How likely is an inner ear infection to become meningitis?

That is, if untreated - does it just go to the brain?

That isn’t meningitis, it’s sometimes a cranial infection/abscess (carried from the inner ear through the skull, into the brain cavity) which may require a mastoidectomy and some brain surgery (not very invasive and usually only to relieve the abscess around the surface of the frontal lobe). Also heavy doses of antibiotics will be needed. The procedure may need to be repeated, hearing loss is inevitable although may be quite minor if you’re lucky, immediate side effects include sporadic, temporary epilepsy.

The likelihood of it occurring is 1/20,000 of the general population. Therefore the potential for misdiagnosis is high.

First, a bit of a nitpick. I think you mean to be saying ‘middle ear’ and not ‘inner ear’. Look here for a good picture of the anatomy. The inner ear is encased by bone and is unlikely to get infected. On the other hand, middle ear infections (otitis media) are really, really common (anyone with a little kid knows that).

Referring to the linked image, you’ll see that the middle ear is separated from the outer ear (and thence to the world) by a mere membrane - the tympanic membrane, or ear drum. I mention this because if there is an infection in the middle ear, the natural ‘cure’ is for the infection to pierce through the ear drum and thereby drain to the outside. Although it’s not always the case, the ear drum usually heels after that happens.

How likely does a middle ear infection lead to meningitis? Well, according to this page of the Medscape article on complications of otitis media the overall incidence of ALL intracranial complications (i.e. including meningitis AND other complications developing within the skull) is 0.36 percent; about one in three hundred. So, the rate of meningitis is even less than that. In fact, the original article on which that figure was based says that meningitis occurred in 43 out of the 87 patients who developed an intracranial complication. In other words, about half of them. Bottom line, then, is that around 0.18 percent of people with otitis media get meningitis. Phrased differently, that’s about 1 in 550 people.

(Note that much of the data in the article I cited above is over 30 years old. And, it’s from Thailand. I suspect that more contemporary figures based on data from the developed world would yield a much lower rate of meningitis following otitis media.)

I’ve just spent a few minutes looking over a number of more recent papers, from European centres, describing the rate of meningitis and other intracranial infections following otitis media. Unfortunately, I can’t link to any of them.

What is clear from looking at them is that the rate of meningitis following middle ear infection is much, much lower than the 1 in 555 I mentioned above. None give a precise value which I assume is because the actual number of people with otitis media is impossible, or at least very difficult, to determine. Still, a figure for meningitis complicating otitis media of about 1 in 10,000 or even less seems reasonable. In fact, in a previously normal person, who’s never had trauma to the middle ear, and has had no problem with prior complications from otitis media such as cholesteatoma, the rate would seem to be even much less.