Diagnosis of a Perllymph Fistula
Apart from the usual methods of presentation which are well known, we would only like to emphasize the necessity of comparing the bone conduction of the operated with the bone conduction of the unoperated ear at follow-up.
Bone conduction of the operated ear has fallen below B.C. of unoperated ear after an initial good result.
It is well known that the cochlear function of the unoperated ear deteriorates faster than that of the operated ear quite apart from the Carhart correction. This probably is due to disuse, as cochlear otosclerosis and presbyacusis should normally have similar effects on both sides. If at any time we see an audiogram where the B.C. of the operated ear after an initial good result drops below the b.c . of the unoperated ear on long term follow-up, we would explore for fistula.
At surgery, the piston must be removed and if the endothelial tube surrounding the piston is isolated on dry gelfoam under high magnification, a drop of perilymph can be identified emerging from it.
The commonest cause of a fistula and late S.N. loss is a long piston.
The piston should project no more than 0.2 mm into the vestibule. This accuracy is only possible with the small 0.4 mm or medium 0.8 mm fenestra technique as you have the edge of the footplate as a reference point for judging the piston length. Even if the initial measurement is perfect, the piston can be pulled in by contraction of fibrous adhesions from the lenticular process to the promontory.
The second cause of fistula is inadvertent excessive footplate removal.
If a large part of the footplate is hooked out, the rounded edge of the oval window makes this grade of accuracy of measurement very difficult. Late bulging of the blue membrane which forms, which is directly proportional to the extent of footplate removal, will almost inevitably result in late S.N. loss due to a fistula even if soft tissue interposition has been used. So, in case of inadvertent total/subtotal footplate removal, a direct piston technique with or without soft tissue interposition is contra-indicated.
Please note that what you put around the piston whether it is gelfoam or connective tissue is not material, as the perilymph leak that develops is submucosal.
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