2. Opening of the labyrinth should be delayed if Bárány’s and other tests show that the labyrinth is not yet destroyed:—
(a) If, in spite of clinical symptoms pointing to involvement of the labyrinth, pus be found under tension within the tympanic cavity or the mastoid process.
(b) If the symptoms before operation consist only of attacks of vertigo and nystagmus, and on operation merely an erosion of the outer wall of the labyrinth (usually the external semicircular canal) is discovered.
In the above cases, if the symptoms be due to irritation of the labyrinth, a rapid recovery is to be expected as a result of the mastoid operation. If, however, they continue or become progressively worse, then the wound cavity must be reopened and the labyrinthine wall carefully examined and further operation undertaken.
The reader may again be reminded that although exploration of the labyrinth is indicated when it is certain that a suppurative lesion exists, yet it is a very serious mistake to open up a labyrinth not yet infected.
Although a great advance has been made in the last few years with regard to operations on the labyrinth, yet there is still much to be learnt, not only with regard to the indications for operation but the result obtained by operation. Now that operations on the labyrinth have become universal, the general tendency is to operate on the immediate occurrence of symptoms of labyrinthine irritation without waiting to see whether simple opening of the mastoid process will not be sufficient—a matter much to be regretted.
Surgical Anatomy. The facial canal, it will be remembered, extends horizontally backwards above the promontory, and passes downwards superficially to the inferior portion of the vestibule which lies between the fenestra ovalis below and ampullary ends of the external and superior semicircular canals above. The nerve then extends directly downwards towards the stylo-mastoid foramen, being situated deeply within the posterior meatal wall.
Of the semicircular canals the external is the most prominent, and the only one visible during the performance of the ordinary mastoid operation; its outer border forms the inner and lower boundary of the aditus, and can usually be recognized as a white eminence. The superior semicircular canal can only be seen on careful removal of the overlying bone; its ampullary end is found lying just above that of the external canal. It forms the highest point of the labyrinth, becoming fused with the innermost portion of the tegmen tympani, and is in such close relationship with the upper surface of the petrous bone as to cause a smooth elevation on its surface. It is at this point in the operation of removal of the semicircular canal that the greatest risk is encountered of breaking through the petrous bone and of injuring the dura mater.
The posterior semicircular canal lies at right angles to the external canal, and is best exposed by careful removal of bone just posterior to the latter (see [Fig. 240]).
The outer half of the first whorl of the cochlea is formed by the promontory. Anteriorly it is in close relationship with the carotid canal, whilst below it lies the dome of the jugular fossa. Medially the modiolus is only separated from the internal auditory meatus by a very fine rim of brittle bone, which can easily be broken; a mishap which may permit of escape of the cerebro-spinal fluid, and also of possible infection of the meninges through the internal meatus.