FUNCTION -- Facial Movement and Sensation of Taste (Anterior 2/3 of Tongue)
TEST I -- Facial Expression
The test involves simultaneous bilateral contraction of facial muscles. On command, the patient should be able to raise the eyebrows and wrinkle the skin of the forehead. Next, the patient must be able to maintain closure of the eyes while the clinician attempts to open the eyes of the patient using the thumb and forefinger. Subsequently, the patient must puff out the cheeks without permitting air to escape through the lips, even with mild pressure applied to the cheeks by the fingers of the clinician. The patient must show the teeth (perhaps while smiling) and scowl or frown.
INTERPRETATION
Unilateral loss of strength and skill affecting the entire half of the face is often interpreted as a facial nerve palsy. Other common features of this condition are facial asymmetries, particularly affecting the eye and the mouth, with the affected eye commonly appearing to be open more widely than the unaffected eye. Dryness of the eye may also lead to corneal abrasion, and the patient may find perceive sounds as unpleasantly loud when presented to the ear ipsilateral to the affected side of the face. On the other hand, preserved lacrimatory function may culminate in watering of the eye because of the inability of the patient to blink. Because of the weakness of muscles controlling the lips, drooling is also frequently evident. More localized motor deficits affecting the face may stem from isolated injuries to select branches of the facial nerve. Alternatively, cerebral damage may culminate in weakness affecting the muscles controlling lower portions of the face (i.e., below the eye).
PITFALLS AND SAFEGUARDS
Careful assessments of accompanying signs and symptoms and full documentation of the relevant history are vital to accurate diagnosis related to dysfunctional facial muscles. The most significant element of the diagnosis relates to successful differentiation of central versus peripheral pathology.
TEST II -- Gustation
The sense of taste, as it relates to the anterior 2/3 of the tongue. To assess this function, the patient is asked to stick out the tongue, as the clinician announces that he/she will place something on the tongue for identification by the patient.
INTERPRETATION
Failure of the patient to identify the substance placed on the tongue should be followed with an additional test (using a subjectively dissimilar substance). Note that the tongues of tobacco users may be chronically insensitive to flavor. Also, superficial burns of the tongue (e.g., stemming from excessively hot beverages) may transiently impair gustation.
PITFALLS AND SAFEGUARDS
Assessments of sensations of taste from the tongue are relatively insensitive to lateralized dysfunction. The chief reason is that substances are most flavorful when in solution, with the fluid being supplied either exogenously (in the form of a vehicle) or endogenously (from the salivary glands). The fluidic interior of the mouth is not amenable to a restricted assessment of only one side of the tongue. It is best to test the anterior 2/3 of the tongue using sweet or salty substances, with mildly acidic substance also being acceptable but suboptimal. Bitter substances, however, are better detected from receptors located beyond the receptive field of cranial nerve VII (i.e., on the back of the tongue or in the deep recesses of the mouth, even down the throat). Complementing a formal test of the gustatory sense, the history should include questions concerning persistent sensations of taste (e.g., metallic) in the absence of flavorful stimuli.