A middle ear infection affects all structures of the hearing organ situated behind the ear drum and inside the tympanic cavity. The latter is a bony hollw inside the skull bone containing the three ossicles: hammer, anvil and stirrup.
The symptoms are very similar to those of a typical inflammation of the ears. The animal scratches the ears, shakes the head or rubs it against objects. Occasionally, it might attract attention that the animal is hard of hearing. In rare cases, the general condition is impaired. As complication of the inflammation of the middle ear, a damage of the neighboring nerves may evolve what may manifest particularly in form of the Horner's syndrome.
In most cases an inflammation of the middle ear is caused by migrated bacteria which either enter through the eardrum (e.g. after infestation of the auditory canal) into the area of the middle ear, or maybe climbed up from the mouth through the Eustachian tube. The latter may e.g. happen as complication of tonsillitis.
For successful elimination, antibiotics need to be given for a longer period of time. It must be examined whether an inflammation developed as result of another disease. If that is the case, it must be treated where necessary.
In case the dog has the symptoms mentioned before, you should as soon as possible, in case of severe developments and pains, quickest possible consult a veterinarian.