A high riding jugular bulb indicates the dome (roof) of the jugular bulb extends more superiorly in the petrous temporal bone than is typical. The transverse level above which a jugular bulb is considered high riding has been variably defined as the following 1,6,8:
- floor of the internal acoustic meatus (internal auditory canal) 2,4
- 2 mm below the floor of the internal acoustic meatus 3
- inferior rim of the round window niche 4
- inferior rim of the tympanic annulus 5 or floor of the external auditory canal
- superior tympanic annulus 6
- basal turn of the cochlea 7
Depending on the definition, the prevalence in different series varies 6-34%.
It need not be asymmetrically larger than the contralateral bulb, but usually is. The associated enlargement of the jugular foramen can simulate a mass lesion on non-contrast-enhanced CT, although the diagnosis is made easily on contrast-enhanced CT or flow-sensitive MR imaging.
The sigmoid (jugular) plate normally separates the jugular bulb, whether high riding or not, from the hypotympanum of middle ear cavity. This can only be appreciated on thin slice bone algorithm CT and is too thin to appreciate on MRI. If the sigmoid plate is deficient, the bulb is free to protrude into the middle ear cavity and is then known as a dehiscent jugular bulb, a common cause of a retrotympanic vascular mass.
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