How common is a dehiscent jugular due to missing sigmoid? jugular bulges into middle ear. could this be causing my dizziness?

Dizzyness: jugular bulb abnormalities such as bone dehiscence and bulging into the inner ear are common. They are present in up to 11% of individuals and are acquired by age 40. In 1/3 of cases, high riding JB erodes the inner ear, most frequently involving the vestibular aqueduct. In one study, 40% had verigo and nearly half had hearing loss. Tinnitus is also common. You need vestibular testing by ENT.

Jugular bulb: I would also add that you need a scan of that area to really check it out fully

Not common: It could be causing your dizziness. But you need some tests first. Vestibular testing can tell you if the balance center that is in the same side of the dehiscence is causing dizziness. If it us then it might be. I would seek the opinion of an Otologist (ENT who just does ear surgery and has extra training in it)