Lesions of the Petrous Apex: Pictographic Distinction at CT and MR Imaging

The petrous apex is the medial region of the temporal bone, it is divided by the internal auditory canal (IAC), and is surrounded by neurovascular structures, from which, various pathological lesions originate. They can cause severe symptoms by invasion or compression of the cranial nerves, brain stem or internal carotid artery.

This structure is defined laterally by the otic capsule and the petrous carotid artery, it is the floor of the middle cranial fossa and extend from the eminentia arcuata to Meckel's cave anteriorly and from the semicircular canal to the clivus posteriorly. The jugular fossa, the carotid canal and the petrous sinus delimitate the inferior border. The superior petrous sinus marks the limit between the middle cranial fossa and the posterior cranial fossa. The internal auditory canal bisects the petrous apex, this is important to consider surgical techniques.

These lesions can present with several symptoms, the most common that have been reported are hearing loss followed by vestibular dysfunction, headache, tinnitus, facial spasm, diplopia, facial paralysis and otorrhea; this presented months or years before diagnosis. Incidental discovery is not unusual.

The patient is then referred for a guided imaging study, which allows characterizing the lesions detected incidentally or by their symptoms in previous studies. The first line of study is the MR because it allows delineating the extension, demonstrating if there is a compromise of the cavernous sinus, and the relationship of the lesion with the cranial nerves and vessels. It also allows a definitive diagnosis of certain apex lesions that have characteristic signal intensities, such as cholesterol granuloma. On the other hand, CT is useful in characterizing bone involvement better than MR, determining if the lesion is osteolytic or if it results in bone remodeling, which helps intuit the degree of aggressiveness.

To make a pictorial description of the different pathologies that may occur in this region, we performed a retrospective and anonymous review of the images by MR and/or CT of sixteen patients in whom pathologies of the petrosal apex have been identified at the Chilean Institute of Neurosurgery Dr. Asenjo.