The seventh cranial nerve controls the facial muscles on one side of the face, taste to the front portion of the tongue, lacrimal gland secretions (tears) and a tiny muscle inside the ear. The diagnosis of Bell’s palsy is only made when an obvious causative process, such as tumour, Lyme disease or Varicella Zoster (shingles affecting the ear canal) is excluded. It occurs in about 1:70 adults in their lifetime, and is more common in pregnancy and diabetes. The cause is not known but the reason the nerve is so susceptible is that it passes through a restrictive boney canal beneath the ear. It is thought that the process involves swelling of the nerve within the canal, which leads to loss of function. The onset is usually fairly rapid, overnight sometimes, and recovery varies between 21 days and six months. Generally the milder the palsy the better chance there is of full recovery. 90% have a full recovery at one year. In about 5% there is only very poor recovery.
Symptoms
Facial weakness, which is one sided, loss of forehead furrows, inability to raise the eyebrow, weakness around the mouth, hyperacusis (loss of sound response in the ear), ectropion – the lower lid turns out – and loss of taste sensation. The main problem as far as eye doctors are concerned, is the ability to shut the eye properly; leading to exposure of the cornea, which in severe cases, can lead to permanent corneal damage, infection, scarring and loss of sight.
Investigations
Often an MRI scan will be performed to exclude a compressive lesion.
Blood tests will be done to exclude certain viral infections.
Treatment
The mainstay of treatment is Prednisolone, a steroid tablet, which is given starting with a high dose and tapering over ten days. Often an antiviral such as Aciclovir or Valaciclovir will be given.
Managing the eye problems
Sometimes just a lubricating drop or gel during the day and ointment at night will be sufficient until the weakness recovers.
If the cornea is becoming affected taping at night may be effective. If this is ineffective a temporary tarshorraphy may be required; this involves suturing the outer margin of the lids together. It can be reversed after nerve recovery.
If there is persistent weakness, and recovery is not expected, gold weights can be inserted into the eyelid to help with closure. If there is an ectropion and the lid turns out, the lid can be tightened.
The eyebrow can also be surgically lifted.
Complications
Other than failure to recover, the main issue is aberrant nerve regeneration problems. As the nerve heals, new fibers can be misdirected causing anomalies such as the eyelid closing when whistling, eating or talking. If the nerve is misdirected to the lacrimal gland, the eye may water when eating, leading to so called “crocodile tears”.
