PHYSIOLOGY OF THE VESTIBULAR APPARATUS
Fistula Test. This test is of use in cases in which the bony wall of the lateral
semicircular canal has been eroded by disease, with exposure of the endosteum.
The condition is known as 'fistula' of the lateral canal. The reaction to the
fistula test presupposes a live or functioning labyrinth. The test is carried out
as follows: an olive-shaped ear-piece attached by means of rubber tubing to a
valveless Politzer bag, or the ear-piece of a Siegle speculum, is fitted tightly
into the meatus. The patient is directed to look straight forward. By squeezing
Fig. 152. Caloric test, showing thermostatically controlled tanks.
A ^o' -A ^o
Fig. 153. Calorigrams. A, Normal; B, Directional preponderance to left; C, Predominance of left
labyrinth (canal paresis of opposite side).
the rubber bag the air pressure in the meatus is raised, and the pressure is
transmitted to the fluid in the labyrinth through the gap caused by the
erosion. By releasing the bag without removing the ear-piece the pressure in
the meatus is diminished. The direction of the nystagmus produced in this
way cannot always be accurately predicted, but the effect of rarefaction is
always the opposite of that due to condensation. In the 'typical' fistula
symptom, on compression, nystagmus is produced to the diseased side—the
slow movement being to the opposite side. In the 'reversed' fistula symptom,