Look for asymmetry in facial shape or in depth of furrows such as the nasolabial fold. Also look for asymmetries in spontaneous facial expressions and blinking. Ask patient to smile, puff out their cheeks, clench their eyes tight, wrinkle their brow, and so on. Old photographs of the patient can often aid your recognition of subtle changes.
Check taste with sugar, salt, or lemon juice on cotton swabs applied to the lateral aspect of each side of the tongue. Like olfaction, taste is often tested only when specific pathology is suspected, such as in lesions of the facial nerve, or in lesions of the gustatory nucleus (nucleus solitarius).
Facial weakness can be caused by lesions of upper motor neurons in the contralateral motor cortex or descending central nervous system pathways, lower motor neurons in the ipsilateral facial nerve nucleus (CN VII) or exiting nerve fibers, the neuromuscular junction, or the face muscles. Note that the upper motor neurons for the upper face (the upper portions of the orbicularis oculi and the frontalis muscles of the forehead) project to the facial nuclei bilaterally (see Neuroanatomy Through Clinical Cases, Figure 12.12). Therefore, upper motor neuron lesions, such as a stroke, cause contralateral face weakness sparing the forehead, while lower motor neuron lesions, such as a facial nerve injury, typically cause weakness involving the whole ipsilateral face.