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Why Temporal Bone?

Because it is difficult and I am bad at it.
It is important to be familiar with the microanatomy to be able to differentiate between normal anatomy and pathologic conditions.
As you can imagine, like the anatomy, the pathology is also micro so findings may not be as apparent if you are not aware of what normal looks like.
This paper reviews the anatomy and function of the temporal bone aqueducts, canals, clefts, and nerves, as well as the relevant developmental, inflammatory, and neoplastic processes that affect each structure.

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Brief Overview of the “Macroanatomy”

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Vestibular Aqueduct

The Vestibular aqueduct is a funnel-shaped canal that connects the medial surface of the Vestibule to the posterior aspect of the petrous pyramid at the operculum. Within the vestibular aqueduct is the endolymphatic duct. At the terminus of the endolymphatic duct is connected to endolymphatic sac which sits externally to the posterior portion of the petrous bone.

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Enlarged Vestibular Aqueduct

An enlarged vestibular aqueduct (EVA) is a common cause of hearing loss among pediatric patients, occurring in up to 15% of patients with mixed hearing loss
The definition of EVA is based on the Cincinnati criteria: ≥1.0 mm diameter at the vestibular aqueduct midpoint, or ≥2.0 mm at its operculum

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Enlarged Vestibular Aqueduct

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Endolymphatic sac tumors

These are locally aggressive neoplasms that are associated with Von Hippel-Lindau syndrome, although sporadic cases also occur.

The typical presentation is hearing loss, tinnitus, and vestibular complaints; however, more advanced cases may also present with facial paralysis and other cranial neuropathies.

Findings typically include a tumor centered in the external aperture of the vestibular aqueduct, with irregular bony margins

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Endolymphatic Sac Tumors

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Cochlear Aqueduct

This is a canal connecting the subarachnoid space with the scala tympani (part of the cochlea).

Functionally, this likely plays a role in balance of pressure between the inner ear and CSF.

Pathology of this is controversial, but it seems that one clinically relevant hypothesis about the cochlear aqueduct is that this canal is the site of spread from bacterial meningitis in labyrinthitis ossificans.

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Petromastoid Canal

This canal runs through the arcade of semicircular canals.
Within the canal is subarcuate artery and vein, which carry the blood supply to the part of the otic capsule that includes the semicircular canals, mastoid air cell mucosa, and parts of the facial nerve canal (fallopian canal) and vestibule.
On CT, the canal can be identified in 100.0% of cases in which the slice thickness is 1 mm; this drops to 79.2% of cases imaged at 2 mm slice thickness. It has a distinctive “inverted V” appearance on coronal oblique images.
In children, the petromastoid canal can be wider, over 2mm even. It is important not to confuse this with pathology. However, it is clinically significant as it creates a route for infection to get from the mastoid to the cranial vault. The canal achieves its narrow adult form by the age of 2-5 years.

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Petromastoid Canal

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Singular Canal

The singular canal extends from the posterior margin of the internal auditory canal (IAC) to the posterior semicircular canal.
The ampullary nerve aka singular nerve runs through this canal. This nerve transmits afferent signals from the semicircular canals, so it plays a role in balance and position.
The radiologic importance of this canal is that it is sometimes confused for a fracture and it is used as an anatomic landmark for singular neurectomy which can be performed as a treatment option for Benign Paroxysmal Positional Vertigo (BPPV).

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Singular Canal

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Cochlear Cleft

C-shaped lucency in the otic capsule that extends from the cochlea to the promontory in a plane parallel to the basal turn of the cochlea.
Only seen in pediatric patients, typically under the age of 3, and conspicuity decreases with age.
Speculated to be a remnant of petrous apex development.
Important to be aware that this is a benign finding and should not be confused with a fracture or other pathologies.

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Cochlear Cleft

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Anatomic Variants

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Tympanomeningeal Fissure (aka Hyrtl’s Fissure)

An embryologic structure that is continuous with, and provides direct communication between, the posterior cranial fossa and fossa of the round window
Closes at 24-26 weeks in normal development
Incomplete closure is a normal variant that may allow persistent communication between the hypotympanum and the subarachnoid space.
The existence of this communication can be pathologic: spontaneous CSF otorrhea, bacterial meningitis, and meningoceles due to Hyrtl’s fissure have all been reported.

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Tympanomeningeal Fissure (Hyrtl’s Fissure)

AJNR case report of a 6-year-old female with spontaneous CSF otorrhea

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Foramen of Huschke (aka Foramen Typmanicum)

A rare developmental defect in the anteroinferior external auditory canal, posterior to the ipsilateral temporomandibular joint (TMJ).
Due to failed fusion of the anterior and posterior processes that make up the external auditory canal.
Not exactly a “foramen” because no anatomic structures pass through it.
Imaging findings can range from a defined defect to focal thinning of the bone
Affected patients can be predisposed to otalgia, TMJ pathology, and herniation of soft tissue from the TMJ into the EAC when the patient’s mouth is closed.

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Foramen of Huschke

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Internal Auditory Canal Diverticula

These are cup-shaped, well-defined osseous lucencies along the anteroinferior margin of the lateral IAC (remember Foramen of Huschke was anteroinferior margin of the EAC).

Fairly common – noted to be in approximately 5% of patients.

IAC diverticula are benign normal variants in bone formation, but have been found to have an association with otosclerosis. So identification on MRI may prompt follow up with a CT to rule out otosclerosis

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Internal Auditory Canal Diverticula

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Cranial Nerve Branches

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Facial Nerve Overview

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Facial Nerve Branches

Along the facial nerve course, multiple branches arise: the greater superficial petrosal nerve (GSPN), lesser petrosal nerve, and external petrosal nerve from the geniculate ganglion, and the nerve to the stapedius muscle, sensory branch of the facial nerve, and chorda tympani nerve from the mastoid segment.
Many pathologies of the facial nerve are commonly observed, including Bell’s palsy, schwannomas, and geniculate venous malformations but similar pathologic findings can occur along the facial nerve branches, so being aware of the typical location branch points can be helpful for localizing these pathologies.

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Jacobson’s Nerve

The first branch of the glossopharyngeal nerve is the tympanic branch, or Jacobson’s Nerve, which transmits secretory innervation to the parotid gland.
Jacobson’s nerve emerges from the inferior GPN ganglion, immediately below the jugular foramen outlet (pars nervosa), and follows a vertical/superior course and into cochlear promontory. There, it joins with the lesser petrosal nerve to supply the Otic ganglion. Other branches of Jacobson’s nerve innervate the oval and round windows and Eustachian tube.
Pathologically, it is best known for its predilection of paragangliomas (glomus tympanicum). These are histologically benign, but can cause local destruction. So soft tissue mass along the promontory with bony erosion and “salt and pepper” appearance on T2 = Glomus Tympanicum.
Care must be taken not to mistake these for aberrant ICA’s which course along the promontory.

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