Causes and Risk Factors
Bell’s palsy has historically been considered an idiopathic condition (ie, a condition that arises spontaneously or from an obscure or unknown cause), but it is now known to be closely associated with exposure to certain viruses (Ferri 2014; Holland 2004; Ronthal 2013a). The virus most closely associated with Bell’s palsy is herpes simplex virus type 1 (HSV-1). One study found that 11 of 14 patients with Bell’s palsy (79%) had evidence of infection with HSV-1 in the region affected compared to none of the controls (Murakami 1996).
HSV-1 and other members of the herpes family of viruses are notable for having two distinct stages in their life cycles. During the first stage, also known as productive infection, the virus is actively replicating and making new copies of itself. Afterward, the virus is able to go into the second phase, known as latent infection. During this time, copies of the virus that are not actively replicating remain in the sensory neuron and cause no symptoms of infection. However, the virus may become reactivated and give rise to an active infection (Taylor 2002). Several studies suggest the reactivation of latent infection with herpes viruses, including HSV-1 and herpes zoster virus (chickenpox), represents a major initiating factor of Bell’s palsy (Holland 2004). Other infectious agents may be involved in some cases as well, including adenovirus, Coxsackievirus, cytomegalovirus, Epstein-Barr virus, mumps, and rubella (Ferri 2014; Morgan 1992; Zandian 2014).
Some people may have a genetic predisposition for Bell’s palsy as well (Ronthal 2013a). Reports of families with multiple cases of Bell’s palsy among immediate relatives suggest there may be some hereditary aspects of Bell’s palsy (Desanto 1969; Hageman 1990; Clement 2000).
Bell’s palsy can occur at any age, but it most commonly affects people between the ages of 15 and 50 (Monini 2010; Peitersen 2002; Zandian 2014). Diabetes increases the risk of developing Bell’s palsy and is present in 5-10% of people with Bell’s palsy (Zandian 2014; Ronthal 2013a). Pregnancy as well as recent upper respiratory ailments or a weakened immune system can also increase the risk of Bell’s palsy (Baugh 2013; Ferri 2014; Monini 2010).