PATHOLOGY
The primary cause of Bell's palsy is an ischaemia of the facial nerve near the stylomastoid foramenprobably caused by arteriolospasm
As a result of this, various structural alterations take place in the facial nerve between the lateral semicircular canal and the stylomastoid foramen and in some cases also in the surrounding bone. The extent of these alterations depends upon the severity of the interruption of blood supply, which may be short and transient, or prolonged. In the following the
pathological anatomy, the pathogenesis and the etiology will be surveyed separately.
Pathological anatomy
Owing to the markedly benign nature of the disease it was not until 1932, when Ballance and Duel 4 proposed decompression of the facial nerve in cases of Bell's palsy, opening the Fallopian canal from the stylomastoid foramen to the lateral semicircular canal and splitting the nerve sheath, that an opportunity was offered to inspect the nerve and its surroundings
in view. By this examination new ways were paved for the study of the disorder.
The alterations observed in the nerve, in the mastoid cells, and in the Fallopian canal shall be described separately.
a) The facial nerve, in fresh and relapsing palsies, becomes unduly constricted at the stylomastoid foramen and edematous proximal to this point. Cawthorne describes how, in 4 0 out 42 cases, tiny haemorrhagic streaks were seen running longitudinally upwards within the nerve sheath.
On splitting of the sheath the nerve may be seen to bulge like a hernia through the slit.
In only 4 cases so far described has the opportunity been offered to examine microscopically the facial nerves of patients suffering from Bell's palsy,