A positive finding usually indicates the patient has eroded horizontal semicircular canal, through which the pressure is being transmitted to the labyrinthine fluid and stimulating macula and ampullary crests.
Why is specifically horizontal semicircular canal affected?
Horizontal semicircular canal lies in close relation to the tympanic cavity or the middle ear cavity, so it’s wall is more vulnerable to erosion and fistula formation than other parts of the labyrinth due to disease processes or cholesteatomas in the tympanic cavity.
But a negative finding here doesn’t exclude the possibility of fistula. It can be negative also in the case of a non-responsive labyrinth or dead labyrinth so that large change in pressure also can’t stimulate it to cause a nystagmus or vertigo.
What can cause non-responsive or dead labyrinth?
Among many causes most common are –
2. Treatment of Meniere’s disease
B. There are two exceptions related to fistula test which appear as false positive findings –
- In congenital syphilis, the stapes footplate is very mobile. So it acts as a piston and allows large fluctuation of pressure in the tympanic cavity to get transmitted to the labyrinth.
- In Ménière’s disease, fibrous bands are formed inside the labyrinth and some of these bands connect the stapes footplate to the macular structures. When stapes moves they stimulate the macula.
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