|ECR 2019 / C-3474|
|Petrous apex lesions: pictorial review and algorithmic diagnostic approach|
The petrous apex is a pyramid-shaped structure that is formed by the medial portions of the temporal bone. It is obliquely positioned within the skull base, with its apex pointing anteromedially and its base located posterolaterally.
What makes the petrous apex anatomically complex is its medial location in the skull base and its intimate relationship to other clinically important structures.
The petrous apex is bounded by the inner ear structures laterally, the petro-occipital fissure medially, the petrosphenoidal fissure anteriorly, and the posterior cranial fossa behind.
The superior surface is formed by the middle cranial fossa. Along the inferior surface are the jugular bulb and inferior petrosal sinus. The internal auditory canal (IAC) bisects the petrous apex into a large anterior portion that typically contains bone marrow and a smaller posterior portion that is derived from the otic capsule.
A number of identifiable vascular and neural channels are contained within the petrous apex. The petrous carotid canal and IAC are the largest channels traversing or bordering the petrous apex, but the Dorello canal, subarcuate canal, singular canal, and Meckel cave are smaller channels that are also seen reliably on high-resolution thin-section CT or MR images.
The IAC is located within the midportion of the petrous apex and houses the vestibulocochlear and facial nerves. It is generally directed anterolaterally from the cerebellopontine angle cistern, with its cisternal opening (the porus acusticus) located along the posteromedial edge of the petrous bone (Fig 1).
The petrous carotid canal lies within the anterior portion of the petrous apex and contains the horizontal portion of the petrous segment of the internal carotid artery (ICA), which passes over the foramen lacerum.
The Dorello canal extends through the posteromedial portion of the petrous apex and contains the abducens nerve (cranial nerve VI).
The subarcuate canal (also referred to as the petromastoid canal) contains the subarcuate artery, which supplies blood to the bony labyrinth, facial canal, and mastoid antrum.
The singular canal is a small channel that contains the singular nerve, which is a division of the inferior vestibular nerve that innervates the ampulla of the posterior semicircular canal.
The Meckel cave is a dura-lined diverticulum that contains the trigeminal (or gasserian) ganglion and the rootlets of the trigeminal nerve (cranial nerve V).
The anterior portion of the petrous apex is filled with marrow in approximately 60% of temporal bones (Fig 2), pneumatized in 33%, and sclerotic in 7%.
Lesions arising in or spreading to the petrous apex cause varied and occasionally severe clinical squealer. Given this variability, petrous apex lesions cannot be diagnosed accurately on the basis of clinical findings alone.
Cross-sectional imaging with computed tomography and magnetic resonance (MR) imaging plays an important role in diagnosis and characterization of lesions occurring there.
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