Typically, they have high T2- and low T1-weighted signal on conventional MR imaging sequences. Diffusion restriction is characteristic and helpful for distinguishing from mucocele (9). Half-Fourier acquisition single-shot turbo spin-echo DW imaging has high sensitivity and specificity in primary diagnosis and follow-up of possible residual/recurrence following surgery (10). They also may show subtle peripheral contrast enhancement (Figures 7, 8) (5).
Imaging plays a pivotal role in assessing petrous apex lesions and normal variants. An understanding of the variants, pitfalls, and patterns of disease processes of the petrous apex facilitates diagnosis and staging. HRCT and MR imaging including DW sequences allow detailed evaluation of this region.
Peer-review: Externally peer-reviewed.
Author Contributions: Concept - S.I., I.B., M.K.; Design - S.I., I.B., M.K.; Supervision - M.K.; Data Collection and/or Processing - S.I., I.B., M.K..; Analysis and/or Interpretation - S.I., I.B., M.K.; Literature Search - S.I., I.B., M.K.; Writing - S.I., I.B., M.K.; Critical Review - M.K.
Conflict of Interest: The authors have no conflicts of interest to declare.
Financial Disclosure: The authors declared that this study has received no financial support.
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(9.) Larson TL, Wong ML. Primary mucocele of the petrous apex: MR appearance. AJNR Am J Neuroradiol 1992; 13: 203-4.
(10.) Ilica AT, Hidir Y, Bulakbasi N, Satar B, Guvenc I, Arslan HH, et al. HASTE diffusion-weighted MRI for the reliable detection of cholesteatoma. Diagn Interv Radiol 2012; 18: 153-8.
Selami Ince (1) [iD], Ilke Beyitler (2) [iD], Murat Kocaoglu (3) [iD]
(1) Clinic of Radiology, Beytepe Murat Erdi Eker State Hospital, Ankara, Turkey
(2) Department of Pediatrics, Near East University School of Medicine, Nicosia, Cyprus
(3) Department of Radiology, Near East University School of Medicine, Nicosia, Cyprus
ORCID IDs of the authors: S.I. 0000-0002-6611-4962; I.B. 0000-0002-7758-9015; M.K. 0000-0002-4674-8634.
Corresponding Author: Ilke Beyitler
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|Author:||Ince, Selami; Beyitler, Ilke; Kocaoglu, Murat|
|Publication:||Cyprus Journal of Medical Sciences|
|Date:||Dec 1, 2018|
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