Answer:
Etiologic tests include routine mastoid films may show destructive lesions, opacification of the mastoid air cells, or widening of the internal auditory canal. High resolution CT may show intracranial, intratemporal, or extratemporal tumors in addition to the fine bony detail within the facial nerve canal within the temporal bone. MRI with gadolinium may help differentiate pathologic (acute inflammation/edema) and non-pathologic changes within the facial nerve.
Prognostic tests include the nerve excitability test (NET) with a nerve stimulator. The test is subjective. The maximal stimulation test (MST) uses the same equipment, but the stimulus is increased until the patient experiences discomfort – also subjective with 30% false positive rate and 10% false negative. Electroneurography (ENOG) involves delivering a current to the stylomastoid area to maximally stimulate the facial muscles. The evoked potentials are recorded at the nasolabial fold. The test is objective and records nerve conduction block as it is occurring. It is the most accurate test for predicting return of function, although it is expensive and time consuming. EMG does not become positive until 14 to 21 days from the onset of paralysis. In cases of delayed recovery and for late prognostication in complete nerve paralysis, serial EMG’s may have a role.
Topographic tests help to localize the intratemporal site or extent of involvement of the facial nerve, and are indicated when a tumor is suspected and directed radiographic studies of a specific segment of the nerve or needed, or preop to determine whether the lesion is distal (approached thru the mastoid) or proximal (middle/posterior cranial fossa or translabyrinthine approach) to the geniculate ganglion. These include the Schirmer test for lacrimation, stapedial reflex, and CT or MRI.