Clinically, Bell's palsy is primarily characterized by the dysfunction of peripheral seventh cranial nerve (i.e. facial nerve), leading to a complete or partial facial paralysis. The classic sign of the disease is the involvement of only one side of the face with a sudden onset. Both sides of the face are simultaneously affected in less than 1% of cases. Symptoms usually peak in less than 48 hours and pain behind the ear sometimes occurs as a preceding symptom.
The paralysis includes the forehead and lower aspect of the face. The forehead loses the ability to furrow, and the corner of the mouth droops on the affected side. The eyelids will not close and when the patient attempts to close them, the eyeball can roll upward (this is known as Bell's phenomenon). Although tear production decreases, the loss of lid control allows tears to spill freely from the eye, thus creating the appearance of excessive tearing. Loss of sensation along the external auditory canal sometimes also occurs, which is known as a positive Hitselberger sign.
Parts of the cranial nerve that supply the ear and the tongue can also be affected, resulting in inappropriate reaction to loud noises (hyperacusis) and a loss of taste on the frontal two-thirds of the tongue. Normal lip movement is usually heavily affected by Bell's palsy, resulting in the inability to purse the lips and show the teeth on the affected side. Facial spasm can rarely occur, more likely as a result of fatigue or stress, and usually in patients in their fifth or sixth decades of life.
A large proportion of patients report numbness on the side of the face affected by paralysis. Although this symptom could be attributed to the lack of mobility of the facial muscles, it could also indicate a secondary involvement of the trigeminal nerve. General (albeit mild) contracture of the facial muscles can result in a narrower fissure of the eyelids of the affected side when compared to the opposite part. This manifestation usually occurs after several months.
It is of utter most importance to differentiate Bell’s palsy from a stroke. Both of these conditions can cause unilateral paralysis; however, presence of other symptoms can be used to distinguish between them. Bell's palsy affects only the facial nerve; hence the symptoms are limited to facial symptoms (muscle paralysis or weakness, taste problems and decreased tearing). On the other hand, stroke can cause additional problems in active speaking or understanding others, paralyze the arm or leg on the same side and severely impair vision quality. In essence, Bell's palsy affects the peripheral nerve and causes weakness of the eyelid and (most importantly) forehead, while stroke represents a problem with central nervous system and typically affects only muscles of the lower face.