Bell's palsy is the most common cause of acute unilateral facial paralysis, accounting for approximately 60-75% of such cases. The right side is generally affected more often, i.e. 63% of the time. Although bilateral facial paralysis can also occur, the occurrence rate is less than 1% when compared to unilateral Bell's palsy, and it accounts for 23% of all conditions presenting with bilateral paralysis of the face. The condition can also be recurrent in 4-14% of affected individuals.
The majority of large population studies reveal a yearly incidence of 15-30 cases per 100.000 persons. The annual incidence of Bell's palsy in the United States is approximately 23 cases per 100.000 persons, and in the United Kingdom 20 cases per 100.000 persons. Nevertheless, certain countries show greater variability in the incidence of this condition; the highest number of cases was found in a study conducted in Ehime Prefecture in Japan, while Sweden had the lowest reported incidence. The condition is more characteristic for winter season, largely because lower temperatures are associated with an overall higher risk.
Bell's palsy affects both sexes equally, although the condition is more frequent in younger women (aged 10-19) compared to the same age group of men. Pregnancy can increase the risk threefold, and in pregnant women Bell's palsy most commonly appears in the third trimester. Preeclampsia (disorder of pregnancy manifesting with high blood pressure and proteins in the urine) is also shown to increase the risk.
The condition is generally more common in adults. Peak ages are between 20 and 40 years of age, although this disease occurs in children and elderly population over 70 years as well. The rate basically increases with age up to the fourth decade, and then remains steady until the old age, when growth of incidence is noted again. Hence a somewhat higher prevalence rate is found in people older than 65 years of age (59 cases per 100.000 persons). In children younger than 13 years of age the incidence rate is markedly lower (13 cases per 100,000 persons).
Diabetes and prediabetes are conditions frequently associated with Bell's palsy. The risk of being affected by this condition is 29% higher in diabetic patients than in healthy individuals. Impaired glucose tolerance and insulin resistance are also significantly more common in patients with Bell's palsy, thus an early stage of diabetes can occasionally be detected upon the onset of this condition. Patients with hypothyroidism and other autoimmune processes of the thyroid gland also represent a susceptible group.
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