It is easy to diagnose facial palsy when fully developed, but sometimes extremely difficult to detect it at the beginning. This is true especially when the condition develops slowly—for instance, in cases of a tumor of the angle, disease of the pons or insidiously progressive lesions of the pyramidal tract. Also, it is sometimes difficult to state how completely a previous facial palsy has subsided or to find the last traces of an old facial palsy and thus to prove that such a palsy existed at all. In either instance, the common signs of facial palsy may fail to help, as they are not sufficiently fine.
Especially, all tests of motor function of the facial muscles (strength, rapidity and range of movement) give results that are difficult to appraise. Slight asymmetry of the face or the presence of contractures further increases the difficulties.
More information can sometimes be obtained by
Wartenberg R. A SIGN OF FACIAL PALSY. Arch NeurPsych. 1939;41(3):586-588. doi:10.1001/archneurpsyc.1939.02270150160015