FISTULA TEST
This is the test done to detect a fistula in the bony labyrinthine wall of the inner ear.
Principle:
Pressure changes in the external auditory canal are transmitted to the labyrinth via the fistula which produces nystagmus.
Fistula Procedure
Pressure changes in the external auditory canal are brought by applying intermittent pressure on the tragus or by Siegel’s speculum. In a normal person the test is negative because the pressure changes cannot be transmitted into the labyrinth (when there is no fistula).
Interpretation:
- In fistula over the dome of lateral semicircular canal increased pressure causes conjugate horizontal deviation of eye to the normal side. As pressure is maintained jerk nystagmus develops with the fast component towards the affected ear. As pressure is released, eyes return to normal.
- Fistula of lateral semicircular canal (anterior to the ampulla) causes deviation of eyes to the affected side.
- Vestibular erosion causes rotatory horizontal nystagmus towards the diseased ear.
- Fistula of the posterior semicircular canal causes vertical movements of eyes.
- A positive fistula also implies that the labyrinth is still functioning.
- Fenestration surgery (Post-stapedotomy).
- Round window rupture.
False positive fistula test: Here the test is positive inspite of absence of fistula:
- Congenital syphilis- here stapes footplate is hypermobile.
- Meniere’s disease (Hennebert’s sign)- It is due to the fibrous bands connecting utricular macula to the stapes footplate.
- Hypermobile stapes footplate.
False negative fistula test: Inspite of fistula being present the test is negative or normal in cases of:
- Cholesteatoma covering the fistula- does not allow pressure changes to be transmitted to the labyrinth.
- Dead labyrinth.
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